📚 Cellular Respiration: Energy Transfer in Cells | 细胞呼吸：细胞中的能量转移
Respiration is the set of enzyme-controlled metabolic reactions by which cells release chemical energy from organic fuel molecules such as glucose and transfer that energy into the terminal phosphate bonds of ATP. It is not the same as breathing; in biology, breathing is ventilation, while cellular respiration is an intracellular process that occurs in every living cell.
呼吸作用是一系列由酶控制的代谢反应，细胞通过这些反应从葡萄糖等有机燃料分子中释放化学能，并将能量转移到 ATP 的末端磷酸键中。它不同于日常所说的呼吸；在生物学中，呼吸指通气，而细胞呼吸是发生在每个活细胞内的过程。
Aerobic respiration requires oxygen and achieves the complete oxidation of glucose to carbon dioxide and water, releasing a large amount of ATP. Anaerobic respiration occurs without oxygen, partially breaks down glucose, and produces lactate or ethanol while regenerating NAD⁺. Both pathways begin with glycolysis in the cytoplasm.
有氧呼吸需要氧气，将葡萄糖完全氧化为二氧化碳和水，释放大量 ATP。无氧呼吸在无氧条件下进行，葡萄糖仅被部分分解，产生乳酸或乙醇，同时再生 NAD⁺。两条途径都始于细胞质中的糖酵解。
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
1. Overview of Respiration and ATP | 呼吸作用与 ATP 概述
ATP is the universal energy currency of cells. When ATP is hydrolysed to ADP and inorganic phosphate (Pᵢ), the terminal phosphate bond is broken and about 30.5 kJ mol⁻¹ of energy is released for cellular work such as active transport, muscle contraction, and biosynthesis.
ATP 是细胞的通用能量货币。当 ATP 水解为 ADP 和无机磷酸（Pᵢ）时，末端磷酸键断裂，释放约 30.5 kJ mol⁻¹ 的能量，用于主动运输、肌肉收缩和生物合成等细胞活动。
ATP → ADP + Pᵢ + energy
Respiration involves four main stages in aerobic eukaryotes: glycolysis in the cytoplasm, the link reaction and Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation on the inner mitochondrial membrane. Reduced coenzymes such as NADH and FADH₂ carry electrons and protons to the electron transport chain.
真核生物的有氧呼吸包括四个主要阶段：细胞质中的糖酵解、线粒体基质中的连接反应和克雷布斯循环，以及线粒体内膜上的氧化磷酸化。还原型辅酶如 NADH 和 FADH₂ 将电子和质子传递到电子传递链。
2. Glycolysis | 糖酵解
Glycolysis takes place in the cytoplasm and does not require oxygen. It is the first stage of both aerobic and anaerobic respiration. Glucose, a six-carbon sugar, is phosphorylated by two ATP molecules to form hexose bisphosphate, which is then split into two molecules of triose phosphate.
糖酵解发生在细胞质中，不需要氧气。它是有氧呼吸和无氧呼吸的第一阶段。葡萄糖是一种六碳糖，被两分子 ATP 磷酸化形成己糖二磷酸，随后裂解为两分子磷酸丙糖。
Each triose phosphate is oxidised by the removal of hydrogen, which reduces NAD⁺ to NADH. Substrate-level phosphorylation then transfers phosphate groups to ADP, producing ATP. At the end of glycolysis, each glucose molecule has yielded a net gain of 2 ATP, 2 NADH, and 2 molecules of pyruvate.
每分子磷酸丙糖通过脱氢被氧化，使 NAD⁺ 还原为 NADH。随后底物水平磷酸化将磷酸基团转移给 ADP，生成 ATP。糖酵解结束时，每分子葡萄糖净生成 2 ATP、2 NADH 和 2 分子丙酮酸。
3. Link Reaction | 连接反应
The link reaction occurs in the mitochondrial matrix. Pyruvate produced by glycolysis is actively transported across the inner mitochondrial membrane. Each pyruvate loses a carbon dioxide molecule by decarboxylation and is oxidised by the removal of hydrogen, which reduces NAD⁺ to NADH.
连接反应发生在线粒体基质中。糖酵解产生的丙酮酸通过线粒体内膜被主动运输进入基质。每分子丙酮酸经脱羧作用失去一分子二氧化碳，并通过脱氢氧化，使 NAD⁺ 还原为 NADH。
The remaining two-carbon acetyl group combines with coenzyme A to form acetyl-CoA. Since one glucose produces two pyruvate molecules, the link reaction occurs twice per glucose, yielding 2 acetyl-CoA, 2 CO₂, and 2 NADH.
剩余的二碳乙酰基与辅酶 A 结合形成乙酰辅酶 A。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，生成 2 乙酰辅酶 A、2 CO₂ 和 2 NADH。
4. Krebs Cycle | 克雷布斯循环
The Krebs cycle, also called the citric acid cycle, takes place in the mitochondrial matrix. Acetyl-CoA enters the cycle by donating its two-carbon acetyl group to oxaloacetate, a four-carbon compound, producing citrate, a six-carbon compound.
克雷布斯循环又称柠檬酸循环，发生在线粒体基质中。乙酰辅酶 A 进入循环，将其二碳乙酰基提供给四碳化合物草酰乙酸，生成六碳化合物柠檬酸。
Citrate is then decarboxylated and dehydrogenated in a series of enzyme-controlled reactions. Two molecules of CO₂ are released per turn, and hydrogen atoms are transferred to NAD⁺ and FAD, forming NADH and FADH₂. One ATP is produced directly by substrate-level phosphorylation, and oxaloacetate is regenerated.
柠檬酸随后在一系列酶控反应中脱羧和脱氢。每轮循环释放两分子 CO₂，氢原子传递给 NAD⁺ 和 FAD，形成 NADH 和 FADH₂。通过底物水平磷酸化直接生成一分子 ATP，同时草酰乙酸得到再生。
Because each glucose molecule yields two acetyl-CoA molecules, the Krebs cycle turns twice per glucose. The total products from the Krebs cycle per glucose are 4 CO₂, 2 ATP, 6 NADH, and 2 FADH₂.
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运行两轮。每分子葡萄糖在克雷布斯循环中的总产物为 4 CO₂、2 ATP、6 NADH 和 2 FADH₂。
5. Oxidative Phosphorylation: Electron Transport Chain | 氧化磷酸化：电子传递链
Oxidative phosphorylation occurs on the inner mitochondrial membrane, which is folded into cristae. The electron transport chain is a series of protein complexes and mobile electron carriers embedded in this membrane. Reduced NAD and reduced FAD donate electrons to the chain.
氧化磷酸化发生在线粒体内膜上，内膜折叠形成嵴。电子传递链是一系列嵌入该膜的蛋白质复合物和流动电子载体。还原型 NAD 和还原型 FAD 将电子提供给电子传递链。
As electrons flow down the chain through carriers with successively higher electronegativity, energy is released. This energy is used to pump protons from the mitochondrial matrix into the intermembrane space, creating a proton gradient. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water.
电子沿传递链流经电负性依次升高的载体时释放能量。该能量用于将质子从线粒体基质泵入膜间隙，形成质子梯度。氧气作为最终电子受体，与电子和质子结合生成水。
6. Chemiosmosis and ATP Synthase | 化学渗透与 ATP 合酶
The proton gradient across the inner mitochondrial membrane stores potential energy. Protons cannot diffuse freely through the lipid bilayer, so they return to the matrix through ATP synthase, a membrane protein that acts as a channel and an enzyme.
线粒体内膜两侧的质子梯度储存了势能。质子无法自由穿过脂质双分子层，因此必须通过 ATP 合酶返回基质，ATP 合酶是一种既作为通道又作为酶的膜蛋白。
This flow of protons down their electrochemical gradient is called chemiosmosis. As protons pass through ATP synthase, the enzyme rotates and catalyses the phosphorylation of ADP to ATP. Most of the ATP produced during aerobic respiration is generated by this mechanism.
质子沿电化学梯度流动的过程称为化学渗透。当质子通过 ATP 合酶时，该酶发生旋转并催化 ADP 磷酸化为 ATP。有氧呼吸产生的大部分 ATP 都是通过这一机制生成的。
The cristae greatly increase the surface area of the inner membrane, allowing many electron transport chains and ATP synthase complexes to function simultaneously. This explains why mitochondria in metabolically active cells, such as muscle and liver cells, have abundant cristae.
嵴大大增加了内膜的表面积，使大量电子传递链和 ATP 合酶复合物能够同时工作。这解释了为什么代谢活跃的细胞（如肌肉细胞和肝细胞）中线粒体具有丰富的嵴。
7. Anaerobic Respiration: Lactate and Ethanol Pathways | 无氧呼吸：乳酸途径与乙醇途径
When oxygen is unavailable, the electron transport chain cannot operate because oxygen is not present to accept electrons. Cells still require a supply of NAD⁺ for glycolysis to continue, so reduced NAD must be reoxidised by alternative pathways.
当没有氧气时，电子传递链无法运行，因为没有氧气作为电子受体。细胞仍需持续供应 NAD⁺ 以维持糖酵解，因此还原型 NAD 必须通过其他途径重新氧化。
In mammals, pyruvate is reduced to lactate by the enzyme lactate dehydrogenase. Reduced NAD donates its hydrogen to pyruvate, regenerating NAD⁺. This occurs in vigorously contracting muscle cells when oxygen supply is temporarily insufficient. Lactate can be oxidised back to pyruvate in the liver after oxygen becomes available.
在哺乳动物中，丙酮酸被乳酸脱氢酶还原为乳酸。还原型 NAD 将氢提供给丙酮酸，再生 NAD⁺。这一过程发生在氧气供应暂时不足的剧烈收缩肌肉细胞中。氧气恢复后，乳酸可在肝脏中被氧化回丙酮酸。
In yeast and some plant tissues, pyruvate is first decarboxylated to ethanal, releasing CO₂. Ethanal is then reduced to ethanol by alcohol dehydrogenase, using reduced NAD and regenerating NAD⁺. This pathway is exploited in brewing and baking.
在酵母和某些植物组织中，丙酮酸首先脱羧生成乙醛并释放 CO₂。乙醛随后被乙醇脱氢酶还原为乙醇，消耗还原型 NAD 并再生 NAD⁺。该途径被用于酿酒和烘焙。
| Feature | Lactate fermentation | 乳酸发酵 | Ethanol fermentation | 乙醇发酵 |
|---|---|---|
| Organism | Mammals, contracting muscle | Yeast, some plants |
| Products | Lactate | Ethanol + CO₂ |
| ATP yield per glucose | 2 ATP | 2 ATP |
Anaerobic respiration yields only the 2 ATP produced during glycolysis. This is much less than aerobic respiration, but it allows ATP production to continue when oxygen is scarce.
无氧呼吸只产生糖酵解过程中生成的 2 ATP。这远低于有氧呼吸，但它能在氧气稀缺时维持 ATP 的生成。
8. ATP Yield and Efficiency | ATP 产量与效率
The complete aerobic oxidation of one glucose molecule can be summarised by adding the products of glycolysis, the link reaction, and the Krebs cycle. Most ATP is not made directly; it is synthesised later by oxidative phosphorylation using the electrons carried by reduced NAD and reduced FAD.
一分子葡萄糖的完全有氧氧化可以通过糖酵解、连接反应和克雷布斯循环的产物相加来总结。大多数 ATP 不是直接生成的，而是在氧化磷酸化过程中利用还原型 NAD 和还原型 FAD 携带的电子合成的。
| Stage | 阶段 | Direct ATP | 直接 ATP | Reduced NAD | 还原型 NAD | Reduced FAD | 还原型 FAD | CO₂ |
|---|---|---|---|---|
| Glycolysis | 2 | 2 | 0 | 0 |
| Link reaction | 0 | 2 | 0 | 2 |
| Krebs cycle | 2 | 6 | 2 | 4 |
| Total per glucose | 4 | 10 | 2 | 6 |
If each reduced NAD leads to the synthesis of about 2.5 ATP and each reduced FAD about 1.5 ATP, oxidative phosphorylation produces roughly 28 ATP. Adding the 4 ATP made directly gives a net yield of about 32 ATP per glucose in many eukaryotic cells. Older sources may quote 36–38 ATP because they use different values for NADH transport and proton leakage.
如果每分子还原型 NAD 约合成 2.5 ATP，每分子还原型 FAD 约合成 1.5 ATP，则氧化磷酸化约产生 28 ATP。加上直接生成的 4 ATP，许多真核细胞中每分子葡萄糖的净产量约为 32 ATP。旧资料可能引用 36–38 ATP，因为它们对 NADH 转运和质子泄漏采用了不同的数值。
The efficiency of aerobic respiration is around 32% under standard conditions. This is calculated by comparing the energy stored as ATP with the total energy released by the complete oxidation of glucose, which is about 2870 kJ mol⁻¹.
在标准条件下，有氧呼吸的效率约为 32%。这是通过将储存在 ATP 中的能量与葡萄糖完全氧化释放的总能量（约 2870 kJ mol⁻¹）进行比较计算得出的。
9. Respiratory Substrates and Respiratory Quotient | 呼吸底物与呼吸商
Carbohydrates, lipids, and proteins can all be used as respiratory substrates. They enter respiration at different points: carbohydrates through glycolysis, fatty acids through beta-oxidation into acetyl-CoA, and amino acids after deamination into various intermediates of the Krebs cycle.
碳水化合物、脂质和蛋白质都可以作为呼吸底物。它们在不同的位置进入呼吸途径：碳水化合物通过糖酵解进入，脂肪酸通过 β-氧化形成乙酰辅酶 A，氨基酸脱氨后进入克雷布斯循环的各个中间产物。
The respiratory quotient, RQ, is the ratio of carbon dioxide produced to oxygen consumed over a given period. It provides information about the type of substrate being respired.
呼吸商 RQ 是一定时间内产生的二氧化碳与消耗的氧气的比值。它可以提供正在被氧化的底物类型的信息。
RQ = CO₂ produced ÷ O₂ consumed
| Substrate | 底物 | RQ | 呼吸商 | Example | 示例 |
|---|---|---|
| Carbohydrate | 1.0 | Glucose |
| Lipid | ~0.7 | Palmitic acid |
| Protein | ~0.9 | Amino acids |
Lipids have a lower RQ than carbohydrates because their molecules contain proportionally less oxygen and more hydrogen. Consequently, lipid oxidation consumes more oxygen relative to the carbon dioxide released. Lipids also release more energy per gram than carbohydrates, making them efficient long-term energy stores.
脂质的呼吸商低于碳水化合物，因为脂质分子含氧比例较低、含氢比例较高。因此，脂质氧化相对于释放的二氧化碳消耗更多氧气。脂质每克释放的能量也高于碳水化合物，使其成为高效的长期能量储存物质。
10. Investigating Respiration | 呼吸作用的实验研究
A respirometer is used to measure the rate of aerobic respiration by recording oxygen consumption. The organism is placed in a sealed chamber, and carbon dioxide produced is absorbed by potassium hydroxide solution. As oxygen is used up, the pressure inside the chamber decreases, causing a coloured liquid in the manometer to move.
呼吸计用于通过记录氧气消耗量来测量有氧呼吸速率。将生物体置于密封室
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