A-Level Biology: Cellular Respiration — A Complete Guide | A-Level 生物：细胞呼吸 — 完整指南

Introduction | 引言

Cellular respiration is one of the most fundamental biochemical processes in living organisms. It is the process by which cells break down organic molecules — primarily glucose — to produce adenosine triphosphate (ATP), the universal energy currency of life. For A-Level Biology students, understanding cellular respiration in depth is essential, as it underpins topics ranging from metabolism and energy transfer to exercise physiology and mitochondrial diseases.

细胞呼吸是生物体中最基本的生化过程之一。它是细胞分解有机分子（主要是葡萄糖）以产生三磷酸腺苷（ATP）——生命的通用能量货币——的过程。对于A-Level 生物学学生来说，深入理解细胞呼吸至关重要，因为它是从新陈代谢和能量转移到运动生理学和线粒体疾病等主题的基础。

In this comprehensive guide, we will walk through the four stages of aerobic respiration — glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation — and then examine anaerobic respiration. Every concept is presented in both English and Chinese to support bilingual learners preparing for their A-Level examinations.

在这份全面指南中，我们将逐一讲解有氧呼吸的四个阶段——糖酵解、连接反应、克雷布斯循环和氧化磷酸化——然后探讨无氧呼吸。每个概念都以中英双语呈现，以帮助准备 A-Level 考试的双语学习者。

1. Overview of Cellular Respiration | 细胞呼吸概述

The overall equation for aerobic respiration is:

有氧呼吸的总方程式为：

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP)

Glucose is oxidised to carbon dioxide and water, releasing energy that is used to synthesise ATP from ADP and inorganic phosphate (P_i). This process is not a single reaction but a metabolic pathway consisting of four interconnected stages:

葡萄糖被氧化为二氧化碳和水，释放出的能量用于从 ADP 和无机磷酸盐（P_i）合成 ATP。这个过程不是单一反应，而是由四个相互关联的阶段组成的代谢途径：

1. Glycolysis — occurs in the cytoplasm | 糖酵解 — 发生在细胞质中
2. The Link Reaction — occurs in the mitochondrial matrix | 连接反应 — 发生在线粒体基质中
3. The Krebs Cycle (Citric Acid Cycle) — occurs in the mitochondrial matrix | 克雷布斯循环（柠檬酸循环）— 发生在线粒体基质中
4. Oxidative Phosphorylation — occurs on the inner mitochondrial membrane | 氧化磷酸化 — 发生在线粒体内膜上

The mitochondrion is the powerhouse of the cell, and its structure is perfectly adapted for respiration. It is bound by a double membrane: the outer membrane is smooth and permeable, while the inner membrane is highly folded into cristae, which greatly increase the surface area for the electron transport chain and ATP synthase enzymes. The matrix contains enzymes for the link reaction and Krebs cycle, along with mitochondrial DNA and ribosomes.

线粒体是细胞的能量工厂，其结构完美地适应了呼吸作用。它由双层膜包裹：外膜光滑且具有渗透性，而内膜高度折叠形成嵴，大大增加了电子传递链和 ATP 合酶的表面积。基质中含有连接反应和克雷布斯循环所需的酶，以及线粒体 DNA 和核糖体。

2. Glycolysis | 糖酵解

Glycolysis is the first stage of both aerobic and anaerobic respiration. It takes place in the cytoplasm and does not require oxygen. The word “glycolysis” literally means “sugar splitting,” and the process involves the breakdown of one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound).

糖酵解是有氧呼吸和无氧呼吸的第一个阶段。它发生在细胞质中，不需要氧气。”糖酵解”这个词的字面意思是”糖的分解”，该过程涉及将一个葡萄糖分子（六碳糖）分解为两个丙酮酸分子（三碳化合物）。

Key Steps of Glycolysis | 糖酵解的关键步骤

Phosphorylation | 磷酸化

Glucose is phosphorylated by two molecules of ATP, forming hexose bisphosphate. This phosphorylation makes the glucose molecule more reactive and prevents it from diffusing out of the cell through glucose transport channels. The investment of ATP at this stage means the net gain is lower than the total produced — a concept A-Level examiners love to test.

葡萄糖被两个 ATP 分子磷酸化，形成己糖二磷酸。这种磷酸化使葡萄糖分子更具反应性，并防止它通过葡萄糖转运通道扩散出细胞。在此阶段投入 ATP 意味着净增益低于总产量——这是 A-Level 考官喜欢测试的概念。

Lysis | 裂解

Hexose bisphosphate is split into two molecules of triose phosphate (TP), each containing three carbon atoms. This is the “splitting” step that gives glycolysis its name.

己糖二磷酸被分裂为两个磷酸三碳糖分子，每个含有三个碳原子。这是赋予糖酵解名称的”裂解”步骤。

Oxidation and ATP Formation | 氧化与 ATP 形成

Each triose phosphate molecule is oxidised to pyruvate. During this oxidation, hydrogen atoms are removed and transferred to the coenzyme NAD⁺, reducing it to NADH (reduced NAD). Simultaneously, four ATP molecules are produced by substrate-level phosphorylation — where a phosphate group is directly transferred from a substrate molecule to ADP. Since two ATP were used in the phosphorylation step, the net gain of glycolysis is 2 ATP per glucose molecule.

每个磷酸三碳糖分子被氧化为丙酮酸。在此氧化过程中，氢原子被移除并转移到辅酶 NAD⁺，将其还原为 NADH（还原型 NAD）。同时，通过底物水平磷酸化产生四个 ATP 分子——即磷酸基团直接从底物分子转移到 ADP。由于在磷酸化步骤中消耗了两个 ATP，因此糖酵解的净增益为每个葡萄糖分子 2 个 ATP。

Products of Glycolysis (per glucose) | 糖酵解的产物（每分子葡萄糖）

Product | 产物	Quantity | 数量
Pyruvate | 丙酮酸	2
ATP (net) | ATP（净）	2
Reduced NAD (NADH) | 还原型 NAD	2

3. The Link Reaction | 连接反应

If oxygen is available, pyruvate enters the mitochondrial matrix, where the link reaction occurs. This reaction “links” glycolysis to the Krebs cycle. During this step, each pyruvate molecule is decarboxylated (loses a carbon atom as CO₂) and dehydrogenated (loses hydrogen atoms).

如果有氧气存在，丙酮酸进入线粒体基质，在那里发生连接反应。该反应将糖酵解”连接”到克雷布斯循环。在此步骤中，每个丙酮酸分子被脱羧（以 CO₂ 形式失去一个碳原子）和脱氢（失去氢原子）。

The hydrogen atoms are accepted by NAD⁺, forming reduced NAD (NADH). The remaining two-carbon acetyl group combines with coenzyme A (CoA) to form acetyl coenzyme A (acetyl-CoA). This is a crucial intermediate that feeds into the Krebs cycle.

氢原子被 NAD⁺ 接受，形成还原型 NAD（NADH）。剩余的二碳乙酰基与辅酶 A 结合，形成乙酰辅酶 A（acetyl-CoA）。这是进入克雷布斯循环的关键中间体。

Summary per pyruvate | 每个丙酮酸的总结： One CO₂ released, one NADH produced, one acetyl-CoA formed. Since each glucose yields two pyruvate molecules, the link reaction occurs twice per glucose, producing 2 CO₂, 2 NADH, and 2 acetyl-CoA.

由于每个葡萄糖产生两个丙酮酸分子，连接反应每个葡萄糖发生两次，产生 2 个 CO₂、2 个 NADH 和 2 个乙酰辅酶 A。

4. The Krebs Cycle | 克雷布斯循环

The Krebs cycle — also known as the citric acid cycle or the tricarboxylic acid (TCA) cycle — is a series of enzyme-controlled reactions that take place in the mitochondrial matrix. It was discovered by Sir Hans Krebs in 1937, earning him the Nobel Prize in Physiology or Medicine in 1953.

克雷布斯循环——也称为柠檬酸循环或三羧酸（TCA）循环——是在线粒体基质中发生的一系列酶控反应。它由汉斯·克雷布斯爵士于 1937 年发现，为他赢得了 1953 年的诺贝尔生理学或医学奖。

Key Events in the Krebs Cycle | 克雷布斯循环的关键事件

1. Formation of citrate | 柠檬酸的形成： The two-carbon acetyl-CoA combines with a four-carbon compound called oxaloacetate to form a six-carbon compound — citrate (citric acid). Coenzyme A is released and recycled. | 二碳乙酰辅酶 A 与称为草酰乙酸的四碳化合物结合，形成六碳化合物——柠檬酸。辅酶 A 被释放并循环使用。
2. Decarboxylation and dehydrogenation | 脱羧与脱氢： Through a series of reactions, citrate is progressively oxidised. Two carbon atoms are lost as CO₂ (decarboxylation), and hydrogen atoms are removed (dehydrogenation) and accepted by NAD⁺ and FAD, forming reduced NAD (NADH) and reduced FAD (FADH₂). | 通过一系列反应，柠檬酸逐渐被氧化。两个碳原子以 CO₂ 形式失去（脱羧），氢原子被移除（脱氢）并被 NAD⁺ 和 FAD 接受，形成还原型 NAD 和还原型 FAD。
3. Regeneration of oxaloacetate | 草酰乙酸的再生： The cycle ends with the regeneration of oxaloacetate, allowing the cycle to continue. | 循环以草酰乙酸的再生结束，使循环得以继续。
4. ATP production | ATP 的产生： One ATP molecule is produced directly by substrate-level phosphorylation (specifically, GTP is produced and converted to ATP). | 一个 ATP 分子通过底物水平磷酸化直接产生（具体来说，产生 GTP 并转化为 ATP）。

Products of the Krebs Cycle (per acetyl-CoA) | 克雷布斯循环的产物（每个乙酰辅酶 A）

Product | 产物	Quantity | 数量
CO2	2
Reduced NAD (NADH)	3
Reduced FAD (FADH2)	1
ATP (as GTP)	1

Since two acetyl-CoA molecules enter the Krebs cycle per glucose molecule, the total yield doubles: 4 CO₂, 6 NADH, 2 FADH₂, and 2 ATP.

由于每个葡萄糖分子有两个乙酰辅酶 A 进入克雷布斯循环，总产量翻倍：4 个 CO₂、6 个 NADH、2 个 FADH₂ 和 2 个 ATP。

5. Oxidative Phosphorylation | 氧化磷酸化

Oxidative phosphorylation is the final and most significant stage of aerobic respiration in terms of ATP yield. It takes place on the inner mitochondrial membrane and consists of two coupled processes: the electron transport chain (ETC) and chemiosmosis.

氧化磷酸化是有氧呼吸中 ATP 产量最大、最重要的最后阶段。它发生在线粒体内膜上，由两个偶联过程组成：电子传递链和化学渗透。

The Electron Transport Chain (ETC) | 电子传递链

The reduced coenzymes NADH and FADH₂ — produced during glycolysis, the link reaction, and the Krebs cycle — donate their hydrogen atoms to the electron transport chain. Here, the hydrogen atoms are split into protons (H⁺) and electrons (e⁻).

在糖酵解、连接反应和克雷布斯循环中产生的还原型辅酶 NADH 和 FADH₂ 将它们的氢原子提供给电子传递链。在这里，氢原子被分裂为质子（H⁺）和电子（e⁻）。

The electrons pass along a series of electron carriers — protein complexes embedded in the inner mitochondrial membrane — each at a progressively lower energy level. As electrons move through the chain, energy is released. This energy is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, establishing a proton gradient (a higher concentration of protons in the intermembrane space than in the matrix).

电子沿着嵌入线粒体内膜的一系列电子载体（蛋白质复合物）传递，每个载体的能级逐渐降低。当电子通过传递链时，能量被释放出来。这些能量被用来将质子（H⁺）从线粒体基质泵入膜间隙，建立一个质子梯度（膜间隙中的质子浓度高于基质中的浓度）。

At the end of the chain, the electrons are accepted by oxygen (O₂), which combines with protons to form water. Oxygen is the final electron acceptor. This is why aerobic respiration requires oxygen — without it, the electrons would back up along the chain and the entire process would halt.

在传递链末端，电子被氧（O₂）接受，氧与质子结合形成水。氧是最终的电子受体。这就是有氧呼吸需要氧气的原因——没有氧气，电子会在传递链中堵塞，整个过程将停止。

Chemiosmosis | 化学渗透

The proton gradient created by the electron transport chain represents stored potential energy — a proton-motive force. Protons can only diffuse back into the matrix through specialised protein channels called ATP synthase (also known as stalked particles). As protons flow through ATP synthase, the enzyme rotates and uses the energy from this flow to phosphorylate ADP, producing ATP. This is called chemiosmosis — the coupling of proton diffusion to ATP synthesis.

电子传递链建立的质子梯度代表了储存的势能——质子动力。质子只能通过称为ATP 合酶（也称为柄状颗粒）的特殊蛋白质通道扩散回基质中。当质子通过 ATP 合酶流动时，该酶旋转并利用此流动的能量磷酸化 ADP，产生 ATP。这称为化学渗透——质子扩散与 ATP 合成的偶联。

This mechanism was proposed by Peter Mitchell in 1961, a theory so elegant yet controversial that it earned him the Nobel Prize in Chemistry in 1978. The chemiosmotic theory is one of the most important concepts in all of biochemistry.

这一机制由彼得·米切尔于 1961 年提出，这一理论如此优雅却又充满争议，以至于为他赢得了 1978 年的诺贝尔化学奖。化学渗透理论是整个生物化学中最重要的概念之一。

ATP Yield of Oxidative Phosphorylation | 氧化磷酸化的 ATP 产量

Each reduced NAD (NADH) that enters the electron transport chain yields approximately 2.5 ATP molecules. Each reduced FAD (FADH₂) yields approximately 1.5 ATP, because it enters the chain at a later point and pumps fewer protons across the membrane.

进入电子传递链的每个还原型 NAD（NADH）产生约 2.5 个 ATP 分子。每个还原型 FAD（FADH₂）产生约 1.5 个 ATP，因为它从传递链的较晚位置进入，泵送的质子较少。

6. Total ATP Yield | 总 ATP 产量

Let us summarise the total ATP produced from one molecule of glucose during aerobic respiration:

让我们总结一个葡萄糖分子在有氧呼吸中产生的总 ATP：

Stage | 阶段	ATP Produced (per glucose) | 产生的 ATP（每分子葡萄糖）
Glycolysis | 糖酵解	2 (substrate-level) | 2（底物水平）
Krebs Cycle | 克雷布斯循环	2 (substrate-level, as GTP) | 2（底物水平，以 GTP 形式）
Oxidative Phosphorylation (from 10 NADH) | 氧化磷酸化（来自 10 个 NADH）	25
Oxidative Phosphorylation (from 2 FADH2) | 氧化磷酸化（来自 2 个 FADH2）	3
Total | 总计: ~32 ATP

Note: The theoretical maximum is often quoted as 38 ATP, but modern textbooks use ~30–32 ATP because the proton gradient is slightly “leaky” and some energy is used to transport pyruvate and NADH into the mitochondria.

注：理论最大值通常被引用为 38 个 ATP，但现代教科书使用约 30–32 个 ATP，因为质子梯度略有”泄漏”，且部分能量用于将丙酮酸和 NADH 运输到线粒体中。

7. Anaerobic Respiration | 无氧呼吸

When oxygen is not available, the electron transport chain cannot function because there is no final electron acceptor. However, glycolysis can still occur — and organisms have evolved strategies to regenerate NAD⁺ from the NADH produced during glycolysis, allowing glycolysis to continue.

当氧气不可用时，电子传递链无法运作，因为没有最终的电子受体。然而，糖酵解仍然可以进行——生物体已经进化出策略来从糖酵解中产生的 NADH 再生 NAD⁺，使糖酵解得以继续。

Anaerobic Respiration in Animals (Lactate Fermentation) | 动物中的无氧呼吸（乳酸发酵）

In animal cells, such as human muscle cells during intense exercise, the pyruvate produced by glycolysis is reduced to lactate (lactic acid) by the enzyme lactate dehydrogenase. This reaction oxidises NADH back to NAD⁺, allowing glycolysis to continue producing 2 ATP per glucose — a small but crucial supply when oxygen delivery cannot keep up with demand.

在动物细胞中，例如剧烈运动期间的人类肌肉细胞，糖酵解产生的丙酮酸被乳酸脱氢酶还原为乳酸。该反应将 NADH 氧化回 NAD⁺，使糖酵解得以继续产生每分子葡萄糖 2 个 ATP——虽少但在氧气供应跟不上需求时至关重要。

Lactate accumulation was once thought to cause muscle fatigue and cramping, but modern research suggests that lactate may actually be a useful fuel source that the body recycles rather than merely a waste product.

乳酸的积累曾被认为会导致肌肉疲劳和痉挛，但现代研究表明，乳酸实际上可能是一种有用的燃料来源，身体会回收利用而不仅仅是废物。

Anaerobic Respiration in Yeast and Plants (Alcoholic Fermentation) | 酵母和植物中的无氧呼吸（酒精发酵）

In yeast and some plant cells, pyruvate is decarboxylated to ethanal (acetaldehyde) by the enzyme pyruvate decarboxylase, releasing CO₂. Ethanal is then reduced to ethanol by alcohol dehydrogenase, using NADH and regenerating NAD⁺. This process is exploited in the production of bread (where the CO₂ causes dough to rise) and alcoholic beverages (where ethanol is the desired product).

在酵母和一些植物细胞中，丙酮酸被丙酮酸脱羧酶脱羧为乙醛，释放 CO₂。然后乙醛被乙醇脱氢酶还原为乙醇，使用 NADH 并再生 NAD⁺。这一过程被用于面包生产（CO₂ 使面团膨胀）和酒精饮料生产（乙醇是目标产物）。

Feature | 特征	Lactate Fermentation | 乳酸发酵	Alcoholic Fermentation | 酒精发酵
Occurs in | 发生在	Animal cells, some bacteria | 动物细胞、一些细菌	Yeast, some plant cells | 酵母、一些植物细胞
End product | 终产物	Lactate | 乳酸	Ethanol + CO2 | 乙醇 + CO2
CO2 released? | 是否释放 CO2？	No | 否	Yes | 是
Reversibility | 可逆性	Reversible (lactate can be converted back to pyruvate when O2 is available) | 可逆（当有 O2 时，乳酸可转化回丙酮酸）	Irreversible (ethanol cannot be converted back) | 不可逆（乙醇不能转化回去）
ATP yield per glucose | 每分子葡萄糖的 ATP 产量	2	2

8. Respiratory Substrates | 呼吸底物

While glucose is the primary respiratory substrate, cells can also respire other molecules. Lipids, particularly triglycerides, yield more ATP per gram than carbohydrates because they are more reduced (contain more hydrogen atoms per carbon atom). When hydrolysed, a triglyceride is broken down into glycerol (which enters glycolysis) and fatty acids (which are broken down by β-oxidation into acetyl-CoA units that enter the Krebs cycle).

虽然葡萄糖是主要的呼吸底物，细胞也可以利用其他分子进行呼吸。脂质，特别是甘油三酯，每克产生的 ATP 比碳水化合物多，因为它们还原程度更高（每个碳原子含有更多的氢原子）。水解后，甘油三酯分解为甘油（进入糖酵解）和脂肪酸（通过 β-氧化分解为乙酰辅酶 A 单元，进入克雷布斯循环）。

Proteins are used as respiratory substrates only during starvation, when carbohydrate and lipid reserves are depleted. Amino acids are deaminated (the amino group is removed) and the remaining carbon skeleton is fed into the respiratory pathway at various points.

蛋白质仅在饥饿期间、碳水化合物和脂质储备耗尽时才被用作呼吸底物。氨基酸被脱氨基（移除氨基），剩余的碳骨架在不同点进入呼吸途径。

9. Respiratory Quotient (RQ) | 呼吸商

The respiratory quotient is a useful concept for determining which substrate is being respired. It is defined as:

呼吸商是确定正在呼吸哪种底物的有用概念。它定义为：

RQ = Volume of CO₂ produced / Volume of O₂ consumed
RQ = 产生的 CO₂ 体积 / 消耗的 O₂ 体积

Substrate | 底物	RQ Value | RQ 值
Carbohydrate | 碳水化合物	1.0
Lipid | 脂质	~0.7
Protein | 蛋白质	~0.9

An RQ value greater than 1.0 indicates that anaerobic respiration is occurring, as CO₂ is produced without O₂ consumption.

RQ 值大于 1.0 表示正在发生无氧呼吸，因为在没有消耗 O₂ 的情况下产生了 CO₂。

10. Exam Tips and Common Pitfalls | 考试技巧与常见陷阱

Key terms to use precisely | 需精确使用的关键术语

• Phosphorylation (adding a phosphate group, as in glucose → glucose-6-phosphate) versus substrate-level phosphorylation (direct ATP synthesis from a phosphorylated substrate) versus oxidative phosphorylation (ATP synthesis coupled to the electron transport chain). | 磷酸化（添加磷酸基团，如葡萄糖 → 葡萄糖-6-磷酸）vs 底物水平磷酸化（从磷酸化底物直接合成 ATP）vs 氧化磷酸化（与电子传递链偶联的 ATP 合成）。
• Decarboxylation (removal of CO₂) versus dehydrogenation (removal of hydrogen). Both occur in the link reaction and Krebs cycle. | 脱羧（移除 CO₂）vs 脱氢（移除氢）。两者都发生在连接反应和克雷布斯循环中。
• Know that NAD and FAD are coenzymes — they are not enzymes themselves, but they assist enzymes by carrying hydrogen atoms. | 了解 NAD 和 FAD 是辅酶——它们本身不是酶，但通过携带氢原子来辅助酶。

Common mistakes students make | 学生常犯的错误

• Saying glucose is “converted to energy” — it is oxidised to release energy used for ATP synthesis. | 说葡萄糖”转化为能量”——它是被氧化以释放用于 ATP 合成的能量。
• Confusing where each stage occurs. Glycolysis: cytoplasm. Link reaction and Krebs cycle: matrix. Oxidative phosphorylation: inner mitochondrial membrane. | 混淆每个阶段发生的位置。糖酵解：细胞质。连接反应和克雷布斯循环：基质。氧化磷酸化：线粒体内膜。
• Forgetting that the link reaction occurs twice per glucose molecule (once per pyruvate). | 忘记连接反应每个葡萄糖分子发生两次（每个丙酮酸一次）。
• Stating that ATP is produced in the Krebs cycle by oxidative phosphorylation — the ATP produced in the Krebs cycle is by substrate-level phosphorylation. | 声称克雷布斯循环中的 ATP 是通过氧化磷酸化产生的——克雷布斯循环中产生的 ATP 是通过底物水平磷酸化。
• Saying that oxygen is used in the Krebs cycle — oxygen is only used as the final electron acceptor in oxidative phosphorylation. | 说氧气在克雷布斯循环中使用——氧气仅作为氧化磷酸化中的最终电子受体使用。

11. Summary | 总结

Cellular respiration is a beautifully orchestrated sequence of biochemical reactions that converts the chemical energy stored in organic molecules into the universal energy currency of the cell — ATP. The four stages — glycolysis (cytoplasm), the link reaction (mitochondrial matrix), the Krebs cycle (mitochondrial matrix), and oxidative phosphorylation (inner mitochondrial membrane) — work together to maximise ATP yield from glucose. When oxygen is unavailable, anaerobic pathways provide a short-term, lower-yield alternative that sustains life until aerobic conditions return.

细胞呼吸是一系列精心编排的生化反应，将储存在有机分子中的化学能转化为细胞的通用能量货币——ATP。四个阶段——糖酵解（细胞质）、连接反应（线粒体基质）、克雷布斯循环（线粒体基质）和氧化磷酸化（线粒体内膜）——协同工作，以最大化从葡萄糖中获得的 ATP 产量。当氧气不可用时，无氧途径提供了短期的、较低产量的替代方案，在有氧条件恢复之前维持生命。

Mastering cellular respiration is not just about memorising the steps — it is about understanding why each step exists, what would happen if it failed, and how the entire system is elegantly regulated. Good luck with your A-Level Biology revision!

掌握细胞呼吸不仅仅是记忆步骤——而是要理解每个步骤为何存在、如果失败会发生什么，以及整个系统如何被优雅地调控。祝你的 A-Level 生物学复习顺利！