📚 A-Level Edexcel Biology: High-Frequency Key Points Summary | A-Level Edexcel 生物：高频考点总结
This article summarises the most frequently tested concepts in A-Level Edexcel Biology, covering molecular biology, energetics, genetics, and ecology. A strong grasp of these topics is essential for high marks.
本文总结了 A-Level Edexcel 生物课程中最常考查的概念，涵盖了分子生物学、能量学、遗传学和生态学。牢固掌握这些主题是取得高分的关键。
1. Protein Structure and Levels of Organisation | 蛋白质结构与组织层次
Proteins have four structural levels: primary (sequence of amino acids linked by peptide bonds), secondary (α-helices and β-pleated sheets stabilised by hydrogen bonds), tertiary (further folding due to hydrophobic interactions, ionic bonds, disulfide bridges, and hydrogen bonds), and quaternary (assembly of multiple polypeptide chains).
蛋白质具有四个结构层次：一级结构（通过肽键连接的氨基酸序列）、二级结构（由氢键稳定的 α-螺旋和 β-折叠）、三级结构（因疏水作用、离子键、二硫键和氢键进一步折叠）以及四级结构（多条多肽链的组装）。
Haemoglobin is a textbook example of quaternary structure, with two α and two β subunits displaying cooperative oxygen binding, giving a sigmoidal dissociation curve.
血红蛋白是四级结构的经典例子，含有两条 α 与两条 β 亚基，表现出协同氧合作用，呈现 S 形解离曲线。
Collagen, another fibrous protein, showcases the importance of repeated glycine‑proline sequences and covalent cross‑links for tensile strength.
另一种纤维蛋白——胶原蛋白，凸显了甘氨酸‑脯氨酸重复序列和共价交联对抗张强度的重要性。
2. Enzyme Kinetics and Michaelis‑Menten Equation | 酶动力学与米氏方程
The Michaelis‑Menten model relates reaction rate V to substrate concentration [S]:
V = Vₘₐₓ[S] / (Kₘ + [S])
米氏模型关联反应速率 V 与底物浓度 [S]：
V = Vₘₐₓ[S] / (Kₘ + [S])
Vₘₐₓ is the maximum rate achieved at enzyme saturation, and Kₘ (Michaelis constant) is the substrate concentration at half Vₘₐₓ, indicating enzyme‑substrate affinity.
Vₘₐₓ 是酶饱和时的最大速率，Kₘ（米氏常数）是达到半 Vₘₐₓ 时的底物浓度，反映酶与底物的亲和力。
| Inhibitor 抑制剂 | Effect on Vₘₐₓ | Effect on Kₘ |
|---|---|---|
| Competitive 竞争性 | Unchanged 不变 | Increases (apparent) 升高（表观） |
| Non‑competitive 非竞争性 | Decreases 降低 | Unchanged 不变 |
Competitive inhibitors resemble the substrate and bind to the active site, while non‑competitive inhibitors bind elsewhere, altering enzyme conformation.
竞争性抑制剂与底物结构类似并占据活性位点，而非竞争性抑制剂结合于别处，改变酶构象。
3. Cellular Respiration: Glycolysis to Oxidative Phosphorylation | 细胞呼吸：糖酵解到氧化磷酸化
Glycolysis in the cytoplasm converts glucose (C₆H₁₂O₆) to pyruvate, producing a net gain of 2 ATP and 2 NADH via substrate‑level phosphorylation.
细胞质中的糖酵解将葡萄糖 (C₆H₁₂O₆) 转化为丙酮酸，通过底物水平磷酸化净生成 2 ATP 和 2 NADH。
The link reaction in the mitochondrial matrix decarboxylates pyruvate, generating acetyl CoA, CO₂, and 2 NADH per glucose.
线粒体基质中的连接反应使丙酮酸脱羧，产生乙酰辅酶 A、CO₂，每分子葡萄糖生成 2 NADH。
The Krebs cycle fully oxidises acetyl CoA, yielding per glucose: 6 NADH, 2 FADH₂, 2 ATP (GTP), and 4 CO₂.
克雷布斯循环彻底氧化乙酰辅酶 A，每葡萄糖产生：6 NADH、2 FADH₂、2 ATP（GTP）和 4 CO₂。
During oxidative phosphorylation, electrons from NADH and FADH₂ pass along the electron transport chain, creating a proton gradient that drives ATP synthase (chemiosmosis).
氧化磷酸化中，NADH 和 FADH₂ 的电子沿电子传递链传递，形成质子梯度，驱动 ATP 合酶工作（化学渗透）。
4. Photosynthesis: Light-Dependent and Calvin Cycle | 光合作用：光反应与卡尔文循环
Non‑cyclic photophosphorylation involves both photosystems: light energy splits water (photolysis), releasing O₂, H⁺, and electrons; electrons flow through an electron transport chain, producing ATP and NADPH.
非环式光合磷酸化涉及两个光系统：光能裂解水（光解），释放 O₂、H⁺ 和电子；电子经传递链，产生 ATP 和 NADPH。
Cyclic photophosphorylation uses only PSI, cycling electrons back to produce ATP without NADPH or O₂, adjusting the ATP:NADPH ratio.
环式光合磷酸化仅用 PSI，将电子循环回传，生成 ATP 而不产生 NADPH 或 O₂，以调节 ATP:NADPH 比例。
he Calvin cycle in the stroma uses ATP and NADPH to fix CO₂: carboxylation of RuBP → two 3‑phosphoglycerate molecules, then reduction to glyceraldehyde 3‑phosphate (G3P), and regeneration of RuBP.
基质中的卡尔文循环用 ATP 和 NADPH 固定 CO₂：RuBP 羧化生成两个 3‑磷酸甘油酸，随即还原为甘油醛 3‑磷酸 (G3P)，最后再生 RuBP。
5. Nervous System: Action Potential and Synaptic Transmission | 神经系统：动作电位与突触传递
The resting membrane potential (−70 mV) is maintained by the Na⁺/K⁺ pump (3 Na⁺ out, 2 K⁺ in) and more K⁺ leak channels, making the inside negative relative to outside.
静息膜电位（−70 mV）由 Na⁺/K⁺ 泵（出 3 Na⁺，入 2 K⁺）和较多的 K⁺ 泄漏通道维持，使膜内相对膜外为负。
An action potential begins when voltage‑ larisation) → membrane potential rises to +40 mV. Then Na⁺ channels inactivate and voltage‑gated K⁺ channels open, causing repolarisation and a brief hyperpolarisation.
动作电位始于电压门控 Na⁺ 通道开放 → Na⁺ 内流（去极化）→ 膜电位升至 +40 mV。随后 Na⁺ 通道失活，电压门控 K⁺ 通道开放，导致复极化及短暂超极化。
At the synapse, calcium ions (Ca²⁺) enter the presynaptic knob, triggering vesicles to release neurotransmitter (e.g., acetylcholine) into the cleft; binding to postsynaptic receptors generates an EPSP or IPSP.
在突触处，钙离子 (Ca²⁺) 进入突触前末梢，触发囊泡释放神经递质（如乙酰胆碱）至间隙；与突触后受体结合产生兴奋性或抑制性突触后电位。
6. Muscle Contraction: Sliding Filament Theory | 肌肉收缩：滑动丝理论
Skeletal muscle fibres contain myofibrils with repeating sarcomeres. Thick filaments consist of myosin; thin filaments contain actin, tropomyosin, and troponin.
骨骼肌纤维含有带重复肌节的肌原纤维。粗肌丝由肌球蛋白构成；细肌丝含肌动蛋白、原肌球蛋白和肌钙蛋白。
Upon Ca²⁺ release from the sarcoplasmic reticulum, Ca²⁺ binds to troponin, causing tropomyosin to shift and expose myosin‑binding sites on actin.
当肌质网释放 Ca²⁺，Ca²⁺ 与肌钙蛋白结合，使原肌球蛋白移位，暴露肌动蛋白上的肌球蛋白结合位点。
Myosin heads bind actin, perform a power stroke pulling thin filaments toward the centre, then detach using ATP hydrolysis; the cycle repeats, shortening the sarcomere.
肌球蛋白头与肌动蛋白结合，进行摆动，将细肌丝拉向中心，随后利用 ATP 水解释放；循环重复，肌节缩短。
7. DNA Replication and Protein Synthesis | DNA复制与蛋白质合成
DNA replication is semiconservative: helicase unwinds the double helix, single‑strand binding proteins stabilise strands, and DNA polymerase III adds nucleotides in the 5' to 3' direction, forming leading and lagging (Okazaki fragments) strands.
DNA 复制是半保留的：解旋酶解开双螺旋，单链结合蛋白稳定链，DNA 聚合酶 III 按 5' 到 3' 方向添加核苷酸，形成前导链和后随链（冈崎片段）。
Transcription: RNA polymerase binds to the promoter, unwinds DNA, and synthesises a complementary mRNA strand, which in eukaryotes undergoes splicing to remove introns.
转录：RNA 聚合酶与启动子结合，解开 DNA，合成一段互补 mRNA 链，真核细胞中 mRNA 经剪接去除内含子。
Translation on ribosomes: mRNA codons pair with tRNA anticodons; tRNAs carry specific amino acids, which are joined by peptide bonds, elongating the polypeptide until a stop codon is reached.
核糖体上的翻译：mRNA 密码子与 tRNA 反密码子配对；tRNA 携带特定氨基酸，通过肽键连接，延伸多肽链，直到遇见终止密码子。
8. Genetic Engineering: Restriction Enzymes and PCR | 基因工程：限制酶与PCR
Restriction endonucleases cut DNA at specific palindromic sequences, often producing 'sticky ends' for ligation with complementary ends using DNA ligase.
限制性内切酶在特定回文序列处切割 DNA，常产生“粘性末端”，便于用 DNA 连接酶与互补末端连接。
Gel electrophoresis separates DNA fragments by size: negatively charged DNA moves toward the anode; smaller fragments travel faster, allowing size estimation against a ladder.
凝胶电泳按大小分离 DNA 片段：带负电的 DNA 移向阳极；小片段迁移更快，对照标记可估算大小。
Polymerase chain reaction (PCR) amplifies DNA via repeated cycles: denaturation (95°C), primer annealing (50‑65°C), and extension by Taq polymerase (72°C).
聚合酶链式反应 (PCR) 通过重复循环扩增 DNA：变性 (95°C)、引物退火 (50‑65°C) 和 Taq 聚合酶延伸 (72°C)。
9. Inheritance and Hardy‑Weinberg Principle | 遗传与哈迪‑温伯格定律
Monohybrid crosses involving dominant/recessive or codominant alleles predict genotypic and phenotypic ratios. Sex‑linked traits, such as haemophilia, show different patterns in males and females.
涉及显隐性或共显性等位基因的单基因杂交可预测基因型与表型比例。伴性性状（如血友病）在男性和女性中表现不同。
The Hardy‑Weinberg principle states that allele and genotype frequencies remain constant from generation to generation in the absence of disturbing factors:
p² + 2pq + q² = 1 and p + q = 1
哈迪‑温伯格定律指出，在没有干扰因素时，等位基因频率和基因型频率代代保持恒定：
p² + 2pq + q² = 1 且 p + q = 1
Conditions include large population size, random mating, no mutation, no migration, and no natural selection. These equations help estimate carrier frequencies for genetic disorders.
条件包括大种群、随机交配、无突变、无迁移、无自然选择。这些方程有助于估计遗传病携带者频率。
10. Ecology: Energy Flow and Nutrient Cycles | 生态学：能量流动与物质循环
Energy enters ecosystems via photosynthesis and flows through trophic levels; typically only about 10% of energy is transferred to the next level, with the rest lost as heat from respiration, movement, and undigested materials.
能量通过光合作用进入生态系统，沿营养级流动；通常仅约 10% 的能量传递到下一级，其余以呼吸热、运动及未消化物质的形式散逸。
The carbon cycle circulates carbon among the atmosphere (CO₂), producers (photosynthesis), consumers (feeding), decomposers (decomposition), and fossil fuel combustion.
碳循环使碳在大气 (CO₂)、生产者（光合作用）、消费者（摄食）、分解者（分解）和化石燃料燃烧之间循环。
In the nitrogen cycle, nitrogen‑fixing bacteria (e.g., Rhizobium) convert N₂ into ammonia; nitrifying bacteria oxidise ammonia to nitrite then nitrate; denitrifying bacteria return N₂ to the atmosphere under anaerobic conditions.
在氮循环中，固氮菌（如根瘤菌）将 N₂ 转化为氨；硝化细菌将氨氧化为亚硝酸盐再至硝酸盐；反硝化细菌在厌氧条件下将 N₂ 送回大气。
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