📚 Core Topics Summary for CIE A-Level Biology (Year 13) | CIE A-Level 生物核心知识点梳理
The second year of CIE A-Level Biology builds on AS concepts, delving into energetics, homeostasis, inheritance, evolution and modern biotechnology. This article summarises the essential topics to help Year 13 students consolidate their understanding and prepare for examinations.
CIE A-Level生物的第二年课程在AS基础上深化，涉及能量学、稳态、遗传、进化以及现代生物技术。本文梳理核心知识点，帮助13年级学生巩固理解、备战考试。
1. Energy and Respiration | 能量与呼吸作用
Respiration is the process by which cells release energy from organic molecules through a series of enzyme-controlled reactions. The universal energy currency is ATP, synthesised by substrate-level phosphorylation or oxidative phosphorylation. Aerobic respiration consists of four stages: glycolysis in the cytoplasm, the link reaction, the Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation across the inner mitochondrial membrane.
呼吸作用是细胞通过一系列酶控反应从有机分子中释放能量的过程。通用能量货币是ATP，可通过底物水平磷酸化或氧化磷酸化合成。有氧呼吸分为四个阶段：细胞质中的糖酵解、线粒体基质中的连接反应与克雷伯斯循环，以及线粒体内膜上的氧化磷酸化。
Glycolysis splits glucose (C₆H₁₂O₆) into two pyruvate molecules, yielding a net gain of 2 ATP and 2 reduced NAD. The link reaction converts pyruvate to acetyl CoA, releasing CO₂ and reducing NAD. The Krebs cycle completes the oxidation of acetyl groups, generating ATP, reduced NAD and reduced FAD. Finally, the electron transport chain uses these reduced coenzymes to create a proton gradient that drives chemiosmosis, producing up to approximately 32 ATP molecules per glucose.
糖酵解将葡萄糖（C₆H₁₂O₆）分解为两分子丙酮酸，净生成2个ATP和2个还原性NAD。连接反应将丙酮酸转化为乙酰辅酶A，释出CO₂并还原NAD。克雷伯斯循环完成乙酰基的氧化，产生ATP、还原性NAD和还原性FAD。最终，电子传递链利用这些还原性辅酶建立质子梯度以驱动化学渗透，每分子葡萄糖最多产生约32个ATP。
sumed) indicates the respiratory substrate; for carbohydrates it is 1.0, for lipids about 0.7, and for proteins around 0.9.
总方程式为：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量。呼吸商（RQ = 产生的CO₂ / 消耗的O₂）可以反映呼吸底物类型：糖类的RQ为1.0，脂质约0.7，蛋白质约0.9。
2. Photosynthesis | 光合作用
gives electrons, protons and O₂: H₂O → 2H⁺ + 2e⁻ + ½O₂. Light energy excites electrons from chlorophyll, which pass along an electron transport chain, generating ATP by chemiosmosis and reducing NADP to NADPH. The light-independent stage (Calvin cycle) in the stroma uses ATP and NADPH to fix CO₂ onto ribulose bisphosphate (RuBP), catalysed by the enzyme Rubisco. The resulting triose phosphate can be converted to glucose, regenerating RuBP.
光合作用利用光能合成有机物。类囊体膜上进行光反应，水的光解产生电子、质子和氧气：H₂O → 2H⁺ + 2e⁻ + ½O₂。光能激发叶绿素电子，电子沿传递链传递，通过化学渗透生成ATP，并将NADP还原为NADPH。暗反应（卡尔文循环）在基质中进行，利用ATP和NADPH将CO₂固定到核酮糖二磷酸（RuBP）上，由Rubisco酶催化。生成的三碳糖磷酸可转化为葡萄糖，同时再生RuBP。
Limiting factors of photosynthesis include light intensity, carbon dioxide concentration and temperature. The compensation point is the light intensity at which carbon dioxide uptake equals carbon dioxide production by respiration. C₄ and CAM plants possess adaptations to minimise photorespiration and improve efficiency in hot, dry conditions.
光合作用的限制因素包括光照强度、二氧化碳浓度和温度。补偿点是CO₂吸收量与呼吸产生量相等时的光照强度。C₄植物和CAM植物具备在干热条件下减少光呼吸、提高光合效率的适应特征。
3. Homeostasis | 稳态
Homeostasis maintains a stable internal environment through negative feedback mechanisms. Blood glucose concentration is regulated by insulin and glucagon: insulin stimulates glucose uptake and glycogenesis, whereas glucagon promotes glycogenolysis and gluconeogenesis. Disruption leads to diabetes mellitus, characterised by hyperglycaemia and glucose in urine.
稳态通过负反馈机制维持内环境稳定。血糖浓度由胰岛素和胰高血糖素调节：胰岛素促进葡萄糖摄取和糖原生成，胰高血糖素则促进糖原分解和糖异生。失调会引发糖尿病，特征为高血糖和糖尿。
The kidney plays a vital role in osmoregulation and excretion. Ultrafiltration in the Bowman's capsule produces glomerular filtrate. In the proximal convoluted tubule, most nutrients and water are reabsorbed. The loop of Henle creates a countercurrent multiplier system to maintain a steep osmotic gradient in the medulla. Antidiuretic hormone (ADH) increases water reabsorption in the distal convoluted tubule and collecting duct by promoting aquaporin insertion.
肾脏在渗透调节和排泄中起关键作用。肾小囊中的超滤作用形成肾小球滤液。在近曲小管，大部分营养物质和水分被重吸收。亨勒袢通过逆流倍增系统维持髓质高渗透梯度。抗利尿激素（ADH）促进远端小管和集合管插入水通道蛋白，增加水分重吸收。
4. Control and Coordination | 调控与协调
Neuronal communication relies on changes in membrane potential. Resting potential is maintained around -70 mV by the sodium-potassium pump and selective membrane permeability. A stimulus opens voltage-gated sodium channels, causing depolarisation; if threshold is reached, an action potential is generated. Repolarisation occurs as potassium channels open and sodium channels inactivate. The action potential propagates along the axon, with saltatory conduction in myelinated neurones dramatically increasing speed.
神经通讯依赖膜电位变化。静息电位由钠钾泵和膜的选择通透性维持在约-70 mV。刺激使电压门控钠通道开放，引发去极化；达到阈电位则产生动作电位。钾通道开放、钠通道失活导致复极化。动作电位沿轴突传导，有髓神经元的跳跃传导极大提高速度。
Synaptic transmission involves the release of neurotransmitters from the presynaptic vesicle into the synaptic cleft, binding to receptors on the postsynaptic membrane and generating excitatory or inhibitory postsynaptic potentials. Summation allows integration of signals. In the mammalian nervous system, the endocrine system works alongside neural control, with hormones such as adrenaline triggering rapid physiological responses via second messengers such as cyclic AMP.
突触传递涉及突触前囊泡释放神经递质到突触间隙，与突触后膜受体结合并产生兴奋性或抑制性突触后电位。叠加作用可整合信号。在哺乳动物中，内分泌系统与神经控制协同，肾上腺素等激素通过环腺苷酸（cAMP）等第二信使触发快速生理反应。
5. Inherited Change | 遗传变异
Meiosis produces genetically varied haploid gametes through two successive divisions. Crossing over during prophase I and independent assortment of homologous chromosomes in metaphase I generate new allele combinations. The random fusion of gametes further increases genetic variation. Monohybrid inheritance follows Mendel's law of segregation, with a 3:1 phenotypic ratio in the F₂ generation for a dominant-recessive cross.
减数分裂通过两次连续分裂产生有遗传差异的单倍体配子。前期I的交叉互换和中期I同源染色体的独立分配产生新的等位基因组合。配子的随机融合进一步增加遗传变异。单基因杂交遵循孟德尔分离定律，显隐性杂交F₂代表型比为3:1。
her observed results fit expected ratios. Alleles on the same chromosome show linkage and do not assort independently unless crossing over occurs. Sex-linked traits, such as haemophilia, appear more frequently in males.
体现自由组合的双因子杂交产生9:3:3:1的表型比例。卡方检验（χ² = Σ (O - E)² / E）用于评估观测结果是否符合预期比例。同一染色体上的等位基因表现出连锁，除非发生交叉互换否则不自由组合。伴性遗传性状（如血友病）在男性中更常见。
| Phenotype | Ratio |
| Round, Yellow | 9 |
| Round, Green | 3 |
| Wrinkled, Yellow | 3 |
| Wrinkled, Green | 1 |
表型比例（圆黄:圆绿:皱黄:皱绿）= 9:3:3:1。
6. Selection and Evolution | 选择与进化
Natural selection acts on phenotypic variation, leading to differential reproductive success. Stabilising selection favours intermediate phenotypes, directional selection favours one extreme, and disruptive selection favours both extremes. Evolution occurs when allele frequencies change over generations in a population.
自然选择作用于表型变异，导致繁殖成功率差异。稳定化选择偏好中间表型，定向选择偏好某一极端，分裂选择则偏好两个极端。当种群等位基因频率逐代改变，进化便发生。
selection remain constant. The equation p² + 2pq + q² = 1 can be used to estimate allele frequencies. Speciation requires reproductive isolation, which may be allopatric (geographic) or sympatric (without physical barriers). Natural selection also explains antibiotic resistance and industrial melanism.
哈代-温伯格定律指出，在无限大、随机交配、无迁移、无突变和无选择的种群中，等位基因与基因型频率保持恒定。方程式 p² + 2pq + q² = 1 可用于估算等位基因频率。物种形成需要生殖隔离，可分为异域（地理隔离）或同域（无物理屏障）。自然选择也可以解释抗生素抗性和工业黑化现象。
7. Biodiversity, Classification and Conservation | 生物多样性、分类与保护
Biodiversity encompasses species diversity, genetic diversity and ecosystem diversity. Classification systems organise organisms into hierarchical taxa: domain, kingdom, phylum, class, order, family, genus, species. The three-domain system separates Archaea, Bacteria and Eukarya, reflecting molecular phylogeny. Species are defined using the biological species concept or morphological criteria.
生物多样性包括物种多样性、遗传多样性和生态系统多样性。分类系统将生物划归为域、界、门、纲、目、科、属、种等层级。三域系统将古菌域、细菌域和真核生物域分开，反映分子系统发生关系。物种可依据生物学物种概念或形态学标准定义。
Biodiversity can be assessed using Simpson' through random sampling. Reasons for conservation include ethical, aesthetic, economic and ecological values. Endangered species are protected by international agreements such as CITES. In situ conservation preserves wild habitats, while ex situ methods involve seed banks, captive breeding and botanical gardens.
可用辛普森指数（D = 1 - Σ(n/N)²）评估生物多样性，并通过随机抽样估算。保护的原因包括伦理、审美、经济和生态价值。濒危物种受CITES等国际协定保护。就地保护维护野生栖息地，异地保护方法包括种子库、人工繁育和植物园。
8. Genetic Technology | 基因技术
Genetic technologies involve the manipulation and analysis of DNA. The polymerase chain reaction (PCR) amplifies DNA fragments using repeated cycles of denaturation (95 °C), annealing (50–65 °C) and extension (72 °C) catalysed by Taq polymerase. Gel electrophoresis separates DNA fragments by size through an agarose gel matrix under an electric field. Restriction endonucleases cut DNA at specific recognition sites, leaving sticky or blunt ends, which can be ligated into plasmid vectors by DNA ligase.
基因技术涉及DNA的操作与分析。聚合酶链式反应（PCR）通过Taq聚合酶催化变性（95 °C）、退火（50–65 °C）和延伸（72 °C）的循环，扩增DNA片段。凝胶电泳利用电场使DNA片段在琼脂糖基质中按大小分离。限制性内切酶在特定识别位点切割DNA，产生黏性末端或平末端，可被DNA连接酶连接至质粒载体。
Genetically modified organisms are produced by inserting recombinant plasmids into host cells using methods like heat shock, electroporation or gene guns. Marker genes (e.g. antibiotic resistance or fluorescence) enable selection of transformed cells. Microarrays and bioinformatics allow genome-wide gene expression analysis. Gene therapy aims to correct defective alleles, but faces challenges of delivery and ethical concerns. CRISPR-Cas9 technology now permits precise genome editing.
通过热激、电穿孔或基因枪等方法将重组质粒导入宿主细胞，可产生转基因生物。标记基因（如抗生素抗性或荧光基因）用于筛选转化细胞。微阵列与生物信息学可实现全基因组表达分析。基因治疗旨在修正缺陷等位基因，但面临递送和伦理挑战。CRISPR-Cas9技术现已实现精准的基因组编辑。
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