Cellular Respiration — 细胞呼吸

📚 Cellular Respiration | 细胞呼吸

English: Cellular respiration is the metabolic pathway through which cells break down glucose and other organic molecules to release energy in the form of adenosine triphosphate (ATP). This process is fundamental to all aerobic organisms, from single-celled bacteria to complex multicellular life forms. In A-Level Biology, understanding respiration means mastering the detailed sequences of glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation — each tightly coordinated to maximise energy extraction.

中文：细胞呼吸是细胞分解葡萄糖和其他有机分子、以三磷酸腺苷（ATP）形式释放能量的代谢途径。这一过程对从单细胞细菌到复杂多细胞生命形式的所有需氧生物都至关重要。在A-Level生物学中，理解呼吸作用意味着掌握糖酵解、连接反应、克雷布斯循环和氧化磷酸化的详细序列——每一步都紧密协调，以最大化能量提取效率。

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

English: Cellular respiration can be divided into four main stages: glycolysis, the link reaction, the Krebs cycle (also known as the citric acid cycle or TCA cycle), and oxidative phosphorylation. Glycolysis occurs in the cytoplasm, while the remaining three stages take place inside mitochondria. The overall balanced equation for aerobic respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP (theoretical maximum)

中文：细胞呼吸可分为四个主要阶段：糖酵解、连接反应、克雷布斯循环（又称柠檬酸循环或TCA循环）和氧化磷酸化。糖酵解发生在细胞质中，其余三个阶段发生在线粒体内。有氧呼吸的总平衡方程式为：

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 38 ATP（理论最大值）

English: It is critical to understand that ATP is the energy currency of the cell. When ATP is hydrolysed to ADP and inorganic phosphate (Pi), approximately 30.5 kJ mol^-1 of energy is released to drive endergonic reactions. The controlled, multi-step nature of respiration allows the cell to capture this energy incrementally rather than in a single, wasteful burst.

中文：理解ATP是细胞的能量货币至关重要。当ATP水解为ADP和无机磷酸（Pi）时，约释放30.5 kJ mol^-1的能量来驱动吸能反应。呼吸作用受控的多步骤特性使细胞能够逐步捕获能量，而不是一次性浪费性地释放。

2. Glycolysis | 糖酵解

English: Glycolysis is the first stage of cellular respiration and occurs in the cytoplasm. It does not require oxygen and therefore occurs in both aerobic and anaerobic organisms. One molecule of glucose (a six-carbon sugar) is phosphorylated twice using 2 ATP molecules, making it more reactive. The resulting phosphorylated hexose is then split into two molecules of triose phosphate (TP), each containing three carbon atoms.

中文：糖酵解是细胞呼吸的第一阶段，发生在细胞质中。它不需要氧气，因此在需氧和厌氧生物中都会发生。一分子葡萄糖（六碳糖）使用2个ATP分子进行两次磷酸化，使其更具反应性。然后，生成的磷酸化己糖被分裂成两分子磷酸三碳糖（TP），每分子含有三个碳原子。

English: Each triose phosphate is subsequently oxidised in a series of enzyme-catalysed reactions. During this oxidation, hydrogen atoms are removed and transferred to the coenzyme NAD⁺, reducing it to NADH (reduced NAD). Four ATP molecules are also produced by substrate-level phosphorylation across the two triose phosphate molecules. The net yield of glycolysis per glucose molecule is therefore: 2 ATP (4 produced minus 2 invested), 2 NADH, and 2 pyruvate molecules.

中文：每分子磷酸三碳糖随后在一系列酶催化反应中被氧化。在氧化过程中，氢原子被移除并转移给辅酶NAD⁺，将其还原为NADH（还原型NAD）。两分子磷酸三碳糖通过底物水平磷酸化还产生4个ATP分子。因此，每分子葡萄糖在糖酵解中的净产量为：2 ATP（产生4个减去投入2个）、2 NADH和2个丙酮酸分子。

English: The key enzymes in glycolysis include hexokinase (which catalyses the first phosphorylation), phosphofructokinase (PFK — the rate-limiting enzyme), and pyruvate kinase. PFK is allosterically inhibited by high levels of ATP and citrate, providing feedback regulation: when energy is abundant, glycolysis slows down.

中文：糖酵解中的关键酶包括己糖激酶（催化第一次磷酸化）、磷酸果糖激酶（PFK——限速酶）和丙酮酸激酶。PFK受高水平ATP和柠檬酸的别构抑制，提供了反馈调节：当能量充足时，糖酵解减慢。

3. The Link Reaction | 连接反应

English: Following glycolysis, pyruvate enters the mitochondrial matrix through active transport via specific carrier proteins in the inner mitochondrial membrane. Once inside, each pyruvate molecule undergoes the link reaction — a decarboxylation and oxidation process catalysed by the multi-enzyme pyruvate dehydrogenase complex.

中文：糖酵解后，丙酮酸通过线粒体内膜上的特定载体蛋白经主动运输进入线粒体基质。一旦进入，每分子丙酮酸经历连接反应——一个由多酶丙酮酸脱氢酶复合体催化的脱羧和氧化过程。

English: In the link reaction, pyruvate (3C) is converted to acetyl coenzyme A (acetyl-CoA, 2C) with the release of one molecule of carbon dioxide (CO₂). During this oxidation, NAD⁺ is reduced to NADH. The acetyl-CoA then enters the Krebs cycle. Since one glucose yields two pyruvate molecules, the link reaction happens twice per glucose, producing 2 CO₂, 2 NADH, and 2 acetyl-CoA.

中文：在连接反应中，丙酮酸（3C）被转化为乙酰辅酶A（acetyl-CoA，2C），并释放一分子二氧化碳（CO₂）。在此氧化过程中，NAD⁺被还原为NADH。乙酰辅酶A随后进入克雷布斯循环。由于一分子葡萄糖产生两分子丙酮酸，连接反应每分子葡萄糖发生两次，产生2 CO₂、2 NADH和2 acetyl-CoA。

4. The Krebs Cycle | 克雷布斯循环

English: The Krebs cycle occurs in the mitochondrial matrix and is a cyclic series of oxidation-reduction reactions. Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C), catalysed by citrate synthase. Through a series of decarboxylation and dehydrogenation steps, citrate is gradually oxidised back to oxaloacetate, regenerating the starting molecule for the next turn.

中文：克雷布斯循环发生在线粒体基质中，是一系列氧化还原反应的循环。乙酰辅酶A（2C）与草酰乙酸（4C）结合形成柠檬酸（6C），由柠檬酸合酶催化。通过一系列脱羧和脱氢步骤，柠檬酸逐渐被氧化回草酰乙酸，为下一轮循环再生起始分子。

English: For each turn of the Krebs cycle (per acetyl-CoA), the following are produced: 3 NADH, 1 FADH₂ (reduced FAD), 1 ATP (via substrate-level phosphorylation, as GTP in some organisms), and 2 CO₂. Since each glucose generates two acetyl-CoA molecules, the Krebs cycle turns twice per glucose, yielding: 6 NADH, 2 FADH₂, 2 ATP, and 4 CO₂.

中文：克雷布斯循环每转一圈（每分子乙酰辅酶A），产生以下产物：3 NADH、1 FADH₂（还原型FAD）、1 ATP（通过底物水平磷酸化，某些生物体中为GTP）和2 CO₂。由于每分子葡萄糖产生两分子乙酰辅酶A，克雷布斯循环每分子葡萄糖转动两圈，产生：6 NADH、2 FADH₂、2 ATP和4 CO₂。

5. Oxidative Phosphorylation — Electron Transport Chain | 氧化磷酸化——电子传递链

English: Oxidative phosphorylation takes place on the inner mitochondrial membrane. This is where the vast majority of ATP is synthesised. The reduced coenzymes NADH and FADH₂ donate their electrons to a series of protein complexes embedded in the inner membrane, collectively known as the electron transport chain (ETC).

中文：氧化磷酸化发生在线粒体内膜上。这是绝大多数ATP被合成的地方。还原型辅酶NADH和FADH₂将它们的电子捐赠给嵌入内膜的一系列蛋白质复合体，统称为电子传递链（ETC）。

English: The ETC consists of four main complexes: Complex I (NADH dehydrogenase), Complex II (succinate dehydrogenase — also part of the Krebs cycle), Complex III (cytochrome bc1 complex), and Complex IV (cytochrome c oxidase). Electrons flow from NADH → Complex I → ubiquinone → Complex III → cytochrome c → Complex IV → O₂. Electrons from FADH₂ enter at Complex II. At each transfer, the electrons fall to a lower energy level, releasing energy used to pump protons (H⁺) from the matrix into the intermembrane space.

中文：电子传递链由四个主要复合体组成：复合体I（NADH脱氢酶）、复合体II（琥珀酸脱氢酶——也是克雷布斯循环的一部分）、复合体III（细胞色素bc1复合体）和复合体IV（细胞色素c氧化酶）。电子从NADH → 复合体I → 泛醌 → 复合体III → 细胞色素c → 复合体IV → O₂流动。来自FADH₂的电子在复合体II进入。每次传递中，电子降至较低的能级，释放的能量用于将质子（H⁺）从基质泵入膜间隙。

English: Oxygen serves as the final electron acceptor at Complex IV, combining with electrons and protons to form water: ½O₂ + 2e^– + 2H⁺ → H₂O. Without oxygen, the entire chain would back up because electrons would have nowhere to go — this explains why aerobic organisms depend on oxygen for efficient ATP production.

中文：氧气在复合体IV处作为最终电子受体，与电子和质子结合形成水：½O₂ + 2e^– + 2H⁺ → H₂O。没有氧气，整个链条会阻塞，因为电子无处可去——这解释了为何需氧生物依赖氧气进行高效的ATP生产。

6. Chemiosmosis and ATP Synthase | 化学渗透与ATP合酶

English: The proton pumping by Complexes I, III, and IV creates an electrochemical gradient — a proton-motive force — across the inner mitochondrial membrane. Protons accumulate in the intermembrane space, creating both a pH gradient (higher H⁺ concentration outside) and an electrical potential (the intermembrane space becomes positively charged relative to the matrix).

中文：复合体I、III和IV的质子泵送在线粒体内膜两侧形成了电化学梯度——质子动力。质子在膜间隙中积累，同时产生pH梯度（外部H⁺浓度更高）和电位差（膜间隙相对于基质带正电荷）。

English: Protons can only flow back into the matrix through specialised channel proteins called ATP synthase (Complex V). As protons flow down their electrochemical gradient through ATP synthase, the enzyme rotates — a mechanical process known as rotational catalysis. This rotation drives the phosphorylation of ADP to ATP: ADP + Pi → ATP. This process is called chemiosmosis, a term coined by Peter Mitchell, who won the 1978 Nobel Prize for this theory.

中文：质子只能通过称为ATP合酶（复合体V）的特殊通道蛋白流回基质。当质子沿着电化学梯度流经ATP合酶时，酶发生旋转——一个称为旋转催化的机械过程。这种旋转驱动ADP磷酸化为ATP：ADP + Pi → ATP。这一过程称为化学渗透，由彼得·米切尔提出，他因此理论获得了1978年诺贝尔奖。

English: Each NADH that donates electrons to the ETC powers enough proton pumping to synthesise approximately 2.5 ATP molecules. Each FADH₂, which enters at Complex II (bypassing Complex I), contributes to approximately 1.5 ATP molecules. This difference is crucial for calculating the total ATP yield of respiration.

中文：每个向电子传递链贡献电子的NADH驱动的质子泵送足以合成约2.5个ATP分子。每个FADH₂在复合体II处进入（绕过复合体I），贡献约1.5个ATP分子。这一差异对于计算呼吸作用的ATP总产量至关重要。

7. Complete ATP Yield Summary | 完整ATP产量总结

Stage / 阶段	Location / 位置	ATP (substrate-level) / 底物水平	NADH	FADH₂	CO₂
Glycolysis / 糖酵解	Cytoplasm / 细胞质	2	2	0	0
Link Reaction / 连接反应	Matrix / 基质	0	2	0	2
Krebs Cycle / 克雷布斯循环	Matrix / 基质	2	6	2	4
Oxidative Phosphorylation / 氧化磷酸化	Inner membrane / 内膜	~28	-10	-2	0
TOTAL / 总计	—	32	—	—	6

English: The theoretical maximum of 38 ATP assumes perfect efficiency. In reality, the cost of transporting cytoplasmic NADH into the mitochondria (via the glycerol phosphate shuttle or malate-aspartate shuttle) reduces the net yield. Most textbooks now quote 30-32 ATP per glucose as the realistic figure. The 2 NADH produced in glycolysis are cytoplasmic and their electrons must be shuttled into the mitochondrion — depending on the shuttle used, each yields either 1.5 or 2.5 ATP.

中文：38 ATP的理论最大值假设完美效率。实际上，将细胞质NADH运入线粒体的成本（通过甘油磷酸穿梭或苹果酸-天冬氨酸穿梭）降低了净产量。现在大多数教科书将每分子葡萄糖30-32 ATP作为实际数值。糖酵解中产生的2个NADH位于细胞质中，其电子必须被转运入线粒体——根据所用穿梭方式，每个产生1.5或2.5个ATP。

8. Anaerobic Respiration | 无氧呼吸

English: When oxygen is unavailable, cells cannot carry out the link reaction, Krebs cycle, or oxidative phosphorylation. The electron transport chain backs up, and NADH accumulates. Glycolysis would also stop because NAD⁺ is needed to accept hydrogen atoms during the oxidation of triose phosphate. To keep glycolysis running and produce at least some ATP, cells use anaerobic pathways to regenerate NAD⁺.

中文：当氧气不可用时，细胞无法进行连接反应、克雷布斯循环或氧化磷酸化。电子传递链阻塞，NADH积累。糖酵解也会停止，因为在磷酸三碳糖氧化过程中需要NAD⁺来接收氢原子。为了保持糖酵解继续进行并至少产生一些ATP，细胞使用无氧途径再生NAD⁺。

English: In animals and some bacteria, lactate fermentation occurs: pyruvate is reduced to lactate by lactate dehydrogenase, using electrons from NADH. This regenerates NAD⁺ so glycolysis can continue. The net yield is only 2 ATP per glucose. The accumulation of lactate in muscles during intense exercise causes the familiar burning sensation and contributes to muscle fatigue.

中文：在动物和一些细菌中，发生乳酸发酵：丙酮酸被乳酸脱氢酶使用来自NADH的电子还原为乳酸。这再生了NAD⁺，使糖酵解得以继续。净产量仅为每分子葡萄糖2 ATP。剧烈运动期间肌肉中乳酸的积累导致熟悉的灼烧感并导致肌肉疲劳。

English: In plants and yeast, ethanol fermentation takes place: pyruvate is first decarboxylated to ethanal (acetaldehyde) by pyruvate decarboxylase, releasing CO₂. Ethanal is then reduced to ethanol by alcohol dehydrogenase, using electrons from NADH, regenerating NAD⁺. This process is exploited in baking (the CO₂ makes dough rise) and brewing (ethanol production).

中文：在植物和酵母中，发生乙醇发酵：丙酮酸首先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使用来自NADH的电子还原为乙醇，再生NAD⁺。这一过程被用于烘焙（CO₂使面团发酵膨胀）和酿造（乙醇生产）。

9. Respiratory Substrates and Respiratory Quotient | 呼吸底物与呼吸商

English: While glucose is the primary respiratory substrate, cells can also respire lipids, proteins, and other carbohydrates when necessary. The respiratory quotient (RQ) is the ratio of CO₂ produced to O₂ consumed:

RQ = CO₂ produced / O₂ consumed

中文：虽然葡萄糖是主要的呼吸底物，但细胞在必要时也可以呼吸脂质、蛋白质和其他碳水化合物。呼吸商（RQ）是产生的CO₂与消耗的O₂之比：

RQ = 产生的CO₂ / 消耗的O₂

English: The RQ value reveals which substrate is being respired:

Carbohydrates: RQ = 1.0 (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, so 6/6 = 1.0)
Lipids: RQ ≈ 0.7 (lipids are more reduced, requiring more O₂ per CO₂ released)
Proteins: RQ ≈ 0.8-0.9 (varies depending on amino acid composition)
Anaerobic respiration: RQ is very high or undefined (no O₂ consumed but CO₂ is produced)

中文：RQ值揭示了正在被呼吸的底物类型：

碳水化合物：RQ = 1.0（C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O，即6/6 = 1.0）
脂质：RQ ≈ 0.7（脂质还原度更高，每释放一分子CO₂需要更多O₂）
蛋白质：RQ ≈ 0.8-0.9（因氨基酸组成而异）
无氧呼吸：RQ非常高或未定义（不消耗O₂但产生CO₂）

10. Factors Affecting Respiration Rate | 影响呼吸速率的因素

English: Several factors influence the rate of cellular respiration. Temperature affects enzyme kinetics — respiration rate increases with temperature up to an optimum (around 37-40 °C in mammals), beyond which enzymes denature. Oxygen concentration directly limits the electron transport chain; below a critical threshold, cells switch to anaerobic pathways. Substrate availability (glucose, fatty acids) determines the raw material supply. ADP concentration is often the rate-limiting factor — when ATP demand is high, ADP accumulates and stimulates respiration through respiratory control.

中文：多种因素影响细胞呼吸速率。温度影响酶动力学——呼吸速率随温度升高而增加，达到最适温度（哺乳动物约37-40°C），超过此温度酶会变性。氧气浓度直接限制电子传递链；低于临界阈值时，细胞转为无氧途径。底物可用性（葡萄糖、脂肪酸）决定原料供应。ADP浓度通常是限速因素——当ATP需求高时，ADP积累并通过呼吸控制刺激呼吸作用。

English: Respiratory inhibitors can block specific steps of the ETC. Cyanide ions (CN^–) bind irreversibly to cytochrome c oxidase (Complex IV), blocking electron transfer to oxygen — this is a lethal poison. Carbon monoxide (CO) also competes with oxygen for binding at Complex IV. Uncouplers like 2,4-dinitrophenol (DNP) create proton leaks in the inner membrane, dissipating the proton gradient so that electron transport continues but ATP is not synthesised — energy is released as heat instead.

中文：呼吸抑制剂可以阻断电子传递链的特定步骤。氰离子（CN^–）不可逆地与细胞色素c氧化酶（复合体IV）结合，阻断电子向氧气的传递——这是一种致命的毒物。一氧化碳（CO）也与氧气竞争复合体IV的结合位点。像2,4-二硝基苯酚（DNP）这样的解偶联剂在线粒体内膜上产生质子泄漏，消散质子梯度，使得电子传递继续但ATP无法合成——能量以热量形式释放。

11. Exam Tips for A-Level Biology | A-Level生物学考试技巧

English: When answering respiration questions, keep these points in mind:

Be precise with locations: Always state whether a reaction occurs in the cytoplasm, mitochondrial matrix, or inner mitochondrial membrane. Marks are often allocated for correct compartmentalisation.
Distinguish NAD from NADH: NAD⁺ is the oxidised form; NADH is the reduced form. Examiners expect precise terminology — do not write “NAD” when you mean NADH.
Substrate-level vs oxidative phosphorylation: The ATP from glycolysis and the Krebs cycle is made by substrate-level phosphorylation (direct enzyme-catalysed transfer of a phosphate group). The ATP from the ETC is made by oxidative phosphorylation (chemiosmosis). Know the difference.
Explain enzyme roles: Be prepared to name key enzymes such as phosphofructokinase, citrate synthase, and ATP synthase. Understanding regulation (feedback inhibition) demonstrates deeper knowledge.
Use the correct electron carriers: Remember that NADH and FADH₂ carry electrons to the ETC at different entry points, resulting in different ATP yields (2.5 vs 1.5).
Draw labelled diagrams: Practise drawing the mitochondrion with cristae, matrix, inner and outer membranes clearly labelled. A well-annotated diagram can earn multiple marks.

中文：回答呼吸作用问题时，请牢记以下几点：

位置要精确：始终说明反应发生在细胞质、线粒体基质还是线粒体内膜。分数通常分配给正确的区室化。
区分NAD和NADH：NAD⁺是氧化形式；NADH是还原形式。考官期望精确的术语——当指NADH时不要写”NAD”。
底物水平磷酸化 vs 氧化磷酸化：糖酵解和克雷布斯循环的ATP是通过底物水平磷酸化（酶直接催化磷酸基团转移）产生的。电子传递链的ATP是通过氧化磷酸化（化学渗透）产生的。了解其区别。
解释酶的作用：准备好命名关键酶，如磷酸果糖激酶、柠檬酸合酶和ATP合酶。理解调控（反馈抑制）展示更深层的知识。
使用正确的电子载体：记住NADH和FADH₂在不同入口点将电子带入电子传递链，导致不同的ATP产量（2.5 vs 1.5）。
绘制标记清晰的图表：练习绘制线粒体并清楚标记嵴、基质、内膜和外膜。一幅注释良好的图表可以赢得多个分数。

12. Summary | 总结

English: Cellular respiration is a masterpiece of biochemical engineering — four coordinated stages that extract energy from glucose with remarkable efficiency. Glycolysis provides a modest ATP return but produces key intermediates. The link reaction and Krebs cycle strip electrons from carbon compounds and load them onto electron carriers. Oxidative phosphorylation converts the energy of those electrons into a proton gradient, which ATP synthase harnesses to produce the majority of the cell’s ATP. Understanding this pathway not only prepares students for A-Level examinations but also provides a foundation for studying bioenergetics, mitochondrial diseases, and metabolic disorders at university level.

中文：细胞呼吸是生化工程的杰作——四个协调阶段以卓越的效率从葡萄糖中提取能量。糖酵解提供适度的ATP回报，但产生了关键中间产物。连接反应和克雷布斯循环从碳化合物中剥离电子并将其加载到电子载体上。氧化磷酸化将这些电子的能量转化为质子梯度，ATP合酶利用该梯度产生细胞大部分的ATP。理解这一途径不仅为学生准备A-Level考试，还为大学阶段学习生物能量学、线粒体疾病和代谢紊乱奠定了基础。

Cellular Respiration — 细胞呼吸