Biology Paper 2 AQA NEW SPEC

The regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes.

Homeostasis maintains optimal conditions for enzyme action and all cell functions.

In the human body, these include control of:

• blood glucose concentration
• body temperature
• water levels

These automatic control systems may involve nervous responses or chemical responses.

All control systems include:

• cells called receptors, which detect stimuli (changes in the environment)
• coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from receptors
• effectors, muscles or glands, which bring about responses which restore optimum levels.

CNS:

• In vertebrates (animals with backbones), this consists of the brain and spinal cord.

SENSORY NEURONES:

• The neurones that carry information as electrical impulses from the receptors to the CNS.

MOTOR NEURONES:

• The neurons that carry electrical impulses from the CNS to effectors.

EFFECTORS:

• All your muscles and glands, which respond to nervous impulses.

• Organisms need to respond to stimuli (changes in the environment) in order to survive.
• A single-celled organism can just respond to its environment, but the cells of multicellular organisms need to communicate with each other first.
• So, as multicellular organisms evolved, they developed nervous and hormonal communication systems.

Receptors:

• They are the cells that detect stimuli.
• They are found IN the sensory organs for eg. taste receptors are found on the tongue or sound receptors found in the ears.
• Can form part of larger, complex organs, e.g. the retina of the eye is covered in light receptors.

Effectors:

• Respond to nervous impulses and bring about a change.
• Muscles and glands are known as effectors.
• Muscles contract and glands secrete hormones.

stimulus -> receptor -> sensory neurone -> CNS -> motor neurone -> effector -> response

• Negative feedback counteracts changes, bringing the conditions back to normal.
• For eg:
• Receptor detects a stimulus and the level is too high. The coordination centre recieves and processes the information and organises a response. The effector produces a response which counteracts the change and restores the optimum level - the level decreases. As a result, the receptor once again detects a stimulus where the level is too low and the cycle repeats again.
• Automatic.

• Synapses connect neurones, they are the gap between neurones.
• The nerve impulse passes across this gap through chemicals that diffuse across the gap.
• The electrical signal travels along an axon and triggers the release of chemical transmitters from the nerve ending of the first neurone.
• These diffuse across the gap and make the second neurone re-transmit the electrical signal.

• Reflex actions are automatic and rapid; they do not involve the conscious part of the brain.
• When a stimulus is detected by the receptors, impulses are sent along a sensory neurone to the CNS.
• When the impulses reach a synapse between the sensory neurone and the relay neurone, they trigger a chemical transmitter to be released. These chemicals cause impulses to be sent along the relay neurone.
• The same occurs at the synapse between the relay neurone and a motor neurone.
• The impulses then travel along a motor neurone to the effector. 
• The muscle then contracts.

• receptor detects a stimulus - change in the environment -> sensory neurone sends signal to relay neurone -> motor neurone sends signal to effector -> effector produces a response

1) Use your weaker hand. Sit down on the chair with good upright posture and eyes looking across the room.

2) Place the forearm of your weaker arm across the table with your hand overhanging the edge of the table.

3) Partner holds a ruler vertically with the bottom end (0cm) in between your thumb and first finger. Practice holding the ruler with those two fingers.

4) Partner takes hold of the ruler and asks you to remove fingers.

5) Partner will hold the ruler so the 0 mark is level with top of your thumb. Prepare to catch.

6) Parner drops the ruler without telling you. You must catch as fast as possible. Look at the number level with the top of your thumb. Continue to repeat experiment. Swap with partner.

• Using computers.
• More precise, they reduce the possibility of a human error. 
• Record time in milliseconds, more accurate.

• The brain controls complex behaviour.
• It is made of billions of interconnected neurones and has different regions that carry out different functions.

Cerebral cortex:

• Consciousness
• Intelligence
• Memory
• Language

Cerebellum:

• Muscle coordination and balance

Medulla:

• Unconscious activities like breathing and heartbeat - heart rate and breathing rate

Hypothalamus:

• regulating centre for temperature and water balance

Studying patients with brain damage:

• if a small part of the brain has been damaged, the effect this has on the patient can tell you a lot about what the damaged part of the brain does.

Electrically stimulating the brain:

• can be stimulated electrically by pushing tiny electrode ino the tissue.
• observing what stimulating different parts of the brain does, an idea of what the different parts does can be obtained.

MRI Scanes:

• magnetic resonance imaging scanner produces detailed picture of the brain structures.
• use it to find out what areas of the brain are active when people are doing things like listening to music and trying to recall a memory.

• The complexity and delicacy of the brain makes investigating and treating brain disorders very difficult.

• SCLERA - tough, supporting wall/layer of the eye.
• CORNEA - transparent outer layer found at the front of the eye, refracts light into the eye.
• IRIS - contains muscles that allow it to control the pupil and how much light enters the eye.
• PUPIL - the space between the iris.
• LENS - focuses the light onto the retina.
• RETINA - contains receptor cells which are sensitive to light intensity and colour.
• CILIARY MUSCLES AND SUSPENSORY LIGAMENTS - control the shape of the lens.
• OPTIC NERVE - carries impulses from the receptors in the retina to the brain.

• When light receptors in the eye detect a very bright light, a reflex is triggered that makes the pupil smaller.
• The circular muscles in the iris contract and the radial muscles relax.
• This reduces the amount of light that can enter the eye.
• The opposite occurs in dim light where the circular muscles relax and the radial muscles contract, making the pupil wider.

Near:

• the ciliary muscles contract
• the suspensory ligaments loosen
• the lens is then thicker and refracts light rays strongly.

Distant: 

• the ciliary muscles relax
• the suspensory ligaments are pulled tight
• the lens is then pulled thin and only slightly refracts light rays

As you get older, your eyes' lens loses flexibility so it can't easily spring back to a round shape. This means that light can't be focused well for near viewing, so older people often have reading glasses.

• Two common defects of the eyes are myopia (short sightedness) and hyperopia (long sightedness) in which rays of light do not focus on the retina.
• Hyperopia: lens is in the wrong shape and doesn't refract light enough or the eyeball is too short. Images of nar objects are brought into focus behind the retina. You can use glasses with a convex lens (curves outwards) to correct it. The lens refracts the light rays so they focus on the retina. 
• Mytopia: lens is in the wrong shape and refracts the light too much or eyeball is too long. Images of distant objects are brought into focus in front of the retina. You can use glasses with a concave lense (curves inwards) to correct it, so the light rays focus on the retina.

Contact lenses:

• Thin lenses that sit on the surface of the eye and are shaped to compensate for the fault in focusing. 
• Popular: lightweight, almost invisible, convenient in sports.
• Two types, hard lenses and soft lenses. Soft lenses are more comfortable but carry a higher risk of eye infections than hard lenses.

Laser eye surgery:

• Laser can be used to vaporise tissue, changing the shape of the cornea and how strongly light is refracted. 
• Slimming it down can make it less powerful and can improve short sight. Changing the shape can make it more powerful and improve long sight.
• Risk of complications.

Replacement lense surgery:

• Natural lense of the eye is removed and an artificial lense, made of clear plastic, is inserted in its place.
• Higher risks than laser eye surgery, like possible damage to the retina leading to loss of sight.

• Body temperature is monitored and controlled by the thermoregulatory centre in the brain.
• The thermoregulatory centre contains receptors sensitive to the temperature of the blood.
• The skin contains temperature receptors and sends nervous impulses to the thermoregulatory centre. 

• Temperature receptors detect that core body temperature is too high.
• Thermoregulatory centre acts as the coordination centre, receiving information from the temperature receptors and triggering the effectors automatically. 
• Effectors eg. sweat glands produce sweat and counteract the change.
• Body cools down.
• The process repeats again because the temperature receptors detect that the core body temperature is too low.

• Antagonistic effectors oppose each other's actions.
• Eg. where one effector heats and the other cools - work at the same time to achieve a very precise temperature.
• Allows for a more sensitive response.

• Hairs lie flat.
• Blood vessels dilate (vasodilation), so more blood flows close to the surface of the skin.
• Sweat is produced from the sweat glands and evaporates.
• Both these mechanisms cause a transfer of energy from the skin to the environment.

• Hairs stand up to trap an insulating layer of air, they erect.
• Blood vessels constrict (vasoconstriction).
• Sweating stops and skeletal muscles contract (shiver). This needs respiration which transfers energy to warm the body.

• Hormones are chemical messengers that travel through the bloodstream.
• They are carried in the blood to other parts of the body, but only affects particular cells in particular organs (target organs).
• Hormones control things in organs and cells that need constant adjustment.
• Hormones are produced in and secreted by various glands, called endocrine glands.

• Pituitary glands - in the brain is a ‘master gland’ which secretes several hormones into the blood in response to body conditions. These hormones in turn act on other glands to stimulate other hormones to be released to bring about effects.
• Thyroid gland - in the throat produces thyroxine, which is involved in regulating things like the rate of metabolism, heart rate and temperature.
• Ovaries (females) - produces oestrogen, which is involved in the menstural cycle.
• Testes (males) - produces testosterone, which controls puberty and sperm production in males.
• Adrenal gland - in kidneys produces adrenaline, which is used to prepare the body for 'fight or flight'. 
• Pancreas - produces insulin which is used to regulate the blood glucose level.

Hormones:

• Slower action.
• Transmitted chemically through the blood.
• Affects any organ (widespread).
• Long-lasting effect.
• Can bring about specific chemical changes and regulates metabolism.

Nerves:

• Faster action.
• Transmitted through electrical impulses.
• Acts on a very precise area: the particular muscles or glands.
• Short-term effect.

• Quick response is nervous. Some information needs to be passed on to the effectors really quickly (e.g. pain signals or information from your eyes telling you about a lion heading your way), so it's no good using hormones to carry the message as they're too slow.
• Long time responses are hormonal. For example, when you get a shock, a hormone called adrenaline is released into the body (causing the fight-or-flight response, where your body is hyped up ready for action. You can tell it's a hormonal response because you feel a bit wobbly afterwards.

• Too much thyroxine is released into the blood.
• Raises BMR.
• Causes an increase in the formation of glucose.
• Increased rate of respiration.

• Blood glucose concentration is monitored and controlled by the pancreas.
• If the blood glucose concentration is too high, the pancreas produces the hormone insulin that causes glucose to move from the blood into the cells.
• In liver and muscle cells excess glucose is converted to glycogen for storage.
• Insulin and glycogen control blood sugar level in a negative feedback cycle.
• If blood glucose level is too high, insulin is added. Glucose moves from blood into liver and muscle cells.
• If blood glucose level is too low, glucagon is added. Glucose is released into the blood by the liver.
• If the blood glucose concentration is too low, the pancreas produces the hormone glucagon that causes glycogen to be converted into glucose and released into the blood.

• Type 1 diabetes is a disorder in which the pancreas fails to produce sufficient insulin.
• This means a person's blood glucose level can rise to a level that can ill them.
• It is characterised by uncontrolled high blood glucose levels and is normally treated with insulin injections most likely at mealtimes.
• This makes sure that glucose is removed from the blood quickly once the food has been digested, stopping the level getting too high.
• Effective treatment.
• The amount of insulin that needs to be injected depends on the person's diet and how active they are.
• As well as insulin therapy, people with Type 1 diabetes need to think about limiting the intake of food rich in simple carbohydrates, e.g. sugars (which cause the blood glucose to rise rapidly) and taking regular exercise (which helps to remove excess glucose from the blood.

• In Type 2 diabetes the body cells no longer respond to insulin produced by the pancreas.
• A person becomes resistant to their own insulin.
• Can cause blood sugar level to rise to a dangerous level.
• A carbohydrate controlled diet and an exercise regime are common treatments, as well as getting regular exercise.
• Obesity is a risk factor for Type 2 diabetes.

• The kidneys make urine by taking waste products out of your blood.
• Substances are filtered out of the blood as it passes through the kidneys. This process is called filtration.
• Useful substances like glucose, some ions and the right amount of water are then absorbed back into the blood. This process is called selective reabsorption.

• Water leaves the body via the lungs during exhalation.
• Water, ions and urea are lost from the skin in sweat.
• There is no control over water, ion or urea loss by the lungs or skin.
• Excess water, ions and urea are removed via the kidneys in the urine.

• The digestion of proteins from the diet results in excess amino acids which need to be excreted safely.
• In the liver these amino acids are deaminated - when the excess amino acids are converted into fats and carbohydrates which can be stored - form ammonia.
• Ammonia is toxic and so it is immediately converted to urea for safe excretion from the kidneys as urine.

• Ions such as sodium are taken into the body in food, and then absorbed into the blood.
• If the ion (or water) content of the boody is wrong, this could upset the balance between ions and water, meaning too much or too little water is drawn into the cells by osmosis.
• Having the wrong amount of volume can damage cells or mean they don't work as normal.
• Some ions are lost in sweat, however, this amount is not regulated, so the right balance of ions in the body must be maintained by the kidneys.
• The right amount of ions is reabsorbed into the blood after filtration and the rest is removed from the body in urine.

• The body has to constantly balance the water coming in and against the water going out.
• We lose water from the skin in sweat and from breathing out.
• We can't control how much we lose in these ways, so the amount of water is balanced by the amount we consume and the amount removed by the kidneys in urine.

• The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water.
• As blood passes through the capillary at the start of the nephron, small molecules are filtered out and pass into the nephron tubule. These small molecules include glucose, urea, ions and water. However, large molecules, such as blood proteins, are too big to fit through the capillary wall and remain in the blood.

• Having filtered out small molecules from the blood - many of which are essential to the body - the kidneys must reabsorb the molecules which are needed, while allowing those molecules which are not needed to pass out in the urine. Therefore, the kidneys selectively reabsorb only those molecules which the body needs back in the bloodstream.
• The reabsorbed molecules include:
• all of the glucose which was originally filtered out
• as much water as the body needs to maintain a constant water level in the blood plasma
• as many ions as the body needs to maintain a constant balance of water and mineral ions in the blood plasma
• The reabsorption of water takes place by osmosis. The reabsorption of glucose and mineral ions - from the nephron to the blood capillary - takes place by active transport.
• The cells which make up the wall of the nephron are adapted by having a folded membrane (providing a large surface area) and a large number of mitochondria (to supply the energy for active transport).

• The molecules which are not selectively reabsorbed (the urea and excess water and ions) continue along the nephron tubule as urine . This eventually passes down to the bladder.
• In carrying out these processes, the kidney is able to fulfil its functions of regulating the water and ion balance of the blood plasma, as well as keeping the level of urea low.

• The water level in the body is controlled by the hormone ADH which acts on the kidney tubules.
• ADH is released by the pituitary gland when the blood is too concentrated and it causes more water to be reabsorbed back into the blood from the kidney tubules.
• The brain monitors the water content of the blood and instructs the pituitary gland to release ADH into the blood according to how much is needed.
• This is controlled by negative feedback.

Water content is too high:

• A receptor in the brain detects that the water content is too high.
• The coordination centre in the brain receives the information and coordinates a response.
• The pituitary gland releases less ADH, so less water is reabsorbed from the kidney tubules.
• Water content decreases.

Water content is too low:

• A receptor in the brain detects that the water content is too low.
• The coordination centre in the brain receives the information and coordinates a response.
• The pituitary gland releases more ADH, so more water is reabsorbed by the kidney tubules.
• Water content increases.

• In a dialysis machine, the person's blood flows between the partially permeable membranes, surrounded by a dialysis fluid. 
• The membranes are permable to things like ions and waste substances, but not to big molecules like proteins, like the membranes in the kidney.
• The dialysis fluid has the same concentration of dissolved ions and glucose as healthy blood.
• This means that useful dissolved ions and glucose won't be lost from the blood during dialysis. 
• Only waste substances like urea and excess ions and water diffuse across the barrier.

• At the moment, the only cure for kidney failure is to have a kidney transplant.
• Healthy kidneys are usually transplanted from people who have died suddenly.
• The person who died has to be on the organ donor register or carry a donor card.

Advantages:

• Available to all kidney patients (no shortage)
• No need for immune-suppressant drugs
• Can be used until donor organ is found.

Disadvantages:

• Patient must limit their salt and protein intake between dialysis sessions
• Expensive for the NHS
• Regular dialysis sessions to keep the concentration of dissolved substances in the blood at normal levels – impacts on the patient’s lifestyle. 3 times a week, 3-4 hours.
• Can cause blood clots or infections.

Advantages:

• Patients can lead a more normal life without having to watch what they eat and drink
• Cheaper for the NHS overall

Disadvantages:

• Must take immune-suppressant drugs which increase the risk of infection
• Shortage of organ donors
• Kidney only lasts 8-9 years on average
• Any operation carries risks
• Long waiting lists

• At puberty, your body starts releasing sex hormones that trigger off secondary sexual characteristics.
• In men, the main reproductive hormone is testosterone, produced by the testes and stimulates sperm production.
• In women, the main reproductive hormone is oestrogen, produced by the ovaries, as well as bringing about physical canges, oestrogen is also involved in the menstrual cycle.
• At puberty eggs begin to mature and one is released approximately every 28 days. This is called ovulation.

Stage 1, days 1-4:

• Menstruation starts. The lining of the uterus breaks down.

Stage 2, day 4-14:

• The lining of the uterus builds up again into a thick spongy layer full of blood vessels, ready to receive a fertilised egg.

Stage 3, day 14:

• An egg develops and is released. Ovulation.

Stage 4, day 14-28:

• The wall is then maintained.
• If no fertilised egg has landed on the uterus wall by day 28, the spongy lining starts to break down and the whole cycle begins again.

• FSH - produced in the pituitary glands, causes an egg to mature in one of the ovaries - follicles - and stimulates the ovaries to produce oestrogen.
• Oestrogen - produced in the ovaries, maintains the lining of the uterus and stimulates the release of LH (causes release of egg) and inhibits the release of FSH.
• LH - produced by the pituitary gland, stimulates the release the release of an egg at day 14 (ovulation).
• Progesterone - produced in the ovaries by the remains of the follice after ovulation, maintains the lining of the uterus until the second half of the cycle, when the level of progesterone falls, the lining breaks down and inhibits the release of LH and FSH.

• Fertility can be controlled by a variety of hormonal and non-hormonal methods of contraception.
• Oestrogen can be used to prevent the release of an egg - so it can be used as a method of contraception.
• This may seem kind of strange since naturally oestrogen helps stimulate the release of eggs. But if oestrogen is taken every day to keep the level of it permanently high, it inhibits the production of FSH, and after a while egg development and production stop and stay stopped.
• Progesterone also reduces fertility, e.g. by stimulating the production of thick mucus which prevents any sperm getting through and reaching an egg.

• An oral pill containing oestrogen and progesterone.
• It's over 99% efficient at preventing pregnancy, but it can cause side effects like headaches and nausea and it doesn't protect against STDs.
• There is also the progesterone-only pill - it has fewer side effects, just as effective.

• The contraceptive patch contains oestrogen and progesterone. It's a small patch that's stuck to the skin. Each patch lasts one week.
• The contraceptive implant is inserted under the skin of the arms. It releases a continuous amount of progesterone, which stops the ovaries releasing eggs, makes it hard for sperm to swim to the egg, and stops any fertilised egg implanting in the uterus. An implant can last for three years.
• The contraceptive injection also contains progesterone. Each dose lasts 2 to 3 months.
• An intrauterine device (IUD) is a T-shaped device that is inserted into the uterus to kill sperm and prevent implantation of a fertilised egg. There are two main types - plastic IUDs that release progesterone and copper IUDs that prevent the sperm from surviving in the uterus.

• Non-hormonal forms of contraception are designed to stop the sperm from getting to the egg. 
• Condoms are worn over the penis during intercourse to prevent the sperm from entering the vagina.
• There are also female condoms that are worn inside the vagina.
• Condoms are the only form of contraception that will protect against STDs.
• A diaphgram is a shallow plastic cup that fits over the cervix (entrance to the uterus) to form a barrier. 
• It has to be used with spermicide (a substance that disables or kills the sperm).
• Spermicide can be used alone as a form as contraception, but it is not as effective.

• Sterilisation - this involves cutting or tying the fallopian tubes (connect the ovaries to the uterus) in a female or the sperm duct (the tube between the testes and penis) in males. This is a permanent procedure. However, there is a very small chance that the tubes can rejoin.
• 'Natural methods' - pregnncy may be avoided by finding out when in the menstrual cycle the woman is most fertile and avoiding sexual intercourse on those days. It's popular with people who think that hormonal and barrier methods are unnatural, but it's not very effective.
• Abstinence - the only way to be completely sure that the sperm and egg don't meet is not to have intercourse.

• Some women have high levels of FSH that are too low to cause their eggs to mature. 
• This means that no eggs are released and the woman can't get pregnant.
• The hormones FSH and LH can be given to women as a fertility drug to stimulate ovulation.
• Pro: it helps a lot of women get pregnant when they couldn't before.
• Cons: it doesn't always work, some women may have to do it many times which could be expensive. Too many eggs could be stimulated, resulting in unexpected multiple pregnancies (twins, triplets, etc.).

• Involves collecting eggs from the woman's ovaries and fertilising them in a lab using the man's sperms.
• IVF treatment can also involve a technique called Intra-Cytoplasmic Sperm Injection, where the sperm is injected directly into an egg. It's useful if the man has a very low sperm count.
• The fertilised eggs are then grown into the embryos in a laboratory incubator.
• Once the embryos are tiny balls of cells, one or two of them are transferred to the woman's uterus to improve the chance of pregnancy.
• FSH and LH are given before egg collection to stimulate several eggs to mature, so more than one egg can be collected.

• Fertility treatment can give an infertile couple a child.
• Best way for a woman to have a baby using her own eggs.
• Increases the chances of older women conceiving.
• Reasonably safe.
• Can help single women and same sex couples who want children.
• Unused embryos can be donated for research.

• Expensive (£5000 per cycle).
• Not always successful. The average success rate in the UK is about 26%. This makes the process incredibly stressful and often upsetting, especially if it ends with multiple failures.
• Emotionally and physically stressful.
• Increased risk of multiple pregnancies.
• Fertility drugs have side effects.

• Advances in microscope techniques have helped to improve the techniques and therefore the success rate of IVF.
• Specialised micro-tools have been developed to use on the eggs and sperm under the microscope. They're also used o remove single cells from the embryo for genetic testing to see it is healthy.
• More recently, the development of time-lapse imaging (using a microscope and camera built into the incubator) means that the growth of the embryos can be continuously monitored to help identify those that are more likely to result in a successful pregnancy.

• The process of IVF often results in used embryos that are eventually destroyed. Because of this, some people think it is unethical because each embryo is a potential human life.
• The genetic testing of embryos before implantation also raises ethical issues as some people think it could lead to the selection of preferred characteristics, such as gender or eye colour.

• Adrenaline is produced by the adrenal glands, which are just above the kidneys, in times of fear or stress.
• It increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’. 

• Thyroxine from the thyroid gland, in the neck, stimulates the basal metabolic rate (the amount of energy per unit time that a person needs to keep the body functioning at rest).
• It plays an important role in growth and development. 
• If thyroxine levels increase, the pituitary gland reduces the production of thyroid stimulating hormone (TSH).
• Thyroxine levels drop and return to normal.
• If thyroxine levels decrease, the pituitary gland increases the production of thyroid stimulating hormone (TSH).
• Thyroxine levels increase and return to normal.

• Auxin is a plant hormone that controls growth near the tips of shoots and roots.
• It controls the growth of a plant in response to light - phototropism - and gravity - gravitropism/geotropsim).
• Auxin is produced in the tips and moves backwards to stimulate the cell elongation (englargement) process which occurs in the cells just behind the tips.
• If the tips of a shoot is removed, no auxin is available and the shoot may stop growing.
• Extra auxin promotes growth in the shoot but inhibits growth in the roots - producing the desired results.

• When a shoot tip is exposed to light, more auxin accumulates on the side that's in the shade than the side that's in the light.
• This makes the cells grow faster on the shaded side, so the shoot bends towards the light for photosynthesis.

• When a shoot is growing sideways, gravity produces an unequal distribution of auxin in the tip, with more auxin on the lower side.
• This causes the lower side to grow faster bending the shoot upwards.

• A root growing sideways will also have more auxin on its lower side.
• But in a root, the extra auxin inhibits growth. This means the cells on top elongate faster, and the root bends downwards.

1) Put 10 cress seeds into 3 different petri dishes, each lines with moist filter paper.

2) Shine a light onto one of the dishes from above and two of the dishes from different directions.

3) Leave your cress seeds alone for one week until you can observe their responses - and you'll find that the seedlings grow towards the light.

You can also investigate the effect of gravity on plant growth. Just place four seedlings on damp cotton wool in a petri dish, each with their roots pointing in different directions, and store the petri dish vertically for a few days in the dark. You should find that the roots of each seedling grow downwards.

• Number of seeds - use the same number.
• Type of seed - use seeds that all come from the same packet.
• Temperature - keep your petri dish in a place where the temperature is stable.
• Water - use a measuring cylinder to make sure you add the same amount of water.
• Light intensity - keep the distance between the bulb and dish the same.

• Killing weeds - most weeds growing in fields of crops or in a lawn are broad-leaved, in contrast to grasses and cereals which have very narrow leaves. Selective weedkillers have been developed using auxins, which only affects the broad-leaved plants. They totally disrupt their normal growth patterns, which soon kills them, whilst leaving the grass and crops untouched.
• Growing from cuttings with rooting powder - a cutting is a part of a plant that has been cut off it, like the end of a branch with a few leaves on it. Normally, if you stick cuttings in the soil, they won't grow, but if you add rooting powder, which contains auxins, they will produce roots rapidly and start growing as new plants. This enables growers to produce lots of clones of a really good plant very quickly.
• Growing cells in tissue culture - tissue culture can be used to grow clones of a plant from a few of its cells. To do this, hormones such as auxins need to be added to the growth medium along with nutrients to stimulate the cells to divide to form both roots and shoots.

• Gibberellin is another type of plant growth hormone. It stimulate seed germination, stem growth and flowering. Uses include:
• Controlling dormancy - lots of seeds won't germinate until they've been through certain conditions (e.g. a period of cold or of dryness). This is called dormancy. Seeds can be treated with giberellin to alter dormancy and make them germinate at times of year that they wouldn't normally. It also helps to make sure all the seeds in a batch germinate at the same time.
• Inducing flowering - some plants require certain conditions to flower, such as longer days or low temperatures. If these plants are treated with gibberellin, they will flower without any change in their environment. Gibberellin can also be used to grow bigger flowers.
• Growing larger fruit - seedless varieties of fruit (e.g. seedless grapes) often do not grow as large as seeded fruit. However, if gibberellin is added to these fruit, they will grow larger to match the normal types.

• Ethene is a gas produced by aging parts of a plant. It influences the growth of the plant by controlling cell division. It also stimulates enzymes that cause fruit to ripen. 
• Commercially, it can be used to speed up the ripening of fruits - either while they are still on the plant, or during transport to the shops.
• This means that fruit, such as bananas, can be picked while they are still unripe and therefore firmer and less easily damaged. The gas is then added to the fruit on the way to the supermarket so that it will be perfect just as it reaches the shelves.
• Ripening can also be delayed while the fruit is in storage by adding chemicals that block ethene's effect on the fruit or reduce the amount of ethene that the fruit can produce. Alternatively, some chemicals can be used that react with ethene to remove it from the air.

Nucleus -> Chromosomes -> DNA -> Genes

• DNA is the cehmical that all of the genetic material in a cell is made up of.
• It contains coded information - basically all the instructions to put an organism together and make it work.
• What is in your DNA determines what inherited characteristics you have.
• DNA is found in the nucleus of animal and plant cells in really long structures called chromosomes.
• Chromosomes usually come in pairs. DNA is a polymer, it is made up of two strands coiled together in the shape of a double helix.

• A gene is a small section of DNA found on a chromosome.
• Each gene codes for a particular sequence of amino acids which are put together to make a specific protein.
• Only 20 amino acids are used, but they make up thousands of different proteins.
• Genes simply tell cells in what order to put the amino acids together. 
• DNA also determines what proteins the cell produces, e.g. haemoglobin, keratin.
• That in turn determines what type of cell it is, e.g. red blood cell, skin cell.

• The genome of an organism is the entire genetic material of that organism.
• The whole human genome has now been studied and this will have great importance for medicine in the future.
• It allows scientists to identify genes in the genome that are linked to different types of disease.
• Knowing which genes are linked to inherited diseases could help us to understand them better and could help us to develop effective treatments for them.
• Scientists can look at genomes to trace the migration of certain populations of people around the world. All modern humans are descended from a common ancestor who lived in Africa, but humans can now be found all over the planet. 
• The human genome is mostly identical in all individuals, but as different populations of people migrated away from Africa, they gradually developed tiny differences in their genomes.
• By investigating these differences, scientists can work out when new populations split of in a different direction and what route they took.

• DNA strands are polymers made up of lots of repeating units called nucleotides.
• Each nucleotide consists of one sugar molecule, one phosphate molecule and one 'base'.
• The sugar and phosphate molecules in the nucleotides form a 'backbone' to the DNA strands.
• The sugar and phosphate molecules alternate.
• One of four different bases - GCAT - join to each sugar.
• Each base links to a base on the opposite strand in the helix.
• A always pairs up with T, and C always pairs up with G. This is called complementary base pairing. 
• It's the order of bases in a gene that decides the order of amino acids in a protein. 
• Each amino acid is coded for by a sequence of three bases in the gene.
• The amino acids are joined together to make various proteins, depending on the order of the gene's bases.
• There are parts of DNA that don't code for proteins. Some of these non-coding parts switch genes on and off, so they control whether or not a gene is expressed.

• Proteins are made in the cell cytoplasm on tiny structures called ribosomes.
• To make proteins, ribosomes use the code in the DNA. 
• DNA is found in the cell nucleus and can't move out of it because it's really big. So the cell needs to get the code from the DNA to the ribosome.
• This is done using a molecule called mRNA - which is made by copying the code from DNA.
• The mRNA acts as a messenger between the DNA and the ribosome - it carries the code between the two.
• The correct amino acids are brought to the ribosomes in the correct order by carrier molecules.

• Proteins are synthesised on ribosomes, according to a template. Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order.
•  When the protein chain is complete it folds up to form a unique shape. This unique shape enables the proteins to do their job as enzymes, hormones or forming structures in the body such as collagen.
• E.g.
• Enzymes - act as biological catalysts to speed up chemical reactions in the body.
• Hormones - used to carry messages around the body, e.g. insulin is a hormone released into the blood by the pancreas to regulate the blood sugar level.
• Structural proteins - are physically strong. E.g. collagen is a structural protein that strengthens connective tissues like ligaments and cartilage.

• Occasionally, a gene may mutate. A mutation is a random change in an organism's DNA. They can sometimes be inherited.
• Mutations occur continuously. They can occur spontaneously, e.g. when a chromosome isn't quite replicated properly. However, the chance of mutation is increased by exposure to certain substance or some types of radiation.
• Mutations change the sequence of DNA bases in a gene, which produces a genetic variant (different form of the gene). As the sequence of DNA bases codes for the sequence of amino acids that make up a protein, mutations to a gene sometimes lead to changes in the protein that it codes for. 
• Most mutations have very little or no effect on the protein. Some will change it to such a small extent that its function or appearance is unaffected.
• A few mutations code for an altered protein with a different shape. An enzyme may no longer fit the substrate binding site or a structural protein may lose its strength.
• Not all parts of DNA code for proteins. Non-coding parts of DNA can switch genes on and off, so variations in these areas of DNA may affect how genes are expressed.

• Insertions - where a new base is inserted into the DNA base sequence where it shouldn't be. You should remember that every three bases in DNA codes for a particular amino acid. An insertion changes the way the groups of three bases are 'read', which can change the amino acids that they code for. Insertions can change more than one amino acid as they have a knock-on effect on the bases further on in the sequence.
• Deletions - when a random base is deleted from the DNA base sequence. Like insertions, they change the way that the base sequence is 'read' and have knock-on effects further down the sequence.
• Substitutions - substitution mutations are when a random base in the DNA base sequence is changed to a different base.

• Sexual reproduction is where genetic information from two organisms (a father and a mother) is combined to produce offspring which are genetically different to either parent.
• In sexual reproduction, the mother and father produce gametes by meiosis, e.g. egg and sperm cells in animals. Pollen and egg cells in flowering plants.
• In humans, each gamete contains 23 chromosomes - half the number of chromosomes in a normal cell. Instead of having two of each chromosome, a gamete has just one of each.
• The egg (from the mother) and the sperm cell (from the father) then fuse together through fertilisation to form a chell with the full number of chromosomes. 
• Sexual reproduction involves the fusion of male and female gametes. This is because there are two parents, the offspring contains a mixture of their parents' genes.
• This is why the offspring inherits features from both parents - it's received mixture of chromosomes from the mum and dad. This mixture produces variation in the offspring.
• Flowering plants can reproduce in this way too. They have egg cells but their version of a sperm cell is pollen.

• In asexual reproduction, there's only one parent so the offspring are genetically identical to that parent.
• Asexual reproduction happens by mitosis - an ordinary cell make a new cell by dividing into two.
• The new cell has exactly the same genetic information as the parent cell - clone.
• In asexual reproduction there's only one parent. There's no fusion of gametes, no mixing of chromosomes and no genetic variation between parent and offspring. The offspring are genetically identical to the parent, they are clones.
• Bacteria, some plants and some animals reproduce asexually.

• Sexual:
• Involves two organisms.
• Gene mixing causes variation.
• Used by larger more complex organisms e.g. mammals.
• Slow.
• High energy requirement.
• High risk (competition for mate).
• Asexual:
• Involves one organism.
• No variation.
• Used by small, simple organisms e.g. bacteria.
• Quick.
• Low energy requirement.
• Low risk (no competition).

• Gametes only have one copy of each chromosome, so that when gamete fusion takes place, you get the right amount of chromosomes again, two copies of each.
• To make gametes have half the original number of chromosomes, cells divide by meiosis. This process involves two cell divisions. In humans, it only happens in the reproductive organs.

• Before the cell starts to divide, it duplicates its genetic information, forming two armed chromosomes - one arm of each chromosome is an exact copy of the other arm. After replication, the chromosomes arrange themselves into pairs.
• In the first division in meiosis, the chromosome pairs line up in the centre of the cell.
• The pairs are then pulled apart so each new cell only has one copy of each chromosome. Some of the father's chromosomes and some of the mother's chromosomes go into each new cell.
• In the second division, the chromosomes line up again in the centre of the cell. The arms of the chromosomes are pulled apart.
• You get 4 gametes, each with a single set of chromosomes in it. Each of the gametes is genetically different from the other's because the chromosomes all get shuffled up during meiosis and each gamete only gets half of them, at random.

• After 2 gametes have fused during fertilisation, the resulting new cell divides by mitosis to make a copy of itself.
• Mitosis repeats many times to produce lots of new cells in an embryo.
• As the embryo develops, these cells then start to differentiate into the different types of specialised cell that make up a whole organism.

• produces variation in the offspring.
• if the environment changes variation gives a survival advantage by natural selection.
• breed successfully due to being better adapted to the environment and able to breed.
• natural selection can be speeded up by humans in selective breeding to increase food production.

• only one parent needed
• more time and energy efficient as do not need to find a mate
• faster than sexual reproduction
• many identical offspring can be produced when conditions are favourable.

• Mosquitoes - malaria is caused by a parasite that's spread by mosquitoes. When a mosquito carrying the parasite bites a human, the parasite can be transferred to the human. The parasite reproduces sexually in the mosquito and asexually in the human host.
• Fungi - many species of fungus can reproduce both ways. These species release spores, which can become new fungi when they land in a suitable place. Spores can be produced sexually and asexually. Sexually-produced spores introduce variation and are often produced in response to an unfavourable change in the environment, increasing the chance that the population will survive the change.
• Plants - loads of species of plants produce seeds asexually, but can also reproduce asexually. Asexual reproduction can take place in different ways.
• For e.g. strawberry plants produce 'runners'. These are stems that grow horizontally on the surface of teh soil away from a plant. At various points along the runner, a new strawberry plant forms that is identical to the original plant.
• Another e.g. in plants that grow from bulbs, e.g. daffodils. New bulbs can form from the main bulb and divide off. Each new bulb can grow into a new identical plant.

• Leads to genetic variation.
• Variation will allow some organisms to survive environmental changes.
• Variation increases the chances of a population surviving.

• Mitosis:
• Used in asexual reproduction.
• Used to produce body cells.
• 1 nuclear division.
• No variation.
• Diploid.
• 2 genetically identical daughter cells.
• Meiosis:
• Used in sexual reproduction.
• Used to produce gametes.
• 2 nuclear divisions.
• Variation.
• Haploid.
• 4 genetically different daughter cells.

• There are 23 pairs of chromosomes in every human body cell.
• Of these, 22 are matched pairs of chromosomes that just control characteristics.
• The 23rd pair are labelled ** for female sex chromosomes or XY for male sex chromosomes.
• They're the two chromosomes that decide your sex - whether you are male or female.

• Males have an XY chromosome. The Y chromosome causes male characteristics.
• Females have an ** crhomosome. The ** combination allows female characteristics to develop.
• When making sperm, the X and Y chromosomes are drawn apart in the first division of meiosis. There's a 50% chance each sperm cell gets an X chromosome and a 50% chance it gets a Y chromosome.
• A similar thing happens when making eggs. But the original cell has two X-chromosomes, so all the eggs have one X-chromosome.

• The possible combinations of gametes.
• To find the probability of getting a boy or a girl, you can draw a genetic diagram.
• Genetic diagrams are just models that are used to show all the possible genetic outcomes when you cross together different genes or chromosomes.

The male or female reproductive cell that contains half the genetic material of the organism. 

A strand of DNA that is encoded with genes.

A part of the DNA in a cell that controls the physical development, behaviour, etc.

• What genes you inherit control what characteristics you develop. Different genes control different characteristics. Some characteristics are controlled by a single gene, e.g. mouse fur colour and red-green colour blindness in humans.
• However, most characteristics are controlled by several genes interacting.
• A version of a particular gene is an allele which are represented by letters in genetic diagrams.
• You have two versions, alleles, of every gene in your body - one on each chromosome in a pair.
• If an organism has two alleles for a particular gene that are the same, then it's homozygous for that trait. If it's two alleles for a particular gene are different, then it's heterozygous.
• If the two alleles are different, only one can determine what characteristic is present. The allele for the characteristic that's shown is called the dominant allele, capital letter. The other one is recessive, lower case letter.
• For an organism to display a recessive characteristic, both of its alleles must be recessive. But to display a dominant characteristic the organism can be either CC or Cc, because the dominant allele overrules the recessive one if the organism is heterozygous.
• Your genotype is the combination of alleles you have. Your alleles work at a molecular level to determine what characterists you have - your phenotype.

• Genotype - the genetic makeup of an individual for a particular characteristic.
• Phenotype - the physical appearance of an individual for a particular characteristic. 

When you cross two parents to look at just one characteristic.

When there's one dominant allele is crossed with two recessive alleles. Hh and hh.

• Cystic fibrosis is a genetic disorder of the cell membranes. It results in the body producing a lot of thick sticky mucus in the air passages and in the pancreas.
• The allele that causes cystic fibrosis is recessive, carried by about 1 person in 25. 
• Because they're recessive, people with only one copy of the allele will not have the disorder - they're known as carriers.
• For a child to have the disorder, both parents must be either carriers or have the disorder themselves.
• 1 in 4 chance of a child having the disorder if both parents are carriers.

• Genetic disorder where a baby's born with extra fingers or toes. It doesn't usually cause any other problems, so isn't life threatening.
• The disorder is caused by a dominant allele, so it can be inherited if just one parent carries the defective allele.
• The parent that has the defective allele will have the condition too since the allele is dominant.
• There's a 50% chance of the child having the disorder if one parent has one dominant allele.

An allele that causes the phenotype to be expressed even if only one is inherited.

An allele that causes the phenotype to be expressed even if both alleles are inherited.

• Homozygous: an individual with two identical alleles for a characteristic e.g. DD or dd.
• Heterozygous: an individual with two different alleles for a characteristic e.g. Dd.

• During IVF, embryos are fertilised in a laboratory, and then implanted into the mother's womb.
• Before being implanted, it's possible to remove a cell from each embryo and analyse its genes.
• Many genetic disorders can be detected this way such as cystic fibrosis.
• It's also possible to get DNA from an embryo in the womb and test that for disorders.
• There are lots of ethical, social and economic concerns surrounding embryo screening.
• Embryonic screening is quite controversial because fo the decisions it can lead to.
• For embryos produced by IVF - after screening, embryos with 'bad' alleles would be destroyed. For embryos in the womb - screening could lead to the decision to terminate the pregnancy.

• Allows healthy embryos to be identified.
• Reduces health care and government costs.
• Helps detect known genetic disorders.
• Reduces the risk of multiple pregnancies.

• Increased risk of miscarriage.
• False results possible.
• Expensive.
• Involves making difficult decisions.
• Healthcare for children with genetic disorders can be expensive.
• Designer babies.

• Gregor Mendel was an Austrian monk who trained in mathematics and natural history at the University of Vienna.
• On his garden plot at the monastery in the mid 19th century, Mendel noted how characteristics in plants were passed on from one generation to the next.
• The results of his research were published in 1866 an eventually became the foundation of modern genetics.
• One of his observations was that the inheritance of each characteristic is determined by ‘units’ that are passed on to descendants unchanged. 
• In the late 19th century behaviour of chromosomes during cell division was observed.
• In the early 20th century it was observed that chromosomes and Mendel’s ‘units’ behaved in similar ways. This led to the idea that the ‘units’, now called genes, were located on chromosomes.
• In the mid-20th century the structure of DNA was determined and the mechanism of gene function worked out. This scientific work by many scientists led to the gene theory being developed.

First cross = a tall pea plant and a dwarf pea plant -> all tall pea plants.

Second cross = 2 tall pea plants from first cross -> 3 tall pea plants and 1 dwarf pea plant.

• Characteristics in plants are determined by 'hereditary units'
• Hereditary units are passed on to offspring from both parents, one unit from each parent.
• Hereditary units can be dominant or recessive - dominant and recessive, dominant will be expressed.

• Work was cutting edge and new to the scientists of the day.
• They didn't have the background knowledge to properly understand his findings - they had no idea about genes, DNA and chromosomes.
• It wasn't until after his death that people realised how significant his work was.
• Using Mendel's work as a starting point, the observation of many different scientisits have contributed to the understanding of genes that we have today. 

Differences in the characteristics of individuals in a population is called variation and may be due to differences in:

• the genes they have inherited (genetic causes)
• the conditions in which they have developed (environmental causes)
• a combination of genes and the environment.

• All plants and animals have characteristics that are in some ways similar to their parents'.
• This is because an organisms' characteristics are determined by the genes inherited from their parents. 
• These genes are passed on in sex cells, from which the offspring develops.
• Most animals get some genes from the mother and some from the father.
• The combining of genes from two parents causes genetic variation - no two of the species are genetically identical.
• Some characteristics are determined only by genes:
• Eye colour
• Blood group
• Inherited disorders like cystic fibrosis.

• The environment, including the conditions that organisms live and grow in, also causes differences between members of the same species - this is called environmental variation.

• Most characteristic (e.g. weight, height, skin colour, condition of teeth, academic or athletic prowess etc.) are determined by a mixture of genetic and environmental factors.
• For example, the maximum heightthat an animal or plant can grow to is determined by its genes. But whether it actually grows that tall depends on the environment (e.g. how much food it gets).

• Mutations are changes to the sequence of bases in DNA.
• Multations can lead to changes in the protein that a gene codes for.
• Most mutations have no effect on the protein the gene codes for, so most mutations have no effect on the organism's phenotype. Some have a small influence on phenotype, and so only alter the characteristics slightly. However, although it's very rare, mutations can result in a new phenotype being seen in a species.
• If the environment changes, and the new phenotype makes an individual more adapted to the new environment, it can become common throughout the species relatively quickly by natural selection.

A change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species.

The theory of evolution by natural selection states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.

• Darwin knew that organisms in a species show variation within a population in their characteristics (phenotype variation). He also knew that organisms had to compete for limited resources in an ecosystem.
• Darwin concluded that the organisms with the suitable characteristics for the environment would be more successful competitors and would be more likely to survive. This is called 'survival of the fittest'.
• The successful organisms that survive are likely to reproduce and pass on the alleles that made them successful to their offspring.
• The organisms that are less well adapted would be less likely to survive and reproduce, their unfavourable alleles are bred out of the population.
• Over time/millions of years, beneficial characteristics become more common in the population and the species changes, it evolves.

• Darwin's theory wasn't perfect. Because the relevant scientific knowledge wasn't avaible at the time, he couldn't give a good explanation for why the characteristics appeared or exactly how individual organisms passed genes on beneficial adaptations to their offspring.
• We now know that phenotype is controlled by genes. New phenotype variations arise because of genetic variants produced by mutations (changes in DNA). Beneficial variations are passed on to future generations in the genes that parents contribute to their offspring.

• The development of a new species.
• Over a long period of time, the phenotype of organisms can change so much because of natural selection that a completely new species is formed. This is called speciation.
• Speciation happens when populations of the same species change enough to become productively isolated - this means that they can't interbreed and produce fertile offspring.
• If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring they have formed two new species.

When no individuals of a species remain.
Reasons:

• The environment changes too quickly (e.g. destruction of habitat).
• A new predator kills them all (e.g. humans hunting them).
• A new disease kills them all.
• They can't compete with another (new) species for food.
• A catastrophic event happens that kills them all (e.g. a volcanic eruption or a collision with an asteroid.

Example:

• Dodos are now extinct.
• Humans not only hunted them, but introduced them to other animals that ate all their eggs, and we destroyed the forest where they lived...

Darwin published his ideas in On the Origin of Species (1859). There was much controversy surrounding these revolutionary new ideas. The theory of evolution by natural selection was only gradually accepted because:

• the theory challenged the idea that God made all the animals and plants that live on Earth
• there was insufficient evidence at the time the theory was published to convince many scientists
• the mechanism of inheritance and variation was not known until 50 years after the theory was published.

• Other theories, including that of Jean-Baptiste Lamarck, are based mainly on the idea that changes that occur in an organism during its lifetime can be inherited.
• We now know that in the vast majority of cases this type of inheritance cannot occur
• Jean-Baptiste Lamarck argued that changes an organisms aquired during its lifetime will be passed on to its offspring - e.g. he thought that if a characteristic was used a whole lot by an organism, then it would become more developed during its lifetime, and the organism's offspring would inherit the aquired characteristic.
• For example, using this theory, if a rabbit used its leg to run a lot (to escape predators), then its legs would get longer. The offspring of that rabbit would then be born with longer legs.

• Often scientists come up with different hypotheses to explain similar observations.
• Scientists might develop different hypotheses because they have different beliefs or they have been influenced by different people, or they just think differently.
• The only way to find out whose hypothesis is right is to find evidence to support or disprove each one:
• Lamarck's hypothesis was eventually rejected because experiments didn't support his hypothesis. You can see it for yourself, e.g. if you dye a hamster with bright pink, its offspring will still be born with the normal fur colour because the new characteristics won't have been passed on.
• The discovery of genetics supported Darwin's idea because it provided an explanation of how organisms born with beneficial characteristics can pass them on. Other evidence was also found by looking a fossils of different ages, this allows you to see how changes in an organism developed slowly over time. The relatively recent discovery of how bacteria are able to evolve to become more resistant to antibiotics also further supports evolution by natural selection.
• There's so much evidence for Darwin's idea that it's now an accepted hypothesis, a theory.

• Selective breeding (artificial selection) is the process by which humans breed plants and animals for particular genetic characteristics.
• Humans have been doing this for thousands of years since they first bred food crops from wild plants and domesticated animals.
• Selective breeding involves choosing parents with the desired characteristic from a mixed population. They are bred together. From the offspring those with the desired characteristic are bred together. This continues over many generations until all the offspring show the desired characteristic.
• The characteristic can be chosen for usefulness or appearance:
• Disease resistance in food crops.
• Animals which produce more meat or milk.
• Domestic dogs with a gentle nature.
• Large or unusual flowers.
• From existing stock, select the ones which have the characteristics you're after.
• Breed them together. Select the best of the offspring, and breed them together.
• Continue this process over several generations, and the desirable traits gets stronger and stronger. Eventually, all the offspring will have the characteristic.

• Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects.
• It reduces the gene pool - the number of different alleles in a population. This is because the farmer keeps breeding from the 'best' animals or plants - which are all closely related - inbreeding.
• Inbreeding can cause health problems because there's more chance of the organisms inheriting harmful genetic defects when the gene pool is limited. Some dog breeds are particularly succeptible to certain defects because of inbreeding, e.g. pugs often have breathing problems.
• There can also be serious problems if a new disease appears, because there's not much variation in the population. All the stock are closely related to each other, so if one of them is going to be killed by a new disease, the others are also likely to succumb to it. 
• Selective breeding -> reduction in the number of different alleles -> less chance of any resistant alleles being present in the population.

• A process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic.
• In genetic engineering, genes from the chromosomes of humans and other organisms can be ‘cut out’ and transferred to cells of other organisms.
• Enzymes are used to isolate the required gene; this gene is inserted into a vector, usually a bacterial plasmid or a virus
• The vector is used to insert the gene into the required cells
• Genes are transferred to the cells of animals, plants or microorganisms at an early stage in their development so that they develop with desired characteristics.
• In some cases, the transfer of the gene is carried out at an early stage of development. This means that the organism develops with the characteristic coded for by the gene.

• Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes.
• Plant crops have been genetically engineered to be resistant to diseases or to produce bigger better fruits. These are known as Genetically Modified crops or GM crops. GM crops include ones that are resistant to insect attack or to herbicides. GM crops generally show increased yields.
• Sheep have been genetically engineered to produce substances, like drugs, in their milk that can be used to treat human diseases.
• Scientists are researching genetic modification treatments for inherited diseases caused by faulty genes, e.g. by inserting working genes into people with the disease. This is known as gene therapy.

• Genetic engineering is an exciting new area of scence, which has the potential for solving many of our problems (e.g. treating disorders, more efficient food production, etc.) but not everyone thinks that it's a great idea.
• There are worries about the long-term effects of genetic engineering - that changing an organism's genes might accidentally create unplanned problems, which could get passed on to future generations.

• GM crops provide the best way to solve the world’s hunger crisis. E.g. gloden rise is a GM rice crop that contains beta-carotene - lack of this substance causes blindness.
• Improved growth rates of plants and animals.
• Increased food value of crops, GM crops produce much bigger yields than normal crops.
• Crops can be designed to grow well in all conditions.
• Crops can be designed to produce their own pesticide or are resistant to herbicides used to control weeds

• Concerns about GM crops include the effect on populations of wild flowers and insects.
• Some people feel the effects of eating GM crops on human health have not been fully explored. Effects of eating GM produced food are unknown.
• Genes from GM plants and animals might spread into the wildlife of the countryside.
• GM crops were originally made infertile - farmers in poor countries had to buy new seed each year.
• The spreading of infertility genes could cause major problems in the environment.

• Modern medical research is exploring the possibility of genetic modification to overcome some inherited disorders.

• Tissue culture
• Cuttings

• Tissue culture: using small groups of cells from part of a plant to grow identical new plants. This is important for preserving rare plant species or commercially in nurseries.
• This where a few plant cells are put in a growth medium with hormones, and they grow into new plants - clones of the parent plant.
• They can be made very quickly, in very little space and can be grown all year. 

• Cuttings: an older, but simple, method used by gardeners to produce many identical new plants from a parent plant.
• Gardeners can take cuttings from good parent plants, and then plant them to produce identical copies of the parent plant.
• These plants can be produced quickly and cheaply.

• Embryo transplants: splitting apart cells from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers.
• Example, best bull and cow:
• Sperm cells are taken from a prize bull and egg cells are taken from a prize cow. The sperm are then used to artificially fertilise an egg cell. The embryo that develops is then split many times (to form clones) before any cells become specialised.
• These cloned embryos can then be implanted into lots of other cows where they grow into baby calves (which will all be genetically identical to each other.
• Hundreds of 'ideal' offspring can be produced every year from the best bull and cow.

• The nucleus is removed from an unfertilised egg cell.
• The nucleus from an adult body cell, such as a skin cell, is inserted into the egg cell.
• An electric shock stimulates the egg cell to divide to form an embryo.
• These embryo cells contain the same genetic information as the adult skin cell.
• When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development. (Dolly the sheep).

• Cloning gets you lots of 'ideal' offspring. But you also get a 'reduced gene pool' - this means there are fewer alleles in a population. If a population are all closely related and a new disease appears, they could all be wiped out - there may be no allele in a the population, giving resistance to it. 
• But the study of animal clones could lead to greater understanding of the development of the embryo, and of ageing and age-related disorders.
• Cloning could also be used to help preserve endangered species.
• However, its possible that cloned animals may not be as healthy as normal ones, e.g. Dolly the sheep had arthritis, which tends to occur in older sheep.
• Some people worry that humans might be cloned in the future. If it was allowed, any success may follow many unsuccessful attempts, e.g. children born severly disabled.

Fossils are the ‘remains’ of organisms from millions of years ago, which are found in rocks. Fossils may be formed:

• from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent
• when parts of the organism are replaced by minerals as they decay
• as preserved traces of organisms, such as footprints, burrows and rootlet traces.

From gradual replacement by minerals:

• Things like teeth, shells, bones etc., which won't decay easily can last a long time when buried. They're eventually replaced by minerals as they decay, forming a rock-like substance shaped like the original hard part. The surrounding sediments also turn to rock, but the fossil stays distinct inside the rock and eventually someone digs it up.

From casts and impressions:

• Sometimes, fossils are formed when an organism is buried in a soft material like clay. The clay later hardns around it and the organism decays, leaving a cast of itself. An animal's burrow or a plant's roots (rootlet traces) can be preserved as casts. Things like footprints can also be pressed into these materials when soft, leaving an impression when it hardens.

From preservation when decay happens:

• In amber (a clear yellow stone made from fossilised resin) and tar pits, there's no oxygen or moisture so decay microbes can't survive. In glaciers to cold for microbes. Peat bogs, to acidic.

• Many early forms of life were soft-bodied, which means that they have left few traces behind.
• What traces there were have been mainly destroyed by geological activity.
• This is why scientists cannot be certain about how life began on Earth.

We can learn from fossils how much or how little different organisms have changed as life developed on Earth.

• A species is a group of similar organisms that can reproduce to give fertile offspring.
• Speciation is the development of a new species.
• Speciation occurs when populations of the same species become so different that they can no longer successfully interbreed to produce fertile offpsring.

• Isolation and natural selection.
• Isolation is where populations of a species are separated. It can occur due to a physical barrier. E.g. floods and eathquakes can cause barriers that geographically isolate some individuals from the main population,
• Conditions on either side of the barrier will be slightly different, e.g. they may have different climates. Because the environment is different on each side, different characteristics will become more common in each population due to natural selection operating differently on the populations:
• Each population shows genetic variation because they have a wide range of alleles.
• In each population, individuals with characteristics that make them better adapted to their environment have a better chance of survival and so are more likely to breed successfully.
• So the alleles that control the beneficial characteristics are more likely to be passed on to the next generation.
• Eventually, individuals from different populations will have changed so much that they won't be able to breed with one another to produce fertile offspring.
• The two groups will have become seperate species.

Two populations of the same species are present in different areas -> physical barriers separate populations -> populations adapt to new environments -> development of a new species.

• Alfred Russel Wallace independently proposed the theory of evolution by natural selection.
• He published joint writings with Darwin in 1858 which prompted Darwin to publish On the Origin of Species (1859) the following year.
• Wallace worked worldwide gathering evidence for evolutionary theory.
• He is best known for his work on warning colouration in animals and his theory of speciation. For example, he realised that warning colours are used by some species like butterflies to deter predators from eating them and that this was an example of a beneficial characteristic that had evolved by natural selection.
• Alfred Wallace did much pioneering work on speciation but more evidence over time has led to our current understanding of the theory of speciation.

• Bacteria can evolve rapidly because they reproduce at a fast rate.
• Mutations of bacterial pathogens produce new strains.
• Some strains might be resistant to antibiotics, and so are not killed.
• They survive and reproduce, so the population of the resistant strain rises.
• The resistant strain will then spread because people are not immune to it and there is no effective treatment.
• MRSA is resistant to antibiotics = 'superbug'.

To reduce the rate of development of antibiotic resistant strains:

• doctors should not prescribe antibiotics inappropriately, such as treating non-serious or viral infections
• patients should complete their course of antibiotics so all bacteria are killed and none survive to mutate and form resistant strains
• the agricultural use of antibiotics should be restricted.

The development of new antibiotics is costly and slow. It is unlikely to keep up with the emergence of new resistant strains.

• Traditionally living things have been classified into groups depending on their structure and characteristics in a system developed by Carl Linnaeus.
• Linnaeus classified living things into kingdom, phylum, class, order, family, genus and species. Organisms are named by the binomial system of genus and species.
• Katie Please Come Over For Great Snacks
• Kingdom Phylum Class Order Family Genus Species

As evidence of internal structures became more developed due to improvements in microscopes, and the understanding of biochemical processes progressed, new models of classification were proposed.

Due to evidence available from chemical analysis there is now a ‘threedomain system’ developed by Carl Woese. In this system organisms are divided into:

• archaea (primitive bacteria usually living in extreme environments).
• bacteria (true bacteria). E.g. E. coli. Althrough they often look similar to Archaea, there are lots of biochemical differences between them.
• eukaryota (which includes protists, fungi, plants and animals).

These are then subdivided into smaller groups - kingdom, phylum, class, order, family, genus, species.

• Organisms are named according to the binomial system.
• The first part refers to the genus that the organism belongs to. This gives you information on the organism's ancestry. The second part refers to the species. E.g. Humans are known as Homo Sapiens. 'Homo' is the genus and 'sapiens' is the species.
• The binomial system is used worldwide and means that scientists in different countries or who speak different languages all refer to a particular species by the same name - avoiding potential confusion.

• Evolutionary trees are a method used by scientists to show how they believe organisms are related.
• They use current classification data for living organisms and fossil data for extinct organisms.
• They show common ancestors and relationships between species. The more recent the common ancestor, the more closely related the two species and the more characteristics they're likely to share.

An ecosystem is the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment.

The place where an organism lives.

All the organisms of one species living in a habitat.

The populations of different species living in a habitat.

Non-living factors of the environment.

Living factors of the environment.

• To survive and reproduce, organisms require a supply of materials from their surroundings and from the other living organisms there.
• Plants in a community or habitat often compete with each other for light and space, and for water and mineral ions from the soil.
• Animals often compete with each other for food, mates and territory.

Within a community each species depends on other species for food, shelter, pollination, seed dispersal etc. If one species is removed it can affect the whole community. This is called interdependence.

The dependence of two or more people or things on each other.

A stable community is one where all the species and environmental factors are in balance so that population sizes remain fairly constant.

• Moisture level
• Light intensity
• Temperature
• Carbon dioxide levels - plants
• Wind intensity and direction
• Oxygen level - aquatic animals
• Soil pH and mineral content

• A change in the environment could be an increase or decrease in an abiotic factor, e.g. an increase in temperature. These changes can affect the sizes of populations in a community.
• This means they can also affect the population sizes of other organisms that depend on them.
• For example, animals depend on plants for food, so a decrease in plant population could affect animal species in a community.
• Example:
• A decrease in light intensity, temperature or level of carbon dioxide could decrease the rate of photosynthesis in a plant species. This could affect plant growth and cause a decrease in the population size.
• A decrease in the mineral content of the soil (e.g. lack of nitrates) could cause nutrient deficiencies. This could affect plant growth and cause a decrease in the population size.

• New predators arriving.
• Competition - one species may outcompete another so that numbers are too low to breed.
• New pathogens
• Availability of food

• A change in the environment could be the introduction of a new biotic factor, e.g. a new predator or pathogen.
• These changes affec the sizes of a population in a community, which can have knock-on effects because of interdependence. 
• Examples:
• A new predator could cause a decrease in the prey population.
• Red and grey squirrels live in the same habitat and eat the same food. Grey squirrels outcompete the red squirrels - so the population of red squirrels is decreasing.

• Interspecific competition: Competition between animals of different species.

• Intraspecific competition: Competition between animals of the same species.

• Organisms have features (adaptations) that enable them to survive in the conditions in which they normally live. These adaptations may be structural, behavioural or functional. 
• Structural - features of an organisms body structure
• Behavioural - the ways that organisms behave. Many species migrate to warmer climates during the winter to avoid the problems of living in cold conditions.
• Functional - the things that go on inside the organism's body that can be related to processes like reproduction and metabolism (all the chemical reactions happening in the body).

Structural:

• Artic animals like the Arctic Fox have white fur so they're camouflaged against the snow. This helps them avoid predators and sneak up on prey.
• Animals that live in cold places (like whales) have a thick layer of blubber (fat) and a low surface area to volume ratio to help them retain heat.
• Animals that live in hot places (like camels) have a thin layer of fat and a large surface area to volume ratio to help them lose heat.

Functional:

• Desert animals conserve water by producing very little sweat and small amounts of concentrated urine.
• Brown bears hibernate over winter. They lower their metabolism which conserves energy, so they don't have to hunt when there's not much food about.

• Some organisms live in environments that are very extreme, such as at high temperature, pressure, or salt concentration.
• These organisms are called extremophiles.
• Bacteria living in deep sea vents are extremophiles. 
• Hot: specially adapted enzymes that do not denature at these high temperatures.
• Salty: adaptations in the cytoplasm so that water doesn’t move out of their cells by osmosis.

• Show what's eaten by what.
• Starts with a producer. Produces make their own food using energy from the sun.
• Producers are usually green plants or algae - they make glucose by photosynthesis.
• When a green plant produces glucose, some of it is used to make other biological molecules in the plant.
• These biological molecules are the plant's biomass - the mass of living material.
• Biomass can be thought of as energy stored in a plant.
• Energy is transferred through living organisms in an ecosystem when organisms eat other organisms.
• Producers are eaten by primary consumers. Primary consumers are then eaten by secondary consumers which are eaten by tertiary consumers.

Consumers that hunt and kill other animals are called predators, and their prey are what they eat. In a stable community containing prey and predators:

• The population of any species is limited to the amount of food available.
• If the population of the prey increases, then so will the population of the predators.
• However, as the population of the predators increase, the population of the prey will decrease.
• Predator-rey cycles are always out of phase with each other. This is because it takes a while for one population to respond to changes in the other population.

• Habitats are where organisms live.
• The distribution of an organism is where an organism is found.
• Where an organism is found is affected by environmental factors. An organism might be more common in one area than another due to differences in environmental factors between the two areas. For example, daisies are more common in the open than under trees because there's more light available in the open.
• There are a couple of ways to study the distribution of an organism. You can:
• measure how common an organism is in two sample areas using quadrats and compare them.
• study how the distribution changes across an area, e.g. by placing quadrats along a transect line.
• Both of these methods give quantitative data (numbers) about the distribution.

• A quadrat is a square frame enclosing a known area. To compare how common an organism is in two sample areas:
• Place a 1 square metre quadrat on the ground at a random point within the first sample area.
• Count all the organisms within the quadrat.
• Repeat the first 2 steps as many times as possible.
• Work out the mean number of organisms per quadrat. Total number of organisms divided by the number of quadrats.
• Repeat steps 1-4 in the second sample area.
• Compare the two means.

• Work out the mean number of organisms per square m.
• Then multiply the mean by the total area.

• Put the 30 m tape measure across a trampled area of the school field to form a transect line.
• Put the 1 m2 quadrat against the transect line.  One corner of the quadrat should touch the
• 0 m mark on the tape measure.
• Count the number of daisy plants within the quadrat.
• Record the number of daisies counted within the quadrat 
• Move the quadrat 5 m up the transect line and count the number of daisy plants again.  Record in the table.
• Continue to place the quadrat at 5 m intervals and count the number of daisy plants in 
• each quadrat.
• Calculate the mean number of daisy plants per m2 for the trampled area.
• Move the 30 m tape measure to an un-trampled area of the school field to form the new transect line.
• Repeat steps 2‒7 for the un-trampled transect line.
• Compare the population size of daisies in the trampled and un-trampled areas of the field.

• Count the number of squares.
• Make this into a % by dividing the number of species by the total number of squares in the quadrat.
• Do the same for another organism.

Environmental changes affect the distribution of species in an ecosystem. A change in distribution means a change in where the organism lives. These changes include:

• temperature - the distribution of bird species in Germany is changing because of a rise in average temperature. E.g. the European bee-eater bird is a Mediterranean species but is now present in parts of Germany.
• availability of water - the distribution of some animals and plant species in the tropics changes between the wet and dry seasons - i.e. the times of year where there is more or less rainfall, and so more or less water available. Each year in Africa, large numbers of giant wildebeest migrate, moving north and then back south as the rainfall patterns change.
• composition of atmospheric gases - the distribution of some species changes in areas where there is more air pollution. Some species of lichen can't grow in areas where sulfur dioxide is given out by certain industrial processes.

The changes may be seasonal, geographic or caused by human interaction. For example, the rise in average temperatures is due to global warming, which has been caused by human activity.

The water cycle provides fresh water for plants and animals on land before draining into the seas. Water is continuously evaporated and precipitated.

• Energy from the sun makes water evaporate from the land and sea, turning it into water vapour. Water also evaporates from plants - this is known as transpiration.
• The warm water vapour is carried upwards (as warm air rises). When it gets higher up it cools and condenses to form clouds.
• Water falls from the clouds as precipitation (usually rain, but sometimes snow or hail) onto land, where it provides fresh water for plants and animals.
• It then drains into the sea and the whole process begins again.

• Living things are made up of materials they take from the world around them. E.g. plants turn elements like carbon, oxygen, hydrogen and nitrogren from the soil and the air into the complex compounds (carbohydrates, proteins and fats) that make up living organisms. These get passed up the food chain.
• These materials are returned to the environment in waste products, or when the organisms die and decay.
• Materials decay because they're broken down by microorganisms. This happens faster in warm, moist, aerobic conditions because microorganisms are more active in these conditions. 
• Decay recycles nutrients and mineral ions, things that plants need to grow, back into the soil.
• In a stable community, the materials are taken out of the soil and used by plants and more are balanced by those that are put back in. There's a constant cycle occuring.

The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide to be used by plants in photosynthesis.

• Carbon dioxide is removed from the atmosphere by green plants and algae during photosynthesis. The carbon is used to make glucose, which can be turned into carbohydrates, fats and proteins that make up the bodies of plants and algae.
• When the plants and algae respire, some carbon is returned to the atmosphere as carbon dioxide.
• When plants and algae are eaten by animals, some carbon becomes part of the fats and proteins in their bodies. The carbon then moves through the food chain.
• When animals respire, some carbon is returned to the atmosphere as carbon dioxide.
• When plants, algae and animals die, other animals called detritus feeders and microorganisms feed on their remains. When these organisms respire, carbon dioxide is returned to atmosphere.
• Animals also produce waste that is broken down by detritus feeders and microorganisms.
• The combustion of wood and fossil fuels also releases carbo dioxide back into the air.
• So the carbon and energy is being constantly recycled.

• Compost is decomposed organic matter that is used as a natural fertiliser for crops and garden plants.
• Farmers and gardeners try to provide the ideal conditions for quick decay to make compost.

Microorganisms such as bacteria and fungi as well as detritus feeders are responsible for decompsition. Factors that affect this:

• Temperature - warmer temperatures make things decompose quicker because they increase the rate that the enzymes involved in decomposition work at. 
• Water availability - decay takes place faster in moist environments because the organisms involved in decay need water to carry out biological processes.
• Oxygen availability - many organisms need oxygen to respire, which they need to do to survive. The microorganisms involved in anaerobic decay don't need oxygen though.
• Number of decay organisms - the more microorganisms and detritus feeders there are, the faster the decomposition happens.

• Anaerobic decay produces methane gas. Biogas generators can be used to produce methane gas as a fuel.
• Biogas made in a simple fermenter called a digester or generator.
• Biogas generators need to be kept at a constant temperature to keep the microorganisms respiring away.
• Biogas can't be stored as a liquid so it has to be used straight away - for heating, cooking, lighting or to power a turbine to generate electricity.

• Batch generators - make biogas in small batches. They're manually loaded up with waste, which is left to digest, and the by-products are cleared away at the end of each session.
• Continuous generators - make biogas all the time. Waste is continuously ged in, and biogas is produced at a steady rate. Continuous generators are more suited to large-scale biogas projects.

Whether it's continuous or batch generator, it needs to have:

• an inlet for waste material to be put in.
• an outlet for the digested material to be removed through.
• an outlet so that the biogas can be piped to where it is needed.

• The practical looks at how temperature affects the rate of decay. In it, an indicator called phenolphthalein is used - it has a pink colour when the pH is around 10 but becomes colourless when the pH falls below 8.3.

1) Measure out 5 cubic cm of lipase solution and add it to a test tube. Label 'L' for lipase.

2) Measure out 5 cubic cm of milk and add it to a different test tube.

3) Add 5 drops of phenolphthalein indictator to the test tube containing milk.

4) Then measure out 7 cubic cm of sodium carbonate solution and add it to the tube containing milk and phenolphtalein. This makes the solution in the tube alkali, so it should turn pink.

5) Put both tubes into a water bath set to 30 degrees celsius and leae them to reach the temperature of the water bath. You should stick a thermometer into the milk tube to check this.

6) Once the tubes have reached the temperature, use a pipette to put 1 cubic cm of lipase solution into the milk tube and start a stopwatch straight away.

7) Stir the contents of the tube with a glass rod. The enzyme will start to decompose the milk.

8) As soon as the solution loses its pink colour, stop the stopwatch and record how long it took for the colour to change. Record in a table.

9) Repeat the experiment at a range of different temperatures. Make sure to try the experiment 3 times at each temperature, then calculate the mean time taken for the colour change to occur at each temperature.

10) You can use your results to calculate the rate of decay using this formula : rate = 1000 divided by time. The units would be s to the power of -1, since rate is given per unit of time.

• Biodiversity is the variety of all the different species of organisms on earth, or within an ecosystem.
• A great biodiversity ensures the stability of ecosystems by reducing the dependence of one species on another for food, shelter and the maintenance of the physical environment.
• The future of the human species on Earth relies on us maintaining a good level of biodiversity.
• Many human activities are reducing biodiversity and only recently have measures been taken to try to stop this reduction.

Rapid growth in the human population and an increase in the standard of living mean that increasingly more resources are used and more waste is produced. Unless waste and chemical materials are properly handled, more pollution will be caused.

Pollution can occur:

• in water, from sewage, fertiliser or toxic chemicals - sewage and toxic chemicals from industry can pollute lakes, rivers and oceans, affecting the plants and animlas that rely on them for survivial. And the chemicals used on land can be washed into water.
• in air, from smoke and acidic gases - smoke and acidic gases released into the atmosphere can pollute the air, e.g. sulfur dioxide can cause acid rain.
• on land, from landfill and from toxic chemicals - we use toxic chemicals for farming. We also bury nuclear waste underground and we dump a lot of household waste in landfill sites.

Pollution kills plants and animals which can reduce biodiversity. 

• The temperature of the Earth is a balance between the energy it gets from the sun and the energy it radiates back out into space.
• Gases in the atmosphere naturally act like an insulating layer. They absorb most of the energy that would normally be radiated out into space, and re-radiate it in all directions including back to Earth. This increases the temperature of the planet.
• If this didn't happen, then at night, there'd be nothing to keep any energy in, and we'd quickly get cold. But recently, we've started to worry that this effect is getting a bit out of hand.
• There are several different gases in the atmosphere which help keep the energy in. They're called greenhouse gases, and the main ones whose levels we worry about are carbon dioxide and methane. 
• The Earth is gradually heating up because of the increasing levels of greenhouse gases - this is global warming. Global warming is a type of climate change and causes other types of climate change.

• Sea levels rising - sea water exapnds, ice melts, sea levels rise, can lead to flooding.
• Changes in species distribution - species can become more widely distributed, e.g. species that need warmer temperatures may spread further as the conditions they thrive in exist over a wider area, vice versa.
• Changes in migration patterns - some birds may migrate to the north = warmer.
• Reduction in biodiversity - species may be unable to survive a change in the climate = extinct.

Humans reduce the amount of land available for other animals and plants by building, quarrying, farming and dumping waste.

• The destruction of peat bogs, and other areas of peat to produce garden compost, reduces the area of this habitat and thus the variety of different plant, animal and microorganism species that live there (biodiversity).
• The decay or burning of the peat releases carbon dioxide into the atmosphere.
• Peat bogs are areas of land that are acidic or waterlogged. Plants that live in bogs don't fully decay when they die because there's not engough oxygen. The partly-rotted plants gradually build up to form a peat.
• So the carbon in the plants is stored in the peat instead of being released into the atmosphere.
• However, peat bogs are often drained so that the area can be used as farmland, or the peat is cut up and dried to use as fuel. It's also sold to gardeners as compost. Peat is being used faster than it forms.
• When peat is drained, it comes into more contact with air and some microorganisms start to decompose it.
• When these microorganisms respire, they release carbon dioxide contributing to global warming. Carbon dioxide is also released when peat is burned as a fuel.

Large-scale deforestation in tropical areas has occurred to:

• provide land for cattle and rice fields
• grow crops for biofuels.

• Less carbon dioxide taken in - photosynthesis is reduced. Trees 'lock up' some of the carbon that they absorb during photosynthesis in their wood, which can remove it from the atmosphere for hundreds of years. Removing trees means that less is locked up.
• More carbon dioxide in the atmosphere - carbon dixide is released when trees are burnt to clear land. Microorganisms feeding on bits of dead wood release carbon dioxide as a product of respiration.
• Less biodiversity - biodiversity is the variety of species. Habitats like forests can contain a large number of different species of plants and animals, so when they are destroyed, there is a danger of many species becoming extinct, biodiversity is reduced.

• breeding programmes for endangered species - these are where animals are bred in captivity to make sure the species survives if it dies out in the wild. Individuals can sometimes be released into the wild to boost or re-establish a population.
• protection and regeneration of rare habitats and reintroduction of field margins and hedgerows in agricultural areas where farmers grow only one type of crop - there are programmes to to reintroduce hedgerows and field margins around fields on farms where only a single type of crop is grown. Field margins are areas of land around the edges of fields where wild flowers and grasses are left to grow. Hedgerows and field margins provide a habitat for a wider variety of organisms that could survive in a single crop habitat.
• reduction of deforestation and carbon dioxide emissions by some governments 
• recycling resources rather than dumping waste in landfill

• The cost of programmes - protecting biodiversity costs money, keeping watch on whether the regulations designed to maintain biodiversity are being followed costs money. There can be conflict between protecting biodiversity and saving money.
• The effect on the local economy - protecting biodiversity can affect livelihoods. People who usually cut trees can become unemployed, economy can be affected.
• Protecting food security - sometimes certain organisms are seen as pests and are killed to potect crops and livestock so that more food can be produced. Food chain and biodiversity are affected.
• The development of society - development is important but it affects the environment. Many people want to protect biodiversity in the face of development, but sometimes land is in such high demand that previously untouched land with high biodiversity has to be used for development. 

• Trophic levels can be represented by numbers, starting at level 1 with plants and algae. Further trophic levels are numbered subsequently according to how far the organism is along the food chain.
• Level 1: Plants and algae make their own food and are called producers.
• Level 2: Herbivores eat plants/algae and are called primary consumers.
• Level 3: Carnivores that eat herbivores are called secondary consumers.
• Level 4: Carnivores that eat other carnivores are called tertiary consumers. Apex predators are carnivores with no predators.

• Decomposers break down dead plant and animal matter by secreting enzymes into the environment.
• Broken down into small soluble food molecules then diffuse into the microorganism.

• There's less energy and biomass every time you move up a trophic level in a food chain. 
• There are usually fewer organisms every time you move up a trophic level.
• Pyramids of biomass can be constructed to represent the relative amount of biomass in each level of a food chain. Trophic level 1 is at the bottom of the pyramid.
• Each bar on the pyramid of biomas shows the relative mass of living material at a trophic level - basically how much all the organisms at each level would 'weigh' if you put them all together.
• The order of organisms in the pyramid must follow the order of the food chain. Each bar must be labelled. 

Producers are mostly plants and algae which transfer about 1% of the incident energy from light for photosynthesis. 

Only approximately 10% of the biomass from each trophic level is transferred to the level above it. Losses of biomass are due to:

• not all the ingested material is absorbed, some is egested as faeces
• some absorbed material is lost as waste, such as carbon dioxide and water in respiration and water and urea in urine.

Large amounts of glucose are used in respiration. 

Biomass transferred to the next level can be calculated using:

efficiency = (biomass transferred to the next level / biomass available at the previous level) x 100

Food security is having enough food to feed a population. Biological factors which are threatening food security include:

• the increasing birth rate has threatened food security in some countries
• changing diets in developed countries means scarce food resources are transported around the world
• new pests and pathogens that affect farming
• environmental changes that affect food production, such as widespread famine occurring in some countries if rains fail
• the cost of agricultural inputs
• conflicts that have arisen in some parts of the world which affect the availability of water or food

Sustainable methods must be found to feed all people on Earth so that enough food can be made to feed everyone now and in the future. Sustainable production means making enough food without using resources faster than they renew.

• Fish stocks in the oceans are declining.
• It is important to maintain fish stocks at a level where breeding continues or certain species may disappear altogether in some areas.
• Control of net size and the introduction of fishing quotas play important roles in conservation of fish stocks at a sustainable level.
• Fishing quotas - there are limits on the number and size of fish that can be caught in certain areas. This prevents certain species from being overfished.
• Net size - there are different limits of the mesh size of the fish net, depending on what's being fished. This is to reduce the number of 'unwanted' and discarded fish - the ones that are accidentally caught, e.g. shrimp caught along with cod. Using a bigger mesh size will let the 'unwanted' species escape. It also means that younger fish will slip through the net, allowing them to reach breeding age.

• The efficiency of food production can be improved by restricting energy transfer from food animals to the environment.
• This can be done by limiting their movement and by controlling the temperature of their surroundings.
• Some animals are fed high protein foods to increase growth.
• Some factory farming methods are controversial. Because animals are kept so close together, disease can spread through them easily. 
• There are also ethical objections, as some people think that making animals livein unnatural and uncomfortable conditions is cruel.

• A plasmid (loop of DNA) is removed from a bacterium.
• The insulin gene is cut out of a human chromosome using a restriction enzyme. Restriction enzymes recognise specific sequences of DNA and cut the DNA at these points. The cut leaves one of the DNA strands with unpaired bases - this is called a 'sticky end'.
• The plasmid is cut open using the same restriction enzyme - leaving the same sticky ends.
• The plasmid and the human insulin are mixed together.
• Lipase (an enzyme) is added. This joins the sticky ends together to produce recombinant DNA (two different bits of DNA stuck together).
• The recombinant DNA is inserted into a bacterium.
• The modified bacterium is grown in a vat under controlled conditions. You end up with millions of bacteria that produce insulin. The insulin can be harvested and purified to treat people with diabetes.

• Using modern technology techniques, large amounts of microorganisms can be cultured industrially under controlled conditions in large vats for use as a food source.
• Mycoprotein is used to make high-protein meat subsitutes for vegetarian meals like Quorn.
• It's made from the fungus Fusarium which is grown in aerobic conditions on glucose syrup, which it uses as food. The fungal biomass is harvested and purified to produce the mycoprotein.

• A genetically modified bacterium produces human insulin.
• When harvested and purified this is used to treat people with diabetes.
• GM crops could provide more food or food with an improved nutritional value such as golden rice.
• But not everyone agrees...
• Many people argue that people go hungry because they can't afford tob buy food, not because there isn't enough food about. So they argue that you need to tackle poverty first.
• There are fears that countries may become dependent on companies who sell GM seeds.
• Sometimes poor soil is the main reason why crops fail, and even GM crops won't survive.