AQA · GCSE · Biology

• 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.

Specialised cells relate structure to function (learn one feature ↔ one advantage each):

Differentiation 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 why):

• 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.

• The nucleus contains chromosomes 染色体 made of DNA; each chromosome carries many genes; body cells have chromosomes in pairs (humans: 23 pairs).
• The 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 and risk/objection):

• Medicine: 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, temperature, surface area of the membrane.

Surface area to volume ratio: 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.

細胞は組織を形成し、組織は器官を形成し、器官は器官系を形成する — 消化器系、血液、植物はそれぞれこのパターンを示している。この参考書は AQA GCSE生物学 8461、トピック 4.2 組織化 を扱っている。

試験におけるこのトピックの扱い方：

• パーザー1には「組織化」が含まれる。必須実験：RP4（食品検査）およびRP5（pHとアミラーゼの反応速度）。
• 心疾患の治療法や幹細胞・ライフスタイルのリスクデータを評価し、リスク要因のグラフや表を解釈できること。
• 正確に暗記すること：酵素の表、心臓の血管、血液成分、植物組織、蒸散作用の要因。

細胞 → 組織 → 器官 → 器官系 → 個体。

• 組織とは、似通った構造と機能を持つ細胞の集団である。
• 器官とは、特定の機能を行う組織の集まりである。
• 器官系は、個体全体を構成するために協力して機能する。

消化器系は器官系の具体的な例として挙げられる。

酵素は生体触媒であり、活性部位を持つタンパク質で、一つの基質に形状が合う（鍵と錠前モデル）。温度やpHの変化により反応速度が変わり、最適条件から大きく外れると酵素は変性し（活性部位の形状が変わる）、機能失調を起こす。

消化酵素は食物を吸収可能な小さな可溶性分子に変換する。生成物は新しい炭水化物、脂質、タンパク質の合成に使われ、一部のブドウ糖は呼吸過程で利用される。

胆汁: 肝臓で産生され、胆嚢に貯蔵される。アルカリ性であり、胃からの塩酸を中和し、脂肪を微小な液滴に乳化して表面積を増加させる。これらによってリパーゼによる脂肪分解の速度が向上する。

RP4 食品検査（試薬、色の変化、物質を学習すること）：

RP5 (pHとアミラーゼ): pH値の緩衝液を用い、アミラーゼと澱粉を混合し、ヨウ素を用いてスポットタイル上で連続的サンプリングを行い、澱粉が消滅する時間を記録する。最短時間が最適pHにおける最速反応速度を示す。

• 心臓は二重循環系で血液を送り出す：右心室は肺へ血液を送り（ガス交換）、左心室は体全体へ血液を送り出す。
• 確認すべき血管：大動脈、上下静脈、肺動脈、肺静脈、冠動脈。
• 安静時心拍数は右心房内の細胞によって設定される——これが天然のペースメーカーであり、人工ペースメーカーは不整脈を矯正する。
• 肺: 気管 → 気管支 → 肺胞、毛細血管に囲まれています。肺胞は、広い表面積、薄い壁、豊富な血液供給、換気によって適応しています。

血液は組織です：血漿（溶解した糖、アミノ酸、CO₂、尿素、ホルモン、熱を運ぶ）、赤血球（核なし、ヘモグロビンが詰まった双凹円盤形 — 酸素輸送のための広い表面積）、白血球（病原体を食胞作用するまたは抗体を作る）、血小板（凝固）。

CADでは、冠動脈内壁に脂肪性の材料が堆積し、管腔が狭くなります — 血流が減少するため、心筋への酸素供給も減少します。各治療法について、利益とリスクを評価してください：

健康とは身体的・精神的な WELL-BEING の状態であり、単に疾病がないことではありません。栄養、ストレス、生活環境は両方に影響を与え、異なる疾病は相互に関係します（免疫欠陥 → 感染症の増加；ウイルス → 一部のがん；免疫反応 → アレルギー性喘息の発症；重篤な身体疾患 → うつ病）。

リスク因子は疾病の発生率の上昇と関連しています — ライフスタイルや体内・環境中の物質の側面です。因果メカニズムが証明されているものもあれば、そうでないものもあります：食事・喫煙・運動 → 心血管疾患；肥満 → 2型糖尿病；アルコール → 肝障害および脳機能低下；喫煙 → 肺疾患および肺癌；喫煙とアルコール → 胎児への影響；発がん物質（電離放射線を含む） → がん。リスク因子データは相関として解釈され、因果メカニズムが証明された場合にのみ因果関係となります。

がんは、細胞の変化によって制御不能な増殖と分裂が生じることで発症します：

• 良性腫瘍: 特定の領域内に限定され、通常は膜に包まれており、浸潤しません。
• 悪性腫瘍（がん）: 隣接組織に浸潤し、血液を介して転移して** secondary tumours（転移巣）**を形成します。

リスク要因には生活習慣が含まれ、一部のがんでは遺伝子も関与します。

• 表皮組織: 保護被覆；上部にワックス質の角質層。
• 掌状薄壁組織: クロロプラストが密集しており、光合成の大部分を担う。
• 海綿状薄壁組織: 気体交換のための空隙。
• 維管束（木部）: リグニンによって補強された中空の管——蒸散流において根から葉へ水と無機イオンを運ぶ。
• 篩管（韧皮部）: 篩管——光合成産物である溶解した糖を葉から植物全体へ運ぶ（移動）。
• 分生組織: 茎や根の成長端に存在する（幹細胞）。
• 保衛細胞: 気孔を取り囲み、気体交換と水分喪失を調節する。
• 根毛細胞: 表面積が大きく、浸透圧によって水が取り込まれ、能動輸送によって無機イオンが取り込まれる。

蒸散とは、葉からの水蒸気の放出（主に気孔を通じて）であり、蒸散流を引き起こす。その速度は以下のように増加する：温度↑、空気の流れ↑、光強度↑（気孔が開く）；湿度↑では減少する。ポトメーターを用いて水の吸収量で速度を測定する。

• 細胞→組織→器官→系 の順序をそれぞれ1例ずつ示す。
• 酵素表：名前、場所、基質、生成物；鍵と錠前モデル；変性。
• 胆汁：2つの機能とその理由；RP4のテストと色；RP5の手順。
• 心臓を5つの名付いた血管すべてでラベル付け；二重循環；ペースメーカー。
• 動脈/静脈/毛細血管の構造機能；血液成分4つとその適応。
• 虚血性心疾患：原因＋各治療法の特長とリスク。
• リスク要因と因果メカニズム；良性腫瘍と悪性腫瘍の違い。
• 植物組織とその機能；木部と篩部の違い；蒸散作用の4つの要因。

Pathogens are microorganisms 病原体 causing infectious disease: viruses, bacteria, protists, fungi. They infect plants or animals and spread by direct contact, water or air. Bacteria may produce toxins 毒素 that damage tissues; viruses live and reproduce inside cells.

Reducing spread: hygiene (hand washing), sterilising wounds, clean water, sewage treatment, using condoms, vector control (mosquito nets, draining water).

Non-specific defences: skin (barrier, scabbing); nose (hairs, mucus); trachea and bronchi (mucus, cilia); stomach (hydrochloric acid).

The immune system destroys pathogens that enter. White blood cells defend by:

1. phagocytosis — engulfing 吞噬 pathogens;
2. antibody production — proteins 抗体 that bind the specific antigen;
3. antitoxin production — neutralising 抗毒素 toxins.

Vaccination 疫苗: small quantities of dead or inactive pathogen stimulate white blood cells to make antibodies; on re-infection, memory cells respond quickly with the correct antibodies, preventing infection. Herd effect: immunising a large proportion reduces the spread for everyone. Evaluate: benefits (epidemic control, e.g. measles) vs rare side effects.

• Antibiotics 抗生素 (e.g. penicillin) cure bacterial disease by killing bacteria inside the body — specific antibiotics for specific bacteria. They cannot kill viruses (viruses live inside cells; killing them would damage body tissue). Resistance is a major concern — the race for new antibiotics.
• Painkillers treat symptoms but do not kill pathogens.

Drug development: traditionally extracted from plants/microorganisms — digitalis from foxgloves (heart), aspirin from willow, penicillin from Penicillium mould (Fleming). Most new drugs are chemically synthesised, but starting points may still be plant chemicals.

Testing stages: preclinical (cells, tissues, live animals) → clinical trials on healthy volunteers (low dose, safety) then patients (optimum dose, effectiveness) → double-blind trials with a placebo; results peer-reviewed.

Monoclonal antibodies 单克隆抗体 are produced from a single clone of cells, specific to one binding site on one protein antigen. Producing them: stimulate mouse lymphocytes to make the antibody → fuse with a tumour cell to make a hybridoma 杂交瘤 — it divides AND makes the antibody → clone the hybridoma → collect and purify large amounts of one specific antibody.

Uses: pregnancy tests; measuring hormone/chemical levels in blood; detecting pathogens; research — fluorescent antibodies locate molecules; cancer treatment — antibody bound to a radioactive/toxic substance delivers it only to cancer cells.

Detection (HT): stunted growth, spots on leaves, areas of decay, growths, malformed stems/leaves, discolouration, presence of pests.

Identification (HT): gardening manual/website; laboratory analysis; testing kits using monoclonal antibodies.

Named plant problems: TMV (viral), rose black spot (fungal), aphids 蚜虫 (insects).

Ion deficiencies: too little nitrate → stunted growth (nitrate needed for amino acids/protein); magnesium deficiency → chlorosis (yellow leaves 缺绿 — magnesium needed to make chlorophyll).

Defence responses: physical — cellulose cell walls, waxy cuticle, bark/dead layers; chemical — antibacterial chemicals, poisons; mechanical — thorns, hairs, drooping/curling leaves, mimicry.

Photosynthesis is an endothermic 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).

Respiration is an exothermic 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 increase 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.

• Homeostasis 稳态 is the regulation of the internal conditions of a cell or organism, to maintain optimum conditions for enzyme action and all cell functions, in response to internal and external changes.
• The body controls blood glucose concentration, body temperature and water levels.
• Every automatic control system has: receptors (detect stimuli), coordination centres (brain, spinal cord, pancreas — receive and process information) and effectors (muscles or glands — bring about the response that restores optimum levels).
• Nervous responses are fast and short-lived; hormonal responses are slower but act for longer.

The pathway: stimulus → receptor → coordinator (CNS: brain + spinal cord) → effector (muscle contracts / gland secretes) → response.

Information passes along cells called neurones 神经元 as electrical impulses. The CNS coordinates the response of effectors.

The reflex arc — automatic and rapid protection that does not involve the conscious part of the brain:

stimulus → receptor → sensory neurone → synapse → relay neurone (in CNS) → synapse → motor neurone → effector → response.

At a synapse 突触 the electrical impulse arrives at the end of a neurone and a chemical is released; it diffuses across the gap and triggers an electrical impulse in the next neurone.

RP7 — reaction time: plan and investigate the effect of a factor (e.g. caffeine) on human reaction time. Drop a ruler between a partner's finger and thumb; measure the catch distance; convert to time (or use a computer test); control variables (same hand, same height); repeat and take a mean. Interpret reaction-time data from graphs and tables.

The brain (HT extra)

Identify on a diagram: cerebral cortex (consciousness, memory, language), cerebellum (balance, coordination of movement), medulla (heartbeat, breathing).

(HT) Difficulties of investigating and treating the brain: it is complex and delicate, easily damaged, and hard to access. Mapping methods: studying patients with brain damage, electrically stimulating regions, and MRI scanning. Evaluate benefits and risks of brain procedures.

The eye

Identify and explain: retina 视网膜 (receptor cells sensitive to light intensity and colour), optic nerve (carries impulses from retina to brain), sclera (strong outer coat), cornea (refracts light), iris (coloured muscles controlling pupil size — reflex adaptation to dim/bright light), ciliary muscles and suspensory ligaments (control lens shape).

Accommodation 视觉调节 — changing lens shape to focus:

Defects: myopia 近视 (short sight — distant objects focus in front of the retina; corrected with a concave lens), hyperopia 远视 (long sight — near objects focus behind the retina; convex lens). Treatments: spectacle lenses, contact lenses, laser surgery to reshape the cornea, replacement lens.

Control of body temperature

The thermoregulatory centre in the brain has receptors sensitive to blood temperature; skin receptors send impulses to it.

• Too hot: blood vessels dilate (vasodilation 血管舒张) — more blood near the surface transfers energy to the environment; sweat 汗液 produced — evaporating sweat transfers energy away.
• Too cold: blood vessels constrict (vasoconstriction 血管收缩); sweating stops; skeletal muscles contract (shivering — respiration transfers energy to warm the blood).

(HT) Explain in context how each mechanism lowers or raises temperature back to the optimum.

The endocrine system is glands that secrete hormones 激素 directly into the bloodstream; blood carries them to target organs. Slower than nerves but longer-lasting. Identify on a body diagram: pituitary gland (brain's 'master gland'), pancreas, thyroid, adrenal gland, ovary, testes.

Blood glucose is monitored and controlled by the pancreas:

• Too high → pancreas secretes insulin 胰岛素 → glucose moves from blood into cells; in liver and muscle cells excess glucose is converted to glycogen 糖原 for storage.
• (HT) Too low → pancreas secretes glucagon 胰高血糖素 → glycogen converted back to glucose, released into the blood. Insulin–glucagon is a negative feedback 负反馈 cycle.

Diabetes: Type 1 — pancreas fails to produce enough insulin; uncontrolled high blood glucose; treated with insulin injections. Type 2 — body cells no longer respond to insulin; treated with a carbohydrate-controlled diet and exercise regime; obesity is a risk factor. Interpret graphs of blood glucose with and without diabetes.

Water and nitrogen balance — biology only

Water leaves via the lungs (exhalation) and the skin (sweat, with ions and urea) — no control over these. Excess water, ions and urea are removed by the kidneys in urine. Cells that lose or gain too much water by osmosis do not work efficiently. (HT) Excess amino acids from the diet are deaminated in the liver: ammonia (toxic) → urea 尿素 for safe excretion.

The kidneys filter the blood and selectively reabsorb glucose, some ions and water. (HT) If the blood is too concentrated, the pituitary releases ADH 抗利尿激素, which makes the kidney tubules more permeable, so more water is reabsorbed — negative feedback again.

Kidney failure: treated by transplant or dialysis — blood flows between partially permeable membranes in dialysis fluid with the right water/ion concentrations, so waste diffuses out while glucose and useful substances are retained; evaluate mechanical device vs transplant.

Hormones in human reproduction

Oestrogen 雌激素 (ovary) — main female hormone; at puberty eggs mature and one is released about every 28 days (ovulation 排卵). Testosterone 睾酮 (testes) — stimulates sperm production.

(HT) The menstrual cycle:

(HT) Interpret hormone-level graphs across the cycle: FSH starts follicle growth → oestrogen rises → LH surge triggers ovulation (about day 14) → progesterone keeps the lining thick until it falls and the cycle restarts.

Contraception — evaluate hormonal and non-hormonal methods: oral contraceptives (inhibit FSH so no eggs mature), slow-release progesterone injection/implant/patch, barrier methods (condoms, diaphragm), intrauterine devices, spermicides, abstaining when an egg may be in the oviduct, sterilisation.

Infertility (HT): 'fertility drugs' (FSH + LH) stimulate eggs to mature. IVF: FSH and LH stimulate several eggs → eggs collected and fertilised with sperm in the laboratory → embryos grow to tiny balls of cells → one or two transferred to the uterus. Evaluate: emotionally/physically stressful, low success rates, risk of multiple births.

Negative feedback (HT): adrenaline (adrenal glands, fear/stress) increases heart rate and delivery of oxygen and glucose to brain and muscles — 'fight or flight'. Thyroxine (thyroid) stimulates the basal metabolic rate and is important in growth and development; its level is controlled by negative feedback. Interpret simple negative-feedback diagrams.

Plants produce hormones to coordinate and control growth and responses to light (phototropism 向光性) and gravity (gravitropism 向地性/geotropism). Unequal distribution of auxin 生长素 causes unequal growth rates in roots and shoots — shoots bend towards light; roots grow downwards.

RP8: investigate the effect of light or gravity on newly germinated seedlings — record as length measurements and careful labelled biological drawings.

(HT) Gibberellins initiate seed germination; ethene controls cell division and fruit ripening.

(HT) Uses: auxins as weed killers, rooting powders and to promote growth in tissue culture; ethene to control ripening during storage and transport; gibberellins to end seed dormancy, promote flowering and increase fruit size. Consider the effect of weed killers on biodiversity.

Sexual reproduction — 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. Asexual reproduction — 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 twice to form four gametes, each with a single set of chromosomes — all genetically different. Fertilisation 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), dominant 显性 (expressed with one copy), recessive 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) — dominant allele; cystic fibrosis 囊性纤维化 (a disorder of cell membranes) — recessive 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.

Variation 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.

Evolution: 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.

Selective breeding (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.

An ecosystem 生态系统 is the interaction of a community of living organisms (biotic 生物因子) with the non-living (abiotic factor 非生物因子) parts of their environment. Levels of organisation: individual → population → community → ecosystem.

Competition 竞争: plants compete for light, space, water and mineral ions; animals compete for food, mates and territory. Interdependence 相互依赖: each species depends on others for food, shelter, pollination, seed dispersal — remove one species and the whole community can be affected.

Abiotic factors affecting a community: light intensity, temperature, moisture levels, soil pH and mineral content, wind intensity and direction, carbon dioxide levels (plants), oxygen levels (aquatic animals). Biotic factors : availability of food, new predators arriving, new pathogens, one species outcompeting another until numbers fall too low to breed. Explain the effect of a change in any of these from given data.

Adaptations 适应性 — features that enable survival in normal conditions — are structural (shape/body), behavioural (actions) or functional (processes, e.g. camouflage chemistry, venom). Extremophiles 极端微生物 live in extreme environments — high temperature, pressure or salt — e.g. bacteria in deep-sea vents.

Photosynthetic organisms are the producers 生产者 of biomass for life on Earth. Food chains: producer (green plant/alga making glucose by photosynthesis) → primary consumers 初级消费者 → secondary consumers 次级消费者 → tertiary consumers 三级消费者.

Predators kill and eat prey; in a stable community their numbers rise and fall in cycles — interpret the classic predator–prey graph (prey rises first, predator follows).

Sampling (RP9): transects and quadrats measure the distribution and abundance of species; calculate mean, mode, median of abundance; plot graphs with suitable scales.

Carbon cycle: returns carbon from organisms to the atmosphere as carbon dioxide for photosynthesis — respiration (plants, animals, decomposers), decay, combustion. Water cycle: evaporation and precipitation provide fresh water before it drains to the sea. Microorganisms recycle materials — returning carbon to the atmosphere as CO₂ and mineral ions to the soil. The nitrogen cycle is NOT required.

(Bio) Decomposition (RP10): temperature, water and oxygen affect the rate of decay. Gardeners and farmers provide optimum conditions for rapid decay 腐解 of waste into compost — a natural fertiliser. Anaerobic decay produces methane — biogas generators. RP10: effect of temperature on the rate of decay of fresh milk by measuring pH change.

(HT Bio) Environmental change: temperature, water availability, atmospheric-gas composition — changes may be seasonal, geographic or human-caused — shift the distribution of species.

Biodiversity 生物多样性 is the variety of all different species on Earth or within an ecosystem; it stabilises ecosystems by reducing the dependence of one species on another, and our own future relies on maintaining it.

• Waste management: population growth and living standards increase resource use and waste → pollution 污染 in water (sewage, fertiliser, toxic chemicals), air (smoke, acidic gases) and on land (landfill, toxic chemicals) — killing plants and animals.
• Land use: building, quarrying, farming and dumping waste reduce land for other species; destroying peat bogs for garden compost destroys habitat and biodiversity, and decaying or burnt peat releases CO₂.
• Deforestation in tropical areas: land for cattle and rice fields, crops for biofuels.
• Global warming: rising atmospheric CO₂ and methane contribute to global warming; describe biological consequences (distribution shifts, migration changes, biodiversity loss).

Maintaining biodiversity — programmes reducing negative human effects: breeding programmes for endangered species; protection and regeneration of rare habitats; field margins and hedgerows reintroduced in single-crop areas; reduced deforestation and CO₂ emissions; recycling rather than landfill.

Trophic levels: 1 producers (plants, algae); 2 primary consumers (herbivores); 3 secondary consumers (carnivores eating herbivores); 4 tertiary consumers (carnivores eating carnivores). Apex predators have no predators. Decomposers secrete enzymes onto dead material and absorb the small soluble food molecules.

Pyramids of biomass: trophic level 1 at the bottom; construct from data. Transfer of biomass: producers transfer ~1 % of incident light energy into biomass; only ~10 % of biomass passes from each level to the next — losses from egested faeces (not all ingested material is absorbed) and waste (CO₂ and water from respiration, water and urea in urine); glucose used in respiration. Calculate efficiency by percentage or fraction of mass.

Food security 粮食安全 — having enough food for the population — is threatened by: rising birth rate; changing diets in developed countries; new pests and pathogens; environmental changes (famine when rains fail); the cost of agricultural inputs; conflicts affecting water or food availability.

Farming techniques improve efficiency by restricting energy transfer from food animals to the environment — limiting movement and controlling temperature; high-protein feeds increase growth. Weigh these against ethical objections to intensive farming.

Sustainable fisheries: maintain fish stocks where breeding continues — control net size and fishing quotas .

Role of biotechnology: culturing microorganisms for food — Fusarium fungus grown on glucose syrup in aerobic conditions produces mycoprotein 菌蛋白 (protein-rich, vegetarian); GM bacteria produce insulin; GM crops such as golden rice add food or nutritional value.