📚 Core Knowledge for Year 13 Cambridge Biology | Year 13 剑桥生物核心知识梳理
Year 13 Cambridge Biology (A2) builds on AS foundations to explore the intricate mechanisms that sustain life. From the molecular details of respiration and photosynthesis to the systems that maintain internal balance, coordinate responses, and drive inheritance, evolution and biodiversity, students develop a deep, interconnected understanding of living processes. This article reviews the essential core concepts, linking theory to application and preparing learners for the demands of the final examinations.
Year 13 剑桥生物（A2）在 AS 基础上深入探究维持生命的复杂机制。从呼吸作用和光合作用的分子细节，到维持内稳态、协调反应、驱动遗传、进化与生物多样性的系统，学生将对生命过程形成深刻而联通的理解。本文梳理核心必备知识点，将理论与应用相连接，帮助学习者应对最终大考的要求。
1. Energy and Respiration | 能量与呼吸
Respiration is the process by which cells harvest energy from organic molecules. The complete aerobic oxidation of glucose occurs in four stages: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. Glycolysis takes place in the cytoplasm, splitting glucose (6C) into two molecules of pyruvate (3C) and yielding a net gain of 2 ATP and 2 reduced NAD. Pyruvate then enters the mitochondrial matrix, where the link reaction converts it to acetyl CoA, releasing CO₂ and generating reduced NAD.
呼吸作用是细胞从有机分子中获取能量的过程。葡萄糖的彻底有氧氧化分为四个阶段：糖酵解、连接反应、克雷伯斯循环和氧化磷酸化。糖酵解发生在细胞质中，将葡萄糖（6C）分解为两分子丙酮酸（3C），净生成 2 个 ATP 和 2 个还原型 NAD。丙酮酸随后进入线粒体基质，连接反应将其转化为乙酰辅酶 A，同时释放 CO₂ 并产生还原型 NAD。
The Krebs cycle oxidises acetyl CoA completely to CO₂ in the mitochondrial matrix, producing ATP, reduced NAD and reduced FAD. The reduced coenzymes then donate electrons to the electron transport chain on the inner mitochondrial membrane. Chemiosmosis couples electron flow to the pumping of protons into the intermembrane space, creating a proton motive force. Protons flow back through ATP synthase, driving the synthesis of up to 28 ATP molecules per glucose. The final electron acceptor is oxygen, forming water.
克雷伯斯循环在线粒体基质中将乙酰辅酶 A 完全氧化为 CO₂，产生 ATP、还原型 NAD 和还原型 FAD。随后还原辅酶将电子传递给线粒体内膜上的电子传递链。化学渗透将电子传递与质子泵入膜间隙相偶联，形成质子动势。质子通过 ATP 合酶回流，驱动每分子葡萄糖合成最多 28 个 ATP。最终电子受体是氧气，生成水。
| Stage | Location | Key Inputs | Key Outputs (per glucose) |
|---|---|---|---|
| Glycolysis | Cytoplasm | Glucose, 2 ATP, 2 NAD⁺ | 2 pyruvate, 4 ATP (net 2), 2 NADH |
| Link reaction | Mitochondrial matrix | 2 pyruvate, 2 NAD⁺, 2 CoA | 2 acetyl CoA, 2 CO₂, 2 NADH |
| Krebs cycle | Mitochondrial matrix | 2 acetyl CoA, 6 NAD⁺, 2 FAD, 2 ADP + Pᵢ | 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP |
| Oxidative phosphorylation | Inner mitochondrial membrane | 10 NADH, 2 FADH₂, O₂, 28 ADP + Pᵢ | 28 ATP, H₂O |
d. The respiratory quotient (RQ = CO₂ produced / O₂ consumed) indicates the respiratory substrate: a value of 1.0 typically indicates carbohydrate, while 0.7 suggests lipid.
在缺氧条件下，某些细胞可进行无氧呼吸。酵母产生乙醇和 CO₂，而哺乳动物肌肉则生成乳酸。呼吸商（RQ = 产生 CO₂ / 消耗 O₂）反映呼吸底物类型：数值为 1.0 时通常指示糖类，0.7 则提示脂类。
2. Photosynthesis | 光合作用
Photosynthesis converts light energy into chemical energy stored in organic molecules. The light-dependent reactions occur in the thylakoid membranes of chloroplasts. Photosystem II absorbs light, splitting water (photolysis) to release O₂, electrons and protons. The electrons pass through an electron transport chain, generating ATP via chemiosmosis, and are then passed to Photosystem I, where they are re-energised and used to reduce NADP⁺ to NADPH. Cyclic photophosphorylation involves only PSI and produces ATP without NADPH.
光合作用将光能转化为储存在有机分子中的化学能。光依赖反应发生在叶绿体的类囊体膜上。光系统 II 吸收光能，分解水（光解）释放 O₂、电子和质子。电子经电子传递链传递，通过化学渗透生成 ATP，随后传递至光系统 I，在那里重新获得能量并用于将 NADP⁺ 还原为 NADPH。环式光合磷酸化仅涉及 PSI，生成 ATP 而不产生 NADPH。
The light-independent reactions (Calvin cycle) take place in the stroma. Ribulose bisphosphate (RuBP, a 5C compound) acts as the CO₂ acceptor, catalysed by the enzyme rubisco. The initial product, an unstable 6C intermediate, splits into two molecules of glycerate 3-phosphate (GP, 3C). GP is then reduced to triose phosphate (TP) using ATP and NADPH from the light-dependent stage. Some TP is used to regenerate RuBP, while the rest is available for synthesising glucose, sucrose, starch, amino acids and lipids.
光不依赖反应（卡尔文循环）发生在基质中。1,5-二磷酸核酮糖（RuBP，5C 化合物）作为 CO₂ 接受体，由 rubisco 酶催化。最初产生的不稳定 6C 中间体裂解为两分子 3-磷酸甘油酸（GP，3C）。GP 再利用光反应产生的 ATP 和 NADPH 还原为磷酸丙糖（TP）。部分 TP 用于再生 RuBP，其余则用于合成葡萄糖、蔗糖、淀粉、氨基酸和脂质。
Key limiting factors for photosynthesis are light intensity, carbon dioxide concentration and temperature. At low light, the rate is limited by the light-dependent reactions; at low CO₂, the Calvin cycle becomes rate-limiting; and at low temperatures, enzyme activity and membrane fluidity restrict the overall rate.
光合作用的主要限制因子是光强度、二氧化碳浓度和温度。弱光下，速率受光反应限制；低 CO₂ 时，卡尔文循环成为限速步骤；低温则通过影响酶活性和膜流动性限制整体速率。
3. Homeostasis | 稳态
Homeostasis is the maintenance of a relatively constant internal environment despite external fluctuations. Core control systems rely on negative feedback, in which any deviation from a set point triggers a corrective response that restores the original condition. The general components include receptors that detect change, a coordination centre (often the brain or pancreas) and effectors (muscles or glands) that bring about the response.
稳态是指尽管外部环境波动，机体仍能维持相对恒定的内环境。核心控制系统依赖负反馈，即任何偏离设定点的变化都会触发纠正性反应，使状态恢复原状。基本组成包括检测变化的感受器、协调中心（通常为脑或胰腺）以及产生反应的效应器（肌肉或腺体）。
Thermoregulation in mammals involves both physiological and behavioural responses. When core temperature rises, skin arterioles vasodilate, sweat glands increase secretion and metabolic rate may be lowered. When temperature falls, vasoconstriction reduces heat loss, shivering generates heat, and hormones such as thyroxine and adrenaline may raise metabolic rate. The hypothalamus acts as the thermostat, integrating signals from peripheral and central thermoreceptors.
哺乳动物的体温调节涉及生理和行为反应。当核心温度升高时，皮肤小动脉舒张，汗腺分泌增加，代谢率可能降低。当温度下降时，血管收缩减少散热，寒战产热，甲状腺素和肾上腺素等激素可能提高代谢率。下丘脑充当恒温器，整合来自外周和中枢温度感受器的信号。
Blood glucose concentration is regulated by the pancreas. When glucose levels rise after a meal, pancreatic β cells secrete insulin, which stimulates target cells to take up glucose and promotes glycogen synthesis in the liver. When glucose falls, α cells secrete glucagon, activating glycogenolysis and gluconeogenesis. The action of insulin involves binding to membrane receptors, triggering a signalling cascade that leads to the translocation of GLUT4 transporters to the plasma membrane.
血糖浓度由胰腺调控。餐后血糖升高，胰岛 β 细胞分泌胰岛素，刺激靶细胞摄取葡萄糖并促进肝内糖原合成。血糖下降时，α 细胞分泌胰高血糖素，激活糖原分解和糖异生。胰岛素的作用涉及与膜受体结合，启动信号级联导致 GLUT4 转运蛋白易位至质膜。
4. Nervous Coordination | 神经协调
Neurones are specialised cells adapted for rapid transmission of electrical signals. A motor neurone has a cell body, dendrites and a long axon. The resting potential is typically −70 mV, maintained by the sodium–potassium pump (actively transporting 3 Na⁺ out and 2 K⁺ in) and the differential permeability of the membrane to K⁺. An action potential is generated when a stimulus depolarises the membrane to the threshold value (around −55 mV), opening voltage-gated Na⁺ channels and causing rapid Na⁺ influx.
神经元是适于快速传递电信号的特化细胞。运动神经元具有胞体、树突和长轴突。静息电位通常约为 −70 mV，由钠钾泵（主动运出 3 个 Na⁺、运入 2 个 K⁺）以及膜对 K⁺ 的不同通透性维持。当刺激使膜去极化达到阈值（约 −55 mV）时，电压门控 Na⁺ 通道开放，Na⁺ 快速内流，产生动作电位。
The depolarisation phase is followed by the closure of Na⁺ channels and the opening of voltage-gated K⁺ channels, leading to efflux of K⁺ and repolarisation. A transient hyperpolarisation often occurs before the resting potential is restored. Action potentials are all-or-nothing and propagate along unmyelinated axons by local circuits. In myelinated axons, saltatory conduction occurs, where the impulse jumps from one node of Ranvier to the next, greatly increasing speed.
去极化之后，Na⁺ 通道关闭，电压门控 K⁺ 通道开放，导致 K⁺ 外流和复极化。在静息电位恢复前常出现短暂的超极化。动作电位是全或无的，并沿无髓轴突通过局部电路传播。在有髓轴突中，发生跳跃传导，冲动从郎飞结跳跃至下一个郎飞结，极大提高了传导速度。
At a synapse, the arrival of an action potential opens Ca²⁺ channels in the presynaptic knob, causing vesicles containing neurotransmitter to fuse with the membrane and release their contents into the synaptic cleft. The neurotransmitter binds to receptors on the postsynaptic membrane, opening ligand-gated ion channels and generating an excitatory or inhibitory postsynaptic potential. Summation of multiple inputs determines whether a new action potential will be initiated.
在突触处，动作电位到达后引起突触前末梢的 Ca²⁺ 通道开放，含神经递质的囊泡与膜融合，将递质释放到突触间隙。神经递质与突触后膜受体结合，打开配体门控离子通道，产生兴奋性或抑制性突触后电位。多个输入的总和决定是否会触发新的动作电位。
5. Hormonal Coordination | 激素协调
Hormones are chemical messengers secreted by endocrine glands, travelling in the blood to target cells with specific receptors. Peptide hormones (e.g. insulin, glucagon) are water-soluble and act via cell-surface receptors, often employing second messenger systems such as cyclic AMP (cAMP). Steroid hormones (e.g. oestrogen, testosterone) are lipid-soluble, diffuse across the plasma membrane and bind to intracellular receptors, directly affecting gene transcription.
激素是由内分泌腺分泌的化学信使，随血液运送至带有特定受体的靶细胞。肽类激素（如胰岛素、胰高血糖素）是水溶性的，通过细胞表面受体起作用，常利用环腺苷酸（cAMP）等第二信使系统。类固醇激素（如雌激素、睾酮）是脂溶性的，可扩散通过质膜并与胞内受体结合，直接调控基因转录。
The regulation of blood glucose illustrates hormonal coordination in detail. High blood glucose triggers insulin release; insulin receptors activate tyrosine kinase, initiating a cascade that leads to the movement of GLUT4 vesicles to the cell surface and activation of glycogen synthase. Low blood glucose stimulates glucagon, which binds to hepatocyte receptors, activating adenylate cyclase, raising cAMP and triggering a phosphorylation cascade that activates glycogen phosphorylase and inhibits glycogen synthase.
血糖的调节详细体现了激素协调。高血糖引发胰岛素释放；胰岛素受体激活酪氨酸激酶，启动一系列反应，导致 GLUT4 囊泡移向细胞表面并激活糖原合酶。低血糖刺激胰高血糖素，它与肝细胞受体结合，激活腺苷酸环化酶，提高 cAMP 水平，触发磷酸化级联，激活糖原磷酸化酶并抑制糖原合酶。
Other examples include the control of growth by human growth hormone, the stress response involving adrenaline, and the reproductive hormones that regulate gametogenesis and secondary sexual characteristics. The pituitary gland, often termed the master gland, links the nervous and endocrine systems and releases tropic hormones that control other glands.
其他例子包括人生长激素对生长的控制、涉及肾上腺素的应激反应以及调控配子发生和第二性征的生殖激素。垂体常被称为主腺，它连接神经系统和内分泌系统，并释放控制其他腺体的促激素。
6. Meiosis and Genetic Variation | 减数分裂与遗传变异
Meiosis produces haploid gametes from diploid cells, halving the chromosome number and introducing genetic variation. Two consecutive divisions occur: meiosis I (reductional) and meiosis II (equational). During prophase I, homologous chromosomes pair up and crossing over may occur, exchanging segments of non-sister chromatids. Metaphase I sees bivalents align randomly at the equator, and independent assortment of maternal and paternal chromosomes generates further diversity.
减数分裂将二倍体细胞转变为单倍体配子，染色体数目减半并引入遗传变异。连续发生两次分裂：减数分裂 I（减数分裂）和减数分裂 II（均等分裂）。在前期 I，同源染色体配对并可能发生交叉互换，交换非姐妹染色单体的片段。中期 I 二价体随机排列在赤道板，母源与父源染色体的独立分配进一步增加多样性。
Anaphase I separates homologous chromosomes through the contraction of spindle fibres, and meiosis II resembles mitosis, separating sister chromatids. Genetic variation results from three main sources: crossing over during prophase I, independent assortment of chromosomes during metaphase I, and random fertilization of gametes. Chromosomal mutations, such as non-disjunction, can lead to aneuploidy, as seen in Down syndrome (trisomy 21).
后期 I 通过纺锤丝收缩分离同源染色体，减数分裂 II 类似有丝分裂，分离姐妹染色单体。遗传变异来自三个主要来源：前期 I 的交叉互换、中期 I 染色体的独立分配以及配子的随机受精。染色体突变（如不分离）可导致非整倍体，例如唐氏综合征（21 三体）。
7. Inheritance | 遗传
notypic ratio. A dihybrid cross of two genes on different chromosomes gives a 9:3:3:1 ratio in the F₂ generation. Co-dominance (both alleles expressed) and multiple alleles (e.g. human ABO blood groups) expand these patterns.
经典孟德尔遗传涉及显性和隐性等位基因。杂合子间的单因子杂交 (Aa × Aa) 产生 3:1 的表型比。位于不同染色体上的两对基因的双因子杂交在 F₂ 代表现为 9:3:3:1 的比率。共显性（两个等位基因均表达）和复等位基因（如人类 ABO 血型）扩展了这些模式。
Sex-linkage refers to genes located on sex chromosomes. Males are hemizygous for X-linked traits; thus recessive X-linked disorders such as haemophilia and red-green colour blindness appear more frequently in males. Linkage, where two genes are on the same chromosome, reduces the frequency of recombinant phenotypes unless crossing over separates them. Epistasis occurs when the allele of one gene masks the expression of another, altering expected ratios (e.g. 9:3:4, 12:3:1 or 9:7).
性连锁是指位于性染色体上的基因。男性对于 X 连锁性状是半合子；因此隐性 X 连锁疾病如血友病和红绿色盲在男性中更常见。连锁，即两个基因位于同一染色体上，降低了重组表型的频率，除非交叉互换使它们分离。上位效应指一个基因的等位基因掩盖了另一基因的表达，改变预期比率（如 9:3:4、12:3:1 或 9:7）。
The chi-squared (χ²) test is used to compare observed with expected frequencies and determines whether deviations are due to chance or indicate a significant difference.
χ² = Σ((O – E)² / E)
Degrees of freedom, probability values and critical values from data tables guide the conclusion. A probability of less than 0.05 suggests that the null hypothesis should be rejected.
卡方（χ²）检验用于比较观察值和预期值，判断偏离是由偶然引起还是存在显著差异。
χ² = Σ((O – E)² / E)
自由度、概率值和从数据表中查得的临界值指导结论。概率小于 0.05 意味着应拒绝无效假设。
8. Selection and Evolution | 选择与进化
Natural selection acts on phenotypic variation within populations. Stabilising selection favours the intermediate phenotype and reduces extremes; it operates in stable environments. Directional selection shifts the population mean towards one extreme, often in response to environmental change. Disruptive selection favours both extremes against the intermediate, potentially leading to speciation.
自然选择作用于种群内的表型变异。稳定性选择偏好中间表型，消除极端类型；在稳定环境中起作用。定向选择将种群均值推向一个极端，常响应环境变化。分裂选择偏好两个极端而淘汰中间型，可能导向物种形成。
Speciation requires reproductive isolation. Allopatric speciation occurs when geographical barriers physically separate populations, preventing gene flow and allowing independent evolution. Sympatric speciation happens within the same area, often due to behavioural, temporal or mechanical isolation. Pre-zygotic and post-zygotic isolating mechanisms prevent successful mating or viable offspring between species.
物种形成需要生殖隔离。异域物种形成指地理障碍将种群物理分隔，阻止基因流动并允许独立进化。同域物种形成发生在同一区域，常因行为、时间或机械隔离所致。前交配和后交配隔离机制阻止物种间成功交配或产生可育后代。
The Hardy– absence of evolutionary forces. The equations p + q = 1 and p² + 2pq + q² = 1 allow calculation of carrier frequencies and the testing of evolutionary change.
哈代-温伯格原理预测，在没有进化力量作用的理想大种群中，随机交配下等位基因频率和基因型频率世代间保持恒定。公式 p + q = 1 和 p² + 2pq + q² = 1 可用于计算携带者频率并检验进化变化。
9. Classification, Biodiversity and Conservation | 分类、生物多样性与保护
The three-domain system classifies life into Bacteria, Archaea and Eukarya, based on ribosomal RNA sequences and fundamental differences in membrane lipids, cell wall chemistry and gene organisation. Within Eukarya, the traditional Linnaean hierarchy runs domain, kingdom, phylum, class, order, family, genus and species. A species is defined as a group of organisms capable of interbreeding to produce fertile offspring.
三域系统根据核糖体 RNA 序列以及膜脂、细胞壁化学和基因组织的根本差异，将生物分为细菌域、古菌域和真核生物域。在真核生物域内，传统林奈等级系统为域、界、门、纲、目、科、属、种。物种被定义为能够相互交配并产生可育后代的一组生物。
ecies) and indices that account for abundance, such as Simpson’s index of diversity (D = 1 – ble, resilient ecosystem. Conservation strategies include habitat protection, captive breeding, seed banks and legislation. Economic, ecological and ethical arguments support biodiversity conservation.
生物多样性可以通过物种丰富度（不同物种的数量）以及考虑丰度的指数（如辛普森多样性指数 D = 1 – Σ(n/N)² ）来评估，其中 n 为某一物种的个体总数，N 为所有物种个体总数。高生物多样性通常意味着稳定、有弹性的生态系统。保护策略包括栖息地保护、圈养繁殖、种子库和立法。经济、生态和伦理方面的论证支持保护生物多样性。
10. Genetic Technology | 遗传技术
Recombinant DNA technology allows genes to be transferred between organisms. Restriction endonucleases cut DNA at specific recognition sequences, leaving sticky or blunt ends. DNA ligase seals the complementary sticky ends, inserting the desired gene into a vector such as a bacterial plasmid. The recombinant vector is introduced into host cells (transformation), which can then express the foreign gene.
重组 DNA 技术使得基因能在生物体间转移。限制酶在特定识别序列处切割 DNA，留下黏性末端或平端。DNA 连接酶将互补的黏性末端封接，把目的基因插入载体（如细菌质粒）。重组载体引入宿主细胞（转化），随后宿主可表达外源基因。
Polymerase chain reaction (PCR) rapidly amplifies specific DNA sequences in vitro. Each cycle involves denaturation (separating strands at ~95 °C), annealing of primers (~55–65 °C) and extension by a thermostable DNA polymerase (e.g. Taq polymerase) at ~72 °C. Gel electrophoresis separates DNA fragments by size, using an electric field to move the negatively charged DNA through an agarose gel. DNA profiling with short tandem repeats (STRs) has applications in forensic science and paternity testing.
聚合酶链式反应 (PCR) 可在体外快速扩增特定 DNA 序列。每轮循环包括变性（约 95 °C 解链）、引物退火（约 55–65 °C）和耐热 DNA 聚合酶（如 Taq 聚合酶）在约 72 °C 延伸。凝胶电泳利用电场使带负电的 DNA 在琼脂糖凝胶中迁移，按大小分离片段。利用短串联重复序列 (STR) 的 DNA 图谱分析在法医学和亲子鉴定中有广泛应用。
Gene therapy aims to treat genetic disorders by inserting a functional allele into affected cells. Viral vectors (e.g. adenoviruses, retroviruses) are often
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