📚 Energy and Respiration | 能量与呼吸作用
All living organisms require a continuous supply of energy to drive processes such as active transport, muscle contraction, nerve impulse transmission, biosynthesis, and cell division. This energy is obtained from the chemical potential energy stored in organic molecules, primarily glucose, through a series of enzyme-catalysed reactions known as respiration. Respiration can occur aerobically, using oxygen to completely oxidise substrates, or anaerobically when oxygen is limited or absent. The energy released is captured in high-energy molecules, principally adenosine triphosphate (ATP), which serves as the universal energy currency linking energy-releasing and energy-consuming reactions within the cell.
所有生物体都需要持续的能量供应来驱动主动运输、肌肉收缩、神经冲动传递、生物合成以及细胞分裂等过程。这些能量来源于有机分子（主要是葡萄糖）中储存的化学势能，通过一系列酶促反应——呼吸作用——释放。呼吸作用既可以在有氧条件下将底物彻底氧化，也可以在氧气不足或无氧的条件下进行。释放出来的能量被捕获到高能分子中，主要是腺苷三磷酸（ATP），它作为通用的能量货币，将细胞内的放能反应与需能反应耦合起来。
1. The Need for Energy in Living Organisms | 生物体对能量的需求
Living cells perform a wide variety of energy‑dependent activities. Metabolic reactions, especially anabolic pathways such as protein synthesis, DNA replication, and polysaccharide formation, require an input of free energy. Movement at the cellular level, including chromosome separation during mitosis, beating of cilia and flagella, and contraction of muscle fibres, depends on ATP hydrolysis. The active transport of ions and molecules across cell membranes against their concentration gradients is another major consumer of ATP. Finally, the maintenance of body temperature in endotherms, bioluminescence, and electrical discharge in some organisms also rely on energy released by respiration. Without a steady supply of ATP, these processes would cease, and the cell would lose its ability to maintain homeostasis.
活细胞进行着多种多样的耗能活动。代谢反应，尤其是蛋白质合成、DNA复制和多糖形成等合成代谢途径，需要自由能的输入。细胞层面的运动，如染色体在有丝分裂时的分离、纤毛和鞭毛的摆动以及肌纤维的收缩，都依赖ATP的水解。离子和分子逆浓度梯度跨膜进行的主动运输是另一个消耗ATP的主要过程。此外，恒温动物维持体温、生物发光以及某些生物释放电脉冲也都依赖于呼吸作用释放的能量。如果没有稳定的ATP供应，这些过程会立即停止，细胞将丧失维持稳态的能力。
2. ATP: The Universal Energy Currency | ATP：通用的能量货币
Adenosine triphosphate (ATP) is a nucleotide consisting of the nitrogenous base adenine, the pentose sugar ribose, and a chain of three phosphate groups. The bonds between the phosphate groups, particularly the terminal phosphoanhydride bond, are considered high‑energy bonds because their hydrolysis releases a relatively large amount of free energy. The enzyme‑catalysed hydrolysis of ATP to adenosine diphosphate (ADP) and inorganic phosphate (Pi) releases about 30.5 kJ mol⁻¹ under standard conditions, and even more inside the cell. This energy is directly coupled to endergonic reactions or conformational changes in proteins. ATP is synthesised from ADP and Pi during phosphorylation reactions, which can be substrate‑level phosphorylation, as seen in glycolysis and the Krebs cycle, or oxidative phosphorylation, which occurs in the inner mitochondrial membrane. Because ATP is continuously recycled, a cell may turn over its entire ATP pool several thousand times a day.
腺苷三磷酸（ATP）是一种核苷酸，由含氮碱基腺嘌呤、戊糖核糖以及一条由三个磷酸基团组成的链组成。磷酸基团之间的键，尤其是末端的磷酸酐键，被称为高能键，因为其水解释放的自由能相对较多。在标准条件下，酶催化ATP水解为腺苷二磷酸（ADP）和无机磷酸（Pi）的过程释放约30.5 kJ mol⁻¹，在细胞内实际值更高。这些能量直接与吸能反应或蛋白质的构象变化相偶联。ATP在磷酸化反应中由ADP和Pi合成，这些反应可以是底物水平磷酸化（如糖酵解和克雷布斯循环中），也可以是发生在线粒体内膜上的氧化磷酸化。由于ATP在体内被不断循环利用，一个细胞每天能将其全部ATP库更新数千次。
3. Overview of Respiration | 呼吸作用概述
Respiration is the process by which cells harvest chemical energy from organic fuel molecules. Aerobic respiration uses oxygen as the final electron acceptor and fully oxidises glucose to carbon dioxide and water, releasing a large amount of energy. The overall balanced equation is:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (approx. 2880 kJ mol⁻¹)
This multistep pathway can be divided into four main stages: glycolysis in the cytoplasm, the link reaction and Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation across the inner mitochondrial membrane. During the first three stages, electrons are removed from substrates and transferred to the coenzymes NAD⁺ and FAD, forming reduced NADH and FADH₂. These reduced coenzymes then donate electrons to the electron transport chain, where the energy is used to create a proton gradient that drives ATP synthesis. Anaerobic respiration follows a shorter pathway, regenerating NAD⁺ without involving the electron transport chain, and yields much less ATP.
呼吸作用是细胞从有机燃料分子中获取化学能的过程。有氧呼吸以氧作为最终的电子受体，将葡萄糖彻底氧化为二氧化碳和水，同时释放大量能量。总反应式如上所示。这一多步途径可分为四个主要阶段：细胞质中的糖酵解、线粒体基质中的连接反应与克雷布斯循环，以及线粒体内膜上的氧化磷酸化。在前三个阶段中，电子从底物中脱除，传递给辅酶NAD⁺和FAD，生成还原型的NADH和FADH₂。这些还原辅酶随后将电子传递给电子传递链，利用所释放的能量建立起质子梯度，从而驱动ATP合成。无氧呼吸则走一条较短的路径，在不依赖电子传递链的条件下再生NAD⁺，产生的ATP要少得多。
4. Glycolysis | 糖酵解
Glycolysis occurs in the cytoplasm and is the first stage of both aerobic and anaerobic respiration. It involves the splitting of one molecule of glucose (a 6‑carbon sugar) into two molecules of pyruvate (3‑carbon each). The pathway consists of ten enzyme‑catalysed reactions divided into an energy‑investment phase, where two molecules of ATP are consumed, and an energy‑payoff phase, where four molecules of ATP and two molecules of NADH are produced. Overall, glycolysis yields a net gain of two ATP molecules per glucose by substrate‑level phosphorylation and two reduced NADH molecules:
Glucose + 2NAD⁺ + 2ADP + 2Pi → 2 pyruvate + 2NADH + 2H⁺ + 2ATP + 2H₂O
Glycolysis does not require oxygen and can proceed under anaerobic conditions. However, the NADH produced must be re‑oxidised to NAD⁺ for glycolysis to continue, which is achieved either by the electron transport chain in aerobic respiration or by fermentation pathways under anaerobic conditions. Key regulatory enzymes in glycolysis, such as phosphofructokinase, ensure that the rate of glucose breakdown matches the cell’s energy needs.
糖酵解发生在细胞质中，是有氧呼吸和无氧呼吸共同的第一阶段。它将一分子葡萄糖（六碳糖）裂解为两分子丙酮酸（各三碳）。该途径包含十步酶促反应，分为消耗两分子ATP的投资阶段和生成四分子ATP以及两分子NADH的回报阶段。总体而言，糖酵解通过底物水平磷酸化每分子葡萄糖净产两分子ATP，并生成两分子还原型NADH。糖酵解不需要氧气，可以在无氧条件下进行。然而，生成的NADH必须被重新氧化为NAD⁺才能使糖酵解继续进行，这在有氧呼吸中经由电子传递链实现，在无氧条件下则通过发酵途径完成。糖酵解中的关键调节酶（如磷酸果糖激酶）可确保葡萄糖分解速率与细胞的能量需求相匹配。
5. Link Reaction | 连接反应
When oxygen is available, the pyruvate molecules produced in glycolysis are actively transported into the mitochondrial matrix. There, each pyruvate undergoes oxidative decarboxylation catalysed by the multi‑enzyme pyruvate dehydrogenase complex. In this irreversible reaction, the carboxyl group of pyruvate is removed as carbon dioxide, and the remaining 2‑carbon acetyl group is transferred to coenzyme A (CoA), forming acetyl coenzyme A (acetyl CoA). At the same time, NAD⁺ is reduced to NADH + H⁺. The overall reaction for one pyruvate is:
Pyruvate + CoA + NAD⁺ → acetyl CoA + CO₂ + NADH + H⁺
Since two pyruvate molecules are produced per glucose, the link reaction occurs twice, yielding two molecules of acetyl CoA, two CO₂, and two NADH. Acetyl CoA now enters the Krebs cycle, while the carbon dioxide diffuses out of the mitochondria and is eventually exhaled. No ATP is produced directly in this step, but the generation of NADH is crucial for the subsequent oxidative phosphorylation.
当氧气充足时，糖酵解产生的丙酮酸分子被主动运输进入线粒体基质。在那里，每个丙酮酸在丙酮酸脱氢酶复合体的催化下经历氧化脱羧反应。在此不可逆反应中，丙酮酸的羧基以二氧化碳的形式脱去，剩下的二碳乙酰基转移到辅酶A（CoA）上，形成乙酰辅酶A。与此同时，NAD⁺被还原为NADH + H⁺。由于每分子葡萄糖产生两分子丙酮酸，连接反应发生两次，共生成两分子乙酰辅酶A、两分子CO₂和两分子NADH。此时乙酰辅酶A进入克雷布斯循环，二氧化碳则扩散出线粒体并最终被呼出。该步骤不直接生成ATP，但产生的NADH对后续的氧化磷酸化至关重要。
6. Krebs Cycle | 克雷布斯循环
The Krebs cycle (also called the citric acid cycle) takes place in the mitochondrial matrix and completes the oxidation of acetyl groups. Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). Through a series of decarboxylation, dehydrogenation, and rearrangement reactions, citrate is gradually converted back into oxaloacetate, closing the cycle. During one turn of the cycle, the following products are generated per acetyl CoA:
- Two molecules of CO₂ are released (decarboxylation steps).
- Three molecules of NAD⁺ are reduced to NADH + H⁺.
- One molecule of FAD is reduced to FADH₂.
- One molecule of ATP (or GTP) is produced by substrate‑level phosphorylation.
克雷布斯循环（又称柠檬酸循环）发生在线粒体基质中，完成乙酰基的氧化。乙酰辅酶A（2C）与草酰乙酸（4C）结合生成柠檬酸（6C）。经过一系列脱羧、脱氢和重排反应，柠檬酸逐步转变回草酰乙酸，使循环闭合。每个乙酰辅酶A在循环一圈中生成以下产物：释放两分子CO₂；将三分子NAD⁺还原为NADH + H⁺；将一分子FAD还原为FADH₂；并通过底物水平磷酸化产生一分子ATP（或GTP）。
Because each glucose gives rise to two acetyl CoA molecules, the cycle turns twice per glucose oxidised. In total, the Krebs cycle produces four CO₂, six NADH, two FADH₂, and two ATP per glucose. The CO₂ is a waste product, while the reduced coenzymes now carry high‑energy electrons to the inner mitochondrial membrane for the final stage of respiration.
由于每个葡萄糖生成两个乙酰辅酶A，循环每氧化一分子葡萄糖运转两次。克雷布斯循环总共生成四分子CO₂、六分子NADH、两分子FADH₂和两分子ATP。CO₂是废物，而还原辅酶则携带高能电子进入线粒体内膜，参与呼吸作用的最后阶段。
7. Oxidative Phosphorylation and Chemiosmosis | 氧化磷酸化与化学渗透
Oxidative phosphorylation occurs across the inner mitochondrial membrane and couples the energy released from the transfer of electrons through a series of protein complexes to the synthesis of ATP. The electron transport chain (ETC) consists of four large enzyme complexes (I–IV) and mobile electron carriers (ubiquinone and cytochrome c). NADH donates electrons to Complex I, while FADH₂ donates electrons to Complex II. Electrons then flow through the chain to Complex IV, where they are finally transferred to molecular oxygen, the terminal electron acceptor, reducing it to water:
½O₂ + 2H⁺ + 2e⁻ → H₂O
As electrons move through Complexes I, III, and IV, energy is released and used to pump protons (H⁺) from the matrix into the intermembrane space, creating an electrochemical proton gradient. This gradient generates a proton motive force (PMF) that drives protons back into the matrix through the enzyme ATP synthase. The flow of protons through ATP synthase causes conformational changes that catalyse the phosphorylation of ADP to ATP. This mechanism, known as chemiosmosis, was proposed by Peter Mitchell. The number of ATP molecules produced per reduced coenzyme is not fixed, but traditionally it is estimated that each NADH yields approximately 3 ATP and each FADH₂ yields approximately 2 ATP, although actual values may be closer to 2.5 and 1.5 depending on the shuttle system used by the cell.
氧化磷酸化发生在线粒体内膜上，将电子沿一系列蛋白质复合体传递所释放的能量与ATP的合成相偶联。电子传递链（ETC）由四个巨大的酶复合体（I–IV）以及流动电子载体（泛醌和细胞色素c）组成。NADH将电子传递给复合体I，FADH₂将电子传递给复合体II。电子随后沿链传递至复合体IV，最终传递给末端电子受体分子氧，将其还原为水。当电子经过复合体I、III和IV时，释放的能量被用来将质子（H⁺）从基质泵入膜间腔，建立起电化学质子梯度。该梯度产生质子驱动力（PMF），驱动质子通过ATP合酶流回基质。质子流经ATP合酶引发构象变化，催化ADP磷酸化为ATP。这一机制被称为化学渗透，由彼得·米切尔提出。每个还原辅酶生成的ATP分子数并不固定，传统估算认为每个NADH约生成3个ATP，每个FADH₂约生成2个ATP，但根据细胞所使用的穿梭系统，实际值可能更接近2.5和1.5。
8. Anaerobic Respiration | 无氧呼吸
When oxygen is absent or in short supply, cells cannot pass electrons through the ETC, and NADH is not re‑oxidised by oxygen. To keep glycolysis running and generate a small amount of ATP, cells must regenerate NAD⁺ by alternative pathways. In mammals and some bacteria, pyruvate is reduced directly to lactate by lactate dehydrogenase, oxidising NADH back to NAD⁺:
Pyruvate + NADH + H⁺ → lactate + NAD⁺
This process, called lactic acid fermentation, yields only 2 ATP per glucose and no CO₂. In yeast and some plants, pyruvate is first decarboxylated to ethanal (by pyruvate decarboxylase) and then reduced to ethanol by alcohol dehydrogenase, oxidising NADH in the process:
Pyruvate → ethanal + CO₂; Ethanal + NADH + H⁺ → ethanol + NAD⁺
Ethanol fermentation also produces 2 ATP and releases CO₂. Both pathways serve the sole purpose of regenerating NAD⁺, allowing glycolysis to persist under anaerobic conditions. However, the overall ATP yield is far lower than in aerobic respiration, and the end products (lactate or ethanol) can be toxic if accumulated.
当氧气缺乏或不足时，细胞无法将电子传递给电子传递链，NADH也不能被氧气重新氧化。为使糖酵解继续并生成少量ATP，细胞必须通过替代途径再生NAD⁺。在哺乳动物和某些细菌中，丙酮酸被乳酸脱氢酶直接还原为乳酸，同时将NADH氧化回NAD⁺。该过程称为乳酸发酵，每分子葡萄糖仅产生2个ATP，不释放CO₂。在酵母和某些植物中，丙酮酸先被丙酮酸脱羧酶脱羧生成乙醛，随后乙醛被乙醇脱氢酶还原为乙醇，并在此过程中氧化NADH。酒精发酵同样产生2分子ATP并释放CO₂。两条途径的唯一目的是再生NAD⁺，使无氧条件下糖酵解得以维持。然而，其总ATP产率远低于有氧呼吸，且终产物（乳酸或乙醇）若积累会产生毒性。
9. Respiratory Substrates and Respiratory Quotient | 呼吸底物与呼吸商
Although glucose is the most commonly used respiratory substrate, cells can also oxidise lipids (especially triglycerides) and amino acids for energy. Triglycerides are first hydrolysed to glycerol and fatty acids. Glycerol enters glycolysis, while fatty acids undergo β‑oxidation in the mitochondrial matrix to produce acetyl CoA, along with large quantities of NADH and FADH₂. Because fatty acids are more reduced than carbohydrates, they release more energy per gram. The respiratory quotient (RQ) is a useful indicator of the type of substrate being respired:
RQ = volume of CO₂ produced / volume of O₂ consumed (over the same time period)
r lipids, RQ is typically around 0.7 because fats have a lower proportion of oxygen to carbon and require more O₂ for complete oxidation. For proteins, RQ is approximately 0.8– id composition. Measuring RQ provides insights into metabolic status and fuel utilisation.
虽然葡萄糖是最常被使用的呼吸底物，但细胞也可以氧化脂质（尤其是甘油三酯）和氨基酸来获取能量。甘油三酯首先水解为甘油和脂肪酸。甘油进入糖酵解途径，而脂肪酸在线粒体基质中经β-氧化生成乙酰辅酶A，并伴随大量NADH和FADH₂的产生。由于脂肪酸比碳水化合物的还原度更高，它们每克释放的能量也更多。呼吸商（RQ）是判断被呼吸利用的底物类型的有用指标。碳水化合物的RQ = 1.0；脂质的RQ通常约为0.7，因其氧碳比例较低，完全氧化需消耗更多O₂；蛋白质的RQ约为0.8–0.9，因氨基酸组成而异。测定RQ有助于了解代谢状态与燃料利用情况。
10. Comparison of Energy Yields | 能量产率比较
The total ATP yield from the complete aerobic oxidation of one molecule of glucose is the sum of ATP produced by substrate‑level phosphorylation and by oxidative phosphorylation. In prokaryotic cells, which lack mitochondria and perform the entire process in the cytoplasm, the theoretical maximum can reach 38 ATP per glucose:
| Stage | Direct ATP (substrate-level) | Reduced coenzymes | Estimated ATP from oxidative phosphorylation |
|---|---|---|---|
| Glycolysis | 2 | 2 NADH | 6 (or 4–5 in eukaryotes) |
| Link reaction (×2) | 0 | 2 NADH | 6 |
| Krebs cycle (×2) | 2 | 6 NADH, 2 FADH₂ | 18 (from NADH) + 4 (from FADH₂) = 22 |
| Total (prokaryote) | 4 | 34 | |
| Grand total | 38 ATP per glucose (prokaryote); ~36 ATP in eukaryotes (after subtracting 2 ATP used for NADH shuttle) | ||
In eukaryotic cells,
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