ESAT Biology: Biochemistry & Metabolism | ESAT 生物：生物化学与代谢

📚 ESAT Biology: Biochemistry & Metabolism | ESAT 生物：生物化学与代谢

Biochemistry and metabolism form the molecular foundation of all biological systems. For the ESAT Biology section, a clear understanding of key biomolecules, enzyme function, and the major metabolic pathways is essential for tackling application-style questions with confidence.

生物化学与代谢是所有生物系统的分子基础。在 ESAT 生物考试中，清晰理解关键生物分子、酶的功能以及主要代谢途径，是自信应对应用型题目的关键。

1. Key Biomolecules: Carbohydrates, Lipids, Proteins | 关键生物分子：碳水化合物、脂质、蛋白质

Carbohydrates are classified into monosaccharides, disaccharides, and polysaccharides. Glucose (C₆H₁₂O₆) is the primary energy source, while starch and glycogen serve as storage polysaccharides in plants and animals, respectively. Cellulose provides structural support in plant cell walls.

碳水化合物分为单糖、二糖和多糖。葡萄糖（C₆H₁₂O₆）是主要的能量来源，而淀粉和糖原分别作为植物和动物体内的储存性多糖。纤维素为植物细胞壁提供结构支持。

Lipids are hydrophobic molecules including triglycerides, phospholipids, and steroids. Triglycerides store long-term energy, phospholipids form cell membranes, and steroids such as cholesterol modulate membrane fluidity and serve as hormone precursors.

脂质是疏水性分子，包括甘油三酯、磷脂和类固醇。甘油三酯储存长期能量，磷脂构成细胞膜，而胆固醇等类固醇调节膜流动性并作为激素前体。

Proteins are polymers of amino acids linked by peptide bonds. Their functions range from enzymatic catalysis and transport to immune defense and cell signalling. The sequence of amino acids determines the protein’s three-dimensional structure and ultimately its function.

蛋白质是由氨基酸通过肽键连接而成的聚合物。其功能涵盖酶催化、物质运输、免疫防御和细胞信号传导。氨基酸的序列决定蛋白质的三维结构，并最终决定其功能。

2. Enzyme Structure and Mechanism | 酶的结构与作用机制

Enzymes are biological catalysts that lower the activation energy of reactions without being consumed. Most enzymes are globular proteins with an active site complementary to their specific substrate.

酶是生物催化剂，能够降低反应的活化能而不被消耗。大多数酶是球状蛋白质，其活性位点与特定底物互补。

The induced-fit model describes how the active site changes shape slightly upon substrate binding, stabilising the transition state and facilitating product formation. Enzymes exhibit high specificity due to the precise three-dimensional arrangement of amino acid residues in the active site.

“诱导契合”模型描述了底物结合时活性位点形状发生轻微改变，从而稳定过渡态并促进产物形成的过程。由于活性位点中氨基酸残基的精确三维排布，酶表现出高度的专一性。

Enzymes reduce activation energy by providing an alternative reaction pathway.
酶通过提供替代反应途径降低活化能。
Enzyme-substrate complexes form transiently and dissociate after catalysis.
酶-底物复合物在催化后暂时形成并随即解离。
Enzyme activity is highly sensitive to temperature and pH.
酶活性对温度和 pH 高度敏感。

3. Factors Affecting Enzyme Activity | 影响酶活性的因素

Temperature and pH are the two most critical factors affecting enzyme activity. As temperature rises, molecular kinetic energy increases, accelerating reaction rates until the optimum temperature is reached. Beyond this point, heat denatures the enzyme’s tertiary structure, reducing activity sharply.

温度和 pH 是影响酶活性的两个最关键因素。随着温度升高，分子动能增加，反应速率加快，直至达到最适温度。超过该温度后，高温使酶的三级结构变性，活性急剧下降。

Enzyme concentration and substrate concentration also influence reaction velocity. At low substrate concentrations, the reaction rate increases linearly; at high substrate concentrations, the rate plateaus as the enzyme becomes saturated.

酶浓度和底物浓度也会影响反应速率。在低底物浓度下，反应速率呈线性增加；在高底物浓度下，由于酶达到饱和，速率趋于平稳。

Competitive inhibitors resemble the substrate and bind to the active site, while non-competitive inhibitors bind elsewhere, altering the enzyme’s shape. Understanding these principles allows prediction of metabolic regulation under physiological conditions.

竞争性抑制剂类似底物并占据活性位点，而非竞争性抑制剂结合在其他位置，改变酶的空间结构。理解这些原理有助于预测生理条件下的代谢调控。

4. Respiration: Glycolysis | 呼吸作用：糖酵解

Glycolysis occurs in the cytoplasm and is the first stage of cellular respiration. One glucose molecule (6C) is converted into two molecules of pyruvate (3C), yielding a net gain of two ATP and two NADH molecules.

糖酵解发生在细胞质中，是细胞呼吸的第一阶段。一分子葡萄糖（6C）被转化为两分子丙酮酸（3C），净产生两分子 ATP 和两分子 NADH。

Glucose + 2 NAD⁺ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 ATP + 2 H₂O

葡萄糖 + 2 NAD⁺ + 2 ADP + 2 Pi → 2 丙酮酸 + 2 NADH + 2 ATP + 2 H₂O

Glycolysis does not require oxygen and is therefore the sole energy-producing pathway in anaerobic organisms and in muscle cells during intense exercise. The process is regulated by allosteric enzymes such as phosphofructokinase, which responds to cellular energy status.

糖酵解不需要氧气，因此是厌氧生物以及在剧烈运动中肌肉细胞唯一的产能途径。该过程受磷酸果糖激酶等别构酶调控，这些酶对细胞能量状态敏感。

5. Link Reaction and Krebs Cycle | 连接反应与克雷布斯循环

In aerobic conditions, pyruvate enters the mitochondrial matrix, where the link reaction converts it to acetyl-CoA, releasing carbon dioxide and producing NADH. This irreversible step connects glycolysis to the Krebs cycle.

在有氧条件下，丙酮酸进入线粒体基质，连接反应将其转化为乙酰辅酶 A，释放二氧化碳并产生 NADH。这一不可逆步骤将糖酵解与克雷布斯循环连接起来。

The Krebs cycle, also known as the citric acid cycle, involves a series of enzyme-catalysed reactions that oxidise acetyl-CoA completely to CO₂. For each acetyl-CoA molecule, the cycle produces three NADH, one FADH₂, and one ATP (or GTP) via substrate-level phosphorylation.

克雷布斯循环又称柠檬酸循环，是一系列酶催化反应，将乙酰辅酶 A 完全氧化为 CO₂。每代谢一分子乙酰辅酶 A，循环产生三分子 NADH、一分子 FADH₂ 和一分子 ATP（或 GTP），后者通过底物水平磷酸化生成。

Acetyl-CoA + 3 NAD⁺ + FAD + ADP + Pi → 2 CO₂ + 3 NADH + FADH₂ + ATP + CoA

乙酰辅酶 A + 3 NAD⁺ + FAD + ADP + Pi → 2 CO₂ + 3 NADH + FADH₂ + ATP + CoA

6. Oxidative Phosphorylation and Chemiosmosis | 氧化磷酸化与化学渗透

Oxidative phosphorylation occurs on the inner mitochondrial membrane, where the electron transport chain (ETC) transfers electrons from NADH and FADH₂ to molecular oxygen. This process releases energy to pump protons (H⁺) across the membrane, creating an electrochemical gradient.

氧化磷酸化发生在线粒体内膜上，电子传递链（ETC）将电子从 NADH 和 FADH₂ 传递给分子氧。该过程释放能量，将质子（H⁺）泵过膜，形成电化学梯度。

ATP synthase harnesses the proton-motive force as protons flow back into the matrix through the enzyme’s channel. This coupling of electron transport to ATP synthesis is called chemiosmosis. Approximately 26–28 ATP molecules are produced per glucose molecule under aerobic conditions.

当质子通过 ATP 合酶的通道回流至基质时，该酶利用质子动力合成 ATP。电子传递与 ATP 合成的偶联称为化学渗透。在有氧条件下，每分子葡萄糖约产生 26–28 分子 ATP。

Stage 阶段	Location 位置	Net ATP 净生成 ATP
Glycolysis 糖酵解	Cytoplasm 细胞质	2
Link reaction 连接反应	Mitochondrial matrix 线粒体基质	0
Krebs cycle 克雷布斯循环	Mitochondrial matrix 线粒体基质	2
Oxidative phosphorylation 氧化磷酸化	Inner mitochondrial membrane 线粒体内膜	≈26–28

7. Anaerobic Respiration and Fermentation | 无氧呼吸与发酵

In the absence of oxygen, pyruvate is converted to lactate in animal cells or to ethanol and CO₂ in yeast, regenerating NAD⁺ so that glycolysis can continue. This process yields only 2 ATP per glucose molecule.

在无氧条件下，丙酮酸在动物细胞中被转化为乳酸，或在酵母中被转化为乙醇和 CO₂，从而再生 NAD⁺，使糖酵解得以继续。该过程每分子葡萄糖仅产生 2 分子 ATP。

In mammals, lactate accumulation in muscles during vigorous exercise causes a temporary drop in intracellular pH, contributing to muscle fatigue. The Cori cycle transports lactate to the liver, where it is converted back to glucose or pyruvate.

在哺乳动物中，剧烈运动时肌肉中乳酸的积累会导致细胞内 pH 暂时下降，从而引起肌肉疲劳。Cori 循环将乳酸转运至肝脏，在那里被重新转化为葡萄糖或丙酮酸。

Fermentation in yeast is exploited in brewing and baking industries, where ethanol production and CO₂ release are commercially valuable. The distinction between aerobic and anaerobic pathways is a frequent ESAT examination topic.

酵母发酵被广泛应用于酿造和烘焙行业，乙醇生产和 CO₂ 释放具有重要的商业价值。有氧与无氧途径的区分是 ESAT 考试中的高频考点。

8. Photosynthesis: Light-Dependent Reactions | 光合作用：光依赖反应

Photosynthesis in plants occurs in chloroplasts, with the light-dependent reactions taking place in the thylakoid membranes. Chlorophyll absorbs light energy, exciting electrons that pass along an electron transport chain, generating ATP and reduced NADP (NADPH).

植物的光合作用发生在叶绿体中，光依赖反应在类囊体膜上进行。叶绿素吸收光能，激发电子沿电子传递链传递，生成 ATP 和还原型 NADP（NADPH）。

Water is split by photolysis to replace the lost electrons, releasing oxygen gas as a by-product. The proton gradient established across the thylakoid membrane drives ATP synthase via chemiosmosis, a process identical in principle to mitochondrial oxidative phosphorylation.

水通过光解作用被分解以补充丢失的电子，并释放氧气作为副产物。类囊体膜两侧建立的质子梯度驱动 ATP 合酶进行化学渗透合成 ATP，该原理与线粒体氧化磷酸化一致。

9. Photosynthesis: Light-Independent Reactions (Calvin Cycle) | 光合作用：光不依赖反应（卡尔文循环）

The Calvin cycle occurs in the stroma of the chloroplast and uses ATP and NADPH from the light-dependent reactions to fix CO₂ into organic molecules. The enzyme RuBisCO catalyses the carboxylation of ribulose-1,5-bisphosphate (RuBP), a five-carbon compound, producing two molecules of 3-phosphoglycerate (3-PGA).

卡尔文循环发生在叶绿体基质中，利用光依赖反应产生的 ATP 和 NADPH 将 CO₂ 固定为有机分子。RuBisCO 酶催化五碳化合物核酮糖-1,5-二磷酸（RuBP）的羧化，产生两分子 3-磷酸甘油酸（3-PGA）。

The cycle has three phases: carbon fixation, reduction, and regeneration of RuBP. For every three CO₂ molecules fixed, six molecules of G3P are produced, of which five are used to regenerate RuBP and one is exported to form glucose and other carbohydrates.

该循环包括三个阶段：碳固定、还原和 RuBP 再生。每固定三分子 CO₂，产生六分子 G3P，其中五分子用于再生 RuBP，一分子被输出以合成葡萄糖和其他碳水化合物。

3 CO₂ + 9 ATP + 6 NADPH → G3P + 9 ADP + 8 Pi + 6 NADP⁺

10. Metabolic Regulation and Integration | 代谢调控与整合

Metabolic pathways are tightly regulated to maintain homeostasis. Hormones such as insulin and glucagon coordinate carbohydrate and lipid metabolism, switching between anabolic and catabolic states depending on energy availability.

代谢途径受到严格调控以维持稳态。胰岛素和胰高血糖素等激素协调碳水化合物和脂质代谢，根据能量供应情况在合成代谢与分解代谢状态之间切换。

Allosteric regulation, covalent modification, and gene expression control are the primary mechanisms of metabolic regulation. For example, ATP inhibits phosphofructokinase in glycolysis, while AMP activates it, reflecting the cell’s energy status.

别构调节、共价修饰和基因表达控制是代谢调控的主要机制。例如，ATP 抑制糖酵解中的磷酸果糖激酶，而 AMP 激活它，反映细胞的能量状态。

A deep appreciation of these regulatory principles allows candidates to predict how metabolic flux changes under different physiological conditions, a skill directly tested in ESAT scenario-based questions.

深入理解这些调控原理，考生可以预测不同生理条件下代谢通量的变化，这是 ESAT 情境题中直接考察的能力。

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ESAT Biology: Biochemistry & Metabolism | ESAT 生物：生物化学与代谢