📚 IB & AQA Biology: Respiration Key Revision Points | IB 与 AQA 生物：呼吸作用考点精讲
Cellular respiration is one of the most central topics in both IB and AQA Biology, linking biochemistry, energy transfer, and whole-organism physiology. Mastering the sequence of glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation gives you the tools to explain how cells release energy from glucose. This article breaks down every key stage, highlights common exam pitfalls, and provides the essential facts you need for top marks.
细胞呼吸是 IB 和 AQA 生物学中最核心的主题之一，它将生物化学、能量转移和整体生理学联系在一起。掌握糖酵解、链接反应、克雷布斯循环和氧化磷酸化的顺序，你就能解释细胞如何从葡萄糖中释放能量。本文分解每个关键阶段，指出常见考试错误，并提供获取高分所需的基本要点。
1. Overview of Respiration | 呼吸作用概述
Respiration is the process by which cells break down organic molecules, primarily glucose, to release energy in the form of ATP. It can be aerobic, requiring oxygen, or anaerobic, occurring without oxygen. The overall balanced equation for aerobic respiration is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP). In eukaryotic cells, aerobic respiration takes place in the cytoplasm and mitochondria, whereas anaerobic respiration is restricted to the cytoplasm.
呼吸作用是细胞分解有机分子（主要是葡萄糖）以 ATP 形式释放能量的过程。它可以是需氧的（需要氧气）或无氧的（不需要氧气）。有氧呼吸的总平衡方程为：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量 (ATP)。在真核细胞中，有氧呼吸发生在细胞质和线粒体中，而无氧呼吸仅限于细胞质。
ATP acts as the universal energy currency, and its hydrolysis is coupled to energy-requiring processes. Respiration generates ATP through substrate-level phosphorylation and oxidative phosphorylation. The total theoretical yield of ATP per glucose is about 30–32 molecules in most eukaryotes, though many textbooks cite 38 for prokaryotes.
ATP 作为通用的能量货币，其水解与需要能量的过程相偶联。呼吸作用通过底物水平磷酸化和氧化磷酸化生成 ATP。每分子葡萄糖的理论 ATP 总产量在大多数真核生物中约为 30–32 个分子，尽管许多教科书提到原核生物为 38 个。
2. Glycolysis: The First Step | 糖酵解：第一步
Glycolysis occurs in the cytoplasm and does not require oxygen. It involves the splitting of one molecule of glucose (6C) into two molecules of pyruvate (3C). The process consists of an energy investment phase, where 2 ATP are used, and an energy payoff phase, where 4 ATP and 2 NADH are produced. The net gain is therefore 2 ATP (via substrate-level phosphorylation) and 2 NADH per glucose.
糖酵解发生在细胞质中，不需要氧气。它涉及将一个葡萄糖分子（6C）分解为两个丙酮酸分子（3C）。该过程包括能量投资阶段（消耗 2 个 ATP）和能量回报阶段（产生 4 个 ATP 和 2 个 NADH）。因此，每分子葡萄糖的净收益为 2 个 ATP（通过底物水平磷酸化）和 2 个 NADH。
Key enzymes include hexokinase for the initial phosphorylation and phosphofructokinase (PFK) as the rate-limiting enzyme. NAD⁺ is reduced to NADH, which will later carry electrons to the electron transport chain if oxygen is present. In the absence of oxygen, NADH must be reoxidised to NAD⁺ to allow glycolysis to continue – this is achieved by anaerobic pathways.
关键酶包括用于初始磷酸化的己糖激酶，以及作为限速酶的磷酸果糖激酶（PFK）。NAD⁺ 被还原为 NADH，如果氧气存在，NADH 随后将电子传递给电子传递链。在无氧条件下，NADH 必须被重新氧化为 NAD⁺，以使糖酵解得以继续——这通过无氧途径实现。
- Hexokinase phosphorylates glucose → glucose-6-phosphate.
- Phosphofructokinase phosphorylates fructose-6-phosphate → fructose-1,6-bisphosphate.
- Final steps produce pyruvate, ATP and NADH.
- 己糖激酶使葡萄糖磷酸化 → 6-磷酸葡萄糖。
- 磷酸果糖激酶使6-磷酸果糖磷酸化 → 1,6-二磷酸果糖。
- 最后步骤产生丙酮酸、ATP 和 NADH。
3. Link Reaction: Pyruvate Oxidation | 链接反应：丙酮酸氧化
When oxygen is available, pyruvate enters the mitochondrial matrix via active transport. Here, the link reaction converts each pyruvate into acetyl coenzyme A (acetyl CoA). Pyruvate undergoes decarboxylation, releasing CO₂, and is oxidised, with NAD⁺ accepting electrons to form NADH. The remaining 2-carbon acetyl group binds to coenzyme A. No ATP is produced directly in this step.
当氧气可用时，丙酮酸通过主动运输进入线粒体基质。在这里，链接反应将每个丙酮酸转化为乙酰辅酶 A（乙酰 CoA）。丙酮酸发生脱羧，释放 CO₂，并被氧化，NAD⁺ 接受电子形成 NADH。剩下的 2 碳乙酰基与辅酶 A 结合。此步骤不直接产生 ATP。
Pyruvate + NAD⁺ + CoA → Acetyl CoA + CO₂ + NADH
丙酮酸 + NAD⁺ + CoA → 乙酰 CoA + CO₂ + NADH
For each glucose molecule, two pyruvate molecules are processed, so the link reaction produces 2 CO₂, 2 NADH and 2 acetyl CoA molecules. This reaction is irreversible and commits the carbon atoms to complete oxidation in the Krebs cycle.
对于每个葡萄糖分子，有两个丙酮酸被处理，因此链接反应产生 2 个 CO₂、2 个 NADH 和 2 个乙酰 CoA 分子。该反应不可逆，将碳原子投入克雷布斯循环的完全氧化。
4. The Krebs Cycle | 克雷布斯循环
The Krebs cycle (also called the citric acid cycle or TCA cycle) takes place in the mitochondrial matrix. Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Through a series of reactions, citrate is oxidised back to oxaloacetate, releasing two CO₂ molecules, one ATP (substrate-level phosphorylation), three NADH and one FADH₂ per turn.
克雷布斯循环（也称柠檬酸循环或 TCA 循环）发生在线粒体基质中。乙酰 CoA（2C）与草酰乙酸（4C）结合形成柠檬酸（6C）。通过一系列反应，柠檬酸被氧化回草酰乙酸，每圈释放两个 CO₂ 分子、一个 ATP（底物水平磷酸化）、三个 NADH 和一个 FADH₂。
Because two acetyl CoA molecules are produced from one glucose, the cycle turns twice. Overall, the Krebs cycle yields 4 CO₂, 2 ATP, 6 NADH and 2 FADH₂ per glucose. The cycle is also a source of intermediates for biosynthesis, making it amphibolic. Key enzymes include citrate synthase and isocitrate dehydrogenase.
由于一分子葡萄糖产生两个乙酰 CoA，循环运行两圈。总体而言，克雷布斯循环每分子葡萄糖产生 4 个 CO₂、2 个 ATP、6 个 NADH 和 2 个 FADH₂。该循环也是生物合成的中间体来源，因此具有双功能。关键酶包括柠檬酸合酶和异柠檬酸脱氢酶。
| Product per glucose | Glycolysis | Link Reaction | Krebs Cycle |
| ATP | 2 | 0 | 2 |
| NADH | 2 | 2 | 6 |
| FADH₂ | 0 | 0 | 2 |
| CO₂ | 0 | 2 | 4 |
每分子葡萄糖产品 | 糖酵解 | 链接反应 | 克雷布斯循环 (中译)
5. Oxidative Phosphorylation & the Electron Transport Chain | 氧化磷酸化与电子传递链
Oxidative phosphorylation occurs on the inner mitochondrial membrane (cristae). NADH and FADH₂ donate electrons to a series of protein complexes (I–IV) and mobile carriers. As electrons pass along the chain, protons (H⁺) are pumped from the matrix into the intermembrane space, creating an electrochemical proton gradient.
氧化磷酸化发生在线粒体内膜（嵴）上。NADH 和 FADH₂ 将电子提供给一系列蛋白质复合物（I–IV）和移动载体。当电子沿链传递时，质子（H⁺）从基质被泵入膜间隙，形成电化学质子梯度。
The final electron acceptor is oxygen, which combines with electrons and protons to form water. This is why oxygen is essential for aerobic respiration. The energy released during electron transfer is not used directly to make ATP but to generate the proton motive force.
最终的电子受体是氧，它与电子和质子结合形成水。这就是有氧呼吸需要氧气的原因。电子传递过程中释放的能量并不直接用于生成 ATP，而是用来产生质子动力势。
NADH + H⁺ + ½O₂ → NAD⁺ + H₂O
NADH 被氧化，O₂ 被还原。FADH₂ 进入电子传递链的位点较晚，因此产生的 ATP 较少。
6. Chemiosmosis and ATP Synthase | 化学渗透与 ATP 合酶
The proton gradient across the inner membrane stores potential energy. Protons flow back down their concentration gradient into the matrix through a channel in ATP synthase, a large enzyme complex. This flow drives the rotation of the synthase, catalysing the formation of ATP from ADP and inorganic phosphate (Pi).
跨内膜的质子梯度储存势能。质子沿浓度梯度通过 ATP 合酶（一个大型酶复合物）的通道流回基质。这一流动驱动合酶的旋转，催化 ADP 和无机磷酸（Pi）形成 ATP。
Each NADH typically fuels the production of about 2.5 ATP, and each FADH₂ about 1.5 ATP, due to the different entry points. The coupling of electron transport to ATP synthesis is called chemiosmosis, first proposed by Peter Mitchell. Uncouplers like DNP dissipate the gradient, stopping ATP production and generating heat instead.
由于进入位点不同，每个 NADH 通常驱动生成约 2.5 个 ATP，每个 FADH₂ 约 1.5 个 ATP。电子传递与 ATP 合成的偶联称为化学渗透，最初由 Peter Mitchell 提出。像 DNP 这样的解偶联剂会消散梯度，停止 ATP 生成并转而产热。
- Protons are pumped at Complex I, III, and IV.
- FADH₂ feeds into Complex II, bypassing Complex I, hence fewer protons pumped.
- 质子泵存在于复合物 I、III 和 IV。
- FADH₂ 进入复合物 II，绕过复合物 I，因此泵出的质子较少。
7. Anaerobic Respiration in Animals | 动物的无氧呼吸
When oxygen is insufficient, cells cannot oxidise the NADH generated in glycolysis via the electron transport chain. To regenerate NAD⁺ and allow glycolysis to continue producing ATP, animal cells reduce pyruvate to lactate (lactic acid) using the enzyme lactate dehydrogenase. This process yields only 2 ATP per glucose and leads to muscle fatigue and oxygen debt.
当氧气不足时，细胞无法通过电子传递链氧化糖酵解产生的 NADH。为了再生 NAD⁺ 并使糖酵解继续产生 ATP，动物细胞利用乳酸脱氢酶将丙酮酸还原为乳酸。该过程每分子葡萄糖仅产生 2 个 ATP，并导致肌肉疲劳和氧债。
Pyruvate + NADH → Lactate + NAD⁺
In humans, lactate is transported to the liver, where it can be oxidised back to pyruvate after oxygen becomes available (Cori cycle). This is a temporary measure to maintain energy production during vigorous exercise.
在人类中，乳酸被运输到肝脏，当氧气恢复后，可被氧化回丙酮酸（Cori 循环）。这是在剧烈运动期间维持能量生产的临时措施。
8. Anaerobic Respiration in Plants and Yeast | 植物和酵母的无氧呼吸
In plants and yeast, anaerobic conditions also require the regeneration of NAD⁺ from NADH. Instead of producing lactate, these organisms convert pyruvate to ethanol (ethyl alcohol) and CO₂. The enzyme pyruvate decarboxylase removes a carboxyl group, and alcohol dehydrogenase produces ethanol.
在植物和酵母中，无氧条件同样需要从 NADH 再生 NAD⁺。这些生物不产生乳酸，而是将丙酮酸转化为乙醇（酒精）和 CO₂。丙酮酸脱羧酶移除一个羧基，乙醇脱氢酶生成乙醇。
Pyruvate → Ethanal + CO₂ → Ethanol + NAD⁺
This pathway is irreversible because CO₂ is lost. The overall ATP yield is again only 2 per glucose. The production of ethanol is commercially exploited in brewing and baking industries, where yeast fermentation produces alcohol and the CO₂ causes dough to rise.
该途径不可逆，因为 CO₂ 丢失。总 ATP 产量再次仅为每分子葡萄糖 2 个。乙醇的生产在酿造和烘焙工业中得到商业利用，酵母发酵产生酒精，CO₂ 使面团膨胀。
9. Respiratory Quotient (RQ) | 呼吸商 (RQ)
The respiratory quotient (RQ) is the ratio of the volume of CO₂ produced to the volume of O₂ consumed during respiration. It provides information about the metabolic substrate being used. Carbohydrates have an RQ of 1.0, lipids around 0.7, and proteins about 0.9. Anaerobic respiration yields an RQ above 1.0 because CO₂ is released without oxygen consumption.
呼吸商 (RQ) 是呼吸过程中产生的 CO₂ 体积与消耗的 O₂ 体积之比。它提供有关正在使用的代谢底物的信息。碳水化合物的 RQ 为 1.0，脂质约为 0.7，蛋白质约为 0.9。无氧呼吸的 RQ 高于 1.0，因为释放 CO₂ 而不消耗氧气。
RQ = CO₂ produced ÷ O₂ consumed
Respirometers are used to measure RQ. The device contains a living organism in a sealed chamber with a CO₂ absorbent (e.g., KOH). The change in volume or pressure indicates O₂ consumption. Repeating the experiment without the CO₂ absorbent allows calculation of CO₂ production.
呼吸计用于测量 RQ。该装置在密封室中包含一个活体生物和 CO₂ 吸收剂（如 KOH）。体积或压力的变化显示 O₂ 消耗。在没有 CO₂ 吸收剂的情况下重复实验可计算 CO₂ 产量。
| Substrate | Typical RQ |
| Carbohydrate | 1.0 |
| Lipid | 0.7 |
| Protein | 0.9 |
底物 - 碳水化合物、脂质、蛋白质 | 典型 RQ (中译)
10. Common Mistakes and Exam Tips | 常见错误与考试技巧
Students often confuse the sites of respiration stages: remember that glycolysis is cytoplasmic, while the link reaction, Krebs cycle and oxidative phosphorylation are mitochondrial. Never say that the Krebs cycle directly uses oxygen – oxygen acts only at the end of the electron transport chain. Also, avoid stating that energy is ‘produced’ in respiration; it is more accurate to say that energy is transferred or released, and ATP is synthesised.
学生经常混淆呼吸作用各个阶段的发生场所：记住糖酵解在细胞质中，而链接反应、克雷布斯循环和氧化磷酸化在线粒体中。决不要说克雷布斯循环直接使用氧气——氧气仅在电子传递链末端起作用。此外，避免说呼吸作用“产生”能量；更准确的说法是能量被转移或释放，ATP 被合成。
- Use precise terminology: 'oxidised' and 'reduced' rather than 'added electrons'.
- Know the role of NAD⁺ and FAD as electron carriers and hydrogen acceptors.
- Be able to distinguish substrate-level phosphorylation from oxidative phosphorylation.
- Explain why anaerobic respiration yields far less ATP – no Krebs cycle or oxidative phosphorylation, only glycolysis.
- 记住使用精确术语：“氧化”和“还原”，而非“添加电子”。
- 了解 NAD⁺ 和 FAD 作为电子载体和氢受体的作用。
- 能区分底物水平磷酸化和氧化磷酸化。
- 解释为什么无氧呼吸产生的 ATP 少得多——没有克雷布斯循环或氧化磷酸化，只有糖酵解。
When drawing the electron transport chain, label the complexes and show the movement of protons and the flow of electrons. In AQA and IB papers, you may be asked to calculate RQ or evaluate respirometer data. Practice applying your knowledge to unfamiliar scenarios, such as the effect of cyanide (inhibits complex IV) or uncouplers on respiration rate.
绘制电子传递链时，标注复合物并显示质子的移动和电子的流动。在 AQA 和 IB 试卷中，你可能需要计算 RQ 或评估呼吸计数据。练习将知识应用于陌生情境，如氰化物（抑制复合物 IV）或解偶联剂对呼吸速率的影响。
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