📚 Respiration Key Points for IB and CCEA Biology | IB CCEA 生物：呼吸作用 考点精讲
Respiration is a central metabolic process in all living cells, releasing energy from organic molecules to fuel cellular activities. This article covers the core A-level and IB syllabus requirements for aerobic and anaerobic respiration, including glycolysis, the link reaction, Krebs cycle, the electron transport chain, chemiosmosis, and the factors that affect respiratory rate.
呼吸作用是所有活细胞的核心代谢过程，通过分解有机分子释放能量以驱动细胞活动。本文涵盖 A-level 和 IB 课程中有氧呼吸与无氧呼吸的核心考点，包括糖酵解、连接反应、克雷布斯循环、电子传递链、化学渗透以及影响呼吸速率的因素。
1. Overview of Respiration | 呼吸作用概述
Respiration is the process by which cells break down organic substrates, typically glucose, to release energy in the form of ATP. It can be aerobic (requiring oxygen) or anaerobic (without oxygen). The overall aerobic equation is often summarized as: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP + heat).
呼吸作用是细胞分解有机底物（通常是葡萄糖）以释放 ATP 形式的能量的过程。它可分为有氧呼吸（需氧）和无氧呼吸（无氧）。有氧呼吸的总方程式常概括为：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量（ATP + 热）。
In reality, aerobic respiration is a multi-step pathway occurring in the cytoplasm and mitochondria, and involves oxidation, decarboxylation, and substrate-level and oxidative phosphorylation.
实际上，有氧呼吸是一个多步骤的代谢途径，发生在细胞质和线粒体中，涉及氧化、脱羧、底物水平磷酸化和氧化磷酸化。
The main stages are: glycolysis, the link reaction, the Krebs cycle, and the electron transport chain / oxidative phosphorylation.
主要阶段包括：糖酵解、连接反应、克雷布斯循环以及电子传递链/氧化磷酸化。
2. Glycolysis | 糖酵解
Glycolysis occurs in the cytoplasm and does not require oxygen. It converts one molecule of glucose (6C) into two molecules of pyruvate (3C). The process uses 2 ATP but produces 4 ATP, giving a net gain of 2 ATP per glucose. It also reduces 2 NAD⁺ to 2 NADH + 2 H⁺.
糖酵解发生在细胞质中，不需要氧气。它将一分子葡萄糖（6C）转化为两分子丙酮酸（3C）。该过程消耗 2 ATP，但产生 4 ATP，因此每分子葡萄糖净得 2 ATP。它还将 2 NAD⁺ 还原为 2 NADH + 2 H⁺。
The key steps include phosphorylation of glucose to hexose bisphosphate (using 2 ATP), lysis into two triose phosphates, oxidation via glyceraldehyde 3-phosphate dehydrogenase producing NADH, and substrate-level phosphorylation generating ATP.
关键步骤包括：葡萄糖磷酸化形成己糖二磷酸（消耗 2 ATP）、裂解为两分子磷酸丙糖、通过甘油醛-3-磷酸脱氢酶氧化产生 NADH，以及底物水平磷酸化生成 ATP。
Glycolysis is the same in both aerobic and anaerobic respiration; the fate of pyruvate differs depending on oxygen availability.
糖酵解在有氧和无氧呼吸中相同；丙酮酸的后续去向取决于氧气的可用性。
3. Link Reaction | 连接反应
If oxygen is present, each pyruvate enters the mitochondrial matrix via active transport. Here, the link reaction occurs: pyruvate (3C) is decarboxylated (CO₂ is removed) and oxidized by NAD⁺, producing acetyl-CoA (2C) and NADH + H⁺.
如果有氧气存在，每个丙酮酸通过主动运输进入线粒体基质。在此发生连接反应：丙酮酸（3C）脱羧（移除 CO₂）并被 NAD⁺ 氧化，产生乙酰辅酶 A（2C）和 NADH + H⁺。
The overall equation per pyruvate is: pyruvate + NAD⁺ + CoA → acetyl-CoA + CO₂ + NADH + H⁺. This reaction is catalysed by the pyruvate dehydrogenase complex.
每个丙酮酸的总方程式为：丙酮酸 + NAD⁺ + 辅酶 A → 乙酰辅酶 A + CO₂ + NADH + H⁺。该反应由丙酮酸脱氢酶复合体催化。
Acetyl-CoA then enters the Krebs cycle. No ATP is produced directly in the link reaction.
乙酰辅酶 A 随后进入克雷布斯循环。连接反应本身不直接产生 ATP。
4. Krebs Cycle | 克雷布斯循环
The Krebs cycle (citric acid cycle) takes place in the mitochondrial matrix. Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C), which is then progressively decarboxylated and oxidized through a series of enzyme-controlled reactions, regenerating oxaloacetate.
克雷布斯循环（柠檬酸循环）发生在线粒体基质中。乙酰辅酶 A（2C）与草酰乙酸（4C）结合形成柠檬酸（6C），然后经过一系列酶控反应逐步脱羧和氧化，最终重新生成草酰乙酸。
For each turn of the cycle (per acetyl-CoA), the following are produced: 2 CO₂, 1 ATP (by substrate-level phosphorylation, as GTP), 3 NADH + 3 H⁺, and 1 FADH₂. Since one glucose yields two acetyl-CoA, the cycle turns twice per glucose.
每轮循环（每个乙酰辅酶 A）产生：2 CO₂、1 ATP（通过底物水平磷酸化，以 GTP 形式）、3 NADH + 3 H⁺ 和 1 FADH₂。由于一分子葡萄糖生成两分子乙酰辅酶 A，循环每分子葡萄糖运转两圈。
Thus, per glucose the Krebs cycle yields: 4 CO₂, 2 ATP, 6 NADH, 2 FADH₂. The CO₂ is released as a waste product; the reduced coenzymes carry electrons to the electron transport chain.
因此，每分子葡萄糖的克雷布斯循环总产物为：4 CO₂、2 ATP、6 NADH、2 FADH₂。CO₂ 作为废物释放；还原型辅酶携带电子进入电子传递链。
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 (cristae). The reduced NADH and FADH₂ donate electrons to a series of electron carriers (protein complexes I, II, III, IV) and mobile carriers (ubiquinone, cytochrome c).
电子传递链（ETC）位于线粒体内膜（嵴）上。还原型 NADH 和 FADH₂ 将电子捐赠给一系列电子载体（蛋白质复合体 I、II、III、IV）和可移动载体（泛醌、细胞色素 c）。
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 (electrochemical gradient).
当电子沿链传递时，释放的能量被用于将质子（H⁺）从基质泵入膜间腔，形成质子梯度（电化学梯度）。
Oxygen acts as the final electron acceptor, combining with electrons and H⁺ to form water: ½O₂ + 2e⁻ + 2H⁺ → H₂O. This is why oxygen is essential in aerobic respiration.
氧气作为最终电子受体，与电子和 H⁺ 结合生成水：½O₂ + 2e⁻ + 2H⁺ → H₂O。这就是有氧呼吸必须依赖氧气的原因。
Chemiosmosis: the proton gradient drives H⁺ back into the matrix through ATP synthase (a channel protein and enzyme), and this flow of protons provides the energy for ATP synthesis from ADP + Pi. This process is called oxidative phosphorylation.
化学渗透：质子梯度驱动 H⁺ 通过 ATP 合酶（一种通道蛋白兼酶）流回基质，质子流动提供的能量用于从 ADP + Pi 合成 ATP。这一过程称为氧化磷酸化。
6. Oxidative Phosphorylation and ATP Yield | 氧化磷酸化与 ATP 产量
Oxidative phosphorylation produces the vast majority of ATP in aerobic respiration. The number of ATP molecules generated per reduced coenzyme can vary slightly depending on the efficiency of the system, but typical textbook values are: each NADH yields about 2.5–3 ATP; each FADH₂ yields about 1.5–2 ATP.
氧化磷酸化产生了有氧呼吸中绝大部分的 ATP。每个还原型辅酶生成的 ATP 分子数因系统效率略有差异，但教科书常用数值为：每个 NADH 约产生 2.5–3 ATP；每个 FADH₂ 约产生 1.5–2 ATP。
Total ATP yield per glucose in aerobic respiration is often cited as 30–32 (or 36–38 in older resources). A common breakdown is: Glycolysis (net 2 ATP, 2 NADH → 3–5 ATP), Link reaction (2 NADH → 5 ATP), Krebs cycle (2 ATP, 6 NADH → 15 ATP, 2 FADH₂ → 3 ATP). Sum ≈ 30–32 ATP.
有氧呼吸每分子葡萄糖的总 ATP 产量常被引用为 30–32（旧版资料为 36–38）。常见的分解为：糖酵解（净 2 ATP，2 NADH → 3–5 ATP），连接反应（2 NADH → 5 ATP），克雷布斯循环（2 ATP，6 NADH → 15 ATP，2 FADH₂ → 3 ATP）。合计约 30–32 ATP。
Note: the NADH produced in glycolysis (cytoplasm) may need to be shuttled into the mitochondria, which can cost some ATP, affecting net yield.
注意：糖酵解（细胞质）中产生的 NADH 需要穿梭进入线粒体，这可能消耗一些 ATP，从而影响净产量。
7. Anaerobic Respiration in Animals | 动物的无氧呼吸
When oxygen is limited, animal cells (e.g., muscle cells during vigorous exercise) undergo anaerobic respiration. Pyruvate is reduced by NADH to form lactate (lactic acid), catalysed by lactate dehydrogenase. This regenerates NAD⁺, allowing glycolysis to continue producing 2 ATP per glucose.
当氧气有限时，动物细胞（如剧烈运动时的肌肉细胞）进行无氧呼吸。丙酮酸被 NADH 还原生成乳酸，由乳酸脱氢酶催化。这一过程使 NAD⁺ 再生，从而使糖酵解能继续产生每分子葡萄糖 2 ATP。
The overall equation is: pyruvate + NADH + H⁺ → lactate + NAD⁺. The production of lactate allows quick ATP supply but can lead to muscle fatigue and cramping if lactate accumulates; it is later oxidized back to pyruvate when oxygen is available (oxygen debt).
总方程式为：丙酮酸 + NADH + H⁺ → 乳酸 + NAD⁺。乳酸的产生能快速提供 ATP，但如果乳酸积累可能导致肌肉疲劳和痉挛；当氧气充足时，乳酸可被重新氧化为丙酮酸（氧债）。
No further ATP is produced beyond glycolysis; anaerobic respiration is much less efficient than aerobic respiration.
除糖酵解外不额外产生 ATP；无氧呼吸的效率远低于有氧呼吸。
8. Anaerobic Respiration in Yeast | 酵母的无氧呼吸
Yeast and some plants under anaerobic conditions convert pyruvate to ethanol and CO₂. This is alcoholic fermentation. Pyruvate is first decarboxylated to ethanal (acetaldehyde) by pyruvate decarboxylase, releasing CO₂. Ethanal is then reduced by NADH to ethanol, catalysed by alcohol dehydrogenase, regenerating NAD⁺.
酵母和某些植物在无氧条件下将丙酮酸转化为乙醇和 CO₂，即酒精发酵。首先由丙酮酸脱羧酶将丙酮酸脱羧生成乙醛，释放 CO₂。然后乙醛被 NADH 还原为乙醇，由乙醇脱氢酶催化，再生成 NAD⁺。
Equation: glucose → 2 ethanol + 2 CO₂ + 2 ATP (net). Alcoholic fermentation is commercially important for baking (CO₂ raises dough) and brewing (ethanol production).
方程式：葡萄糖 → 2 乙醇 + 2 CO₂ + 2 ATP（净）。酒精发酵在商业上对烘焙（CO₂ 使面团发起）和酿造（乙醇生产）十分重要。
As in lactate fermentation, only the 2 ATP from glycolysis are gained, and the main purpose is to regenerate NAD⁺ for glycolysis to continue.
与乳酸发酵一样，仅获得糖酵解的 2 ATP，主要目的是再生 NAD⁺ 以便糖酵解继续进行。
9. Mitochondrial Structure | 线粒体结构
The mitochondrion is adapted for aerobic respiration. Key structural features include: outer membrane (permeable to small molecules), inner membrane (folded into cristae, impermeable to most ions, containing ETC proteins and ATP synthase), intermembrane space (low volume for rapid proton gradient build-up), and matrix (contains enzymes for link reaction and Krebs cycle).
线粒体的结构适应有氧呼吸。关键结构特征包括：外膜（对小分子通透）、内膜（向内折叠成嵴，对大多数离子不通透，含有电子传递链蛋白和 ATP 合酶）、膜间腔（体积小以便快速建立质子梯度）和基质（含有连接反应和克雷布斯循环的酶）。
The cristae greatly increase the surface area for oxidative phosphorylation. The matrix also contains mitochondrial DNA and ribosomes, supporting the endosymbiotic theory.
嵴极大增加了氧化磷酸化的表面积。基质还含有线粒体 DNA 和核糖体，支持内共生学说。
ATP synthase is embedded in the inner membrane, and the flow of H⁺ through it drives the rotational mechanism that synthesizes ATP.
ATP 合酶嵌入内膜中，H⁺ 的流动驱动其旋转机制以合成 ATP。
10. Respiratory Quotient (RQ) | 呼吸商
Respiratory quotient (RQ) is the ratio of CO₂ produced to O₂ consumed in a given time: RQ = CO₂ produced / O₂ consumed. It provides information about the substrate being respired.
呼吸商（RQ）是给定时间内产生的 CO₂ 与消耗的 O₂ 之比：RQ = 产生的 CO₂ / 消耗的 O₂。它可提供有关正在被呼吸的底物的信息。
Typical RQ values: carbohydrate = 1.0; lipid (e.g., oleic acid) ≈ 0.7; protein ≈ 0.8–0.9. If RQ > 1.0 it may indicate anaerobic respiration overlapping with aerobic respiration, or synthesis of fat from carbohydrate.
典型的 RQ 值：碳水化合物 = 1.0；脂类（如油酸）≈ 0.7；蛋白质 ≈ 0.8–0.9。如果 RQ > 1.0，可能表明无氧呼吸与有氧呼吸同时发生，或由碳水化合物合成脂肪。
Measuring RQ using a respirometer can help determine metabolic fuel use in organisms or tissues.
利用呼吸计测定 RQ 有助于确定生物体或组织使用的代谢燃料类型。
11. Factors Affecting Respiration | 影响呼吸作用的因素
Respiration rate is influenced by temperature, oxygen concentration, CO₂ concentration, substrate availability, and metabolic demand. Enzymes in respiration have optimum temperatures; rates increase up to an optimum, then decline as enzymes denature.
呼吸速率受温度、氧气浓度、CO₂ 浓度、底物可用性和代谢需求的影响。呼吸作用中的酶都有最适温度；速率随温度升高而增加直至最适点，然后因酶变性而下降。
Oxygen is the final electron acceptor; at very low oxygen levels, the ETC and Krebs cycle are inhibited. In high CO₂, some feedback inhibition may occur. In humans, hormones like thyroxine increase metabolic rate and thus respiration rate.
氧气是最终电子受体；在极低氧水平下，电子传递链和克雷布斯循环受到抑制。在高 CO₂ 下，可能发生某些反馈抑制。在人体中，甲状腺素等激素可提高代谢率，从而增加呼吸速率。
In plants, factors such as light (providing substrate via photosynthesis), water, and oxygen for aerobic respiration in roots are important. In germinating seeds, high respiration rates consume stored nutrients.
在植物中，诸如光照（通过光合作用提供底物）、水分和根系有氧呼吸所需的氧气等因素都很重要。在萌发种子中，高呼吸速率会消耗储存的养分。
12. Comparison and Summary | 比较与总结
The table below summarises the main stages of aerobic respiration:
下表总结了有氧呼吸的主要阶段：
| Stage | Location | Input | Output per glucose | ATP (net) |
|---|---|---|---|---|
| Glycolysis | Cytoplasm | Glucose, 2 NAD⁺, 2 ADP | 2 pyruvate, 2 NADH, 2 H₂O | 2 |
| Link Reaction | Mitochondrial matrix | 2 pyruvate, 2 NAD⁺, CoA | 2 acetyl-CoA, 2 CO₂, 2 NADH | 0 |
| Krebs Cycle | Mitochondrial matrix | 2 acetyl-CoA, 6 NAD⁺, 2 FAD, 2 ADP | 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP | 2 |
| Oxidative Phosphorylation | Inner mitochondrial membrane | 10 NADH, 2 FADH₂, O₂ | H₂O, 26–28 ATP (approx.) | ~26-28 |
| Total | 30-32 |
Key differences between aerobic and anaerobic respiration: location, final electron acceptor, ATP yield, products, and regeneration of NAD⁺.
有氧呼吸与无氧呼吸的主要区别：发生位置、最终电子受体、ATP 产量、产物以及 NAD⁺ 的再生方式。
Understanding respiration is essential for explaining energy flow in ecosystems, cellular function, and the effects of exercise, disease, and environmental change on organisms.
理解呼吸作用对于解释生态系统中的能量流动、细胞功能以及运动、疾病和环境变化对生物体的影响至关重要。
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