📚 Cellular Respiration: From Glycolysis to ATP Synthesis | 细胞呼吸：从糖酵解到 ATP 合成
Respiration is a universal biochemical process that releases chemical energy stored in organic fuel molecules and traps it in the energy‑carrying molecule ATP. In eukaryotic cells, aerobic respiration consumes oxygen and completely oxidises substrates such as glucose to carbon dioxide and water. The overall reaction is the reverse of photosynthesis and is the primary source of metabolic energy for growth, maintenance, and reproduction. A‑Level candidates must understand not only the balanced equation but the location, inputs, outputs, and regulation of each stage.
呼吸作用是普遍存在的生化过程，它释放储存在有机燃料分子中的化学能，并将其捕捉到能量载体 ATP 中。在真核细胞中，有氧呼吸消耗氧气，并将葡萄糖等底物完全氧化为二氧化碳和水。总反应是光合作用的逆反应，也是生长、维持和繁殖所需代谢能量的主要来源。A‑Level 考生不仅需要掌握配平方程，还要理解每个阶段的场所、输入物、输出物及调控机制。
1. Overview of Aerobic Respiration | 有氧呼吸概述
Aerobic respiration consists of four tightly coordinated stages: glycolysis in the cytoplasm, the link reaction in the mitochondrial matrix, the Krebs cycle also in the matrix, and oxidative phosphorylation on the inner mitochondrial membrane. The complete oxidation of one molecule of glucose can yield up to 32 molecules of ATP under ideal conditions, although the actual yield is often slightly lower due to proton leakage and the cost of transporting cytosolic NADH into the matrix.
有氧呼吸由四个紧密协调的阶段组成：发生在细胞质的糖酵解、线粒体基质中的连接反应、同样在基质中的三羧酸循环，以及内线粒体膜上的氧化磷酸化。在理想条件下，一分子葡萄糖完全氧化最多可产生 32 分子 ATP，但由于质子泄漏和将胞质 NADH 转运入基质的能量消耗，实际产量往往略低。
The overall equation, C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP, masks a complex, stepwise oxidation that prevents the cell from releasing the energy of glucose in a single, destructive burst. Instead, energy is transferred in manageable packets via reduced coenzymes (NADH and FADH₂) to the electron transport chain.
总方程 C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP 掩盖了一个复杂的逐步氧化过程，它防止细胞以一次性破坏性爆发的方式释放葡萄糖能量。相反，能量通过还原型辅酶（NADH 和 FADH₂）以可控的“能量包”传递到电子传递链。
2. Structure of the Mitochondrion | 线粒体的结构
The mitochondrion is bounded by a smooth outer membrane and a highly folded inner membrane whose infoldings are called cristae. The space enclosed by the inner membrane is the matrix, which contains the enzymes for the link reaction and the Krebs cycle, as well as mitochondrial DNA and ribosomes. The intermembrane space collects protons pumped out of the matrix, establishing an electrochemical gradient.
线粒体由光滑的外膜和高度折叠的内膜包裹，内膜的内褶称为嵴。内膜包围的空间为基质，其中含有进行连接反应和三羧酸循环的酶，以及线粒体 DNA 和核糖体。膜间隙收集从基质泵出的质子，从而建立电化学梯度。
The cristae dramatically increase the surface area for the electron transport chain and ATP synthase complexes. Proteins of the respiratory chain are organised into four large complexes (I–IV) that transfer electrons vectorially, meaning they pump protons from the matrix into the intermembrane space as electrons flow through them.
嵴极大增加了电子传递链和 ATP 合酶复合物的表面积。呼吸链的蛋白质组织为四个大型复合物（I–IV），它们进行矢量性电子传递，即当电子流经复合物时，它们将质子从基质泵到膜间隙。
3. Glycolysis: The Cytoplasmic Phase | 糖酵解：细胞质阶段
Glycolysis splits one molecule of glucose (a six‑carbon sugar) into two molecules of pyruvate (a three‑carbon compound). It occurs in the cytosol and does not require oxygen, making it the universal first step of both aerobic and anaerobic respiration. The process consists of an energy‑investment phase that consumes 2 ATP and an energy‑payoff phase that generates 4 ATP and 2 NADH, giving a net yield of 2 ATP and 2 NADH per glucose.
糖酵解将一分子葡萄糖（六碳糖）裂解为两分子丙酮酸（三碳化合物）。它发生在细胞质中，不需要氧气，因此是有氧和无氧呼吸共同的起始步骤。该过程包括消耗 2 ATP 的能量投入阶段和产生 4 ATP 与 2 NADH 的能量回报阶段，每分子葡萄糖净得 2 ATP 和 2 NADH。
Key regulatory enzymes include hexokinase (phosphorylating glucose), phosphofructokinase (the rate‑limiting step), and pyruvate kinase. Phosphofructokinase is allosterically inhibited by ATP and citrate and activated by AMP, ensuring that glycolysis speeds up when cellular energy charge is low.
关键调控酶包括己糖激酶（磷酸化葡萄糖）、磷酸果糖激酶（限速步骤）和丙酮酸激酶。磷酸果糖激酶被 ATP 和柠檬酸别构抑制，而被 AMP 激活，从而保证当细胞能荷低时糖酵解加速。
4. The Link Reaction | 连接反应
Pyruvate enters the mitochondrial matrix via a specific carrier protein. Inside the matrix, the pyruvate dehydrogenase complex catalyses its oxidative decarboxylation: a carboxyl group is removed as CO₂, the remaining two‑carbon fragment is oxidised to an acetyl group, and NAD⁺ is reduced to NADH. The acetyl group is then attached to coenzyme A, forming acetyl‑CoA.
丙酮酸通过特定载体蛋白进入线粒体基质。在基质内，丙酮酸脱氢酶复合体催化其氧化脱羧：一个羧基以 CO₂ 的形式被移除，剩下的二碳片段被氧化为乙酰基，同时 NAD⁺ 被还原为 NADH。乙酰基随即与辅酶 A 连接，形成乙酰辅酶 A。
For each glucose molecule, two pyruvates enter the link reaction, producing 2 CO₂, 2 NADH, and 2 acetyl‑CoA. No ATP is made directly, but the abundant NADH feeds into oxidative phosphorylation. The reaction is irreversible and commits the carbon skeleton to complete oxidation.
每分子葡萄糖提供两分子丙酮酸进入连接反应，生成 2 CO₂、2 NADH 和 2 乙酰辅酶 A。此阶段不直接产生 ATP，但大量的 NADH 为氧化磷酸化提供还原力。该反应不可逆，使碳骨架进入彻底氧化。
5. The Krebs Cycle | 三羧酸循环
Acetyl‑CoA enters the Krebs cycle by condensing with oxaloacetate (a four‑carbon compound) to form citrate (six carbons). Through a series of enzyme‑catalysed oxidations and decarboxylations, the cycle regenerates oxaloacetate and releases two CO₂ molecules per acetyl group. In one turn, three NADH, one FADH₂, and one GTP (equivalent to one ATP) are produced.
乙酰辅酶 A 进入三羧酸循环，与草酰乙酸（四碳化合物）缩合形成柠檬酸（六碳）。经过一系列酶催化的氧化和脱羧反应，循环再生草酰乙酸，每个乙酰基释放两分子 CO₂。每一轮循环产生 3 个 NADH、1 个 FADH₂ 和 1 个 GTP（相当于 1 个 ATP）。
Because two acetyl‑CoA molecules are generated from one glucose, the Krebs cycle turns twice per glucose oxidised. The cycle is a metabolic hub: intermediates such as α‑ketoglutarate and oxaloacetate are precursors for amino acid synthesis, illustrating why respiration is interconnected with anabolic pathways.
由于一分子葡萄糖生成两分子乙酰辅酶 A，因此每个葡萄糖氧化时三羧酸循环运转两轮。该循环是代谢枢纽：α‑酮戊二酸和草酰乙酸等中间产物是合成氨基酸的前体，这体现了呼吸作用与合成代谢途径的相互关联。
6. Oxidative Phosphorylation: Electron Transport Chain | 氧化磷酸化：电子传递链
NADH and FADH₂ deposit high‑energy electrons into the electron transport chain (ETC) on the inner mitochondrial membrane. Electrons pass through Complex I (NADH dehydrogenase), Complex II (succinate dehydrogenase; also part of the Krebs cycle), Complex III (cytochrome bc₁), and Complex IV (cytochrome c oxidase). Mobile carriers ubiquinone (coenzyme Q) and cytochrome c shuttle electrons between the complexes.
NADH 和 FADH₂ 将高能电子释放到内线粒体膜上的电子传递链。电子依次流经复合体 I（NADH 脱氢酶）、复合体 II（琥珀酸脱氢酶，同时参与三羧酸循环）、复合体 III（细胞色素 bc₁）和复合体 IV（细胞色素 c 氧化酶）。移动载体泛醌（辅酶 Q）和细胞色素 c 在复合体之间传递电子。
As electrons travel along the chain, their energy is harnessed to pump protons (H⁺) from the matrix into the intermembrane space at Complexes I, III, and IV. This creates a proton gradient across the inner membrane, storing potential energy that will be used for ATP synthesis. Oxygen acts as the final electron acceptor, combining with electrons and H⁺ to form water.
电子沿传递链流动时，其能量被用于在复合体 I、III 和 IV 处将质子（H⁺）从基质泵入膜间隙。这在膜两侧形成质子梯度，储存了可用于合成 ATP 的势能。氧气作为最终的电子受体，与电子和 H⁺结合生成水。
7. Chemiosmosis and ATP Synthesis | 化学渗透与 ATP 合成
ent generated by the ETC is a store of electrochemical potential energy, termed the proton‑motive force. Protons can only re‑ adient, the synthase rotates and catalyses the phosphorylation of ADP to ATP: ADP + Pᵢ → ATP.
电子传递链产生的质子梯度是电化学势能的储存，称为质子动势。质子只能通过嵌入内膜的 ATP 合酶复合体（一个分子涡轮）重新进入基质。当质子顺浓度梯度流动时，合酶旋转并催化 ADP 磷酸化为 ATP：ADP + Pᵢ → ATP。
This coupling of electron transport to ATP synthesis is chemiosmosis. In a typical mitochondrion, about 2.5 ATP are synthesised for every NADH oxidised, while FADH₂, which enters the ETC at a lower energy point, yields about 1.5 ATP. Respiratory inhibitors such as cyanide block Complex IV, halting electron flow and rapidly stopping ATP production.
这种将电子传递与 ATP 合成偶联的机制就是化学渗透。在一个典型的线粒体中，每氧化一个 NADH 约合成 2.5 个 ATP，而 FADH₂ 由于在能量较低处进入电子传递链，只能产生约 1.5 个 ATP。呼吸抑制剂如氰化物阻断复合体 IV，中断电子流并迅速停止 ATP 合成。
8. Anaerobic Respiration in Mammals (Lactic Acid Fermentation) | 哺乳动物的无氧呼吸（乳酸发酵）
When oxygen is limiting, mammalian muscle cells can regenerate NAD⁺ by reducing pyruvate to lactate. The enzyme lactate dehydrogenase catalyses: Pyruvate + NADH → Lactate + NAD⁺. This process keeps glycolysis running, producing a net 2 ATP per glucose, but it does not consume oxygen or generate a proton gradient.
当氧气受限时，哺乳动物肌细胞可通过将丙酮酸还原为乳酸来再生 NAD⁺。乳酸脱氢酶催化：丙酮酸 + NADH → 乳酸 + NAD⁺。该过程使糖酵解得以持续，每葡萄糖净产 2 ATP，但不消耗氧气，也不产生质子梯度。
Lactate accumulation lowers intracellular pH and can contribute to muscle fatigue. After oxygen becomes available again, lactate can be converted back to pyruvate in the liver (Cori cycle) and oxidised aerobically. This explains the oxygen debt repaid during recovery after intense exercise.
乳酸积累会降低细胞内 pH，可能导致肌肉疲劳。当氧气恢复供应后，乳酸可在肝脏中重新转化为丙酮酸（Cori 循环）并进行有氧氧化。这就解释了剧烈运动后偿还氧债的现象。
9. Anaerobic Respiration in Yeast (Alcoholic Fermentation) | 酵母的无氧呼吸（酒精发酵）
In yeast and some other microorganisms, pyruvate is first decarboxylated to ethanal (acetaldehyde), releasing CO₂, and then ethanal is reduced to ethanol using NADH, catalysed by alcohol dehydrogenase: Pyruvate → Ethanal + CO₂ ; Ethanal + NADH → Ethanol + NAD⁺. The net ATP yield is again limited to 2 ATP per glucose.
在酵母及某些其他微生物中，丙酮酸首先脱羧生成乙醛并释放 CO₂，然后乙醛被 NADH 还原为乙醇，由乙醇脱氢酶催化：丙酮酸 → 乙醛 + CO₂；乙醛 + NADH → 乙醇 + NAD⁺。每葡萄糖的净 ATP 同样只有 2 个。
Alcoholic fermentation is economically important in baking (CO₂ raises dough) and brewing (ethanol is the desired product). Unlike lactic acid fermentation, the decarboxylation step is irreversible, so the reaction cannot be easily reversed to feed carbon back into aerobic pathways.
酒精发酵在烘焙（CO₂ 使面团膨发）和酿酒（乙醇为目标产物）中具有重要经济意义。与乳酸发酵不同，脱羧步骤不可逆，因此该反应难以逆转使碳骨架重新进入有氧途径。
| Feature | Lactic Acid Fermentation | Alcoholic Fermentation |
|---|---|---|
| Substrate | Pyruvate | Pyruvate |
| Products | Lactate + NAD⁺ | Ethanol + CO₂ + NAD⁺ |
| Organisms | Mammalian muscle, some bacteria | Yeast, some plant tissues |
| Reversibility | Reversible (Cori cycle) | Decarboxylation is irreversible |
10. Respiratory Substrates and Energy Values | 呼吸底物与能量值
Although glucose is the standard textbook fuel, cells oxidise a wide range of substrates. Lipids yield more than twice the energy per gram compared to carbohydrates because fatty acids are highly reduced. Proteins are used as a respiratory substrate only after deamination, and their energy value depends on the amino acid composition.
尽管葡萄糖是教科书中的标准燃料，但细胞可以氧化多种底物。脂质因脂肪酸高度还原，每克提供的能量是碳水化合物的两倍多。蛋白质只有在脱氨后才被用作呼吸底物，其能量值取决于氨基酸组成。
The respiratory quotient (RQ) is the ratio of CO₂ produced to O₂ consumed: RQ = CO₂ produced ÷ O₂ consumed easuring RQ can indicate which substrate is being predominantly respired, a concept often tested in A‑Level practicals using respirometers.
呼吸商（RQ）是产生的 CO₂ 与消耗的 O₂ 之比：RQ = 产生 CO₂ ÷ 消耗 O₂。纯碳水化合物的 RQ ≈ 1.0，脂质 ≈ 0.71，蛋白质 ≈ 0.9。测定 RQ 可推测正在被呼吸的主要底物，这一概念常出现在 A‑Level 的呼吸计实验中。
11. Control and Integration of Respiration | 呼吸作用的调控与整合
Respiration is tightly regulated to match ATP supply with demand. Phosphofructokinase in glycolysis and pyruvate dehydrogenase in the link reaction are major control points. High levels of ATP, NADH, and citrate inhibit these enzymes, while AMP, ADP, and NAD⁺ stimulate them, achieving feedback regulation that stabilises metabolic flux.
呼吸作用受到严格调控，以使 ATP 供给与需求相匹配。糖酵解中的磷酸果糖激酶和连接反应中的丙酮酸脱氢酶是主要控制点。高水平的 ATP、NADH 和柠檬酸抑制这些酶，而 AMP、ADP 和 NAD⁺ 则激活它们，从而实现反馈调节以稳定代谢通量。
In tissues such as the heart, where oxygen supply is usually sufficient, fatty acids are the preferred fuel because they provide more ATP per carbon atom—albeit with a higher oxygen requirement. The brain, by contrast, relies almost exclusively on glucose aerobically oxidised, highlighting the tissue‑specific integration of respiratory pathways.
在心脏等通常氧气供应充足的组织中，脂肪酸是优先燃料，因为每个碳原子可产生更多 ATP——尽管需氧量更高。而大脑几乎完全依赖有氧氧化的葡萄糖，这突出了呼吸途径的组织特异性整合。
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