GCSE Biology (AQA) key facts

Cell biology

Cell structure and specialisation

• Nucleus: holds the genetic material and controls the cell. Cytoplasm: where most chemical reactions happen. Cell membrane: controls what enters and leaves.
• Mitochondria: aerobic respiration, which releases energy. Ribosomes: where proteins are made.
• Plant cells only: chloroplasts (photosynthesis), permanent vacuole filled with cell sap, cell wall made of cellulose (strengthens the cell).
• Bacterial cell: cytoplasm, cell membrane, cell wall, one loop of DNA, plasmids. No nucleus, no mitochondria, no chloroplasts.
• One order of magnitude = 10 times bigger. 100 times = 10² = 2 orders of magnitude.
• Sperm: tail and many mitochondria. Nerve cell: long, with branched ends. Muscle cell: many mitochondria. Root hair cell: long extension gives a large surface area.
• Xylem: dead hollow tubes with no end walls, strengthened by lignin. Phloem: living cells with pores in the end walls.

Microscopy and culturing microorganisms

• magnification = size of image ÷ size of real object (M = I ÷ A)
• real size = image size ÷ magnification. Put both sizes in the same unit first.
• 1 mm = 1000 µm. 1 µm = 1000 nm. 1 µm = 10⁻⁶ m.
• number of divisions = total time ÷ mean division time. Final number = starting number × 2^{number of divisions}.
• Aseptic technique: sterilise Petri dishes and agar before use, pass the inoculating loop through a flame, tape the lid without sealing it all round, store the dish upside down.
• In school, incubate at no more than 25 °C so that harmful pathogens are less likely to grow.
• area of a colony or clear zone = πr², where r = diameter ÷ 2.

Cell division

• Size order: nucleus, then chromosome, then gene (largest to smallest).
• Mitosis gives two cells that are genetically identical, each with the full number of chromosomes (46 in humans).
• The DNA is copied before mitosis, so the amount of DNA doubles and is then shared equally between the two new cells.
• Mitosis is used for growth, for repair and replacement of cells, and for asexual reproduction.
• Embryonic stem cells can become most types of human cell. Adult bone marrow stem cells can form many types of cell, including blood cells.
• Plant meristems (root and shoot tips) can form any type of plant cell throughout the life of the plant, so plants can be cloned quickly and cheaply.
• Therapeutic cloning: an embryo is made with the same genes as the patient, so its stem cells are not rejected. Risks: transfer of a viral infection; some people have ethical or religious objections.

Transport in cells

• Diffusion is faster with a bigger concentration difference, a higher temperature and a larger membrane surface area.
• For a cube: surface area = 6 × side², volume = side³. Ratio = surface area ÷ volume.
• The bigger the organism, the smaller its surface area to volume ratio, so it needs exchange surfaces and a transport system.
• Good exchange surfaces: large surface area, thin membrane (short diffusion path), good blood supply, and ventilation in animals.
• Animal cell in pure water: swells and may burst. Plant cell in pure water: becomes turgid, because the cell wall stops it bursting.
• Plant cell in a concentrated solution: water leaves the vacuole and the membrane pulls away from the cell wall.
• percentage change in mass = (final mass − starting mass) ÷ starting mass × 100. Where the graph crosses the x-axis there is no net movement of water.

Organisation

Organisation and the digestive system

• Order of size: cell, tissue, organ, organ system, organism.
• Carbohydrases break carbohydrates into simple sugars. Amylase breaks starch into sugars; it is made in the salivary glands, pancreas and small intestine.
• Proteases break proteins into amino acids; made in the stomach, pancreas and small intestine.
• Lipases break lipids (fats) into glycerol and fatty acids; made in the pancreas and small intestine.
• Bile is made in the liver and stored in the gall bladder. It is alkaline, so it neutralises stomach acid, and it emulsifies fat to give a larger surface area for lipase. Bile is not an enzyme.
• Food tests: iodine turns blue-black with starch. Benedict’s solution, when heated, turns from blue to green, yellow or orange-red with reducing sugar.
• Biuret reagent turns from blue to purple with protein. Ethanol gives a cloudy white emulsion with lipids.

The heart, blood vessels and blood

• Body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body.
• Arteries carry blood away from the heart at high pressure: thick walls of muscle and elastic tissue. Veins carry blood back at low pressure: thinner walls, wide lumen, valves.
• Capillaries have walls one cell thick, so substances diffuse quickly between blood and tissues.
• rate of blood flow = volume of blood ÷ time
• Air path: trachea, bronchi, alveoli. Alveoli have a very large surface area, thin walls and a good blood supply.
• Red blood cells: no nucleus, full of haemoglobin, carry oxygen. White blood cells: defend against pathogens. Platelets: help blood to clot. Plasma: carries dissolved substances such as glucose, urea and carbon dioxide.
• An artificial pacemaker is an electrical device that corrects an irregular heart rate. The coronary arteries supply the heart muscle with blood.

Coronary heart disease and non-communicable diseases

• Stent: a mesh tube that holds a coronary artery open. Statins: drugs that lower blood cholesterol, so fatty material builds up more slowly.
• Faulty heart valves may not open fully or may leak. They can be replaced with biological or mechanical valves.
• Heart failure can be treated with a donor heart, which may be rejected by the immune system. An artificial heart can keep a patient alive while waiting.
• Diseases interact: a weak immune system makes infections more likely; some viruses can trigger cancers; severe physical illness can lead to depression.
• Smoking: lung disease, lung cancer and heart disease. Alcohol: liver and brain damage. Obesity: type 2 diabetes. Smoking and alcohol also harm unborn babies.
• Benign tumour: stays in one place, usually inside a membrane; not cancer. Malignant tumour: cancer; invades nearby tissue and spreads in the blood to form secondary tumours.
• Cancer risk factors can be lifestyle (smoking, UV light) or genetic (inherited alleles).

Plant tissues, organs and organ systems

• Upper epidermis: transparent, covered by a waxy cuticle that reduces water loss. Palisade mesophyll: packed with chloroplasts. Spongy mesophyll: air spaces for gases to diffuse.
• Xylem: dead hollow tubes with no end walls, strengthened by lignin. Flow is one way, upwards.
• Phloem: elongated living cells with pores in the end walls, so cell sap can pass from cell to cell.
• Guard cells control the size of the stomata. Turgid guard cells: stomata open. Flaccid guard cells: stomata close and less water is lost.
• Transpiration is faster when it is warmer, when the air is drier (less humid), when the air moves more and when the light is brighter.
• Potometer: volume of water taken up = distance moved by the bubble × cross-sectional area of the tube. Rate = volume ÷ time.

Infection and response

Communicable diseases

• Measles (virus): spread by droplets from coughs and sneezes. Fever and a red skin rash. Can be fatal, so most young children are vaccinated.
• HIV (virus): spread by sexual contact or exchange of body fluids such as blood. Flu-like illness at first, then it attacks immune cells. Controlled with antiretroviral drugs; late stage is AIDS.
• Tobacco mosaic virus: mosaic pattern of discoloured patches on leaves of many plants, including tomatoes. Less photosynthesis, so less growth.
• Salmonella (bacteria): from food that is undercooked or prepared in unhygienic conditions. Fever, abdominal cramps, vomiting, diarrhoea. In the UK poultry are vaccinated.
• Gonorrhoea (bacteria): sexually transmitted. Thick yellow or green discharge and pain when urinating. Treated with antibiotics, but many strains resist penicillin; a condom prevents spread.
• Rose black spot (fungus): purple or black spots on leaves, which turn yellow and drop early. Spread by water or wind. Use fungicides and remove and destroy affected leaves.
• Malaria (protist): carried by mosquitoes, which are the vectors. Repeated fevers; can be fatal. Stop mosquitoes breeding and use mosquito nets.

Human defence systems and vaccination

• Skin: a barrier that also produces antimicrobial secretions. Nose: hairs and mucus trap particles.
• Trachea and bronchi: mucus traps pathogens and cilia sweep the mucus up to the throat.
• Stomach: hydrochloric acid kills most pathogens in food and drink.
• Phagocytosis: a white blood cell engulfs a pathogen and digests it.
• Antibodies bind to the antigens on one type of pathogen and help to destroy it. Antitoxins neutralise the toxins made by bacteria.
• A vaccine contains no antibodies and is not a medicine: it makes the body produce its own antibodies.
• Herd immunity: when a large proportion of people are immune, the pathogen cannot spread easily, so unvaccinated people are also less likely to meet it.

Antibiotics, painkillers and drug development

• Antibiotics kill bacteria only. A specific antibiotic is used for a specific bacterium.
• Painkillers relieve symptoms but do not kill pathogens.
• To slow down resistance: no antibiotics for viral or mild infections, and patients finish the whole course.
• Drug sources: digitalis (a heart drug) from foxgloves, aspirin from willow, penicillin from the Penicillium mould (Alexander Fleming).
• Preclinical testing: on cells, tissues and live animals, before any person takes the drug.
• Clinical trials: very low doses to healthy volunteers first (to check safety), then to patients to find the optimum dose.
• Double blind trial: neither the patients nor the doctors know who has the drug and who has the placebo.

Monoclonal antibodies and plant disease

• Hybridoma cell = mouse lymphocyte (makes the antibody) + tumour cell (divides quickly).
• Uses: pregnancy tests (they bind to a hormone made only in pregnancy), measuring hormones and detecting pathogens in laboratories, finding molecules in a tissue with a fluorescent dye.
• Cancer treatment: the antibody carries a radioactive substance, a toxic drug or a chemical that stops cell division straight to the cancer cells, without harming other cells.
• Disadvantage: they cause more side effects than expected, so they are not yet used as widely as hoped.
• Nitrate deficiency → stunted growth (nitrate is needed to make proteins). Magnesium deficiency → chlorosis, yellow leaves (magnesium is needed to make chlorophyll).
• Identifying a plant disease: a gardening manual or website, laboratory tests, or a testing kit with monoclonal antibodies.
• Defences. Physical: cellulose cell walls, waxy cuticle, bark. Chemical: antibacterial chemicals, poisons. Mechanical: thorns and hairs, leaves that droop or curl when touched, mimicry.

Bioenergetics

Photosynthesis

• carbon dioxide + water → glucose + oxygen (light is needed)
• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
• Inverse square law: light intensity ∝ 1 ÷ distance². Double the distance → a quarter of the intensity.
• Less chlorophyll (for example from a lack of magnesium ions) → less light absorbed → slower photosynthesis.
• Uses of glucose: respiration, insoluble starch for storage, fats or oils for storage, cellulose to strengthen cell walls, amino acids for making proteins.
• To make amino acids from glucose the plant also needs nitrate ions from the soil.

Respiration

• Aerobic: glucose + oxygen → carbon dioxide + water
• C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
• Anaerobic in muscles: glucose → lactic acid
• Anaerobic in plant and yeast cells: glucose → ethanol + carbon dioxide. In yeast this is called fermentation (used to make bread and alcoholic drinks).
• Oxygen debt: the extra oxygen needed after exercise to react with the lactic acid and remove it from the cells.
• Blood carries lactic acid to the liver, where it is converted back into glucose.
• Metabolism = the sum of all the reactions in a cell or the body, for example making lipids from glycerol and fatty acids, or breaking down excess protein to urea.

Homeostasis and response

Homeostasis and the nervous system

• Homeostasis: the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes.
• stimulus → receptor → coordinator → effector → response
• Central nervous system (CNS) = brain and spinal cord.
• Reflex arc: receptor → sensory neurone → relay neurone (in the CNS) → motor neurone → effector.
• Sensory neurones carry impulses from receptors to the CNS. Motor neurones carry impulses from the CNS to effectors.
• Synapse: the gap between two neurones. A chemical diffuses across it and starts an impulse in the next neurone.
• Mean reaction time = sum of the times ÷ number of times. Leave out any anomalous result first.

The brain, the eye and body temperature control

• Cerebral cortex: consciousness, intelligence, memory, language. Cerebellum: muscle coordination and balance. Medulla: unconscious activities such as heartbeat and breathing.
• Retina: receptor cells sensitive to light intensity and colour. Optic nerve: carries impulses to the brain. Sclera: tough outer layer. Iris: controls the size of the pupil (in bright light the circular muscles contract and the pupil gets smaller).
• Near object: ciliary muscles contract → suspensory ligaments loosen → lens thicker, refracts light strongly.
• Distant object: ciliary muscles relax → suspensory ligaments pulled tight → lens thinner, refracts light only slightly.
• Myopia (short sight): light focuses in front of the retina; corrected with a concave lens. Hyperopia (long sight): light focuses behind the retina; corrected with a convex lens.
• Other treatments for sight: contact lenses, laser eye surgery, a replacement lens in the eye.
• Too hot: vasodilation and sweating. Too cold: vasoconstriction, sweating stops, shivering (muscles respire more and transfer energy).

The endocrine system and blood glucose control

• Glands and hormones: pituitary (the master gland, its hormones act on other glands), thyroid (thyroxine), adrenal glands (adrenaline), pancreas (insulin and glucagon), ovaries (oestrogen), testes (testosterone).
• Glucose too high → pancreas releases insulin → glucose moves from the blood into cells → liver and muscle cells store the excess as glycogen.
• Glucose too low → pancreas releases glucagon → glycogen is converted into glucose and released into the blood.
• Type 1 diabetes: too little insulin is produced, so blood glucose stays high. Treated with insulin injections.
• Type 2 diabetes: cells no longer respond to insulin. Treated first with a carbohydrate-controlled diet and regular exercise. Obesity is a risk factor.
• Glucose is the sugar, glycogen is the storage carbohydrate, glucagon is the hormone.

Maintaining water and nitrogen balance

• Filtration: water, glucose, ions and urea pass out of the blood. Proteins are too large to be filtered.
• Selective reabsorption: all of the glucose, some ions and as much water as the body needs go back into the blood. Urine is what is left.
• Excess amino acids are deaminated in the liver → ammonia (toxic) → converted at once to urea, which is safer to carry in the blood.
• Blood too concentrated → pituitary gland releases more ADH → kidney tubules more permeable → more water reabsorbed → small volume of concentrated urine.
• Blood too dilute → less ADH → less water reabsorbed → large volume of dilute urine. This is negative feedback.
• Dialysis fluid has the same glucose and ion concentrations as healthy blood and no urea, so only urea and any excess ions diffuse out of the blood.
• Transplant: no regular dialysis sessions, but donors are few, the kidney may be rejected and immunosuppressant drugs are needed.

Hormones in reproduction and negative feedback

• FSH (pituitary): makes an egg mature in the ovary.
• Oestrogen (ovaries): builds up the uterus lining and inhibits FSH.
• LH (pituitary): triggers ovulation, the release of the egg.
• Progesterone (ovaries): maintains the uterus lining after ovulation and during pregnancy.
• The pill contains hormones that inhibit FSH, so no eggs mature. Implants, injections and patches release progesterone slowly, which stops eggs maturing and being released for months or years.
• IVF: FSH and LH given → eggs collected → fertilised by sperm in the laboratory → embryos develop → one or two inserted into the uterus. It is stressful, success rates are low and multiple births are possible.
• Thyroxine (thyroid) stimulates the basal metabolic rate and is kept steady by negative feedback. Adrenaline (adrenal glands) raises the heart rate and sends more oxygen and glucose to the brain and muscles: 'fight or flight'.

Plant hormones

• In shoots, more auxin → cells elongate more. The side with more auxin grows faster, so the shoot bends away from that side.
• In roots, more auxin → cells elongate less. The lower side grows more slowly, so the root bends downwards.
• Shoot towards light = positive phototropism. Root downwards = positive gravitropism. Shoot upwards = negative gravitropism.
• The tip of the shoot detects the light. If the tip is removed or covered, the shoot does not bend towards light.
• Uses of auxins: selective weedkillers (broad-leaved weeds grow out of control and die), rooting powders for cuttings, promoting growth in tissue culture.
• Uses of gibberellins: end seed dormancy, promote flowering, increase fruit size.
• Use of ethene: ripening fruit during storage and transport, so it is ripe when it reaches the shops.

Inheritance, variation and evolution

Reproduction, meiosis and DNA

• Human body cells have 46 chromosomes (23 pairs); human gametes have 23.
• Meiosis: one cell → four genetically different gametes, each with half the chromosome number.
• Sexual reproduction gives variation, which helps a species survive change. Asexual reproduction is fast, needs one parent and no mate, but gives no variation.
• Genome = all the genetic material of an organism.
• Each nucleotide has a sugar, a phosphate and one of four bases: A, C, G, T. A pairs with T and C pairs with G.
• Three bases code for one amino acid.
• A mutation is a change in the base sequence. Most have little or no effect on the protein; a few change its shape so it no longer works.

Genetic inheritance, inherited disorders and sex determination

• Homozygous = two alleles the same (TT or tt). Heterozygous = two different alleles (Tt).
• Capital letter = dominant allele; small letter = recessive allele.
• Heterozygous × heterozygous gives 3 dominant : 1 recessive phenotype (probability of recessive = 0.25).
• Heterozygous × homozygous recessive gives 1 : 1 (probability 0.5).
• Polydactyly (extra fingers or toes) is caused by a dominant allele. Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
• A carrier has one recessive allele for a disorder but does not have the disorder.
• All eggs carry X. Half the sperm carry X and half carry Y, so the chance of a boy is 0.5 at each birth.

Variation, evolution and genetic technologies

• Natural selection: variation → competition → the best suited survive and breed → their alleles are passed on.
• A new species has formed when two populations can no longer interbreed to produce fertile offspring.
• Selective breeding: choose parents with the desired characteristic, breed them, choose the best offspring, repeat for many generations.
• Risk of selective breeding: inbreeding, which makes some breeds prone to disease or inherited defects.
• Genetic engineering: enzymes cut out the gene → it is inserted into a vector (a bacterial plasmid or a virus) → the vector puts the gene into the target cells at an early stage of development.
• GM crops can resist insects or herbicides and give higher yields. Concerns: effects on wild flowers and insects, and possible effects on human health that are not fully studied.
• Adult cell cloning: the nucleus of an adult body cell is put into an egg cell that has had its nucleus removed; an electric shock makes it divide; the embryo is put into a female's womb.

The development of understanding of genetics and evolution

• Darwin published On the Origin of Species in 1859. Wallace also worked on warning colouration and speciation.
• Speciation: populations become isolated → each has variation → natural selection differs in each place → they become so different that they cannot interbreed to produce fertile offspring.
• Mendel: characteristics are decided by 'units' that are passed on unchanged. These units are now called genes.
• Fossils form when hard parts are replaced by minerals, when decay cannot happen (no oxygen, too cold, too acidic), or as preserved traces such as footprints and burrows.
• The fossil record is incomplete: early life was soft-bodied and many fossils were destroyed by geological activity.
• Extinction can follow a new predator, new disease, new competitor, a change in the environment or a catastrophic event.
• To slow antibiotic resistance: do not prescribe antibiotics for viral or mild infections, complete the whole course, and restrict their use in farming.

Classification of living organisms

• Order of groups: kingdom → phylum → class → order → family → genus → species.
• Binomial name = genus then species, e.g. Homo sapiens: genus Homo, species sapiens.
• Archaea: primitive bacteria, usually living in extreme environments.
• Bacteria: true bacteria.
• Eukaryota: protists, fungi, plants and animals (cells with a nucleus).
• On an evolutionary tree, each branch point is a common ancestor. The more recent the shared branch point, the more closely related the two species are.

Ecology

Adaptations, interdependence and competition

• Abiotic factors: light intensity, temperature, moisture, soil pH and mineral content, wind intensity and direction, carbon dioxide level (plants), oxygen level (aquatic animals).
• Biotic factors: availability of food, new predators, new pathogens, one species outcompeting another.
• Structural adaptation = a feature of the body, e.g. thick fur, large ears, spines.
• Behavioural adaptation = something the organism does, e.g. migrating, hunting at night.
• Functional adaptation = a process inside the body, e.g. making concentrated urine, antifreeze proteins.
• Extremophiles live in extreme conditions: high temperature, high pressure or high salt concentration, e.g. bacteria in deep-sea vents.

Organisation of an ecosystem

• Estimated population = mean number per quadrat × (total area ÷ area of one quadrat).
• Predator–prey cycle: more prey → more predators → fewer prey → fewer predators. The predator peak comes just after the prey peak.
• Carbon cycle: photosynthesis removes carbon dioxide from the air; respiration, decay by microorganisms and combustion return it.
• Decay is fastest when it is warm and moist with plenty of oxygen.
• Biogas (mainly methane) is made when microorganisms decay waste anaerobically (without oxygen) in a biogas generator.
• Changes in temperature, availability of water and atmospheric gases change where species live. These changes may be seasonal, geographic or caused by humans.

Biodiversity and the effect of human interaction on ecosystems

• Water pollution: sewage, fertiliser, toxic chemicals. Air pollution: smoke and acidic gases. Land pollution: landfill and toxic chemicals.
• Deforestation in tropical areas provides land for cattle and rice fields and for crops to make biofuels.
• Fewer trees means less photosynthesis, so less carbon dioxide is removed; burning or decay of the trees releases more.
• Peat stores carbon. When peat is dug up for compost it decays or is burned, releasing carbon dioxide, and the bog habitat is lost.
• Effects of global warming: loss of habitat, changes in distribution of species, changes in migration patterns, reduced biodiversity.
• Protecting biodiversity: breeding programmes for endangered species, protecting and regenerating rare habitats, replanting hedgerows and field margins, reducing deforestation and carbon dioxide emissions, recycling instead of landfill.

Trophic levels in an ecosystem

• Efficiency of transfer = (biomass passed to the higher level ÷ biomass at the lower level) × 100.
• Producers transfer about 1% of the incident light energy into biomass.
• About 10% of the biomass at one level is transferred to the level above.
• Biomass is lost because: not all the food is eaten or absorbed (some is egested as faeces); much glucose is used in respiration, giving waste carbon dioxide and water; water and urea are lost in urine.
• Decomposers (bacteria and fungi) secrete enzymes; the small soluble food molecules then diffuse into the microorganism.
• In a pyramid of biomass, trophic level 1 is always the bottom bar and bar widths are drawn to scale.

Food production

• Threats to food security: rising birth rate, changing diets in developed countries, new pests and pathogens, environmental changes (e.g. failed rains), cost of farming inputs, armed conflict.
• Intensive farming: animals are kept in warm sheds with limited movement, so less energy is transferred to the surroundings and more food becomes biomass. Some are fed high-protein food to speed up growth.
• Ethical objections: the animals cannot behave naturally and disease spreads easily in crowded conditions.
• Fishing quotas limit the mass of fish that may be caught. Larger net mesh lets young fish escape to grow and breed.
• Mycoprotein: Fusarium is grown on glucose syrup in aerobic conditions, then the biomass is harvested and purified.
• GM bacteria make human insulin, which is harvested and purified to treat diabetes.