AQA · GCSE · 生物

• 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.

细胞构成组织，组织构成器官，器官构成系统——消化系统、血液和植物均体现这一模式。本参考涵盖AQA GCSE生物学8461，主题4.2组织结构。

考试如何考察该主题：

• 试卷1包含“组织结构”内容。必修实验：RP4（食物检测）和RP5（pH与淀粉酶速率）。
• 你必须评估心脏病治疗方法及干细胞/生活方式风险数据，并解读风险因素的图表和表格。
• 准确回忆：酶表、心脏血管、血液成分、植物组织、蒸腾作用因素。

细胞 → 组织 → 器官 → 器官系统 → 生物体。

• 组织是结构相似且功能相同的细胞群。
• 器官是由执行特定功能的多种组织聚集而成。
• 器官系统协同工作以构成完整的生物体。

消化系统是器官系统的命名示例。

酶是生物催化剂：具有活性位点的蛋白质，其形状与一种底物相契合——即锁钥模型。温度和pH值改变反应速率；偏离最适条件时，酶会变性（活性位点改变形状）。

消化酶将食物转化为可被吸收入血的小分子可溶性物质。这些产物用于构建新的碳水化合物、脂类和蛋白质；部分葡萄糖在呼吸作用中被消耗。

胆汁：由肝脏产生，储存在胆囊中。它呈碱性，中和来自胃的盐酸，并将脂肪乳化成微小液滴，增加表面积——这两者均提高了脂肪酶对脂肪的分解速率。

RP4食物检测（需掌握试剂、颜色变化及对应物质）：

RP5（pH值与淀粉酶）：在特定pH值的缓冲液中，将淀粉酶与淀粉混合，通过连续取样滴加于斑点板上并加入碘液，记录淀粉完全消失所需时间——最短时间对应最适pH下的最高反应速率。

• 心脏通过双循环系统泵送血液：右心室将血液泵入肺部进行气体交换；左心室将其泵送至全身其余部位。
• 需掌握的血管：主动脉、上下腔静脉、肺动脉、肺静脉、冠状动脉。
• 静息心率由右心房内的细胞设定——即天然起搏点；人工起搏器用于纠正心律不齐。
• 肺：气管 → 支气管 → 肺泡，周围包绕毛细血管；肺泡适应特征包括巨大的表面积、极薄的壁、丰富的血液供应及良好的通气。

血液是一种结缔组织：血浆（运输溶解的糖类、氨基酸、CO₂、尿素、激素及热量），红细胞（无细胞核，富含血红蛋白，呈双凹圆盘状——增大表面积利于氧气运输），白细胞（吞噬病原体或产生抗体），血小板（参与凝血）。

在冠心病（CHD）中，脂肪物质在冠状动脉内壁堆积，导致管腔变窄——血流量减少，从而使心肌获得的氧气减少。请从收益与风险两方面评估每种治疗方案：

健康是身心福祉的状态 — 而不仅仅是没有疾病。饮食、压力和人生境遇都会影响身心健康；不同疾病之间会相互作用（免疫缺陷 → 更多感染；病毒 → 某些癌症；免疫反应诱发哮喘；严重躯体疾病 → 抑郁）。

风险因素危险因素与疾病发病率升高有关 — 涉及生活方式或身体/环境中的物质。部分因素已证实存在致病机制，部分则未证实：饮食/吸烟/运动 → 心血管疾病；肥胖 → 2型糖尿病；酒精 → 肝脏损伤和脑功能受损；吸烟 → 肺病和肺癌；吸烟和饮酒 → 影响胎儿；致癌物包括电离辐射 → 癌症。解读风险因素数据时应视为相关性；证实的机制才构成因果关系。

癌症源于细胞发生变异，导致不受控制的生长和分裂：

• 良性肿瘤：局限于某一区域，通常被膜包裹；不侵入周围组织。
• 恶性肿瘤（癌症）侵入邻近组织，并通过血液扩散形成继发性肿瘤。

风险因素包括生活方式，对于某些癌症还包括基因。

• 表皮组织：保护性覆盖层；表面有蜡质角质层。
• 栅栏组织栅栏组织：紧密排列的叶绿体 — 进行大部分光合作用。
• 海绵组织：气室用于气体交换。
• 木质部木质部：由木质素加强的空心管道 — 在蒸腾流中将水和无机盐离子从根部运输至叶片。
• 韧皮部韧皮部：筛管 — 将溶解的糖分从叶片运输至植物其余部位（运输作用运输）。
• 分生组织：位于茎尖和根尖（干细胞）。
• 保卫细胞保卫细胞包围气孔气孔，控制气体交换和水分散失。
• 根毛细胞：表面积大；水通过渗透作用进入，无机盐离子通过主动运输吸收。

蒸腾作用蒸腾作用是水蒸气从叶片散失的过程（主要通过气孔）；它拉动蒸腾流。其速率随以下因素增加：温度 ↑、空气流动 ↑、光照强度 ↑（气孔张开）；随湿度 ↑ 而降低。可通过测量吸水率使用测蒸腾仪测定速率。

• 按顺序列举细胞 → 组织 → 器官 → 系统，各举一例。
• 酶表：名称、部位、底物、产物；锁钥模型；变性。
• 胆汁：两项功能及原因；RP4测试与颜色；RP5方法。
• 标注心脏所有五条命名血管；双循环；起搏器。
• 动脉/静脉/毛细血管结构 ↔ 功能；四种血液成分及其适应特征。
• 冠心病（CHD）：病因 + 每种治疗的好处和风险。
• 风险因素及其因果机制；良性与恶性肿瘤的区别。
• 植物组织及其功能；木质部与韧皮部；四种蒸腾作用影响因素。

病原体入侵，免疫系统作出反应，药物助力对抗。本参考资料涵盖 AQA GCSE生物 8461，主题4.3 感染与免疫反应。

考试如何考察该主题：

• 试卷1包含此主题；RP2（琼脂上抗生素/防腐剂实验）关联自主题1。
• 单克隆抗体（4.3.2）和 植物病害（4.3.3）属于 仅生物学内容，而单克隆抗体结合植物病害检测为 高阶题（HT only）。
• 准确记忆：每种命名疾病 → 病原体类型、症状及传播途径；防御系统；药物测试阶段。

病原体是引起传染病的微生物：病毒、细菌、原生生物、真菌。它们感染植物或动物，通过直接接触、水或空气传播。细菌可能产生损伤组织的毒素；病毒在细胞内生存并繁殖。

减少传播的方法：卫生措施（洗手）、伤口消毒、清洁饮用水、污水处理、使用安全套、媒介控制（蚊帐、排水）。

非特异性防御：皮肤（屏障、结痂）；鼻腔（毛发、黏液）；气管和支气管（黏液、纤毛）；胃（盐酸）。

免疫系统 消灭入侵的病原体。白细胞 通过以下方式防御：

1. 吞噬作用 — 吞噬 病原体；
2. 抗体产生 — 结合特定抗原的蛋白质 抗体；
3. 抗毒素产生 — 中和 毒素。

疫苗接种：少量 灭活或减毒病原体 刺激白细胞产生抗体；再次感染时，记忆细胞能 迅速 产生正确的抗体，从而防止感染。群体免疫效应：对大部分人群进行免疫接种可减少疾病在所有人中的传播。评估：益处（如控制麻疹等流行病）与罕见副作用之间的权衡。

• 抗生素（例如青霉素）通过 在体内杀死细菌来治愈细菌感染 — 特定抗生素针对特定细菌。它们 无法杀死病毒（病毒生活在细胞内；杀死病毒会损伤人体组织）。耐药性 是一个主要问题 — 研发新抗生素的竞争。
• 止痛药 治疗 症状，但不能杀灭病原体。

新药开发：传统上从植物/微生物中提取 — 强心苷来自 毛地黄（心脏），阿司匹林来自 柳树，青霉素来自 青霉菌（弗莱明）。大多数新药是化学合成的，但起始点可能仍是植物化学物质。

测试阶段：临床前（细胞、组织、活体 动物） → 在 健康志愿者 身上进行临床试验（低剂量，安全性），然后在患者身上进行（最佳剂量，有效性） → 采用 安慰剂 的 双盲 试验；结果经同行评审。

单克隆抗体 由单一细胞克隆产生，特异性地结合一种蛋白质抗原上的一个结合位点。生产方法：刺激小鼠 淋巴细胞 产生抗体 → 与 肿瘤细胞 融合形成 杂交瘤 — 它既能分裂又能产生抗体 → 克隆该杂交瘤 → 收集并纯化大量特定的同一种抗体。

用途：验孕；测量血液中的激素/化学物质水平；检测病原体；研究 — 荧光抗体定位分子；癌症治疗 — 抗体连接放射性/有毒物质，将其 仅递送至癌细胞。

检测（高中拓展）：生长受阻、叶片斑点、腐烂区域、肿块、茎/叶畸形、变色、害虫存在。

鉴定（高中拓展）：园艺手册/网站；实验室分析；使用 单克隆抗体 的检测试剂盒。

已命名的植物病害：烟草花叶病毒（病毒性）、玫瑰黑斑病（真菌性）、蚜虫（昆虫性）。

离子缺乏症：缺 硝酸盐 → 生长受阻（硝酸盐用于合成 氨基酸/蛋白质）；缺 镁 → 缺绿症（叶片发黄 — 镁用于合成 叶绿素）。

防御反应：物理防御 — 纤维素细胞壁、蜡质角质层、树皮/死亡层；化学防御 — 抗菌化学物质、毒素；机械防御 — 刺、毛、叶片下垂/卷曲、拟态。

• 四种病原体类型；三种传播途径；针对每种指定疾病的两种减少传播方法。
• 背诵疾病表：病原体、传播方式、症状、控制措施。
• 四种非特异性防御机制；三种白细胞作用。
• 疫苗接种机制及群体效应；抗生素与止痛药的区别；抗生素为何对病毒无效。
• 药物来源（毛地黄、柳树、青霉菌）以及临床前/临床试验阶段，包括安慰剂和双盲法。
• （高中生物）杂交瘤细胞制备流程及单克隆抗体的四种用途。
• （生物学）植物病害症状、识别方法、硝酸盐/镁缺乏症、三类防御机制。

植物捕获太阳能，每个细胞又将其释放出来。本参考涵盖 AQA GCSE 生物 8461，主题 4.4 生物能量学。

考试如何考察该主题：

• 试卷 1 包含生物能量学内容。RP6：利用水蕴草研究光照强度与光合作用速率的关系。
• 限制因子图（含两个或三个因子）、平方反比定律和温室经济属于 高中(Higher Tier) 内容。
• 方程式为文字方程式——还需掌握符号形式（CO₂, H₂O, O₂, C₆H₁₂O₆）。

光合作用是一种吸热反应：能量通过光从环境传递至叶绿体。

速率随以下因素增加而提高：光照强度、CO₂浓度、温度（至酶的最适温度），以及叶绿素含量。

阅读单因子图：曲线顶部的平台意味着其他因素已成为限制因素。(高中) 在双因子或三因子图中，判断每一点处哪个因素是限制因子。

(高中) 平方反比定律：将灯的距离加倍，光照强度变为原来的四分之一。

(高中) 温室经济：增加热量、光照或CO₂可提高速率，但每项都有成本；种植者会增加最便宜且处于限制状态的因素，直至额外产量不再带来收益为止。

RP6：将水蕴草置于不同距离的灯光下；计数每分钟产生的氧气气泡数（或收集气体）；控制温度和CO₂（碳酸氢钠）；重复实验并取平均值；速率 = 气泡数 ÷ 时间。

例题。 气泡：30个/分钟在10 cm处；在20 cm处光照强度变为四分之一，因此预期约为8个/分钟（若光照仍为限制因素）。

光合作用产生的葡萄糖用于：

• 呼吸作用；
• 转化为不溶性淀粉以储存；
• 脂肪或油以储存；
• 纤维素用于细胞壁；
• 氨基酸用于合成蛋白质——这还需要来自土壤的硝酸根离子。

呼吸作用是一种放热反应，发生在活细胞中且持续进行，释放的能量用于：构建大分子、运动和维持体温。

酵母的无氧呼吸是发酵过程，在面包制作（CO₂使面团膨胀）和酒精饮料生产方面具有重要经济价值。

运动时心率、呼吸频率和潮气量增加，以便向肌肉输送更多富氧血液。若供氧不足，肌肉会转为无氧呼吸：葡萄糖不完全氧化导致乳酸积累→疲劳；同时产生氧债。

(HT) 血液将乳酸运至肝脏，将其转化回葡萄糖。氧债是指运动后为反应并清除积累的乳酸所需额外消耗的氧气。

代谢是指细胞或体内所有反应的总和；呼吸作用为其提供能量。包括：

• 葡萄糖 → 淀粉、糖原、纤维素；
• 甘油 + 3脂肪酸 → 脂类；
• 葡萄糖 + 硝酸根离子 → 氨基酸 → 蛋白质；
• 呼吸作用；
• 过剩蛋白质的分解 → 尿素以供排泄。

• 写出光合作用的文字表达式和符号表达式；说明其为吸热反应 + 光→叶绿体。
• 解释四个速率因素；阅读单因子曲线的平坦部分；（高阶）平方反比定律与温室经济。
• 描述RP6及其控制变量和速率计算方法。
• 列出葡萄糖的五种用途（包括利用硝酸盐合成蛋白质）。
• 对比有氧呼吸与两种无氧呼吸的方程式；命名发酵的两个产物及其用途。
• 练习：三种增加情况；乳酸与氧债；（高阶）肝脏的作用。
• 定义代谢及其五个方面。

• 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.

• 太阳能量流经生态系统；碳和水在生物与非生物世界之间循环。
• 群落之间相互竞争并彼此依赖；适应性使生物适应其生存环境。
• 人类活动——废物、土地利用、森林砍伐、全球变暖——威胁生物多样性，而营养级生物学（仅限生物）解释了食物链为何较短。

生态系统是群落（由生物因子组成的生物体集合）与其环境中非生物因子相互作用的整体。组织层次：个体 → 种群 → 群落 → 生态系统。

竞争: 植物争夺光照、空间、水和无机盐离子；动物争夺食物、配偶和领地。相互依赖: 每个物种都依赖其他物种获取食物、庇护所、授粉和种子传播——移除一个物种可能会影响整个群落。

影响群落的非生物因素：光照强度、温度、湿度、土壤pH值和矿物质含量、风力强度和方向、二氧化碳浓度（植物）、氧气浓度（水生动物）。生物因素：食物供应、新捕食者的出现、新病原体的入侵、一种物种竞争力过强导致另一种物种数量降至无法繁殖的水平。能根据给定数据解释上述任一因素变化的影响。

适应性 — 使生物能在正常条件下生存的特征 — 分为结构型（形状/身体）、行为型（动作）或功能型（生理过程，如伪装化学机制、毒液）。极端微生物生活在极端环境中——高温、高压或高盐——例如深海热液喷口处的细菌。

光合生物是地球上生命的生物量生产者。食物链：生产者（通过光合作用制造葡萄糖的绿色植物/藻类）→ 初级消费者 → 次级消费者 → 三级消费者。

捕食者捕杀并食用猎物；在稳定的群落中，它们的数量呈周期性升降——解读经典的捕食者-猎物曲线图（猎物先升，捕食者后升）。

取样 (RP9)：样带和样方用于测量物种的分布和丰度；计算丰度的平均值、众数和中位数；绘制具有合适刻度的图表。

碳循环：通过呼吸作用（植物、动物、分解者）、腐烂和燃烧，将碳以二氧化碳的形式从生物体返还给大气供光合作用使用。水循环：蒸发和降水提供淡水，随后流入大海。微生物回收物质——将碳作为CO₂返回大气，并将无机盐离子归还土壤。氮循环不在要求范围内。

(生物) 分解作用 (RP10)：温度、水分和氧气影响腐烂速率。园丁和农民提供最佳条件以促进废物的快速腐解成为堆肥——一种天然肥料。厌氧腐烂产生甲烷——沼气发电机。RP10：通过测量pH值变化来研究温度对鲜奶腐烂速率的影响。

(高阶生物) 环境变化：温度、水分 availability、大气成分的变化可能是季节性的、地理性的或人为造成的——这些变化会改变物种的分布。

生物多样性是指地球上或某一生态系统内所有不同物种的多样性；它通过减少物种间的相互依赖来稳定生态系统，而我们的未来也依赖于维持它。

• 废物管理：人口增长和生活水平提高导致资源消耗和废物增加 → 造成水体（ sewage, fertilizer, toxic chemicals）、空气（smoke, acidic gases）和陆地（landfill, toxic chemicals）的污染——杀死植物和动物。
• 土地利用：建筑、采矿、农业和倾倒废物减少了其他物种的生存空间；为园艺堆肥而破坏泥炭沼泽会破坏栖息地和生物多样性，且腐烂或燃烧的泥炭会释放CO₂。
• 热带地区的森林砍伐：用于养牛和稻田，以及生物燃料作物。
• 全球变暖：大气中CO₂和甲烷浓度上升导致全球变暖；描述其生物学后果（分布变化、迁徙改变、生物多样性丧失）。

维持生物多样性——减少人类负面影响的计划：濒危物种繁育计划；保护和稀有生境恢复；在单一作物种植区重新引入田边和树篱；减少森林砍伐和CO₂排放；采用回收而非填埋处理废弃物。

营养级：1 生产者（植物、藻类）；2 初级消费者（草食动物）；3 次级消费者（以草食动物为食的肉食动物）；4 三级消费者（以肉食动物为食的肉食动物）。顶级捕食者没有天敌。分解者向死亡有机物分泌酶，并吸收其中小的可溶性食物分子。

生物量金字塔：营养级1位于底部；根据数据构建。生物量传递：生产者将约1%的入射光能转化为生物量；仅约10%的生物量从每一级传递到下一级——损失来自排出的粪便（并非所有摄入物质都被吸收）和排泄物（呼吸作用产生的CO₂和水，尿液中的水和尿素）；葡萄糖用于呼吸作用。通过质量百分比或分数计算效率。

粮食安全 Food security — 满足人口足够的食物需求 — 受到以下因素威胁：不断上升的出生率；发达国家饮食习惯的改变；新的害虫和病原体；环境变化（降雨不足导致饥荒）；农业投入品成本；影响水资源或粮食供应的冲突。

耕作技术通过限制食物动物向环境的能量转移来提高效率——限制活动范围和控制温度；高蛋白饲料增加生长量。需权衡集约化养殖的伦理争议。

可持续渔业：在繁殖持续进行的地方维持鱼类种群——控制网眼大小和捕捞配额。

生物技术的作用：培养微生物作为食物——Fusarium真菌在葡萄糖糖浆上好氧条件下生长产生菌蛋白 mycoprotein（富含蛋白质，素食来源）；转基因细菌生产胰岛素；转基因作物如黄金大米增加食品或营养价值。

• 定义链：生态系统、群落、非生物/生物因素及示例。
• 植物与动物竞争；相互依存的影响；三种适应类型 + 极端微生物。
• 食物链词汇；从图表读取捕食者-猎物周期；RP9取样法求均值/众数/中位数。
• 碳循环和水循环及分解者的作用；(生物)腐烂因素 + 沼气 + RP10。
• 四种人类威胁 + 五项生物多样性保护计划名称。
• (生物) 营养级、金字塔形状、10%法则及效率计算。
• (生物) 六项粮食安全威胁、高效农业、网眼大小和配额、菌蛋白、黄金大米。