Paper 1

• Plant and animal cells (eukaryotic cells) have a cell membrane, cytoplasm and genetic material enclosed in a nucleus.
• Bacterial cells (prokaryotic cells) are much smaller in comparison.
• They have cytoplasm and a cell membrane surrounded by a cell wall.
• The genetic material is not enclosed in a nucleus.
• It is a single DNA loop and there may be one or more small rings of DNA called plasmids.

Most animal cells have the following parts:

•  a nucleus
• cytoplasm 
• a cell membrane 
• mitochondria
• ribosomes.

In addition to the parts found in animal cells, plant cells often have:

• chloroplasts
• a permanent vacuole filled with cell sap.
• Plant and algal cells also have a cell wall made of cellulose, which strengthens the cell.

Cells may be specialised to carry out a particular function:

• sperm cells, nerve cells and muscle cells in animals
• root hair cells, xylem and phloem cells in plants.

As an organism develops, cells differentiate to form different types of cells.

• Most types of animal cell differentiate at an early stage.
• Many types of plant cells retain the ability to differentiate throughout life.

In mature animals, cell division is mainly restricted to repair and replacement.
As a cell differentiates it acquires different sub-cellular structures to enable it to carry out a certain function.
It has become a specialised cell.

• Limited to the differences in magnification and resolution.
• An electron microscope has much higher magnification and resolving power than a light microscope.
• This means that it can be used to study cells in much finer detail.
• This has enabled biologists to see and understand many more sub-cellular structures.

• Bacteria multiply by simple cell division (binary fission) as often as once every 20 minutes if they have enough nutrients and a suitable temperature.
• Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate.
• Uncontaminated cultures of microorganisms are required for investigating the action of disinfectants and antibiotics.
• Petri dishes and culture media must be sterilised before use 
• inoculating loops used to transfer microorganisms to the media must be sterilised by passing them through a flame
• the lid of the Petri dish should be secured with adhesive tape and stored upside down
• in school laboratories, cultures should generally be incubated at 25°C.

• The nucleus of a cell contains chromosomes made of DNA molecules.
• Each chromosome carries a large number of genes.
• In body cells, the chromosomes are normally found in pairs.

• Cells divide in a series of stages called the cell cycle. 
• During the cell cycle the genetic material is doubled and then divided into two identical cells.
• Before a cell can divide it needs to grow and increase the number of sub-cellular structures such as ribosomes and mitochondria.
• The DNA replicates to form two copies of each chromosome.
• In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides.
• Finally, the cytoplasm and cell membranes divide to form two identical cells. 
• Cell division by mitosis is important in the growth and development of multicellular organisms. 

• A stem cell is an undifferentiated cell of an organism which is capable of giving rise to many more cells of the same type, and from which certain other cells can arise from differentiation.
• Stem cells from human embryos can be cloned and made to differentiate into most different types of human cells.
• Stem cells from adult bone marrow can form many types of cells including blood cells.
• Meristem tissue in plants can differentiate into any type of plant cell, throughout the life of the plant. 
• Treatment with stem cells may be able to help conditions such as diabetes and paralysis.
• In therapeutic cloning an embryo is produced with the same genes as the patient. Stem cells from the embryo are not rejected by the patient’s body so they may be used for medical treatment.
• The use of stem cells has potential risks such as transfer of viral infection, and some people have ethical or religious objections.
• Stem cells from meristems in plants can be used to produce clones of plants quickly and economically.
• - Rare species can be cloned to protect from extinction.
• - Crop plants with special features such as disease resistance can be cloned to produce large numbers of identical plants for farmers.

• Substances may move into and out of cells across the cell membranes via diffusion.
• Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.
• Some of the substances transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange, and of the waste product urea from cells into the blood plasma for excretion in the kidney. 
• Factors which affect the rate of diffusion are:
• - the difference in concentrations (concentration gradient)
• - the temperature
• - the surface area of the membrane.
• A single-celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.
• In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported into and out of cells for the organism’s needs.
• The effectiveness of an exchange surface is increased by:
• - having a large surface area
• - a membrane that is thin, to provide a short diffusion path • (in animals) having an efficient blood supply
• - (in animals, for gaseous exchange) being ventilated.

• Water may move across cell membranes via osmosis.
• Osmosis is the diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane.

• Active transport moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient).
• This requires energy from respiration.
• Active transport allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil.
• Plants require ions for healthy growth.
• It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration.
• Sugar molecules are used for cell respiration.

• Cells are the basic building blocks of all living organisms.
• A tissue is a group of cells with a similar structure and function.
• Organs are aggregations of tissues performing specific functions.
• Organs are organised into organ systems, which work together to form organisms.

• The digestive system is an example of an organ system in which several organs work together to digest and absorb food.
• Enzymes catalyse specific reactions in living organisms due to the shape of their active site.
• Digestive enzymes convert food into small soluble molecules that can be absorbed into the bloodstream.
• Carbohydrases break down carbohydrates to simple sugars. Amylase is a carbohydrase which breaks down starch.
• Proteases break down proteins to amino acids.
• Lipases break down lipids (fats) to glycerol and fatty acids.
• The products of digestion are used to build new carbohydrates, lipids and proteins. Some glucose is used in respiration.
• Bile is made in the liver and stored in the gall bladder.
• It is alkaline to neutralise hydrochloric acid from the stomach.
• It also emulsifies fat to form small droplets which increases the surface area.
• The alkaline conditions and large surface area increase the rate of fat breakdown by lipase.

• The heart is an organ that pumps blood around the body in a double circulatory system.
• The right ventricle pumps blood to the lungs where gas exchange takes place.
• The left ventricle pumps blood around the rest of the body. 
• The natural resting heart rate is controlled by a group of cells located in the right atrium that act as a pacemaker.
• Artificial pacemakers are electrical devices used to correct irregularities in the heart rate.
• The body contains three different types of blood vessel:
• - arteries 
• - veins
• - capillaries.

• Blood is a tissue consisting of plasma, in which the red blood cells, white blood cells and platelets are suspended.

• In coronary heart disease layers of fatty material build up inside the coronary arteries, narrowing them.
• This reduces the flow of blood through the coronary arteries, resulting in a lack of oxygen for the heart muscle.
• Stents are used to keep the coronary arteries open.
• Statins are widely used to reduce blood cholesterol levels which slows down the rate of fatty material deposit.
• In some people heart valves may become faulty, preventing the valve from opening fully, or the heart valve might develop a leak. 
• Faulty heart valves can be replaced using biological or mechanical valves.
• In the case of heart failure a donor heart, or heart and lungs can be transplanted.
• Artificial hearts are occasionally used to keep patients alive whilst waiting for a heart transplant, or to allow the heart to rest as an aid to recovery.

• Health is the state of physical and mental well-being.
• Diseases, both communicable and non-communicable, are major causes of ill health.
• Other factors including diet, stress and life situations may have a profound effect on both physical and mental health.
• Different types of disease may interact.
• - Defects in the immune system mean that an individual is more likely to suffer from infectious diseases.
• - Viruses living in cells can be the trigger for cancers.
• - Immune reactions initially caused by a pathogen can trigger allergies such as skin rashes and asthma
• - Severe physical ill health can lead to depression and other mental illness.

• Risk factors are linked to an increased rate of a disease.
• They can be:
• • aspects of a person’s lifestyle
• • substances in the person’s body or environment.
• A causal mechanism has been proven for some risk factors, but not in others.
• - The effects of diet, smoking and exercise on cardiovascular disease.
• - Obesity as a risk factor for Type 2 diabetes.
• - The effect of alcohol on the liver and brain function.
• - The effect of smoking on lung disease and lung cancer.
• - The effects of smoking and alcohol on unborn babies.
• - Carcinogens, including ionising radiation, as risk factors in cancer.
• Many diseases are caused by the interaction of a number of factors.

• Benign tumours are growths of abnormal cells which are contained in one area, usually within a membrane.
• They do not invade other parts of the body.
• Malignant tumour cells are cancers.
• They invade neighbouring tissues and spread to different parts of the body in the blood where they form secondary tumours.
• Scientists have identified lifestyle risk factors for various types of cancer.
• There are also genetic risk factors for some cancers.

• Plant tissues include:
• - epidermal tissues
• - palisade mesophyll
• - spongy mesophyll
• - xylem and phloem
• - meristem tissue found at the growing tips of shoots and roots.
• The leaf is a plant organ

• The roots, stem and leaves form a plant organ system for transport of substances around the plant.
• Root hair cells are adapted for the efficient uptake of water by osmosis, and mineral ions by active transport.
• Xylem tissue transports water and mineral ions from the roots to the stems and leaves.
• It is composed of hollow tubes strengthened by lignin adapted for the transport of water in the transpiration stream.
• The role of stomata and guard cells are to control gas exchange and water loss.
• Phloem tissue transports dissolved sugars from the leaves to the rest of the plant for immediate use or storage.
• The movement of food molecules through phloem tissue is called translocation.
• Phloem is composed of tubes of elongated cells.
• Cell sap can move from one phloem cell to the next through pores in the end walls.

• Pathogens are microorganisms that cause infectious disease.
• Pathogens may be viruses, bacteria, protists or fungi.
• They may infect plants or animals and can be spread by direct contact, by water or by air.
• Bacteria and viruses may reproduce rapidly inside the body.
• Bacteria may produce poisons (toxins) that damage tissues and make us feel ill.
• Viruses live and reproduce inside cells, causing cell damage.

• Measles is a viral disease showing symptoms of fever and a red skin rash.
• Measles is a serious illness that can be fatal if complications arise.
• For this reason most young children are vaccinated against measles.
• The measles virus is spread by inhalation of droplets from sneezes and coughs.
• HIV initially causes a flu-like illness.
• Unless successfully controlled with antiretroviral drugs the virus attacks the body’s immune cells.
• Late stage HIV infection, or AIDS, occurs when the body’s immune system becomes so badly damaged it can no longer deal with other infections or cancers.
• HIV is spread by sexual contact or exchange of body fluids such as blood which occurs when drug users share needles.
• Tobacco mosaic virus (TMV) is a widespread plant pathogen affecting many species of plants including tomatoes.
• It gives a distinctive ‘mosaic’ pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis.

• Salmonella food poisoning is spread by bacteria ingested in food, or on food prepared in unhygienic conditions.
• In the UK, poultry are vaccinated against Salmonella to control the spread.
• Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins they secrete.
• Gonorrhoea is a sexually transmitted disease (STD) with symptoms of a thick yellow or green discharge from the vagina or penis and pain on urinating.
• It is caused by a bacterium and was easily treated with the antibiotic penicillin until many resistant strains appeared.
• Gonorrhoea is spread by sexual contact.
• The spread can be controlled by treatment with antibiotics or the use of a barrier method of contraception such as a condom.

• Rose black spot is a fungal disease where purple or black spots develop on leaves, which often turn yellow and drop early.
• It affects the growth of the plant as photosynthesis is reduced.
• It is spread in the environment by water or wind.
• Rose black spot can be treated by using fungicides and/or removing and destroying the affected leaves.

• The pathogens that cause malaria are protists.
• The malarial protist has a life cycle that includes the mosquito.
• Malaria causes recurrent episodes of fever and can be fatal.
• The spread of malaria is controlled by preventing the vectors, mosquitos, from breeding and by using mosquito nets to avoid being bitten.

• If a pathogen enters the body the immune system tries to destroy the pathogen.
• White blood cells help to defend against pathogens by:
• - phagocytosis
• - antibody production
• - antitoxin production.

• Vaccination involves introducing small quantities of dead or inactive forms of a pathogen into the body to stimulate the white blood cells to produce antibodies.
• If the same pathogen re-enters the body the white blood cells respond quickly to produce the correct antibodies, preventing infection.

• Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body.
• It is important that specific bacteria should be treated by specific antibiotics.
• The use of antibiotics has greatly reduced deaths from infectious bacterial diseases.
• However, the emergence of strains resistant to antibiotics is of great concern.
• Antibiotics cannot kill viral pathogens.
• Painkillers and other medicines are used to treat the symptoms of disease but do not kill pathogens.
• It is difficult to develop drugs that kill viruses without also damaging the body’s tissues.

• Traditionally drugs were extracted from plants and microorganisms.
• - The heart drug digitalis originates from foxgloves.
• - The painkiller aspirin originates from willow.
• - Penicillin was discovered by Alexander Fleming from the Penicillium mould.
• Most new drugs are synthesised by chemists in the pharmaceutical industry.
• However, the starting point may still be a chemical extracted from a plant.
• New medical drugs have to be tested and trialled before being used to check that they are safe and effective.
• New drugs are extensively tested for toxicity, efficacy and dose.
• Preclinical testing is done in a laboratory using cells, tissues and live animals.
• Clinical trials use healthy volunteers and patients.
• - Very low doses of the drug are given at the start of the clinical trial.
• - If the drug is found to be safe, further clinical trials are carried out to find the optimum dose for the drug.
• - In double blind trials, some patients are given a placebo.

• Monoclonal antibodies are produced from a single clone of cells.
• The antibodies are specific to one binding site on one protein antigen and so are able to target a specific chemical or specific cells in the body.
• They are produced by stimulating mouse lymphocytes to make a particular antibody. The lymphocytes are combined with a particular kind of tumour cell to make a cell called a hybridoma cell.
• The hybridoma cell can both divide and make the antibody.
• Single hybridoma cells are cloned to produce many identical cells that all produce the same antibody.
• A large amount of the antibody can be collected and purified.

• Some examples include:
• - for diagnosis such as in pregnancy tests
• - in laboratories to measure the levels of hormones and other chemicals in blood, or to detect pathogens
• - in research to locate or identify specific molecules in a cell or tissue by binding to them with a fluorescent dye
• - to treat some diseases: for cancer the monoclonal antibody can be bound to a radioactive substance, a toxic drug or a chemical which stops cells growing and dividing. It delivers the substance to the cancer cells without harming other cells in the body.
• Monoclonal antibodies create more side effects than expected.
• They are not yet as widely used as everyone hoped when they were first developed.

• Plant diseases can be detected by:
• - stunted growth
• - spots on leaves
• - areas of decay (rot)
• - growths
• - malformed stems or leaves
• - discolouration
• - the presence of pests.
• Identification can be made by:
• - reference to a gardening manual or website
• - taking infected plants to a laboratory to identify the pathogen
• - using testing kits that contain monoclonal antibodies.
• Plants can be infected by a range of viral, bacterial and fungal pathogens as well as by insects.
• Plants can be damaged by a range of ion deficiency conditions:
• - stunted growth caused by nitrate deficiency
• - chlorosis caused by magnesium deficiency.

• Physical defence responses to resist invasion of microorganisms.
• - Cellulose cell walls.
• - Tough waxy cuticle on leaves.
• - Layers of dead cells around stems (bark on trees) which fall off.
• Chemical plant defence responses.
• - Antibacterial chemicals.
• - Poisons to deter herbivores.
• Mechanical adaptations.
• - Thorns and hairs deter animals.
• - Leaves which droop or curl when touched.
• - Mimicry to trick animals.

• Photosynthesis is represented by the equation:

• These factors interact and any one of them may be the factor that limits photosynthesis.
• Limiting factors are important in the economics of enhancing the conditions in greenhouses to gain the maximum rate of photosynthesis while still maintaining profit.

• The glucose produced in photosynthesis may be:
• - used for respiration
• - converted into insoluble starch for storage
• - used to produce fat or oil for storage
• - used to produce cellulose, which strengthens the cell wall
• - used to produce amino acids for protein synthesis.
• To produce proteins, plants also use nitrate ions that are absorbed from the soil.

• The energy transferred supplies all the energy needed for living processes. Respiration in cells can take place aerobically (using oxygen) or anaerobically (without oxygen), to transfer energy.
• Organisms need energy for:
• • chemical reactions to build larger molecules
• • movement
• • keeping warm.
• Aerobic respiration is represented by the equation:
• glucose + oxygen        carbon dioxide + water 
• Anaerobic respiration in muscles is represented by the equation:
• glucose        lactic acid
• As the oxidation of glucose is incomplete in anaerobic respiration much less energy is transferred than in aerobic respiration.
• Anaerobic respiration in plant and yeast cells is represented by the equation:
• glucose             ethanol + carbon dioxide
• Anaerobic respiration in yeast cells is called fermentation and has economic importance in the manufacture of bread and alcoholic drinks.

• During exercise the human body reacts to the increased demand for energy.
• The heart rate, breathing rate and breath volume increase during exercise to supply the muscles with more oxygenated blood.
• If insufficient oxygen is supplied anaerobic respiration takes place in muscles.
• The incomplete oxidation of glucose causes a build up of lactic acid and creates an oxygen debt.
• During long periods of vigorous activity muscles become fatigued and stop contracting efficiently.
• Blood flowing through the muscles transports the lactic acid to the liver where it is converted back into glucose.
• Oxygen debt is the amount of extra oxygen the body needs after exercise to react with the accumulated lactic acid and remove it from the cells. 

• Metabolism is the sum of all the reactions in a cell or the body.
• The energy transferred by respiration in cells is used by the organism for the continual enzyme controlled processes of metabolism that synthesise new molecules.
• Metabolism includes:
• - conversion of glucose to starch, glycogen and cellulose
• - the formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids - the use of glucose and nitrate ions to form amino acids which in turn are used to synthesise proteins
• - respiration
• - breakdown of excess proteins to form urea for excretion.

• 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.

• The nervous system enables humans to react to their surroundings and to coordinate their behaviour.
• Information from receptors passes along cells (neurones) as electrical impulses to the central nervous system (CNS).
• The CNS is the brain and spinal cord.
• The CNS coordinates the response of effectors which may be muscles contracting or glands secreting hormones.
• stimulus      receptor        coordinator        effector        response.
• Reflex actions are automatic and rapid; they do not involve the conscious part of the brain.

• The brain controls complex behaviour.
• It is made of billions of interconnected neurones and has different regions that carry out different functions.
• Neuroscientists have been able to map the regions of the brain to particular functions by studying patients with brain damage, electrically stimulating different parts of the brain and using MRI scanning techniques.
• The complexity and delicacy of the brain makes investigating and treating brain disorders very difficult.

• The eye is a sense organ containing receptors sensitive to light intensity and colour.
• Accommodation is the process of changing the shape of the lens to focus on near or distant objects.
• To focus on a near object:
• - the ciliary muscles contract
• - the suspensory ligaments loosen
• - the lens is then thicker and refracts light rays strongly.
• To focus on a distant object:
• - the ciliary muscles relax
• - the suspensory ligaments are pulled tight
• - the lens is then pulled thin and only slightly refracts light rays.
• 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.
• - Generally these defects are treated with spectacle lenses which refract the light rays so that they do focus on the retina.
• - New technologies now include hard and soft contact lenses, laser surgery to change the shape of the cornea and a replacement lens in the eye.

• 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.
• If the body temperature is too high, blood vessels dilate (vasodilation) and sweat is produced from the sweat glands.
• Both these mechanisms cause a transfer of energy from the skin to the environment.
• If the body temperature is too low, blood vessels constrict (vasoconstriction), sweating stops and skeletal muscles contract (shiver). 

• The endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream.
• The blood carries the hormone to a target organ where it produces an effect.
• Compared to the nervous system the effects are slower but act for longer.
• The pituitary gland 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.

• 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. 
• Type 1 diabetes is a disorder in which the pancreas fails to produce sufficient insulin. It is characterised by uncontrolled high blood glucose levels and is normally treated with insulin injections.
• In Type 2 diabetes the body cells no longer respond to insulin produced by the pancreas. A carbohydrate controlled diet and an exercise regime are common treatments. Obesity is a risk factor for Type 2 diabetes.
• 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.

• 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.
• If body cells lose or gain too much water by osmosis they do not function efficiently.
• 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 to form ammonia. Ammonia is toxic and so it is immediately converted to urea for safe excretion.
• The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water.
•  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. This is controlled by negative feedback.
• People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis. Students should know the basic principles of dialysis.

• During puberty reproductive hormones cause secondary sex characteristics to develop. Oestrogen is the main female reproductive hormone produced in the ovary.
• At puberty eggs begin to mature and one is released approximately every 28 days.
• This is called ovulation.
• Testosterone is the main male reproductive hormone produced by the testes and it stimulates sperm production.
• Several hormones are involved in the menstrual cycle of a woman.
• - Follicle stimulating hormone (FSH) causes maturation of an egg in the ovary.
• - Luteinising hormone (LH) stimulates the release of the egg.
• - Oestrogen and progesterone are involved in maintaining the uterus lining.

• Fertility can be controlled by a variety of hormonal and non-hormonal methods of contraception.
• These include:
• - oral contraceptives that contain hormones to inhibit FSH production so that no eggs mature
• - injection, implant or skin patch of slow release progesterone to inhibit the maturation and release of eggs for a number of months or years
• - barrier methods such as condoms and diaphragms which prevent the sperm reaching an egg
• - intrauterine devices which prevent the implantation of an embryo or release a hormone
• - spermicidal agents which kill or disable sperm
• - abstaining from intercourse when an egg may be in the oviduct
• - surgical methods of male and female sterilisation.

• This includes giving FSH and LH in a ‘fertility drug’ to a woman. She may then become pregnant in the normal way.
• In Vitro Fertilisation (IVF) treatment.
• - IVF involves giving a mother FSH and LH to stimulate the maturation of several eggs. • The eggs are collected from the mother and fertilised by sperm from the father in the laboratory.
• - The fertilised eggs develop into embryos.
• - At the stage when they are tiny balls of cells, one or two embryos are inserted into the mother’s uterus (womb). 
• Although fertility treatment gives a woman the chance to have a baby of her own:
• - it is very emotionally and physically stressful
• - the success rates are not high
• - it can lead to multiple births which are a risk to both the babies and the mother.

• Adrenaline is produced by the adrenal glands 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 stimulates the basal metabolic rate.
• It plays an important role in growth and development.
• Thyroxine levels are controlled by negative feedback.

• Plants produce hormones to coordinate and control growth and responses to light (phototropism) and gravity (gravitropism or geotropism).
• Unequal distributions of auxin cause unequal growth rates in plant roots and shoots.
• Gibberellins are important in initiating seed germination.
• Ethene controls cell division and ripening of fruits.
• The mechanisms of how gibberellins and ethene work are not required.

• Plant growth hormones are used in agriculture and horticulture.
• Auxins are used:
• - as weed killers
• - as rooting powders
• - for promoting growth in tissue culture.
• Ethene is used in the food industry to control ripening of fruit during storage and transport.
• Gibberellins can be used to:
• - end seed dormancy
• - promote flowering
• - increase fruit size.

• Sexual reproduction involves the joining (fusion) of male and female gametes:
• - sperm and egg cells in animals
• - pollen and egg cells in flowering plants.
• In sexual reproduction there is mixing of genetic information which leads to variety in the offspring.
• The formation of gametes involves meiosis.
• Asexual reproduction involves only one parent and no fusion of gametes.
• There is no mixing of genetic information.
• This leads to genetically identical offspring (clones).
• Only mitosis is involved.

• Cells in reproductive organs divide by meiosis to form gametes.
• When a cell divides to form gametes:
• - copies of the genetic information are made
• - the cell divides twice to form four gametes, each with a single set of chromosomes
• - all gametes are genetically different from each other.
• Gametes join at fertilisation to restore the normal number of chromosomes.
• The new cell divides by mitosis.
• The number of cells increases.
• As the embryo develops cells differentiate. 

• Advantages of sexual reproduction:
• - produces variation in the offspring
• - if the environment changes variation gives a survival advantage by natural selection
• - natural selection can be speeded up by humans in selective breeding to increase food production.
• Advantages of asexual reproduction:
• - 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.
• Some organisms reproduce by both methods depending on the circumstances.
• - Malarial parasites reproduce asexually in the human host, but sexually in the mosquito.
• - Many fungi reproduce asexually by spores but also reproduce sexually to give variation.
• - Many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils.

• The genetic material in the nucleus of a cell is composed of a chemical called DNA.
• DNA is a polymer made up of two strands forming a double helix.
• The DNA is contained in structures called chromosomes.
• A gene is a small section of DNA on a chromosome.
• Each gene codes for a particular sequence of amino acids, to make a specific protein.
• 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.

• DNA is a polymer made from four different nucleotides. Each nucleotide consists of a common sugar and phosphate group with one of four different bases attached to the sugar. DNA contains four bases, A, C, G and T. A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein. The long strands of DNA consist of alternating sugar and phosphate sections. Attached to each sugar is one of the four bases. The DNA polymer is made up of repeating nucleotide units.
• In the complementary strands a C is always linked to a G on the opposite strand and a T to an A.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.
• Mutations occur continuously. Most do not alter the protein, or only alter it slightly so that its appearance or function is not changed. 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.

• Some characteristics are controlled by a single gene, such as: fur colour in mice; and red-green colour blindness in humans.Each gene may have different forms called alleles.
• The alleles present, or genotype, operate at a molecular level to develop characteristics that can be expressed as a phenotype.
• A dominant allele is always expressed, even if only one copy is present. A recessive allele is only expressed if two copies are present (therefore no dominant allele present).
• If the two alleles present are the same the organism is homozygous for that trait, but if the alleles are different they are heterozygous.
• Most characteristics are a result of multiple genes interacting, rather than a single gene.

• Some disorders are inherited.
• These disorders are caused by the inheritance of certain alleles.
• - Polydactyly (having extra fingers or toes) is caused by a dominant allele.
• - Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.

• Ordinary human body cells contain 23 pairs of chromosomes.
• 22 pairs control characteristics only, but one of the pairs carries the genes that determine sex.
• • In females the sex chromosomes are the same (**).
• • In males the chromosomes are different (XY). 

• Evolution is 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.
• 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.

• Evolution is 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.
• 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.

• 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.
• Selective breeding can lead to ‘inbreeding’ where some breeds are particularly prone to disease or inherited defects.

• Genetic engineering as a process which involves modifying the genome of an organism by introducing a gene from another organism to give a desired characteristic.
• Plant crops have been genetically engineered to be resistant to diseases or to produce bigger better fruits.Bacterial cells have been genetically engineered to produce useful substances such as human insulin to treat diabetes.
• In genetic engineering, genes from the chromosomes of humans and other organisms can be ‘cut out’ and transferred to cells of other organisms.Crops that have had their genes modified in this way are called genetically modified (GM) crops. GM crops include ones that are resistant to insect attack or to herbicides. GM crops generally show increased yields.
• 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.Modern medical research is exploring the possibility of genetic modification to overcome some inherited disorders.
• In genetic engineering:
• • 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.