📚 Anaerobic Respiration: Processes and Products | 无氧呼吸过程及其产物
Anaerobic respiration is the process by which cells release energy from glucose in the absence of oxygen. Unlike aerobic respiration, which yields a large amount of ATP, anaerobic respiration produces far less ATP and results in characteristic end products such as lactate or ethanol and carbon dioxide.
无氧呼吸是细胞在无氧条件下从葡萄糖中释放能量的过程。与产生大量ATP的有氧呼吸不同，无氧呼吸产生的ATP要少得多，并生成乳酸或乙醇和二氧化碳等特征性终产物。
1. Overview of Cellular Respiration | 细胞呼吸概览
Cellular respiration encompasses all the biochemical pathways by which cells break down organic molecules—typically glucose—to release energy in the form of ATP. There are two main categories: aerobic respiration, which requires oxygen, and anaerobic respiration, which does not.
细胞呼吸包括细胞分解有机分子（通常是葡萄糖）以ATP形式释放能量的所有生化途径。主要分为两类：需要氧气的有氧呼吸和不需要氧气的无氧呼吸。
In aerobic respiration, glucose is completely oxidised to carbon dioxide and water, releasing a theoretical maximum of approximately 38 ATP molecules per glucose. In anaerobic respiration, however, glucose is only partially oxidised, and the end products still contain significant chemical energy.
在有氧呼吸中，葡萄糖被完全氧化为二氧化碳和水，每分子葡萄糖理论上可释放约38个ATP分子。然而，在无氧呼吸中，葡萄糖仅被部分氧化，终产物中仍含有大量化学能。
2. Where Does Anaerobic Respiration Occur? | 无氧呼吸发生在何处？
In eukaryotic cells, glycolysis occurs in the cytoplasm, and this is the only stage of anaerobic respiration that produces ATP. The subsequent steps—reduction of pyruvate—also take place in the cytoplasm, meaning that the entire anaerobic pathway is cytosolic.
在真核细胞中，糖酵解发生在细胞质基质中，这是无氧呼吸中唯一产生ATP的阶段。后续步骤——丙酮酸的还原——也发生在细胞质基质中，这意味着整个无氧呼吸途径都在细胞质中进行。
This distinction is important: while aerobic respiration involves the mitochondria (link reaction, Krebs cycle, oxidative phosphorylation), anaerobic respiration bypasses these organelles entirely. This explains why cells can produce ATP rapidly even when oxygen supply is limited.
这一区别很重要：有氧呼吸涉及线粒体（连接反应、克雷布斯循环、氧化磷酸化），而无氧呼吸完全绕过了这些细胞器。这就解释了为什么即使氧气供应受限，细胞仍能快速产生ATP。
3. Glycolysis: The Shared First Stage | 糖酵解：共同的第一阶段
Glycolysis is the metabolic pathway that breaks down one molecule of glucose (6-carbon) into two molecules of pyruvate (3-carbon). This pathway is identical in both aerobic and anaerobic respiration and consists of two main phases: the energy-investment phase and the energy-payoff phase.
糖酵解是将一分子葡萄糖（六碳）分解为两分子丙酮酸（三碳）的代谢途径。该途径在有氧呼吸和无氧呼吸中完全相同，包含两个主要阶段：能量投入阶段和能量偿还阶段。
In the energy-investment phase, two ATP molecules are hydrolysed to phosphorylate glucose and convert it to fructose-1,6-bisphosphate. In the energy-payoff phase, the six-carbon intermediate splits into two triose phosphates, each of which is oxidised, producing two NADH and four ATP molecules.
在能量投入阶段，两分子ATP被水解以磷酸化葡萄糖并将其转化为果糖-1,6-二磷酸。在能量偿还阶段，六碳中间物分裂为两个三碳磷酸丙糖，每个都被氧化，产生两个NADH和四个ATP分子。
Glucose + 2 NAD⁺ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O
The net yield from glycolysis is therefore 2 ATP per glucose molecule. The NADH produced must be regenerated to NAD⁺ for glycolysis to continue, and this is precisely what the next stages of anaerobic respiration accomplish.
因此，糖酵解的净产量是每分子葡萄糖产生2个ATP。产生的NADH必须被再生为NAD⁺才能使糖酵解继续，而这正是无氧呼吸后续阶段所要完成的。
4. The Need for NAD⁺ Regeneration | NAD⁺再生的必要性
NAD⁺ acts as a hydrogen carrier in glycolysis. During the oxidation of glyceraldehyde-3-phosphate, NAD⁺ is reduced to NADH. For glycolysis to continue producing ATP, the cell must maintain a steady supply of NAD⁺. Without NAD⁺ regeneration, glycolysis would cease within seconds.
NAD⁺在糖酵解中作为氢载体。在甘油醛-3-磷酸被氧化时，NAD⁺被还原为NADH。为使糖酵解持续产生ATP，细胞必须维持稳定的NAD⁺供应。如果没有NAD⁺再生，糖酵解将在数秒内停止。
Under anaerobic conditions, oxidative phosphorylation is unavailable because oxygen acts as the final electron acceptor in the electron transport chain. Therefore, the cell uses a different strategy: pyruvate is reduced to lactate (in animals) or ethanol and carbon dioxide (in yeast), and in doing so, NADH is oxidised back to NAD⁺.
在无氧条件下，氧化磷酸化无法进行，因为在电子传递链中氧气是最终电子受体。因此，细胞采用不同的策略：丙酮酸被还原为乳酸（动物）或乙醇和二氧化碳（酵母），在此过程中NADH被氧化回NAD⁺。
5. Lactate Fermentation in Animals | 动物中的乳酸发酵
In animal skeletal muscle cells, when oxygen supply cannot meet demand during intense exercise, pyruvate is converted to lactate. This reaction is catalysed by the enzyme lactate dehydrogenase (LDH), and it requires NADH, which is oxidised to NAD⁺.
在动物骨骼肌细胞中，当剧烈运动时氧气供应无法满足需求时，丙酮酸被转化为乳酸。该反应由乳酸脱氢酶（LDH）催化，需要NADH，NADH被氧化为NAD⁺。
Pyruvate + NADH + H⁺ ⇌ Lactate + NAD⁺
Lactate is a three-carbon molecule that retains approximately 85% of the energy originally present in glucose. It diffuses into the bloodstream and is transported to the liver, where it can be converted back to pyruvate via the Cori cycle, or oxidised by the heart and other tissues as a fuel source.
乳酸是一种三碳分子，保留了葡萄糖中原本约85%的能量。它扩散进入血液并被运送到肝脏，在那里通过科里循环转化回丙酮酸，或作为燃料源被心脏和其他组织氧化。
6. Ethanol Fermentation in Yeast | 酵母中的乙醇发酵
Yeast cells (Saccharomyces cerevisiae) perform a distinct type of anaerobic respiration called alcoholic fermentation. This process occurs in two steps, both catalysed by specific enzymes present in the cytoplasm.
酵母细胞（酿酒酵母）进行一种独特的无氧呼吸，称为酒精发酵。该过程分两步进行，每一步都由存在于细胞质中的特定酶催化。
First, pyruvate is decarboxylated by pyruvate decarboxylase, releasing carbon dioxide and producing acetaldehyde (ethanal). This enzyme requires magnesium ions (Mg²⁺) and thiamine pyrophosphate (TPP) as cofactors. Note that this decarboxylation step is irreversible.
首先，丙酮酸被丙酮酸脱羧酶脱羧，释放二氧化碳并产生乙醛（乙醛）。该酶需要镁离子（Mg²⁺）和焦磷酸硫胺素（TPP）作为辅因子。注意此脱羧步骤是不可逆的。
Pyruvate → Acetaldehyde + CO₂
Second, acetaldehyde is reduced to ethanol by alcohol (ethanol) dehydrogenase. This reaction uses the NADH produced during glycolysis, regenerating NAD⁺ in the process.
其次，乙醛被乙醇脱氢酶还原为乙醇。该反应利用糖酵解过程中产生的NADH，在此过程中再生NAD⁺。
Acetaldehyde + NADH + H⁺ → Ethanol + NAD⁺
7. Overall Equations and Comparisons | 总体方程式与比较
The overall equation for anaerobic respiration in animals can be summarised as follows. It is important to note that the entire process yields only 2 ATP per glucose molecule, compared with approximately 38 ATP in aerobic respiration.
动物中无氧呼吸的总体方程式可总结如下。重要的是要注意，整个过程每分子葡萄糖仅产生2个ATP，而有氧呼吸约为38个ATP。
C₆H₁₂O₆ → 2 C₃H₆O₃ (lactate) + 2 ATP
The overall equation for alcoholic fermentation in yeast is:
酵母中酒精发酵的总体方程式为：
C₆H₁₂O₆ → 2 C₂H₅OH (ethanol) + 2 CO₂ + 2 ATP
| Feature | Aerobic Respiration | Anaerobic Respiration |
| Oxygen requirement | Required | Not required |
| Location | Cytoplasm + mitochondria | Cytoplasm only |
| ATP yield per glucose | ~38 ATP | 2 ATP |
| End products | CO₂ and H₂O | Lactate OR ethanol + CO₂ |
| NADH fate | Oxidised via ETC | Oxidised during fermentation |
8. ATP Yield and Efficiency Comparison | ATP产量与效率比较
Anaerobic respiration is markedly less efficient than aerobic respiration. A single glucose molecule contains approximately 2870 kJ of free energy. Aerobic respiration captures about 38 ATP molecules, equivalent to roughly 1160 kJ (≈40% efficiency). In contrast, anaerobic respiration captures only 2 ATP molecules, approximately 61 kJ (≈2% efficiency).
无氧呼吸的效率明显低于有氧呼吸。一分子葡萄糖含有约2870kJ的自由能。有氧呼吸可捕获约38个ATP分子，约相当于1160kJ（约40%效率）。相比之下，无氧呼吸仅捕获2个ATP分子，约61kJ（约2%效率）。
The vast majority of the energy in glucose remains trapped in lactate or ethanol. This explains why fermentation products still serve as excellent fuels: ethanol is readily combusted for energy, and lactate can be further metabolised by the heart and liver.
葡萄糖中的绝大部分能量仍保留在乳酸或乙醇中。这就解释了为什么发酵产物仍然是优良的燃料：乙醇很容易被燃烧释放能量，而乳酸可被心脏和肝脏进一步代谢。
9. Oxygen Debt and Lactate Accumulation | 氧债与乳酸累积
During vigorous exercise, the oxygen supply to muscles may become insufficient to meet the demands of aerobic respiration. Under these conditions, muscle cells switch to lactate fermentation. This produces ATP rapidly, albeit in small amounts, allowing muscle contraction to continue.
在剧烈运动期间，肌肉的氧气供应可能不足以满足有氧呼吸的需求。在这种情况下，肌细胞转向乳酸发酵。这能快速产生ATP，尽管量少，但足以使肌肉收缩继续。
Accumulation of lactate causes muscle fatigue and a decrease in pH within the cells, which can inhibit glycolytic enzymes. After exercise ends, the extra oxygen consumed—known as the oxygen debt or excess post-exercise oxygen consumption (EPOC)—is used to oxidise lactate back to pyruvate via the Cori cycle and to replenish ATP and creatine phosphate stores.
乳酸累积会导致肌肉疲劳和细胞内pH值下降，这可能抑制糖酵解酶。运动结束后，额外消耗的氧气——称为氧债或运动后过量氧耗（EPOC）——用于通过科里循环将乳酸氧化回丙酮酸，并补充ATP和磷酸肌酸储备。
10. Industrial and Commercial Applications | 工业与商业应用
Anaerobic respiration is exploited in several industries. Alcoholic fermentation by yeast is used in brewing and winemaking, where the ethanol produced is the desired end product. In baking, the CO₂ released during fermentation causes bread to rise, while the ethanol evaporates during baking.
无氧呼吸在多个行业中得到了利用。酵母的酒精发酵被用于酿造啤酒和葡萄酒，其中产生的乙醇是期望的终产物。在烘焙中，发酵过程中释放的CO₂使面包膨胀，而乙醇在烘焙过程中蒸发。
Lactate fermentation is similarly important in food production. Lactobacillus species convert lactose and other sugars into lactic acid, which acts as a preservative and flavouring agent. Yoghurt, cheese, sauerkraut, kimchi, and pickles are all produced using this fermentation pathway.
乳酸发酵在食品生产中同样重要。乳酸杆菌属将乳糖和其他糖类转化为乳酸，乳酸起到防腐剂和风味剂的作用。酸奶、奶酪、酸菜、泡菜和腌黄瓜都是利用这种发酵途径生产的。
11. Comparing Lactate and Alcoholic Fermentation | 乳酸发酵与酒精发酵的比较
Both types of fermentation regenerate NAD⁺ from NADH and produce exactly 2 ATP per glucose. However, they differ in several fundamental aspects. Lactate fermentation occurs in animal cells and some bacteria and involves no CO₂ release. Alcoholic fermentation occurs in yeast and some plants and releases CO₂.
两种发酵都从NADH再生NAD⁺，每分子葡萄糖恰好产生2个ATP。然而，它们在几个基本方面存在差异。乳酸发酵发生在动物细胞和某些细菌中，不释放CO₂。酒精发酵发生在酵母和某些植物中，释放CO₂。
A noteworthy point is that some plant tissues, such as rice seedlings submerged in water, can also perform alcoholic fermentation. This allows them to survive temporary oxygen deprivation by producing ATP anaerobically, though their growth rate is greatly reduced.
一个值得注意的观点是，一些植物组织，例如淹在水中的水稻幼苗，也能进行酒精发酵。这使得它们能够在暂时缺氧的情况下通过无氧方式产生ATP来存活，尽管其生长速率会大大降低。
12. Examination Tips and Common Misconceptions | 考试技巧与常见误区
A common misconception is that the Krebs cycle and oxidative phosphorylation are involved in anaerobic respiration. In fact, only glycolysis plus a single fermentation step constitute the anaerobic pathway. Another error is stating that no ATP is produced in anaerobic respiration—it produces precisely 2 net ATP per glucose.
一个常见误区是认为克雷布斯循环和氧化磷酸化参与无氧呼吸。实际上，无氧呼吸途径仅由糖酵解加一步发酵反应构成。另一个错误是声称无氧呼吸不产生ATP——它每分子葡萄糖恰好产生2个净ATP。
When writing the overall equations, ensure that the products are correctly specified: lactate for animals, ethanol and carbon dioxide for yeast. Include the enzyme names (lactate dehydrogenase, pyruvate decarboxylase, alcohol dehydrogenase) when describing mechanisms, as this earns additional marks in extended-response questions.
在写总体方程式时，确保终产物正确写出：动物为乳酸，酵母为乙醇和二氧化碳。在描述机制时包括酶的名称（乳酸脱氢酶、丙酮酸脱羧酶、乙醇脱氢酶），因为在拓展回答题中这能获得额外分数。
Finally, remember the fate of NADH: it is regenerated to NAD⁺ by reducing pyruvate. This is not merely a side reaction—it is the essential purpose of the fermentation step, enabling glycolysis to continue generating ATP under anaerobic conditions.
最后，记住NADH的去向：它通过还原丙酮酸被再生为NAD⁺。这不仅仅是一个副反应——它是发酵步骤的基本目的，使糖酵解在无氧条件下能够持续产生ATP。
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