Biology Core Science - revision notes

Diet and Energy

Diet and metabolic rate D-C

  • People should eat a balanced diet which contains some of these different kinds of nutrients:
    • carbohydrates, fats and protein for energy
    • small amounts of vitamins and mineral ions for keeping healthy.
  • If your diet is not balanced, you may become malnourished (become too fat or too thin, or suffer from deficiency diseases).
  • To lose body mass, people may go on a slimming diet where they eat less. Exercising more also helps. These both lead to more energy being used up than is taken in, and the body is forced to use some of its stored fat for energy.
  • Metabolic rate is the rate at which chemical reactions take place in your cells.
  • The greater the proportion of muscle to fat in the body, the higher the metabolic rate is likely to be. It also increases during exercise.
  • Metabolic rate can be affected by your genes, which you inherit from your parents.

Fatty foods B-A*

  • one gram of fat releases almost twice as much energy as one gram of carbohydrate, or one gram of protein.
  • proteins are not usually a major source of energy for the body because they are used for the more important functions of growth and repair.
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Diet, exercise and health

Diet and cholesterol D-C

  • A high level of cholesterol in the blood increases the risk of developing plaques in the walls of the arteries. Figure 1 shows how this can happen.
  • Sometimes, a clot blocks one of the arteries that take oxygenated blood to the heart muscle. This causes a heart attack - the muscle cannot work, so the heart cannot beat properly.
  • Eating saturated fats (those found in animal products) raises blood cholesterol levels. Unsaturated fats, found in plants, seem to lower blood cholesterol level.
  • Some people's bodies are better than others at keeping low levels of cholesterol in their blood. They have inherited this from their parents.

Good and bad cholesterol B-A*

  • Cholesterol is carried in your blood in two ways, as:
    • low-density lipoprotein (LDL) cholesterol, which is 'bad' and can cause heart disease.
    • high-density lipoprotein (HDL) cholesterol, which is 'good' as it can protect against heart disease by helping to remove cholesterol from the walls of blood vessels.
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Pathogens and infections


  • Microorganisms that cause disease are called pathogens.
  • Bacteria can reproduce rapidly inside the body They may produce toxins (poisons) that make us feel ill.
  • Viruses reproduce inside a body cell then destroy it when they burst out. The viruses then invade other cells.
  • An epidemic occurs when a wide spread of people have a disease. A pandemic is when the disease affects a whole country or goes worldwide.
  • In the 1840's, a doctor called Semmelweis used evidence from the death rates of women to work out that they were dying because doctors were transferring something to them from dead bodies. He made all the doctors wash their hands in chlorine water and, within a very shot time, the death rate plummeted. We now know the infection that killed the women was caused by bacteria.
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Fighting infection

Phagocytosis and lymphocytes D-C

  • Figure 2 shows how a type of white blood cell, called a phagocyte, can surround and ingest bacteria. This activity is called phagocytosis.
  • Lymphocytes produce chemicals called antibodies. The antibodies group round and stick to the pathogen. this may kill it directly, or stick it to other pathogens in clumps so that the phagocytes can destroy them more easily.
  • Some lymphocytes make antitoxins that can stick to the toxins given off by bacteria, and destroy them.
  • Both antibodies and antitoxins are very specific - each kind only works against a particular pathogen or toxin.

More about antibodies B-A*

  • This is an antibody molecule. The bits on the end of the Y arms can come in millions of different shapes. Each lymphocyte can make just one kind. The end bits fit onto molecules on the pathogen. Each shape only fits onto one kind of pathogen.
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Drugs against disease

Antibiotics D-C

  • Antibiotics are drugs that kill bacteria inside your body, without killing your own cells. Examples are penicillin and streptomycin.
  • Antibiotics do not all work equally well against all the kinds of bacteria.
  • Figure 1 shows how we find the best antibiotic to kill a bacterium. Bacteria are spread onto a jelly. Paper discs, socked in different antibiotics, are placed on the jelly and the antibiotics diffuse out. If the antibiotic kills the bacteria, they do not grow around the disc.

Exam tip - You may be shown an image of a test like this and be asked whit it shows. The clear jelly shows no bacteria growth: so the bigger this area, the more effective the antibiotic. in this case, antibiotic E is the most effective.

Prescribing antibiotics B-A*

  • Scientists now know that people must not use antibiotics unnecessarily. Overuse makes it more likely that bacteria will become resistant to them.
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Antibiotic resistance

Reducing the risk D-C

  • Bacteria do not become resistant to antibiotics on purpose. It happens ny natural selection. Figure 2 shows how.
  • As new antibiotic - resistant strains of bacteria emerge, we have to find new antibiotics to kill them.
  • To reduce the chance of new strains forming, we need to reduce the use of antibiotics.
  • Whenever antibiotics are used, it gives an advantage to any mutant bacterium that happens to be resistant to them.
  • If they are not used, then a mutant bacterium does not have any advantage: it is no more likely to reproduce than any other bacterium.

MRSA deaths B-A*

  • MRSA (methicillin-resistant Staphylococcus aureus) is sometimes called a superbug because it is resistant to most antibiotics.
  • The number of deaths from MRSA rose between 1993 and 2006, but is now gradually decreasing as people become aware of how to stop it spreading.
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  • In the UK, children have the MMR vaccination which makes them immune to measles, mumps and rubella.
  • A small amount of the dead or inactive viruses that cause the disease are injected into the blood.
  • The white blood cells attack them, just as they would attack living pathogens. They remember how to make the antibody, so the child is now immune to the disease without having to suffer them first.

New diseases B-A*

  • New infectious diseases appear when mutations occur in bacteria or viruses.
  • For example in 2009, a new kind of flu virus, called swine flu, spread quickly to all parts of the world. As existing flu vaccines would not work against it, new ones had to be developed.
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Growing bacteria

Avoiding contamination

  • The microorganisms growing in the nutrient medium are called a culture.
  • You should use a sterile technique to stop unwanted microorganisms from entering the nutrient medium.
  • All equipment, including the medium, should be sterilised before use.
  • Metal equipment, such as a wire inoculating loop, can be held in a flame.
  • You must not touch the nutrient jelly (agar) with your fingers or breathe over it.
  • The dish containing the agar should be sealed with tape. This prevents microorganisms, from the air, contaminating the culture.
  • You should keep the cultures lower than 25oC. If you keep them warmer then this might encourage the growth of microorganisms that live in the body, which are more likely to be pathogens.

Pathology and industrial laboratories B-A*

  • Bacteria that are causing illness in a patient may be grown in a hospital pathology lab.
  • The Petri dishes, on which the bacteria are growing, will be put into an incubator to keep them warm and to encourage rapid growth.
  • In industrial labs, harmless bacteria are growing commercially, for example, to make enzymes.
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Co-ordination, nerves and hormones

Nerves and hormones

  • Nerves contain special cells called nerve cells, which transmit impules to and from the brain and spinal cord (the central nervous system).
  • Glands secrete chemicals, called hormones, into the blood. The bloodstream carries the hormones around the body.
  • Most hormones affect just a few different organs. These are called their target organs.
  • An example of a hormone is adrenaline, which affects the heart, breathing muscles, eyes and digestive system.

Response duration

  • A nervous response, for example touching a football, is fast and short-lived because impulses travelling along nerves only take a short time.
  • Where a longer-term response is needed, hormones are oftern a more appropriate method of communication.
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  • Information is carried in the nervous system as electrical impulses. The cells that transmit these impulses are called nerve cells or neurones.
  • The neurones that transmit impulses from receptors to the central nervous system are called sensory neurones
  • The neurones that transmit impulses from the central nervous system to effectors are called motor neurones.

Exam tip - make sure you use the correct science words in your answers. Use 'impulses' rather than 'messages'; and 'neurone' if you are talking about a single cell or 'nerve' if you are talking about a bundle of them, as in spinal cord.

Rod cells

  • Rod cells at the back of the eye (retina) are sensitive to light.
  • If only one photon of light falls onto a rod cell, this is enough to make it generate an electrical impulse which is sent along the optic nerve to the brain.
  • The brain uses the pattern of impulses, arriving from different parts of the retina, to construct a 'picture' of the world you are looking at.
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Reflex actions

Impulse pathways

  • A reflex arc is the pathway taken by a nerve impulse as it passes from a receptor, through the central nervous system, and finally to an effector.
  • Figure 4 shows the path the impulses take.
  • It takes a nerve impulse only a fraction of a second to go along this route. Thats is why reflex actions are so quick.
  • The gaps between neurones are called synapses.
  • Electrical impulses cannot jump across synapses. When an impulse gets to the end of a neurone, it causes a chemical to be secreted. The chemical diffuses across th gap and starts an electrical impulse along the next neurone.

Conscious control

  • Synapses enable us to respond to a stimulus in more than one way. For example, the relay neurone, in the spinal cord, will have synapses to other neurones that can carry nerve impulses down from the brain.
  • This allows us to take conscious control of our response to a stimulus.
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Controlling the body

Temperature control

  • You gain water from food and drink. You lose water in your breath, sweat and urine.
  • Your blood has ions dissolved in it, such as those found in salt.
  • The kidneys help to keep the balance of water and ions by varying the amount of water and salt excreted from your body in urine.
  • Human body temperature needs to be kept around 37oC, as this is the temperature at which our enzymes work best.
  • The body loses heat by radiation from the skin, and from the evaporation of sweat.
  • The body also has mechanisms to keep the concentration of sugar in the blood constant.

Survival in the desert

  • An SAS survival manual gives this advice to conserve water: avoid exertion, keep cool and stay in the shade; don't lie on the hot ground; don't eat, because digestion uses up fluids; talk as little as possible; and breathe through your nose rather than your mouth.
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Reproductive hormones

Hormones and the menstrual cycle

  • At the start of the menstrual cycle, the pituitary gland secretes FSH (follicle-stimulating hormone). This causes an egg to mature in one of the woman's ovaries and also stimulates the ovary to secrete oestrogen.
  • Oestrogen makes the inner lining of the uterus grow thicker. High levels of oestrogen stop the production of FSH.
  • Luteinising hormone (LH) stimulates the release of eggs from the ovary.
  • As the level of FSH drops, the ovary stops secreting oestrogen. This cuts off the the inhibition of FSH secretion, so the cycle starts all over again.
  • The contraceptive pill may contain oestrogen. It stops FSH secretion, so the cycle starts all over again.

Co-ordinating the menstrual cycle

  • The change in concentration of oestrogen causes changes in the thickness of the uterus lining. The rise in oestrogen concentration causes the uterus lining to thicken. When oestrogen concentration falls below a certain point, the uterus lining breaks down.
  • The release of an egg from the ovary usually happens at about day 14 of the cycle. It is called ovulation.
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Controlling fertility


  • IVF stands for 'In Vitro Fertilisation'
  • The woman is given hormones, such as FSH, to make her ovaries several eggs.
  • The eggs are removed and are mixed with her partner's sperm for fertilisation to occur.
  • One of the embryos is chosen and placed in the woman's uterus. With luck, it will sink into the uterus lining and develop as a fetus.

Multiple births B-A*

  • Twins or triplets are more likely to have problems developing in the uterus than a single fetus. They are also more likely to be underweight at birth.

Exam tip - Multiple births are more common with women who have used fertility treatments. You should be able to explain why this is.

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Plant responses and hormones

Phototropism and gravitropism D-C

  • A growth response to light is a tropism called phototropism.
  • Auxin is a plant hormone which makes cells in the shoots get longer. When light shines onto a shoot, the auxin builds up on the shady side. This makes the cells on that side get longer. So, the shoot bends towards the light.
  • A growth responses to gravity is called gravitropism of geotropism.
  • Auxin tends to accumulate on the lower side of a root. In roots, auxin reduces the rate of growth. So, the lower side of the shoot grows more slowly than the upper surface. This causes the root to bend downwards.
  • Gardeners dip the base of a cutting into a powder or gel called rooting hormone, which makes the cutting grow roots.
  • Plant hormones are also used as weedkillers. The hormones make the weeds grow very fast and then die. The hormones only affect weeds because they have a different metabolism.

Where are the receptors in a plant? B-A*

  • Scientists in the past carried out experiments to work out how plants detect stimuli
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Dangers of drugs D-C

  • Some people take drugs because they make them feel different. This is called a recreational drug use.
  • Many recreational drugs are legal but still can be harmful, such as alcohol.
  • Some recreational drugs are illegal because of the harm they can cause. Cannabis is an example, it may cause mental illness.
  • A drug addiction can have a dangerous long-term effects. Over time, the lungs, brain and liver can be seriously damaged.
  • If someone is addicted to a drug, they may suffer from very unpleasant withdrawal symptoms if they stop taking it.

Deaths from drug use B-A*

  • Each year, thousands of people in Britain die from misusing drugs.
  • Some deaths are from poisoning. Some happen because drugs can affect the brain, making people behave in a dangerous way.
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Developing new drugs

Drug trials D-C

  • A drug trial, on a potential new medicine, contains three stages:
    • 1. The drugs is tested in a laboratory on human cells or tissue to find out if it is toxic (poisonous). It may be also be tested on live animals.
    • 2. Human volunteers are given different doses, to find out what the highest dose that can be taken to safety. Any side-effects are recorded.
    • 3. In clinical trials, the drug is tested on its target illness. It is given to people who have the illness, to see if it makes them better. Some patients are given placebo or the real drug (a double-blind trial). This helps determine whether the drug really works.

Evaluating Statins B-A*

  • Statins are drugs that help people to reduce their blood cholesterol level and therefore greatly reduce their risk of getting heart disease.
  • When statins were first introduced, trials had shown almost no side-effects.
  • However, since then, side-effects such as painful muscles have been discovered.
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Legal and illegal drugs

Dangers from recreational drugs

  • Alcohol and nicotine cause far more illness and death each year than all the illegal drugs put together.
  • People do not worry so much about them because they have been around for so long, and because so many people use them

Drugs in sport

  • Professional sports people are banned from taking certain performance-enhancing drugs because they could give them an unfair advantage. For example:
    • steroids can stimulate the body to grow larger, stronger muscles
    • beta blockers can help someone to stay calm and steady
    • stimulants can increase the heart rate.
  • These drugs can often cause long-lasting damage or even death.
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Competing D-C

  • Organisms may have to complete for resources if they are in short supply.
  • For example, plants compete for light - the ones that grow tallest win the competition.
  • The individuals best at competing are the most likely to survive. Those not good at getting resources are the most likely to die.
  • If there are not enough females to go around, then males will compete for a mate.
  • Animals may also compete for a territory - a space in which they can find food and a place to breed.

Avoiding competition B-A*

  • Many organisms have become able to live in places where few others can survive. Although these places make survival tough, there is no need to share resources.
  • This can increase chances of surviving and having large numbers of offspring.
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Adaptations for survival - Living in difficult cli

Living in difficult climates

  • Plants that live in dry places usually have:
    • long, wide-spreading roots - the roots grow deep into the soil, to reach water
    • small or no leaves - the smaller the leaf surface area, the less the amount if water evaporating away
    • tissues that can store water.
  • Animals that live in dry places must be able to manage without much water. For example, camel's stomachs can hold over 20 litres of water, they can drink very quickly, store water a fat in their humps, and they produce very little urine.
  • Desert animals often have large ears. A large surface area helps the animal lose body heat and stay cool.
  • Animals that live in very cold places like Arctic, often have thick fur and thick layers of fat. This insulation helps the animal reduce heat loss. They are coloured white, for camouflage against snow.
  • Many plants and animals have thorns, poisons and warning colours to deter predators.
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Adaptations for survival - Extreme environments

Extreme environments

  • Organisms that can live in very difficult environments are called extremophiles. They are usually microorganisms.
  • For most organisms, conditions such as high temperatures and high pressure could be lethal.
  • Extremophiles must have very stable protein molecules that are not affected by these conditions.

Exam tip - Make sure you can describe how the camel and one animal from the Arctic, for example a polar bear, are adapted.

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Environmental change

Causes of change D-C

  • Environmental changes are caused by living and non-living factors. For example:
    • non-living factors include global warming, which has caused rainfall in central Australia to decrease
    • living factors include the introduction of the grey squirrel into Britain, which caused a decrease in the population of the native red squirrel.

The disappearing bees B-A*

  • Honeybees help pollinate flowers that will develop into food crops.
  • in recent years, there has been a decline in the numbers of honeybees.
  • We are not sure of the cause but various suggestions have been put forward to explain it.
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Pollution indicators

Measuring changes in the environment D-C

  • In the UK, the composition of the air and of the water in rivers and streams; and the air temperature and rainfall, are constantly being measured. this makes sure that any changes can be tracked.
  • Oxygen metres measure the concentration of dissolved oxygen in the water. Unpolluted water contains a lot of dissolving oxygen.
  • Thermometers measure temperature. Rain gauges measure rainfall.
  • Scientists can use the distribution of living organisms of find out about pollution. For example:
    • if there is a lot of sulfur dioxide in the air, many species of lichens will not be able to grow
    • if there is not very much oxygen in a river, there will be no oxygen-loving mayfly larvae in the water, instead there will be just rat-tailed maggots and blood worms. 

Sewage pollution and invertebrates

  • Polluted water often contains very little dissolved oxygen. Some species of invertebrate are able to live in this water, but others are not. Figure 1 shows some of these species.

Exam tip - You do have to memorise this diagram but you may be asked to apply the information in contains

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Food chains and energy flow

Energy wastage

  • Green plants capture only a small amount of energy from the light that falls onto them. This is because some light:
    • misses the leaves altogether
    • hits the leaf and reflects back from the leaf surface
    • ho
    • hits the leaf, but goes all the way through without hitting any chlorophyll
    • hits the chlorophyll, but is not absorbed because it is of the wrong wavelength (colour)
  • As a result, very little of the light energy that falls on a plantcan be used for photosynthesis and get transferred into chemical energy in carbohydrates and other substances.


  • Whenever energy is transferred, some of it is wasted.
  • To calculate the efficiency of energy transfer, use the formula:

efficiency= useful energy transferred x 100%

(divide top by bottom) original amount of energy

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Energy losses

  • Figure 3 shows a pyramid of biomass drawn to scale. Its shape is explained by the fact that whenever energy is transferred, some is wasted (not used for useful work). So at each step, there is less energy available for the organisms to use. Less energy means less biomass.
  • The food chain loses energy because:
    • some materials and energy are lost in the waste materials produced by each organism, such as carbon dioxide, urine and faeces
    • respiration in each organism's cells releases energy from nutrients to be used for movement and other purposes, so much of the energy is eventually lost as heat to the surroundings
    • not all of the organism's tissues are eaten, for example the antelope does not eat the roots of the grass as they are under the ground.

More about energy loss

  • Mammals and birds use glucose to provide energy to keep their body temperature high.
  • This means that energy loss from birds and mammals id high.
  • Other animals, such as snakes, frogs and fish, just stay the same temperature as their environment.
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Speeding or slowing decay

  • Most of the bacteria and fungi that carry out decay need:
    • oxygen for aerobic respirtation
    • a warm temperature for their enzymes to work at an optimum rate
    • moisture for reproduction
  • Increasing the temperature of microorganisms slows or stops decay.

Preventing decay

  • If food is not to decay, it can be treated so as to slow down of stop the activity of microorganisms.
  • Examples of the include canning, picking and drying food.

Exam tip - you should be able to appy your knowledge explain how these methods of food preservation help me to slow down decay.

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Recycling and food chains

  • Figure 2 shows how microorganisms fit into a simple food chain.
  • You can see that these decay microorganisms feed on every organism in the chain.
  • They will break down most of the waste material that the plants and animals produce, and then their bodies will be broken down by others when they die.

Dead whales

  • Whole communities of organisms use whale carcasses as food.
  • Crabs, worms and fish eat the whale's body.
  • Microorganisms gradually decay the whales tissues. The whole process can take decades.
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The carbon cycle - Processes in the carbon cycle

Processes in the carbon cycle

  • Animals, plants and decomposers all interact with each other in the carbon cycle (figure 3).
  • Photosynthesis converts carbon dioxide into carbohydrates and other food molecules such as proteins: carbon dioxide + water --> glucose + oxygen
  • When animals eat plants (or another animal) the food goes into their cells and is broken down by respiration: glucose + oxygen --> carbon dioxide + water
  • Carbon dioxide is returned to the air when the animal breathes out.
  • Plants and microorganisms also respire.
  • Some dead organisms do not decay. They become buried and compressed, deep underground and change into fossil fuels.
  • Carbon dioxide is returned to the air when wood or fossil fuels are burnt (combustion).
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The carbon cycle - Energy in the carbon cycle

Energy in the carbon cycle B-A*

  • Energy is transferred in the carbon cycle.
  • During photosynthesis, energy from the sunlight is transferred to energy stores as chemical in carbohydrates.
  • Some of this energy is transferred to other organisms – such as animals or decomposers, when they feed on the plant.
  • Some of the energy is wasted, heating the soil and air.
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Genes and chromosomes

Chromosome numbers D-C

  • Chromosomes are long sections of DNA.
  • Most human cells have 46 chromosomes, that is 23from the gamete of each parent (sperm and egg), which carry about 25000 genes.
  • Each gene contains coded information that controls one characteristic. For example, some genes control hair colour. Other genes control eye colour.
  • Most of these genes come in two or more forms. For example, a gene that controls hair colour might have one form that produces brown hair and a different firm that produces red hair.

Causes of variation B-A*

  • Variation (differences) in organisms may be due to either: - the genes they have inherited (genetic causes) - the condition in which they have developed (environmental causes) - or a combination of both.
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How it works

  • In sexual reproduction, gametes and fertilisation are always involved.
  • The new cell that is reproduced by fertilisation is a zygote. It divides repeatedly to produce a little ball of cells. This develops into an embryo and finally into an adult animal.
  • Sexual reproduction produces a variety in offspring because each zygote has a different mix of genes from its parents and all its brother and sisters.
  • In asexual reproduction, an individual splits in two (as in bacteria) or a part divides off. This is the offspring.
  • There is no variation. The new organisms all have exactly the same genes as their parents, and as each other. They are genetically identical (clones).

Different kinds of fertilisation

  • In bird and mammals, the male sperm are deposited and egg is fertilised inside the female’s body. This is called internal fertilisation.
  • In other animals, such as fish, the male and female shed sperm and eggs into water. This is called external fertilisation. The fertilised eggs develops outside the female’s body.
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Cloning plants and animals

Cloning methods

  • Taking cuttings is a wayof making new plants from one origional plant. Stems are cut from the parent plant; the ends dipped in hormone rooting powder and placed in soi;. The cuttings will grow into the new plants which are genetically identical to each pther parent plant.
  • Tissue culture can also be used to clone plants.
    • a small piece of tissue is taken from a root, stem or leaf of the parent plant. The tissue is then grown on a jelly containing all the nutrients it needs.
    • everything has to be kept sterile, so this is usually done in a laboratory.
    • eventually, each tiny group of cells grows into a complete adult plant.
  • One technique to clone animals is called embryo transplants. This is sometimes done with farm animals, such as cows.
    • egg cells are taken from a cow and fertilised with sperm from a bull.
    • one embryo is chosen and split into two (or more) and then each is placed into a host mother.
    • the calves born are clones of each other as they have the same genes.

Adult-cell cloning

  • Adult-cell cloning can be used to clone just one parent. Figure 1 shows how it is done.
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Genetic engineering

How it works

  • Bacteria have been genetically engineered to make human insulin. Figure 2 shows how this is dine.
  • Farmers spray bean fields with herbicides to kill weeds that compete with soya plants. The spray contains a chemical called glyphosate.
  • Some soya bean varieties have been genetically engineered to give them a gene that makes them resistant to glyphosate.
  • So when a farmer sprays the field with glyphosate, the weeds die but the bean plants do not.

Genetic modification - good or bad?

  • Some GM crop plants are resistant to attack by pests. This can greatly increase the yields and keep prices down. It also reduces the amount of pesticide they has to be sprayed.
  • Some people have concerns about GM crops:
    • genes for a toxin to kill insects could be transferred to a wild plant, which could then disrupt natural food chains
    • there may be effects on humans of eating food from GM plants.
  • GM crops have to be thoroughly tested before they are allowed to be grown on a large scale and there os no evidence that eating GM plants does any harm
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Accepting Darwin's ideas

  • Jean-Baptiste Lamarck suggests that changes in organisms caused by their environment were passed on to their offspring. We now know that this is not correct.
  • Charles Darwin suggests that species gradually changed from one form to another by natural selection. Darwin thought that, in each generation, only the best-adapted individuals survive and reproduce to pass on their characteristics to the next generation.
  • Darwin's ideas challenged the established thinking of the day, so his ideas were not accepted at first. They undermined the idea that God made all animals and plants.
  • In the late 19th century, there was not much scientific evidence to support the theories of evolution and natural selection. At that time, no one even knew genes existed, let alone that way that they were inherited. This was not discovered until 50 years later . However, this is the theory that almost all scientists today believe as there is now a lot of scientific evidence to support it.

Simple to complex? D-C

  • The very earliest forms of life on Earth were almost certainly simple, single-celled organisms. Today, many organisms are much more complicated. Does that mean that evolution produces increasingly complex organisms?
  • Some bacteria living today are almost the same as bacteria that have lived billions of years ago. They were, and still are, supremely well adapted to their environment.
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Natural selection - How natural selection works

How natural selection works

  • This is how natural selection happens:
    • living organisms produce many offspring
    • the offspring vary frm one another, because they have differences in their genes
    • some of them have genes that give them a better chance of survival. They are most likely to reproduce
    • their genes will be passed on to their offspring.
  • Occasionally, unpredictable chances to chromosomes and genes, called mutations happen.
  • Occasionally, the new form of the gene increases an organism's chance of surviving and reproducing. It is therefore very likely to be passed on to the next generation. Over time, the new feature, producing by this gene, becomes more common in this species.
  • The change in colour of the peppered moth from pale to dark is an example of evoltuion occurring because of a mutation.

Exam tip - You may be asked to explain how certain species evolved. Do this by applying the stages of natural selection: variation, competition, survival and reproduction.

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Natural selection - The randomness of mutation

The randomness of mutation

  • Some forms of bacteria have become resistant to antibiotics.
  • This happened as a result of mutation in the bacteria producing form of a gene that helped them survive, even when the antibiotic was present in their environment.
  • This was just chance. The bacteria did not purposefully mutate to become resistant.
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Evidence of evolution

Comparing living organisms

  • You can get clues about evolution by looking carefully at organisms that are alive today.
  • For example, your arm, a bat's wing and a bird's wing all have the same bones in the same places. Similarities like this suggest that humans, bats and birds are quite closely related and that, long ago, an animal lived from which humans, bats and birds have all evolved (a common ancestor).
  • Evoltuionary trees like this one (Figure 1) show the pathway along which different kinds of organisms may have evolved.
  • Organisms that lived longest ago are at the bottom of the tree.
  • Models like this helps to show how different groups of organisms might be related, which helps scientists to classify them.


  • Scientists are still discovering new species which change their ideas about how organisms should be classified.
  • Recently, they have found that there are two distinct groups of these microorganisms. They are different from one another as animals are from bacteria.
  • They have now been split into two big groups - the bacteria anf the archaea.
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Chloe Randall


Omg this was so helpful, thank you :) you saved me so much time and it made me understand it more!

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