📚 A-Level Biology Year 2: Mastering Core Topics | A-Level 生物第二年：掌握核心专题
Year 2 Biology moves beyond descriptive content into quantitative, analytical and evaluative skills. You need to link biochemical pathways, genetics, physiology and ecology, while using mathematical tests and precise terminology. This revision guide covers the most heavily examined A-level Biology Year 2 topics across major boards.
生物第二年从描述性内容转向定量、分析和评价能力。你需要把生化途径、遗传学、生理学和生态学联系起来，同时运用数学检验和精确术语。本复习指南涵盖主要考试局最常考查的 A-level 生物第二年主题。
1. Energy Transfer and Productivity | 能量传递与生产力
Energy enters most ecosystems as sunlight and is converted by producers during photosynthesis. Gross primary production (GPP) is the total chemical energy stored in plant biomass. Net primary production (NPP) is the energy remaining after respiratory losses, and this is the energy available to the next trophic level.
能量大多以阳光形式进入生态系统，并由生产者在光合作用中转化。总初级生产量（GPP）是储存在植物生物量中的总化学能。净初级生产量（NPP）是呼吸损耗后剩余的能量，也是可供下一营养级使用的能量。
NPP = GPP − R
Productivity is usually expressed in kJ m⁻² yr⁻¹. Secondary production refers to energy stored in consumer biomass. Efficiency of energy transfer between trophic levels is often low because energy is lost in excretion, movement, heat and uneaten parts.
生产力通常以 kJ m⁻² yr⁻¹ 表示。次级生产量指储存在消费者生物量中的能量。营养级之间能量传递效率通常较低，因为能量会通过排泄、运动、产热和未食用部分而损失。
2. Aerobic Respiration: Glycolysis to Electron Transport | 有氧呼吸：从糖酵解到电子传递
Aerobic respiration occurs in four main stages: glycolysis in the cytoplasm, the link reaction and Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation on the inner mitochondrial membrane. Glycolysis breaks glucose into two pyruvate molecules, giving a net gain of 2 ATP and 2 reduced NAD.
有氧呼吸分为四个主要阶段：糖酵解发生在细胞质，连接反应和克雷布斯循环发生在线粒体基质，氧化磷酸化发生在线粒体内膜。糖酵解将葡萄糖分解为两个丙酮酸分子，净生成 2 个 ATP 和 2 个还原型 NAD。
The link reaction converts each pyruvate into acetyl coenzyme A, releasing CO₂ and producing reduced NAD. The Krebs cycle then oxidises acetyl groups, generating ATP, reduced NAD and reduced FAD. The electron transport chain uses these reduced coenzymes to create a proton gradient, which drives ATP synthase in chemiosmosis.
连接反应将每个丙酮酸转化为乙酰辅酶 A，释放 CO₂ 并产生还原型 NAD。克雷布斯循环随后氧化乙酰基，生成 ATP、还原型 NAD 和还原型 FAD。电子传递链利用这些还原型辅酶建立质子梯度，在化学渗透中驱动 ATP 合酶。
3. Anaerobic Respiration and Respiratory Substrates | 无氧呼吸与呼吸底物
When oxygen is unavailable, glycolysis can continue if reduced NAD is reoxidised. In animals, pyruvate is reduced to lactate by lactate fermentation. In plants and yeast, pyruvate is decarboxylated to ethanal and then reduced to ethanol, releasing CO₂ in alcoholic fermentation.
当氧气不足时，如果还原型 NAD 能被再氧化，糖酵解就能继续进行。在动物中，丙酮酸通过乳酸发酵被还原为乳酸。在植物和酵母中，丙酮酸先脱羧生成乙醛，再被还原为乙醇，并在酒精发酵中释放 CO₂。
Different respiratory substrates release different amounts of energy. Lipids produce more ATP per gram than carbohydrates because they contain more reduced carbon atoms and fewer oxygen atoms. Proteins must first be deaminated, and the remaining carbon skeleton enters respiration.
不同呼吸底物释放的能量不同。脂质每克产生的 ATP 比碳水化合物多，因为脂质含有更多还原态碳原子和更少氧原子。蛋白质必须先脱氨基，剩余的碳骨架才能进入呼吸过程。
4. Photosynthesis: Light-Dependent Reactions | 光合作用：光依赖反应
The light-dependent reactions occur in the thylakoid membranes. Photosystem II absorbs light energy, exciting electrons that pass along an electron transport chain. Photolysis of water supplies replacement electrons, releasing O₂ and protons.
光依赖反应发生在类囊体膜上。光系统 II 吸收光能，激发电子沿电子传递链传递。水的光解提供替换电子，同时释放 O₂ 和质子。
In non-cyclic photophosphorylation, electrons move from water through Photosystem II and Photosystem I to reduce NADP into reduced NADP. In cyclic photophosphorylation, electrons from Photosystem I return to it, generating ATP only. Both ATP and reduced NADP are used in the Calvin cycle.
在非循环光合磷酸化中，电子从水经过光系统 II 和光系统 I，最终将 NADP 还原为还原型 NADP。在循环光合磷酸化中，来自光系统 I 的电子又回到光系统 I，只生成 ATP。ATP 和还原型 NADP 都用于卡尔文循环。
5. The Calvin Cycle and Limiting Factors | 卡尔文循环与限制因素
The Calvin cycle takes place in the stroma. Rubisco fixes CO₂ to ribulose bisphosphate (RuBP), forming an unstable six-carbon compound that immediately splits into two molecules of glycerate 3-phosphate (GP). ATP and reduced NADP then reduce GP to triose phosphate (TP).
卡尔文循环发生在基质中。Rubisco 将 CO₂ 固定到核酮糖二磷酸（RuBP）上，形成一个不稳定的六碳化合物，并立即分解为两分子甘油酸-3-磷酸（GP）。随后 ATP 和还原型 NADP 将 GP 还原为磷酸丙糖（TP）。
One-sixth of TP is used to make glucose and other organic molecules, while five-sixths are used to regenerate RuBP. The rate of photosynthesis is limited by light intensity, CO₂ concentration and temperature. In glasshouses, farmers manipulate these factors to increase crop yield.
六分之一的 TP 用于合成葡萄糖和其他有机分子，六分之五用于再生 RuBP。光合作用速率受光照强度、CO₂ 浓度和温度限制。在温室中，农民通过调控这些因素来提高作物产量。
6. Inheritance, Linkage and Chi-Squared | 遗传、连锁与卡方检验
Year 2 genetics includes monohybrid and dihybrid inheritance, codominance, multiple alleles and sex linkage. In codominance, both alleles are expressed in the phenotype, as in ABO blood groups where IA, IB and IO interact. Sex-linked traits such as haemophilia are carried on the X chromosome.
第二年遗传学包括单基因和双基因遗传、共显性、复等位基因和伴性遗传。在共显性中，两个等位基因都在表型中表达，例如 ABO 血型中 IA、IB 和 IO 的相互作用。血友病等伴性性状由 X 染色体携带。
Linkage occurs when two genes are located on the same chromosome, so they do not assort independently. The chi-squared test compares observed phenotypic ratios with expected ratios. A significant difference indicates that the data do not fit the expected Mendelian ratio.
当两个基因位于同一染色体上时就会发生连锁，因此它们不能自由组合。卡方检验用于比较观察到的表型比例与预期比例。显著差异表明数据不符合预期的孟德尔比例。
χ² = Σ (O − E)² ÷ E
chi-squared value exceeds the critical value at p = 0.05, the null hypothesis is rejected.
自由度为表型类别数减一。如果计算出的卡方值大于 p = 0.05 时的临界值，则拒绝原假设。
7. Populations and Hardy-Weinberg Equilibrium | 种群与哈迪-温伯格平衡
A population is a group of interbreeding individuals of the same species in one area. The gene pool is the total set of alleles in that population. The Hardy-Weinberg principle predicts that allele frequencies remain constant from generation to generation if certain conditions are met.
种群是同一区域内可相互交配的同种个体群体。基因库是该种群中全部等位基因的总和。哈迪-温伯格原理预测，在满足特定条件时，等位基因频率会逐代保持稳定。
q² = 1, where p is the dominant allele frequency and q is the recessive allele frequency.
这些条件为：种群足够大、随机交配、无突变、无迁移、无自然选择。所用方程为 p + q = 1 和 p² + 2pq + q² = 1，其中 p 为显性等位基因频率，q 为隐性等位基因频率。
p + q = 1 | p² + 2pq + q² = 1
In the second equation, p² represents the frequency of homozygous dominant individuals, 2pq represents heterozygotes, and q² represents homozygous recessive individuals. These calculations are commonly examined with genetic disease carrier frequencies.
在第二个方程中，p² 表示显性纯合子个体频率，2pq 表示杂合子，q² 表示隐性纯合子。这些计算常与遗传病携带者频率一起考查。
8. Gene Expression and Epigenetic Control | 基因表达与表观遗传调控
Not all genes are expressed in every cell. Transcription factors bind to promoter regions and either stimulate or inhibit RNA polymerase. Oestrogen can enter cells, bind to a receptor and activate gene transcription. Small interfering RNA (siRNA) can degrade mRNA and prevent translation.
并非所有基因都在每个细胞中表达。转录因子与启动子区域结合，刺激或抑制 RNA 聚合酶。雌激素可进入细胞，与受体结合并激活基因转录。小干扰 RNA（siRNA）可降解 mRNA 并阻止翻译。
Epigenetic changes do not alter the DNA sequence but affect gene expression. DNA methylation usually suppresses transcription, while histone acetylation loosens chromatin and promotes transcription. These changes can be passed on during cell division and influenced by environment.
表观遗传变化不改变 DNA 序列，但影响基因表达。DNA 甲基化通常会抑制转录，而组蛋白乙酰化使染色质松散并促进转录。这些变化可在细胞分裂中传递，并受环境影响。
Stem cells are undifferentiated cells capable of self-renewal. Totipotent stem cells can form any cell type, while pluripotent stem cells can form most cell types. Induced pluripotent stem cells are reprogrammed adult cells used in research and regenerative medicine.
干细胞是能够自我更新的未分化细胞。全能干细胞可形成任何细胞类型，而多能干细胞可形成大多数细胞类型。诱导多能干细胞是重编程的成体细胞，用于研究和再生医学。
9. Nervous Coordination and Muscle Contraction | 神经协调与肌肉收缩
An action potential is generated when a stimulus depolarises the membrane to threshold. Voltage-gated sodium channels open, Na⁺ enters and the membrane depolarises. Potassium channels then open, K⁺ leaves and repolarisation occurs. The sodium-potassium pump restores resting potential.
当刺激使膜去极化达到阈值时产生动作电位。电压门控钠通道打开，Na⁺ 内流，膜去极化。随后钾通道打开，K⁺ 外流，发生复极化。钠钾泵恢复静息电位。
In myelinated neurones, action potentials jump between nodes of Ranvier by saltatory conduction, which speeds up transmission. At a synapse, an action potential triggers the release of neurotransmitter from vesicles; the neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic membrane.
在有髓神经元中，动作电位通过郎飞结之间的跳跃式传导快速传递。在突触处，动作电位触发突触小泡释放神经递质；神经递质扩散越过突触间隙，并与突触后膜上的受体结合。
Muscle contraction is explained by the sliding filament theory. Calcium ions bind to troponin, causing tropomyosin to move and expose myosin-binding sites on actin. Myosin heads bind, pull actin filaments inward and use ATP to detach and re-attach, shortening the sarcomere.
肌肉收缩可用肌丝滑动学说解释。钙离子与肌钙蛋白结合，使原肌球蛋白移动并暴露肌动蛋白上的肌球蛋白结合位点。肌球蛋白头部结合并拉动肌动蛋白丝向内，利用 ATP 脱离并再次结合，使肌节缩短。
10. Homeostasis, Osmoregulation and Exam Technique | 稳态、渗透调节与应试技巧
Homeostasis involves maintaining a stable internal environment through negative feedback. In blood glucose regulation, insulin lowers blood glucose by increasing cell uptake and glycogenesis, while glucagon raises blood glucose by promoting glycogenolysis and gluconeogenesis.
稳态通过负反馈维持稳定的内环境。在血糖调节中，胰岛素通过增加细胞摄取和糖原生成来降低血糖，而胰高血糖素通过促进糖原分解和糖异生来升高血糖。
Osmoregulation is controlled by antidiuretic hormone (ADH). When blood water potential falls, osmoreceptors in the hypothalamus trigger ADH release from the posterior pituitary. ADH increases the permeability of the collecting duct to water by inserting aquaporins, so more water is reabsorbed.
渗透调节由抗利尿激素（ADH）控制。当血液水势下降时，下丘脑中的渗透压感受器触发垂体后叶释放 ADH。ADH 通过插入水通道蛋白增加集合管对水的通透性，从而重吸收更多水。
In the exam, always use precise biological terminology and refer to data when a question includes a graph or table. Practise calculations such as chi-squared, Hardy-Weinberg and energy transfer efficiencies. Common errors include confusing gene flow with genetic drift, omitting units, and failing to describe control variables in experimental design.
考试中务必使用精确的生物学术语，并在题目给出图表或表格时引用数据。练习卡方、哈迪-温伯格和能量传递效率等计算。常见错误包括混淆基因流与遗传漂变、遗漏单位，以及在实验设计中未能描述控制变量。
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