📚 IB & Edexcel Biology: Cellular Respiration Key Points | IB & Edexcel 生物：呼吸作用考点精讲
Cellular respiration is the controlled release of energy from organic compounds in cells to produce ATP. It occurs in a series of metabolic pathways, primarily in the mitochondria, and is essential for all life processes. For both IB and Edexcel Biology, a thorough understanding of aerobic and anaerobic respiration, the key stages, ATP yield, and the factors influencing the rate of respiration is required. This article consolidates the core examination points across these syllabi.
细胞呼吸是细胞内有机物受控分解释能并生成 ATP 的过程，主要通过线粒体中的一系列代谢途径完成，对一切生命活动至关重要。在 IB 与 Edexcel 生物考试中，均需深入掌握有氧呼吸与无氧呼吸的关键阶段、ATP 产量以及影响呼吸速率的因素。本文整合了不同考纲下的核心考点。
1. Overview of Cellular Respiration | 细胞呼吸概述
tored in the molecule adenosine triphosphate (ATP). The overall balanced equation for aerobic respiration is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (as ATP). Respiration occurs in several stages: glycolysis in the cytoplasm, the link reaction and Krebs cycle in the mitochondrial matrix, and the electron transport chain on the inner mitochondrial membrane. The process uses coenzymes NAD⁺ and FAD to carry hydrogen atoms and electrons.
细胞呼吸是分解代谢途径，将葡萄糖等有机燃料分子分解释放能量，能量暂时储存于腺苷三磷酸 (ATP) 中。有氧呼吸的总平衡方程式为：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量（以 ATP 形式）。呼吸作用分为多个阶段：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，电子传递链位于线粒体内膜。过程中需要辅酶 NAD⁺ 和 FAD 来携带氢原子和电子。
2. Glycolysis | 糖酵解
Glycolysis is the first stage of respiration and takes place in the cytoplasm. It does not require oxygen and is therefore anaerobic. One molecule of glucose (6C) is phosphorylated using 2 ATP and then split into two molecules of triose phosphate (3C). These are then oxidised to form two molecules of pyruvate (3C). This produces a net yield of 2 ATP (substrate-level phosphorylation) and 2 reduced NAD (NADH). The steps involve hexose phosphorylation, lysis, and oxidation. Key enzymes include hexokinase and phosphofructokinase.
糖酵解是呼吸作用的第一阶段，发生在细胞质中，不需要氧气，因此是厌氧过程。一分子葡萄糖（6C）先消耗 2 ATP 进行磷酸化，然后裂解为两分子丙糖磷酸（3C），再被氧化生成两分子丙酮酸（3C）。这一阶段的净产量为 2 ATP（底物水平磷酸化）和 2 个还原态 NAD (NADH)。步骤包括己糖磷酸化、裂解和氧化，关键酶有己糖激酶和磷酸果糖激酶。
3. Link Reaction | 连接反应（丙酮酸氧化脱羧）
Pyruvate enters the mitochondrial matrix via active transport. In the link reaction, each pyruvate (3C) is decarboxylated (CO₂ is removed) and oxidised by NAD⁺ to form an acetyl group (2C). The acetyl group combines with coenzyme A to form acetyl-CoA. No ATP is produced directly. For each glucose molecule, two pyruvate molecules undergo this reaction, yielding 2 CO₂, 2 NADH, and 2 acetyl-CoA. This step links glycolysis to the Krebs cycle.
丙酮酸通过主动运输进入线粒体基质。在连接反应中，每个丙酮酸（3C）脱羧（释放 CO₂）并被 NAD⁺ 氧化，形成乙酰基（2C）。乙酰基与辅酶 A 结合生成乙酰辅酶 A。此步骤不直接产生 ATP。每分子葡萄糖产生两分子丙酮酸，因此连接反应共释放 2 CO₂、生成 2 NADH 和 2 乙酰辅酶 A，将糖酵解与克雷布斯循环连接起来。
4. Krebs Cycle (Citric Acid Cycle) | 克雷布斯循环（柠檬酸循环）
The Krebs cycle takes place in the mitochondrial matrix. Acetyl-CoA (2C) combines with a 4-carbon compound, oxaloacetate, to form citrate (6C). A series of enzyme-controlled reactions then regenerates oxaloacetate. In one turn of the cycle, 2 CO₂ molecules are released, one ATP (or GTP) is produced by substrate-level phosphorylation, and the coenzymes NAD⁺ and FAD are reduced to 3 NADH and 1 FADH₂. Since each glucose yields two acetyl-CoA, the cycle turns twice, producing a total of 4 CO₂, 2 ATP, 6 NADH, and 2 FADH₂ per glucose.
克雷布斯循环发生在线粒体基质中。乙酰辅酶 A（2C）与四碳化合物草酰乙酸结合，形成柠檬酸（6C）。然后经过一系列酶促反应重新生成草酰乙酸。每循环一次，释放 2 分子 CO₂，通过底物水平磷酸化生成 1 ATP（或 GTP），并使 NAD⁺ 和 FAD 还原生成 3 NADH 和 1 FADH₂。因为每个葡萄糖产生两分子乙酰辅酶 A，所以循环运行两次，每分子葡萄糖合计生成 4 CO₂、2 ATP、6 NADH 和 2 FADH₂。
5. Electron Transport Chain and Chemiosmosis | 电子传递链与化学渗透
The electron transport chain (ETC) is located on the inner mitochondrial membrane. Reduced NAD and FAD (NADH and FADH₂) donate electrons to a series of carrier proteins. As electrons pass along the chain, energy is released and used to pump protons (H⁺) from the matrix into the intermembrane space, creating a proton gradient. The protons then flow back through ATP synthase, a process called chemiosmosis, driving the synthesis of ATP from ADP and inorganic phosphate. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. If oxygen is absent, the ETC halts, preventing NAD⁺ regeneration and stopping aerobic respiration.
电子传递链 (ETC) 位于线粒体内膜。还原态 NAD 和 FAD（NADH 和 FADH₂）将电子传递给一系列载体蛋白。电子沿链传递时释放能量，将质子 (H⁺) 从基质泵入膜间隙，形成质子梯度。然后质子通过 ATP 合酶回流，这一化学渗透过程驱动 ADP 和磷酸合成 ATP。氧气作为最终电子受体，与电子和质子结合生成水。若无氧气，电子传递链停止，NAD⁺ 无法再生，有氧呼吸随之终止。
6. ATP Yield in Aerobic Respiration | 有氧呼吸的 ATP 产量
Theoretical ATP yield per glucose in eukaryotes is approximately 30–32 ATP. The breakdown is as follows: glycolysis yields 2 ATP and 2 NADH (each yields about 1.5–2.5 ATP depending on the shuttle system); link reaction yields 2 NADH (each ~2.5 ATP); Krebs cycle yields 2 ATP (substrate-level), 6 NADH (~15 ATP), and 2 FADH₂ (~3 ATP). Total from oxidative phosphorylation is about 28 ATP, plus 4 ATP from substrate-level phosphorylation, giving ~32 ATP. Prokaryotes can yield more, around 38 ATP, because they lack mitochondrial membranes and shuttle costs. The actual yield is often lower due to proton leak and use of the proton gradient for other processes.
真核生物每分子葡萄糖的理论 ATP 产量约为 30–32 ATP。分解如下：糖酵解 2 ATP 和 2 NADH（每个 NADH 视穿梭系统可产生 1.5–2.5 ATP）；连接反应 2 NADH（每个约 2.5 ATP）；克雷布斯循环 2 ATP（底物水平）、6 NADH（约 15 ATP）和 2 FADH₂（约 3 ATP）。氧化磷酸化合计约 28 ATP，加上底物水平磷酸化的 4 ATP，共约 32 ATP。原核生物因无线粒体膜穿梭消耗，产量可达 38 ATP 左右。实际产量通常偏低，存在质子泄漏及梯度其他用途。
7. Anaerobic Respiration | 无氧呼吸
eria, pyruvate is reduced to lactate (lactic acid) by lactate dehydrogenase, oxidising NADH back to NAD⁺. This causes muscle fatigue. In yeast and plants, pyruvate is decarboxylated to ethanal, which is then reduced to ethanol, also regenerating NAD⁺. Both processes yield only 2 ATP per glucose, all from glycolysis. The reactions: C₆H₁₂O₆ → 2 C₃H₆O₃ (lactic acid) or C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂.
氧气受限时，细胞通过发酵丙酮酸再生 NAD⁺，使糖酵解得以持续产生 ATP。在动物和某些细菌中，丙酮酸被乳酸脱氢酶还原为乳酸，同时将 NADH 氧化回 NAD⁺，导致肌肉疲劳。在酵母和植物中，丙酮酸脱羧生成乙醛，再被还原成乙醇，同样再生 NAD⁺。这两种途径每分子葡萄糖仅净产 2 ATP，全部来自糖酵解。反应式：C₆H₁₂O₆ → 2 C₃H₆O₃（乳酸发酵）或 C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂。
8. Respiratory Substrates and Respiratory Quotient (RQ) | 呼吸底物与呼吸商
ed / O₂ consumed. For glucose, RQ = 1.0. For lipids, which are more reduced, RQ is about 0.7. For proteins, RQ is around 0.9. RQ values can indicate which substrate is being used. Decomposition of triglycerides requires more oxygen per carbon, so RQ is lower. In IB and Edexcel practicals, measuring RQ using a respirometer helps infer metabolic fuel.
细胞可以呼吸利用糖类、脂质和蛋白质。呼吸商 (RQ) 是产生的 CO₂ 与消耗的 O₂ 的体积比。RQ = 释放的 CO₂ / 消耗的 O₂。对葡萄糖来说，RQ = 1.0；脂质因高度还原，RQ 约为 0.7；蛋白质的 RQ 约 0.9。RQ 值可以指示正在利用何种底物。甘油三酯的分解每碳需氧量更大，因此 RQ 更低。在 IB 和 Edexcel 实验中，使用呼吸计测定 RQ 可帮助推断代谢燃料。
9. Factors Affecting the Rate of Respiration | 影响呼吸速率的因素
The rate of cellular respiration is influenced by temperature, oxygen concentration, substrate availability, and pH. Enzyme activity increases with temperature up to an optimum, then denatures. Oxygen is the final electron acceptor; its shortage limits the ETC. In many organisms, higher CO₂ levels can reduce respiration rate. Substrate concentration, especially glucose, can be rate-limiting. In plants, light intensity and water potential also affect respiration indirectly. The effect of these factors can be measured using respirometers or dye reduction experiments.
细胞呼吸速率受温度、氧气浓度、底物供应和 pH 影响。酶活性随温度升高而增加至最适点，之后变性。氧气是最终电子受体，缺氧会限制电子传递链。在许多生物中，CO₂ 浓度升高会减慢呼吸速率。底物浓度，特别是葡萄糖，可以成为限制因子。在植物中，光照强度和水势也间接影响呼吸。可使用呼吸计或染料还原实验测量这些因素的影响。
10. Key Experimental Investigations | 重要实验探究
Common investigations include: using a respirometer to measure oxygen uptake in germinating seeds or invertebrates; observing CO₂ production using limewater or hydrogencarbonate indicator; using 2,3,5-triphenyltetrazolium chloride (TTC) to test for dehydrogenase activity; and investigating yeast fermentation rates at different temperatures or sugar concentrations. For IB IA and Edexcel core practicals, students must design experiments, control variables, and justify the reliability of data. Measures of respiratory rate can be linked to metabolic theory and enzyme kinetics.
常见实验包括：使用呼吸计测量萌发种子或无脊椎动物的吸氧量；用石灰水或碳酸氢钠指示剂观察 CO₂ 生成；用 2,3,5-三苯基氯化四氮唑 (TTC) 检测脱氢酶活性；以及研究不同温度或糖浓度下酵母的发酵速率。对 IB 内部评估和 Edexcel 核心实验，学生须设计实验、控制变量并论证数据可靠性。呼吸速率的测量可与代谢理论和酶动力学相结合。
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