📚 Cellular Respiration: Processes and Energy Transformation | 细胞呼吸的过程与能量转化
Cellular respiration is a set of enzyme-controlled reactions that transfer chemical energy from glucose and other organic substrates into ATP. This process involves four main stages: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. Each stage is important for understanding how cells convert stored chemical energy into a usable form.
细胞呼吸是一系列由酶控制的反应，将葡萄糖及其他有机底物中的化学能转化为 ATP。此过程包括四大阶段：糖酵解、连接反应、克雷布斯循环（柠檬酸循环）和氧化磷酸化。每个阶段对理解细胞如何将储存的化学能转化为可用形式都至关重要。
1. Energy Transformation and Redox Principles | 能量转化与氧化还原原理
Energy released during respiration comes from the stepwise oxidation of organic molecules. Each step is catalysed by a specific enzyme, allowing energy to be released in small, manageable packets rather than in one destructive burst.
呼吸作用释放的能量来自有机分子的逐步氧化。每一步由特定酶催化，使能量以微小而可控的“包”逐步释放，而不是一次性爆发释放。
Coenzymes NAD and FAD accept electrons and hydrogen atoms, becoming reduced. NAD is reduced to NADH, and FAD is reduced to FADH₂. These reduced coenzymes later donate electrons to the electron transport chain, where the final ATP yield is achieved.
辅酶 NAD 和 FAD 接受电子和氢原子而被还原。NAD 被还原为 NADH，FAD 被还原为 FADH₂。这些还原型辅酶随后将电子传递给电子传递链，从而最终完成 ATP 的生成。
NAD⁺ + H⁺ + 2e⁻ ⇌ NADH
FAD + 2H ⇌ FADH₂
2. Glycolysis | 糖酵解
Glycolysis occurs in the cytoplasm and does not require oxygen. One six-carbon glucose molecule is converted into two three-carbon pyruvate molecules. It is the first stage of respiration and is common to both aerobic and anaerobic pathways.
糖酵解发生在细胞质中，无需氧气。一分子六碳葡萄糖被转化为两分子三碳丙酮酸。这是呼吸作用的第一阶段，也是有氧呼吸和无氧呼吸共有的途径。
There are four main stages within glycolysis: phosphorylation, lysis, oxidation, and ATP formation.
糖酵解内部主要包括四个阶段：磷酸化、裂解、氧化和 ATP 生成。
First, glucose is phosphorylated by two ATP molecules. This makes the molecule more reactive and prevents it from diffusing out of the cell.
首先，葡萄糖被两分子 ATP 磷酸化。这使分子更容易发生反应，并防止其扩散出细胞。
Second, the six-carbon phosphate ester splits into two three-carbon molecules called triose phosphate. Third, hydrogen is removed from triose phosphate by NAD, forming two NADH. Fourth, four ATP are produced by substrate-level phosphorylation, giving a net gain of two ATP per glucose.
其次，六碳磷酸酯裂解为两个三碳分子，称为丙糖磷酸（三碳糖磷酸）。第三，NAD 从丙糖磷酸脱氢，形成两分子 NADH。第四，通过底物水平磷酸化生成四分子 ATP，每分子葡萄糖净得两分子 ATP。
C₆H₁₂O₆ + 2NAD⁺ + 2ADP + 2Pi → 2CH₃COCOOH + 2NADH + 2H⁺ + 2ATP + 2H₂O
3. Link Reaction | 连接反应
In aerobic cells, pyruvate moves from the cytoplasm into the mitochondrial matrix. Once inside, it undergoes oxidative decarboxylation in the presence of coenzyme A.
在有氧细胞中，丙酮酸从细胞质进入线粒体基质。进入基质后，丙酮酸在辅酶 A 的存在下发生氧化脱羧。
In this reaction, one carbon is removed as carbon dioxide, and NAD is reduced to NADH. The remaining two-carbon acetyl group combines with coenzyme A to form acetyl coenzyme A, usually shortened to acetyl CoA.
在此反应中，一个碳以二氧化碳形式被去除，同时 NAD 被还原为 NADH。剩余的二碳乙酰基与辅酶 A 结合形成乙酰辅酶 A，常缩写为 acetyl CoA。
Pyruvate + NAD⁺ + CoA → Acetyl CoA + CO₂ + NADH + H⁺
For each glucose molecule, two pyruvate molecules are formed in glycolysis, so the link reaction occurs twice. This produces two acetyl CoA, two CO₂ and two NADH per glucose.
每分子葡萄糖在糖酵解中形成两分子丙酮酸，因此连接反应进行两轮。每分子葡萄糖由此产生两分子乙酰辅酶 A、两分子 CO₂ 和两分子 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 donates its two-carbon acetyl group to a four-carbon molecule called oxaloacetate, producing a six-carbon molecule called citrate.
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 将其二碳乙酰基交给四碳的草酰乙酸，生成六碳的柠檬酸。
Citrate is then gradually broken down through a series of enzyme-catalysed steps. Two carbons are released as carbon dioxide by decarboxylation, while hydrogen atoms are transferred to NAD and FAD by dehydrogenation. At the end of the cycle, oxaloacetate is regenerated and can accept another acetyl group.
柠檬酸随后通过一系列酶催化步骤逐步被分解。其中两个碳以二氧化碳形式通过脱羧反应释放，同时氢原子通过脱氢反应转移给 NAD 和 FAD。循环结束时草酰乙酸得以再生，并可接受下一个乙酰基。
Per turn of the cycle, one ATP is made by substrate-level phosphorylation, three NADH and one FADH₂ are produced. Since the cycle turns twice per glucose molecule, the totals are two ATP, six NADH and two FADH₂.
每轮循环通过底物水平磷酸化生成一分子 ATP，同时产生三分子 NADH 和一分子 FADH₂。由于每分子葡萄糖使循环进行两轮，因此总量为两分子 ATP、六分子 NADH 和两分子 FADH₂。
5. Electron Transport Chain | 电子传递链
The electron transport chain is located on the inner mitochondrial membrane. It consists of a series of carrier molecules, each capable of accepting and donating electrons in sequence.
电子传递链位于线粒体内膜上。它由一系列载体分子组成，每个载体分子都能依次接受和传递电子。
NADH and FADH₂ donate electrons to the chain. As electrons pass from one carrier to the next, energy is released. This energy is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space.
NADH 和 FADH₂ 向电子传递链提供电子。当电子从一个载体传递到下一个载体时，能量被释放出来。这些能量被用于将质子（H⁺）从线粒体基质泵入膜间隙。
Molecular oxygen acts as the final electron acceptor. It combines with electrons and protons to form water. This is why oxygen is essential for aerobic respiration; if oxygen is absent, the electron transport chain cannot proceed.
分子氧作为最终电子受体，与电子和质子结合形成水。这就是氧气对有氧呼吸至关重要的原因；如果缺氧，电子传递链将无法继续进行。
½O₂ + 2H⁺ + 2e⁻ → H₂O
6. Chemiosmosis and Oxidative Phosphorylation | 化学渗透与氧化磷酸化
The pumping of protons creates a high proton concentration in the intermembrane space, producing an electrochemical gradient across the inner mitochondrial membrane. This gradient represents stored energy.
质子泵出使膜间隙中质子浓度升高，在线粒体内膜两侧形成电化学梯度。该梯度代表了储存的能量。
Protons flow back into the matrix through ATP synthase, an enzyme complex embedded in the inner mitochondrial membrane. This flow of protons drives the conversion of ADP and inorganic phosphate into ATP. Because this ATP synthesis is coupled to electron transport and requires oxygen, it is called oxidative phosphorylation.
质子通过内膜上的酶复合物 ATP 合酶回流到基质中。这种质子流驱动 ADP 与无机磷酸结合生成 ATP。由于这种 ATP 合成与电子传递相偶联且需要氧气，因此被称为氧化磷酸化。
7. Aerobic Energy Yield | 有氧呼吸的产能统计
The total ATP yield from one glucose molecule depends on how the reduced coenzymes are counted. In traditional calculations, one NADH produces three ATP and one FADH₂ produces two ATP. More modern estimates give lower values, typically between 30 and 32 ATP, because of proton leakage and the cost of transporting molecules into mitochondria.
一分子葡萄糖的 ATP 总产量取决于还原型辅酶的折算方式。在传统计算中，一分子 NADH 产生三分子 ATP，一分子 FADH₂ 产生两分子 ATP。更现代的估算值通常较低，约为 30 至 32 分子 ATP，这是因为存在质子泄漏以及将分子运入线粒体的耗能。
| Stage | 阶段 | Direct ATP | 直接ATP | Reduced coenzymes | 还原型辅酶 | Traditional total ATP | 传统ATP总量 |
|---|---|---|---|
| Glycolysis | 糖酵解 | 2 ATP | 2 NADH | 2 + (2 × 3) = 8 ATP |
| Link reaction | 连接反应 | 0 ATP | 2 NADH | 2 × 3 = 6 ATP |
| Krebs cycle | 克雷布斯循环 | 2 ATP | 6 NADH + 2 FADH₂ | 2 + (6 × 3) + (2 × 2) = 24 ATP |
| Total | 总计 | 4 ATP | 10 NADH + 2 FADH₂ | 38 ATP |
The overall equation for aerobic respiration is therefore:
因此，有氧呼吸的总方程式为：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (approximately 30–38 ATP)
8. Anaerobic Respiration in Mammals | 哺乳动物的无氧呼吸
When oxygen is limiting, some animal cells can still carry out glycolysis only if NADH is recycled back to NAD⁺. In mammals, pyruvate is reduced to lactate by the enzyme lactate dehydrogenase.
当氧气供应受限时，一些动物细胞若要继续进行糖酵解，就必须将 NADH 重新转化为 NAD⁺。在哺乳动物中，丙酮酸在乳酸脱氢酶的作用下被还原为乳酸。
CH₃COCOOH + NADH + H⁺ ⇌ CH₃CH(OH)COOH + NAD⁺
This reaction occurs in the cytoplasm and does not produce additional ATP. Its main purpose is to regenerate NAD⁺ so that glycolysis can continue for a short period. Lactate is carried away in the blood and converted back to pyruvate in the liver when oxygen becomes available.
该反应发生在细胞质中，不额外产生 ATP。其主要目的是再生 NAD⁺，使糖酵解能在短时间内继续运作。乳酸进入血液，当氧气重新充足时，在肝脏中被
Published by TutorHao | A-Level Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导