AQA · GCSE · Biology

Every living thing is built from cells, and the differences between them decide what each can do. This reference covers AQA GCSE Biology 8461, topic 4.1 Cell biology.

How the exam treats this topic:

• Paper 1 (4.1–4.4) carries Cell biology. Required practicals: RP1 (light microscope), RP2 (antiseptics/antibiotics on agar, biology only) and RP3 (osmosis in plant tissue).
• Culturing microorganisms is biology only; standard-form answers for bacterial numbers are HT only.
• Calculations the exam demands: magnification (image ÷ real), real size from image and magnification, area by πr², percentage change in mass, surface-area-to-volume ratio, bacterial division counts.

• Eukaryotic 真核 cells (plant and animal): genetic material enclosed in a nucleus.
• Prokaryotic 原核 cells (bacteria) are much smaller; cytoplasm and cell membrane surrounded by a cell wall; genetic material is not in a nucleus — a single DNA loop plus one or more plasmids (small rings of DNA).

Scale: use the prefixes centi (10⁻²), milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹) and order-of-magnitude comparisons in standard form.

Células especializadas relate structure to function (learn one feature ↔ one advantage each):

Diferenciación 分化: cells acquire different sub-cellular structures to become specialised. Animal cells differentiate mostly at an early stage; plant cells throughout life. In mature animals, cell division is mainly for repair and replacement.

Microscopy: a light microscope uses lenses; an electron microscope has much higher magnification and resolving power 分辨能力, so far more sub-cellular structures could be seen and understood.

Worked example. A cell image is 4.8 cm wide at magnification ×1200.

• Convert first: $4.8\ \text{cm} = 4.8\times10^{-2}$ m $= 48\,000\ \mu\text{m}$.
  $$\text{real size} = \frac{\text{image size}}{\text{magnification}} = \frac{48\,000\ \mu\text{m}}{1200} = 40\ \mu\text{m}$$

RP1: observe, draw and label plant and animal cells with a magnification scale included (e.g. a scale bar or the printed magnification).

Bacteria multiply by binary fission 二分裂 as often as once every 20 minutes with enough nutrients and a suitable temperature. They grow in nutrient broth or as colonies on agar gel.

Aseptic technique 无菌操作 (each point earns a mark — know the por qué):

• sterilise Petri dishes and culture media before use — kills unwanted microorganisms;
• flame the inoculating loop before transferring bacteria — kills contaminants on the loop;
• secure the lid with adhesive tape and store upside down — stops airborne contamination and condensation dripping;
• in schools, incubate at a maximum of 25 °C — prevents growth of pathogens harmful to humans.

Worked example (division count). One bacterium divides every 20 minutes for 3 hours.

• 3 hours = 180 min = 9 division times; number $= 2^9 = 512$.

Worked example (zone area). A clear zone of radius 4 mm around an antibiotic disc.

RP2: effect of antiseptics/antibiotics on bacterial growth — measure zones of inhibition with a ruler (several diameters, mean, ÷2), controls with no substance.

• El nucleus contains chromosomes 染色体 made of DNA; each chromosome carries many genes; body cells have chromosomes in pairs (humans: 23 pairs).
• El cell cycle: DNA replicates → growth (more ribosomes and mitochondria) → mitosis 有丝分裂 (one set of chromosomes pulled to each end; nucleus divides) → cytoplasm and membranes divide → two identical cells.

Mitosis matters for growth and development of multicellular organisms, and repair. Recognise contexts where mitosis is occurring (a growing root tip, healing skin, a tumour).

A stem cell 干细胞 is an undifferentiated cell that can give rise to many more of the same type and, by differentiation, to other cell types.

Uses and evaluation (the credited pairs — benefit y risk/objection):

• Medicina: potential treatment for diabetes and paralysis; therapeutic cloning makes an embryo with the patient's own genes, so cells are not rejected.
• Risks/ethics: transfer of viral infection; ethical and religious objections to using embryos.
• Plants: meristem clones — rare species preserved from extinction; disease-resistant crops cloned quickly and economically.

Rate of diffusion increases with: concentration gradient, temperatura, surface area of the membrane.

Relación superficie a volumen: single-celled organisms have a large SA:V — enough exchange across the surface alone. Multicellular organisms need exchange surfaces and a transport system. Exchange surfaces are effective by: large surface area, thin membrane (short diffusion path), (animals) good blood supply, (animals, gas exchange) ventilation.

Adapted exchange surfaces to know: small intestine and lungs in mammals, gills in fish, roots and leaves in plants.

Worked example (SA:V). A cube of side 2 mm: SA $= 6\times2^2 = 24$ mm², V $= 8$ mm³, SA:V $= 3:1$. A cube of side 4 mm: SA $= 96$, V $= 64$, SA:V $= 1.5:1$ — doubling the side halves the ratio.

RP3 (osmosis): potato pieces (same surface area, blotted dry, exact masses) in a range of sugar or salt solutions; measure percentage change in mass:

Interpretation: gain in dilute solutions (water enters); no change where the solution matches the cell concentration; loss in concentrated solutions (water leaves). Control variables: volume of solution, temperature, time, surface area.

• Label animal, plant and bacterial cells; match every structure to its function.
• Specialised cells: one structural feature linked to one function each.
• Magnification, real size and image size — convert units before dividing.
• Aseptic technique whys; binary-fission counts; πr² zones (RP1, RP2).
• Cell cycle stages and where mitosis occurs; stem cell sources with benefits and risks.
• Diffusion, osmosis, active transport: direction, energy, examples; the three rate factors.
• SA:V calculations; four exchange-surface features; percentage change in mass (RP3).

Cells form tissues, tissues form organs, organs form systems — and the digestive system, blood and plants each show the pattern. This reference covers AQA GCSE Biology 8461, topic 4.2 Organisation.

How the exam treats this topic:

• Paper 1 carries Organisation. Required practicals: RP4 (food tests) and RP5 (pH and amylase rate).
• You must evaluate heart-disease treatments and stem-cell/lifestyle risk data, and interpret graphs and tables of risk factors.
• Recall exactly: the enzymes table, the heart's vessels, blood components, plant tissues, transpiration factors.

Cells → tissues → organs → organ systems → organism.

• A tissue 组织 is a group of cells with a similar structure and function.
• An organ 器官 aggregates tissues performing specific functions.
• Organ systems 器官系统 work together to form the whole organism.

The digestive system is the named example of an organ system.

Enzimas 酶 are biological catalysts: proteins with an active site whose shape fits one substrate — the lock and key model. Temperature and pH change the rate; far from the optimum the enzyme denatures (active site changes shape).

Digestive enzymes turn food into small soluble molecules that can be absorbed into the bloodstream. The products build new carbohydrates, lipids and proteins; some glucose is used in respiration.

Bilis 胆汁: made in the liver, stored in the gall bladder. It is alcalina, neutralising hydrochloric acid from the stomach, and emulsifies fat into small droplets, increasing the surface area — both raise the rate of fat breakdown by lipase.

RP4 food tests (learn reagent, colour change, substance):

RP5 (pH and amylase): buffer solutions at pH values, amylase + starch, continuous sampling onto a spotting tile with iodine, record the time for the starch to disappear — shortest time = fastest rate at the optimum pH.

• The heart pumps blood in a double circulatory system: the right ventricle pumps blood to the lungs (gas exchange); the left ventricle pumps it around the rest of the body.
• Vessels to know: aorta, vena cava, pulmonary artery, pulmonary vein, coronary arteries.
• The resting heart rate is set by cells in the right atrium — the natural pacemaker; artificial pacemakers correct irregular rates.
• Lungs: trachea → bronchi → alveoli 肺泡, surrounded by capillaries; alveoli are adapted by a large surface area, thin walls, good blood supply and ventilation.

Sangre is a tissue: plasma 血浆 (carries dissolved sugars, amino acids, CO₂, urea, hormones, and heat), red blood cells 红细胞 (no nucleus, packed with haemoglobin, biconcave — large surface area for oxygen transport), white blood cells 白细胞 (engulf pathogens or make antibodies), platelets 血小板 (clotting).

In CHD layers of fatty material build up inside the coronary arteries, narrowing them — less blood, so less oxygen for the heart muscle. Evaluate each treatment with benefits and risks:

Health is the state of physical and mental well-being — not just the absence of disease. Diet, stress and life situations affect both; different diseases interact (immune defects → more infections; viruses → some cancers; immune reactions trigger asthma; severe physical illness → depression).

Risk factors 危险因素 are linked to an increased rate of a disease — aspects of lifestyle or substances in the body/environment. A causal mechanism is proven for some, not others: diet/smoking/exercise → cardiovascular disease; obesity → Type 2 diabetes; alcohol → liver damage and brain function; smoking → lung disease and lung cancer; smoking and alcohol → unborn babies; carcinogens including ionising radiation → cancer. Interpret risk-factor data as correlation; proven mechanism makes it causation.

Cancer results from changes in cells producing uncontrolled growth and division:

• Benign tumours: contained in one area, usually within a membrane; do not invade.
• Malignant tumours (cancers) invade neighbouring tissues and spread in the blood to form secondary tumours.

Risk factors include lifestyle and, for some cancers, genes.

• Epidermal tissue: protective covering; waxy cuticle on top.
• Mesófilo en empalizada 栅栏组织: packed chloroplasts — most photosynthesis.
• Mesófilo esponjoso: air spaces for gas exchange.
• Xilema 木质部: hollow tubes strengthened by lignin — carries water and mineral ions from roots to leaves in the transpiration stream.
• Floema 韧皮部: sieve tubes — carries dissolved sugars from leaves to the rest of the plant (translocation 运输).
• Meristem: at the growing tips of shoots and roots (stem cells).
• Guard cells 保卫细胞 surround stomata 气孔, controlling gas exchange and water loss.
• Root hair cells: large surface area; water enters by osmosis, mineral ions by active transport.

Transpiración 蒸腾作用 is the loss of water vapour from the leaves (mostly through stomata); it pulls the transpiration stream. The rate increases with: temperature ↑, air movement ↑, light intensity ↑ (stomata open); decreases with humidity ↑. Measure rate by water uptake with a potometer.

• Order cells → tissues → organs → systems with one example each.
• Enzyme table: name, site, substrate, products; lock and key; denaturing.
• Bile: two functions with reasons; RP4 tests and colours; RP5 method.
• Label the heart with all five named vessels; double circulation; pacemaker.
• Artery/vein/capillary structure ↔ function; four blood components with adaptations.
• CHD: cause + each treatment's benefit and risk.
• Risk factors with causal mechanisms; benign vs malignant tumours.
• Plant tissues and their jobs; xylem vs phloem; four transpiration factors.

Plants capture the Sun's energy; every cell releases it again. This reference covers AQA GCSE Biology 8461, topic 4.4 Bioenergetics.

How the exam treats this topic:

• Paper 1 carries Bioenergetics. RP6: light intensity and the rate of photosynthesis using pondweed.
• Limiting-factor graphs with two or three factors, the inverse square law and greenhouse economics are HT only.
• Equations are word equations — know the symbol forms too (CO₂, H₂O, O₂, C₆H₁₂O₆).

Photosynthesis is an endotérmica 吸热 reaction: energy is transferred from the environment to the chloroplasts by light.

Rate increases with: light intensity, CO₂ concentration, temperature (to the enzyme optimum), and the amount of chlorophyll.

Reading single-factor graphs: the flat top means something else has become the limiting factor 限制因素. (HT) On two- or three-factor graphs, decide which factor is limiting at each point.

(HT) Inverse square law: doubling the distance from the lamp quarters the light intensity:

(HT) Greenhouse economics: adding heat, light or CO₂ raises the rate — but each costs money; the grower adds the cheapest factor that is limiting, up to the point where extra yield no longer pays.

RP6: pondweed in water at different lamp distances; count the oxygen bubbles per minute (or collect the gas); control temperature and CO₂ (sodium hydrogencarbonate); repeat and mean; rate = bubbles ÷ time.

Worked example. Bubbles: 30 per minute at 10 cm; at 20 cm the light intensity is a quarter, so expect about 8 per minute (if light is still limiting).

Glucose from photosynthesis is used for:

• respiration;
• converted to insoluble starch for storage;
• fat or oil for storage;
• cellulose for cell walls;
• amino acids for protein — this also needs nitrate ions from the soil.

Respiration is an exotérmica 放热 reaction occurring continuously in living cells, transferring the energy for: building larger molecules, movement, and keeping warm.

Anaerobic respiration in yeast is fermentation 发酵, economically important in bread (CO₂ makes it rise) and alcoholic drinks.

During exercise the heart rate, breathing rate and breath volume aumentar to deliver more oxygenated blood to muscles. If oxygen supply is insufficient, muscles switch to anaerobic respiration: incomplete glucose oxidation builds up lactic acid 乳酸 → fatigue; an oxygen debt 氧债 builds.

(HT) Blood carries lactic acid to the liver, which converts it back to glucose. The oxygen debt is the extra oxygen needed after exercise to react with and remove the accumulated lactic acid.

Metabolism 代谢 is the sum of all reactions in a cell or body; respiration fuels them. It includes:

• glucose → starch, glycogen, cellulose;
• glycerol + 3 fatty acids → lipids;
• glucose + nitrate ions → amino acids → proteins;
• respiration;
• breakdown of excess protein → urea for excretion.

• Write the photosynthesis word and symbol equations; say endothermic + light→chloroplasts.
• Explain the four rate factors; read flattening single-factor graphs; (HT) inverse square law and greenhouse economics.
• Describe RP6 with its controls and the rate calculation.
• List five uses of glucose (with nitrate for protein).
• Compare aerobic with both anaerobic equations; name fermentation's two products and uses.
• Exercise: three increases; lactic acid and oxygen debt; (HT) the liver's role.
• Define metabolism with its five strands.

• Sexual reproduction mixes genetic information; asexual reproduction clones it. Meiosis halves chromosomes for gametes; fertilisation restores them.
• DNA → genes → alleles → phenotype: the Punnett square turns this into probabilities.
• Natural selection changes inherited characteristics across generations. Selective breeding, genetic engineering and cloning (biology only) use different mechanisms. Fossils and resistant bacteria provide evolutionary evidence; classification uses evidence of relationships.

Reproducción sexual — fusion of male and female gametes (sperm + egg in animals; pollen + egg in flowering plants). Meiosis forms gametes; mixing of genetic information gives variety in offspring. Reproducción asexual — one parent, no fusion, mitosis only: genetically identical offspring (clones 克隆).

Meiosis 减数分裂: in reproductive organs, copies of the genetic information are made first, then the cell divides dos veces to form four gametes, each with a single set of chromosomes — all genetically different. Fecundación restores the normal number; the new cell divides by mitosis and cells differentiate as the embryo develops.

(Bio) Sexual vs asexual: sexual gives variation — a survival advantage if the environment changes, and the variation selective breeding uses; asexual needs no mate (time and energy efficient), is faster, and produces many identical offspring when conditions are favourable. Malarial parasites: asexual in the human host, sexual in the mosquito; many fungi: asexual spores + sexual; strawberry plants: sexual seeds + asexual runners; daffodils: bulb division.

DNA and the genome: DNA is a polymer of two strands forming a double helix, held in the nucleus as chromosomes. A gene 基因 is a small section of DNA that codes for a sequence of amino acids → a specific protein. The genome 基因组 is the entire genetic material of the organism. Importance of the human genome: searching for genes linked to disease; understanding and treating inherited disorders; tracing human migration patterns from the past.

(Bio) DNA structure: a polymer of four different nucleotides 核苷酸 — common sugar + phosphate group + one of four bases — A 腺嘌呤, C 胞嘧啶, G 鸟嘌呤, T 胸腺嘧啶; alternating sugar–phosphate backbone; a sequence of three bases codes for one amino acid, and the base order controls the amino-acid order of the protein.

(HT Bio) Protein synthesis: on ribosomes, from a template; carrier molecules bring specific amino acids in the correct order; the chain folds into a unique shape that lets the protein work as an enzyme, hormone or structural protein (e.g. collagen). In complementary strands C pairs with G, T with A. Mutations change the base sequence: a mutation may leave the protein unchanged, alter it slightly, or change its shape so an enzyme no longer fits its substrate or a structural protein loses strength. Variants in non-coding DNA can switch genes on and off, changing how genes are expressed.

Apply the reproduction and DNA knowledge

Try first — strawberries. Seeds form after fertilisation; runners form without gamete fusion. Identify the two routes and explain which may help a population survive a new disease.

Worked reasoning. Seeds result from sexual reproduction: meiosis makes gametes and fertilisation mixes genetic information. Variation means some offspring may resist the disease. Runners form by mitosis from one parent: clones, apart from mutation. Asexual reproduction needs no mate and is fast in favourable conditions; neither route guarantees survival.

Try first — chromosome number. A human cell has 46 chromosomes before meiosis. Predict the number of gametes, their chromosome number, the number after fertilisation and the division growing the embryo.

Worked reasoning. DNA is copied before two divisions form four gametes, each with 23 chromosomes (one set). DNA copying does not change the count to 92 chromosomes at that point. Fertilisation combines 23 + 23 = 46; mitosis increases embryo cell number and cells differentiate. The diagram models chromosome numbers, not the stages of meiosis.

Try first — scale and code. Distinguish a gene, chromosome and genome. (HT Bio) Write the bases complementary to A C G T and explain whether a mutation must stop an enzyme working.

Worked reasoning. A gene is a DNA section coding an amino-acid sequence for a protein; a chromosome contains a long DNA molecule and many genes; a genome is all the organism’s genetic material. Complementary bases are T G C A. At ribosomes, a template and carrier molecules determine the amino-acid order; the chain folds. A mutation may leave the protein unchanged or change its shape and function. Non-coding variants can change whether genes are expressed.

Learn the terms: gamete 配子 (sex cell), chromosome 染色体 (DNA structure), gene (section coding a protein), allele 等位基因 (a form of a gene), dominante 显性 (expressed with one copy), recesivo 隐性 (expressed only with two), homozygous 纯合 (two same alleles), heterozygous 杂合 (two different), genotype 基因型 (alleles present), phenotype 表现型 (characteristics expressed).

Single-gene examples: fur colour in mice; red-green colour blindness. Most characteristics result from multiple genes interacting.

Punnett squares and crosses: complete a Punnett square, extract ratios (e.g. 3:1, 1:1) and probabilities from genetic crosses and family trees. (HT) Construct a cross from parent genotypes and predict outcomes with probability.

Inherited disorders: polydactyly 多指症 (extra fingers or toes) — dominante allele; cystic fibrosis 囊性纤维化 (a disorder of cell membranes) — recesivo allele. Make informed judgements about the economic, social and ethical issues of embryo screening.

Sex determination: body cells have 23 pairs of chromosomes; 22 pairs control characteristics, one pair carries the sex genes — female XX, male XY. A sex cross (X×X, X×Y) gives a 1:1 ratio, 50 % chance each.

From terms to family inference and probabilities

Try first — terminology. D causes dominant polydactyly; d does not. Classify Dd and predict its phenotype and gamete alleles.

Worked reasoning. Dd is heterozygous (two different alleles); DD and dd are homozygous. Genotype means the alleles present; phenotype means the characteristics expressed. Dd is affected because one D is expressed. “Dominant” does not mean stronger or more common. Gametes contain D or d, not Dd.

Guided family inference — adapted from AQA June 2024 Paper 2H Q06.2. An affected father and unaffected mother have an unaffected son. Deduce the father’s genotype before reading the answer.

Worked reasoning. The mother and son are dd. The father passed d to his son, but must also carry D because he is affected. Therefore he is Dd. The child’s genotype supplies evidence that the father’s phenotype alone cannot provide.

Construct a cross (HT). Two unaffected CF carriers are Cc and Cc. Each produces gametes C or c. Combining them gives CC, Cc, Cc, cc: probability of CF = 1/4; probability of being a carrier = 1/2. Carriers do not have CF in this recessive model. The separate XX × XY cross gives XX, XX, XY, XY: probability 1/2 each in the GCSE sex-determination model.

Exam transfer — adapted from AQA June 2024 Paper 2H Q06.3. An affected mother Dd and unaffected father dd have three unaffected sons. Construct a cross and predict whether the fourth child will be affected.

Worked reasoning. Egg alleles D/d combine with sperm alleles d/d to give Dd, dd, Dd, dd. Two of four equally likely combinations are affected: probability = 2/4 = 1/2. Earlier births do not change this probability. The four boxes describe possible combinations, not four promised children.

Evaluate — teacher-written screening scenario. Screening can identify embryos with the CF genotype, but treatment is costly and some embryos may not be used. A biological benefit is identifying cc embryos to inform the couple’s decisions. Cost can limit access and creates funding choices. Some families object to selecting or not using embryos; others prioritise reducing inherited disease. An informed judgement uses the stated evidence and recognises different values.

Variación 变异 — differences between individuals in a population — comes from genes inherited (genetic), conditions of development (environmental), or a combination. There is usually extensive genetic variation within a population; all genetic variants arise from mutations — most have no effect on the phenotype, some influence it, very few determine it. A rare mutation giving a new phenotype suited to an environmental change can change the species rapidly.

Evolución: a change in the inherited characteristics of a population over time, through natural selection 自然选择, which may result in a new species. All species evolved from simple life forms that first developed over three billion years ago. Natural selection: variation → the phenotype best suited to the environment survives and breeds → those characteristics are passed on. If two populations become so different they cannot interbreed to produce fertile offspring, two new species have formed.

Cría selectiva (artificial selection): choose parents with the desired characteristic from a mixed population → breed → choose the best offspring → repeat over many generations. Uses: disease resistance in food crops; animals with more meat or milk; dogs with gentle natures; large or unusual flowers. Risk: inbreeding 近交 — breeds prone to disease or inherited defects.

Genetic engineering: modifying the genome by introducing a gene from another organism to give a desired characteristic — GM crops resistant to insect attack or herbicides (increased yields); bacteria engineered to make human insulin. (HT) Steps: enzymes isolate the required gene → inserted into a vector (bacterial plasmid or virus) → vector inserts the gene into the required cells → transferred early in development so the organism develops with the desired characteristic. Weigh benefits (medicine, agriculture) against risks (wild-flower and insect populations, unexplored health effects) and ethical objections.

(Bio) Cloning: tissue culture — small groups of plant cells grown into identical new plants (preserving rare species; nurseries); cuttings — simple, older gardeners' method; embryo transplants — splitting unspecialised cells of a developing animal embryo into identical embryos placed in host mothers; adult cell cloning — nucleus removed from an unfertilised egg cell → nucleus from an adult body cell inserted → electric shock makes it divide into an embryo → ball of cells placed in the womb.

Apply variation, selection and biotechnology

Start with variation — teacher-written seedling case. Seedlings differ in inherited disease resistance. Genetically identical plants grow to different heights under different light conditions. Identify genetic and environmental variation, then explain what may happen during a disease outbreak.

Worked reasoning. Resistance alleles provide genetic variation; light conditions can cause environmental differences in height. Phenotype often depends on both. Resistant plants may survive the disease and reproduce more successfully, passing favourable alleles to offspring. Over generations, resistance alleles can become more common. Mutations change DNA; they do not occur because an organism needs a particular change.

Exam transfer — adapted from AQA June 2024 Paper 2H Q01.2. Describe natural selection using insects that already vary in inherited insecticide resistance.

Worked reasoning. The insecticide is a selection pressure: more susceptible insects die. Resistant survivors reproduce and pass resistance alleles to offspring. The population’s inherited characteristics change across generations; an individual does not evolve just by being sprayed. Speciation requires populations to become unable to interbreed to produce fertile offspring.

Compare mechanisms — teacher-written crop case. A grower wants inherited disease resistance without transferring a gene. Select resistant parents from a mixed population, breed them, select resistant offspring and repeat over many generations. This is selective breeding, not natural selection or genetic engineering. Repeatedly breeding close relatives risks inherited defects and reduced genetic variation.

Linked exam explanation — adapted from AQA June 2024 Paper 2H Q09.1. GM soya plants resist glyphosate. Explain how spraying the field can increase yield.

Worked reasoning. Glyphosate kills weeds but the resistant crop survives. Less competition gives the crop more light, water and mineral ions. More light and water can support photosynthesis, producing glucose for respiration and building biomass. Nitrate ions support amino-acid and protein synthesis. Link resources to growth and harvested yield; “GM means higher yield” omits the mechanism. These are authored explanation points checked against the scheme, not an official model or a guaranteed score.

HT — follow a gene. Enzymes isolate a required gene (for example, human insulin). Insert it into a vector such as a bacterial plasmid; use the vector to introduce it into the required cells. The modified cells express the gene and produce the substance. For a developing animal or plant, introduce the gene early so the organism develops with the desired characteristic.

Evaluate — teacher-written GM scenario. An insect-resistant crop suffers less target-pest damage, but a study reports fewer nearby non-target insects without establishing why. Reduced damage may improve harvest. The insect decline raises a food-web concern; compare modified and unmodified fields while accounting for habitat and pesticide use. Association alone does not prove the inserted gene caused the decline. Judge the particular modification and evidence.

Biology — choose and order cloning methods. A nursery can use tissue culture to grow plants from small groups of cells, or cuttings from a parent plant. Embryo transplants split cells before they specialise and place identical embryos into host mothers. Adult cell cloning removes an unfertilised egg’s nucleus, inserts an adult body-cell nucleus, stimulates division with an electric shock, and transfers the ball of cells to a womb. The adult body-cell donor supplies the nuclear genetic information. Environmental differences can still affect a clone’s phenotype.

(Bio) Darwin's theory: wide variation within a species; individuals best suited to the environment survive to breed; the useful characteristics are passed on. Published in On the Origin of Species (1859). Accepted only gradually: it challenged the idea that God made all living things; insufficient evidence at publication; the mechanism of inheritance was unknown for another 50 years. Lamarck's rival theory — that changes acquired during an organism's lifetime are inherited — is now known to be wrong in almost all cases.

(Bio) Wallace: independently proposed evolution by natural selection; joint 1858 publications with Darwin prompted Darwin to publish; best known for warning colouration and pioneering work on speciation — the steps by which new species arise.

(Bio) Mendel and the growth of genetics: mid-19th-century breeding experiments showed each characteristic is inherited through 'units' passed unchanged to descendants; late 19th century — chromosomes observed in cell division; early 20th century — the units behave like chromosomes → genes on chromosomes; mid-20th century — DNA structure determined and gene function worked out. Mendel's work was unrecognised in his lifetime.

(Bio) Evidence for evolution: genes show characteristics pass to offspring; fossils; antibiotic resistance in bacteria.

(Bio) Fossils form when parts do not decay (a decay condition is absent), when parts are replaced by minerals as they decay, or as preserved traces (footprints, burrows, rootlet traces). Early soft-bodied life left few traces, mostly destroyed by geological activity — so science cannot be certain how life began. Extinction means no individuals of a species remain — causes include new diseases, new predators, competition, environmental change, catastrophic events. Evolutionary trees use current classification data and fossil data.

(Bio) Resistant bacteria: mutations produce genetic variants; some are resistant to a particular antibiotic. Susceptible bacteria are killed, while resistant survivors reproduce quickly and pass on resistance genes. The resistant strain becomes more common and can spread (for example, MRSA). This population change is evidence for evolution. Resistance can make infection harder to treat; it does not mean every antibiotic is ineffective or that people have no immune defence. Reducing inappropriate prescribing and unnecessary agricultural antibiotic use reduces avoidable selection pressure. Developing new antibiotics takes time and resources.

Try first — resistance explanation. A population contains susceptible and resistant variants before antibiotic exposure. Explain why resistant bacteria become more common, and correct “the antibiotic makes every bacterium mutate because it needs resistance”.

Worked reasoning. Genetic variation already exists; mutations are not directed by need. Antibiotic exposure selects survivors. Resistant bacteria reproduce and pass resistance genes to offspring, increasing their share of the population. Selection and inheritance explain the change without claiming that every individual becomes resistant.

Use historical and fossil evidence

Explain the history (Biology). Darwin and Wallace independently proposed natural selection; their joint writings appeared in 1858 and Darwin published his book in 1859. Acceptance was gradual: the theory challenged prevailing creation beliefs, some scientists found the evidence insufficient, and inheritance mechanisms were unknown. Mendel’s plant breeding suggested inherited units. His work’s importance was recognised after his death; later chromosome behaviour, genes on chromosomes and DNA structure connected these units to physical mechanisms.

Apply speciation — teacher-written case. A barrier separates a population into different habitats. Different selection pressures favour different variants; survivors reproduce and pass favourable alleles to offspring. Mutations supply genetic variants. Over many generations the populations may diverge. Separation alone does not prove a new species: inability to interbreed to produce fertile offspring is the stated species criterion.

Interpret evidence — teacher-written fossil case. A footprint preserved in rock is a trace fossil; a shell replaced by minerals illustrates mineral replacement. Conditions that prevent decay can also preserve remains. Soft-bodied organisms may leave no trace, and geological activity destroys fossils. An incomplete record still supplies evidence of past organisms and change. A species missing from one site is not necessarily extinct: extinction means no living individuals remain anywhere.

Read the diagram. Trace A and B back to their first shared fork: this is their most recent common ancestor. A and C share the older ancestor of all four species, so A and B are more closely related. This is a schematic without a time scale; do not infer dates from branch lengths or relationships from vertical spacing. Trees use classification and fossil evidence.

Linnaeus: classification by structure and characteristics into kingdom, phylum, class, order, family, genus, species; organisms named by the binomial system (genus + species).

Three-domain system (Carl Woese), from chemical-analysis evidence: archaea (a distinct domain of prokaryotes, including many organisms from extreme environments), bacteria (true bacteria), eukaryota (protists, fungi, plants, animals). Improvements in microscopes and biochemistry drove the new models.

Evolutionary trees show how scientists believe organisms are related — interpret them using classification and fossil data.

Apply classification. In Panthera leo y Panthera tigris, Panthera identifies the shared genus; the second word distinguishes the species within it. The ranks are kingdom → phylum → class → order → family → genus → species. New microscopy and chemical-analysis evidence can reveal relationships that external appearance does not show, leading to revised classification. Woese’s three domains are archaea, bacteria and eukaryota.

Final retrieval. Without the answers, explain (1) how 23-chromosome gametes restore 46 at fertilisation; (2) the next affected-child probability for Dd × dd after three unaffected births; (3) how antibiotic exposure changes a bacterial population. Check: two gamete sets combine; the cross gives a 1/2 probability independently at each birth; resistant survivors reproduce and pass on resistance genes. Then use the earlier worked examples to retrieve gene transfer, cloning order and common ancestry.

• Meiosis (four gametes, halved number) vs mitosis; sexual vs asexual advantages with named organisms.
• All ten genetics terms; Punnett squares with ratios; polydactyly vs cystic fibrosis; XX/XY cross 1:1.
• (Bio) DNA nucleotides and bases; (HT) protein synthesis and mutations.
• Variation three sources; natural selection sequence; selective breeding steps and inbreeding risk; (HT) genetic engineering steps; (Bio) four cloning methods.
• (Bio) Darwin/Wallace/Mendel history, fossil formation and why the record is incomplete, MRSA story, Linnaeus ranks and the three domains.