📚 Respiration without oxygen | 无氧呼吸
Even when oxygen is scarce or absent, living cells are still able to release energy from glucose. This process is called anaerobic respiration. Although it produces far fewer ATP molecules than complete aerobic respiration, it is essential for cells that experience oxygen deprivation, such as vigorously contracting muscle cells or yeast in an oxygen-poor environment. In this article, we explore the pathways, significance, and limitations of respiration without oxygen, closely following the Cambridge International A‑Level Biology syllabus.
即使在氧气稀缺或完全缺氧的情况下，活细胞仍能从葡萄糖中释放能量。这一过程称为无氧呼吸。尽管它产生的ATP分子数量远远少于完整的有氧呼吸，但对于遭遇缺氧的细胞（如剧烈收缩的肌肉细胞或氧气贫乏环境中的酵母）至关重要。本文将严格依据剑桥国际A‑Level生物学教学大纲，探讨无氧呼吸的途径、意义及其局限。
1. What is Anaerobic Respiration? | 什么是无氧呼吸？
Anaerobic respiration is the release of energy from respiratory substrates, such as glucose, without the involvement of oxygen as the terminal electron acceptor. It takes place entirely in the cytoplasm and does not rely on the Krebs cycle or the electron transport chain in mitochondria. Instead, it couples glycolysis with a fermentation step that regenerates oxidised NAD⁺, allowing glycolysis to continue producing a small but vital ATP yield.
无氧呼吸是指在没有氧气作为最终电子受体的情况下，从呼吸底物（如葡萄糖）中释放能量。它完全在细胞质中进行，不依赖线粒体中的克雷布斯循环或电子传递链。它将糖酵解与一个发酵步骤相耦合，再生氧化态NAD⁺，使得糖酵解能够继续产生少量但至关重要的ATP。
2. Glycolysis: The Universal First Step | 糖酵解：通用的第一步
Glycolysis is the initial stage of both aerobic and anaerobic respiration. It occurs in the cytoplasm and begins with the phosphorylation of glucose using two ATP molecules. The 6‑carbon molecule is then split into two 3‑carbon triose phosphate molecules. These are subsequently oxidised, losing hydrogen atoms which are accepted by NAD⁺, forming reduced NAD (NADH). The final product is pyruvate, and there is a net gain of 2 ATP and 2 NADH per molecule of glucose.
糖酵解是有氧呼吸和无氧呼吸的共同起始阶段。它发生在细胞质中，首先消耗两个ATP分子对葡萄糖进行磷酸化。随后6碳分子裂解为两个3碳的磷酸丙糖分子。这些丙糖接着被氧化，失去的氢原子由NAD⁺接受，生成还原态NAD（NADH）。最终产物为丙酮酸，每分子葡萄糖净产生2个ATP和2个NADH。
The overall equation for glycolysis can be summarised as follows:
糖酵解的总反应式可概括如下：
Glucose + 2 NAD⁺ + 2 ADP + 2 Pᵢ → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP
Importantly, glycolysis itself does not require oxygen. The pyruvate produced can then follow different routes depending on the availability of oxygen and the type of organism.
重要的是，糖酵解本身不需要氧气。生成的丙酮酸随后可根据氧气供给情况和生物体类型进入不同代谢途径。
3. NAD⁺ Regeneration: The Crucial Challenge | NAD⁺再生：关键的挑战
For glycolysis to proceed continuously, the cell must maintain a supply of NAD⁺ to accept hydrogen atoms. In an actively respiring cell, NADH would quickly accumulate, and without a mechanism to re‑oxidise it, glycolysis would grind to a halt. In aerobic respiration, this is achieved by the electron transport chain, where NADH donates electrons to oxygen. In the absence of oxygen, however, the cell must use an alternative method to convert NADH back to NAD⁺.
为了使糖酵解持续进行，细胞必须维持NAD⁺的供应来接受氢原子。在活跃呼吸的细胞中，NADH会迅速累积，如果没有将其再氧化的机制，糖酵解就会停滞。在有氧呼吸中，这通过电子传递链实现，NADH将电子传递给氧气。然而在无氧条件下，细胞必须使用替代方法将NADH重新转化为NAD⁺。
In anaerobic respiration, pyruvate itself acts as the electron acceptor, thereby regenerating NAD⁺. This is the fundamental principle behind all fermentation pathways. The specific end product—lactic acid or ethanol—depends on the organism and the enzymes present.
在无氧呼吸中，丙酮酸自身充当电子受体，从而再生NAD⁺。这是所有发酵途径背后的基本原理。具体的终产物——乳酸或乙醇——取决于生物体和存在的酶。
4. Lactate Fermentation in Animals | 动物中的乳酸发酵
In mammalian muscle cells, when the oxygen supply cannot meet demand during strenuous exercise, the pyruvate produced by glycolysis is converted to lactate. The enzyme lactate dehydrogenase catalyses the transfer of hydrogen from NADH to pyruvate, producing lactate and regenerating NAD⁺.
在哺乳动物肌肉细胞中，当剧烈运动时氧气供应无法满足需求，糖酵解生成的丙酮酸会被转化为乳酸。乳酸脱氢酶催化氢从NADH转移至丙酮酸，生成乳酸并再生NAD⁺。
Pyruvate + NADH + H⁺ → Lactate + NAD⁺
Lactate is the ionised form of lactic acid. The accumulation of lactate in muscle tissue leads to a fall in pH, contributing to muscle fatigue and a temporary burning sensation. While this pathway yields only 2 ATP per glucose, it enables fast‑twitch muscle fibres to continue contracting for a short period without oxygen.
乳酸是乳酸的离子形式。乳酸在肌肉组织中的积累会导致pH下降，引起肌肉疲劳和短暂的灼烧感。虽然此途径每分子葡萄糖仅产生2个ATP，但它能使快肌纤维在无氧条件下短时间持续收缩。
5. Alcoholic Fermentation in Yeast and Plants | 酵母和植物中的乙醇发酵
Yeast and some plant cells, particularly under waterlogged conditions, carry out alcoholic (ethanol) fermentation. This two‑step process first decarboxylates pyruvate to form acetaldehyde (ethanal), releasing carbon dioxide. Then acetaldehyde is reduced by alcohol dehydrogenase using NADH, forming ethanol and regenerating NAD⁺.
酵母和某些植物细胞，尤其在浸水条件下，进行乙醇发酵。这个两步过程首先将丙酮酸脱羧形成乙醛，释放二氧化碳。然后乙醛在乙醇脱氢酶作用下被NADH还原，生成乙醇并再生NAD⁺。
Pyruvate → Ethanal + CO₂
Ethanal + NADH + H⁺ → Ethanol + NAD⁺
The overall equation for alcoholic fermentation therefore is:
因此乙醇发酵的总方程式为：
Glucose → 2 Ethanol + 2 CO₂ + 2 ATP
Unlike animal cells, yeast cells are able to tolerate moderate concentrations of ethanol. However, when ethanol accumulates to high levels, it eventually becomes toxic and kills the cells. The carbon dioxide released is vital in baking, causing dough to rise.
与动物细胞不同，酵母细胞能够耐受中等浓度的乙醇。然而，当乙醇累积至高浓度时，最终会对细胞产生毒性并杀死细胞。所释放的二氧化碳在烘焙中至关重要，能使面团膨胀。
6. ATP Yield and Energy Efficiency | ATP产量与能量效率
Anaerobic respiration produces a net gain of only 2 ATP molecules per glucose molecule, all of which come from substrate‑level phosphorylation during glycolysis. This contrasts sharply with the estimated 36–38 ATP produced by the complete aerobic oxidation of glucose. The reason is that the majority of the energy remains trapped in the products—lactate or ethanol—because the Krebs cycle and oxidative phosphorylation are not utilised.
无氧呼吸每分子葡萄糖仅净产2个ATP，全部来自糖酵解过程中的底物水平磷酸化。这与葡萄糖彻底有氧氧化所产生的约36–38个ATP形成鲜明对比。其原因是大部分能量仍滞留在产物——乳酸或乙醇中，因为克雷布斯循环和氧化磷酸化均未参与。
Despite its low efficiency, anaerobic respiration provides a rapid burst of ATP. In a sprint, for example, muscles can generate ATP far more quickly via glycolysis and lactate fermentation than through the slower, but more efficient, aerobic pathway. This trade‑off between speed and yield is a key evolutionary compromise.
尽管效率低下，无氧呼吸却能迅速提供ATP。例如在短跑中，肌肉通过糖酵解和乳酸发酵产生ATP的速度远快于较慢但更高效的有氧途径。这种速度与产量之间的权衡是一项关键的进化妥协。
7. Oxygen Debt and the Cori Cycle | 氧债与科里循环
After a period of intense anaerobic respiration, the body enters a recovery phase where lactate is removed. The extra oxygen consumed during this period is referred to as the oxygen debt. This oxygen is required to convert lactate back into pyruvate and then either oxidise it aerobically or convert it into glucose in the liver (gluconeogenesis).
在经历一段高强度无氧呼吸后，身体进入恢复期，此期间乳酸被清除。此阶段多消耗的氧气被称为氧债。这些氧气需要用于将乳酸重新转化为丙酮酸，然后或被有氧氧化，或在肝脏中转化为葡萄糖（糖异生）。
The Cori cycle describes the process by which lactate produced in muscles travels via the bloodstream to the liver, where it is converted into glucose. This glucose can then be returned to the muscles to serve as a respiratory substrate. The cycle requires energy in the liver but helps regulate lactate levels and replenish glycogen stores.
科里循环描述了肌肉产生的乳酸经血流到达肝脏，并在那里被转化为葡萄糖的过程。这些葡萄糖随后可返回肌肉作为呼吸底物。此循环在肝脏中消耗能量，但有助于调节乳酸水平并补充糖原储备。
8. Anaerobic Respiration in Other Organisms | 其他生物的无氧呼吸
Anaerobic respiration is not limited to animals and yeast. Many prokaryotes use anaerobic respiration with electron acceptors other than oxygen, such as nitrate (NO₃⁻), sulfate (SO₄²⁻), or even carbon dioxide. This type of respiration is part of the nitrogen and sulfur cycles and occurs in environments like deep sediments, waterlogged soils, and the guts of ruminants.
无氧呼吸并不仅限于动物和酵母。许多原核生物使用除氧气以外的电子受体进行无氧呼吸，例如硝酸盐（NO₃⁻）、硫酸盐（SO₄²⁻）甚至二氧化碳。这种呼吸是氮循环和硫循环的一部分，发生在深层沉积物、浸水土壤以及反刍动物的肠道等环境中。
In plants, root cells may be forced to respire anaerobically when soil becomes waterlogged and oxygen diffusion is restricted. Some plant species switch to ethanol fermentation, while others produce lactate. The ability to operate an anaerobic pathway is crucial for survival during temporary flooding.
在植物中，当土壤浸水、氧气扩散受限时，根部细胞可能被迫进行无氧呼吸。有些植物种类转向乙醇发酵，而另一些则产生乳酸。具备无氧途径的能力对于在暂时性水淹中生存至关重要。
9. Industrial and Commercial Applications | 工业与商业应用
The fermentation pathways of various microorganisms are exploited in food and biofuel industries. Yeast anaerobic respiration is the basis of bread making, where the CO₂ produced makes dough rise, and of alcoholic beverage production, where ethanol is the desired product. Lactic acid bacteria are used to produce yogurt, cheese, and sauerkraut, where the acid acts as a preservative and flavour enhancer.
各种微生物的发酵途径被应用于食品和生物燃料工业。酵母的无氧呼吸是面包制作的基础，所释放的二氧化碳使面团膨胀；也是酒精饮料生产的基础，乙醇正是期望的产物。乳酸菌被用于生产酸奶、奶酪和酸菜，其中乳酸起到防腐剂和增味剂的作用。
In biofuel production, yeast ferments sugars from crops such as maize or sugarcane into ethanol, which can be used as a renewable fuel. The process relies on optimising anaerobic conditions to maximise yield while preventing contamination by unwanted microbes.
在生物燃料生产中，酵母将来自玉米或甘蔗等作物的糖类发酵成乙醇，可用作可再生燃料。该过程依赖于优化厌氧条件以最大化产量，并防止杂菌污染。
10. Limitations of Anaerobic Respiration | 无氧呼吸的局限性
Although anaerobic respiration provides an emergency supply of ATP, it has several significant drawbacks. The low ATP yield cannot sustain high metabolic demands over a prolonged period. Furthermore, the accumulation of end products—lactic acid in animals or ethanol in microorganisms—is toxic. Lactate lowers pH, disrupting enzyme activity and muscle function, while ethanol denatures proteins and disrupts membranes when concentrations rise.
尽管无氧呼吸能提供应急的ATP供应，但它存在几个显著缺陷。低ATP产量无法长时间维持高代谢需求。此外，终产物的累积——动物中的乳酸或微生物中的乙醇——是有毒的。乳酸降低pH，干扰酶活性和肌肉功能；而乙醇浓度升高时会使蛋白质变性并破坏膜结构。
For these reasons, organisms must eventually return to aerobic conditions to clear these metabolites and repay the oxygen debt. The ability to withstand anoxia varies among species, but for most eukaryotes, anaerobic respiration is strictly a short‑term survival mechanism.
出于这些原因，生物体最终必须恢复有氧条件以清除这些代谢物并偿还氧债。耐受缺氧的能力因物种而异，但对大多数真核生物而言，无氧呼吸严格来说只是一种短期生存机制。
11. Comparison with Aerobic Respiration | 与有氧呼吸的比较
The table below summarises the key differences between anaerobic and aerobic respiration in eukaryotic cells. Understanding these distinctions is essential for examination success.
下表概括了真核细胞中无氧呼吸与有氧呼吸的关键区别。理解这些差异对考试成功至关重要。
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Location | Cytoplasm and mitochondria | Cytoplasm only |
| Oxygen requirement | Required as terminal electron acceptor | Not used; organic molecule acts as electron acceptor |
| ATP yield per glucose | ~36–38 ATP | 2 ATP (net) |
| Final electron acceptor | Oxygen | Pyruvate (or acetaldehyde) |
| Products | CO₂ and H₂O | Lactate (animals) or ethanol + CO₂ (yeast/plants) |
| NAD⁺ regeneration | Via electron transport chain | Via reduction of pyruvate or ethanal |
| Stages involved | Glycolysis → Link reaction → Krebs cycle → Oxidative phosphorylation | Glycolysis → Fermentation only |
Despite these differences, it is critical to remember that both types of respiration share the initial glycolysis step. The fate of pyruvate is the branching point that determines whether respiration proceeds aerobically or anaerobically.
尽管存在这些差异，但切记两种呼吸类型都共享起始的糖酵解步骤。丙酮酸的命运是决定呼吸进行有氧路线还是无氧路线的分支点。
12. Exam Tips and Common Misconceptions | 考试要点与常见误解
When answering exam questions on anaerobic respiration, avoid stating that it produces 'no ATP' or that it occurs 'instead of' glycolysis. Instead, highlight that ATP is generated only during glycolysis and that fermentation ensures continuation of glycolysis by regenerating NAD⁺. A common error is to confuse lactate with lactic acid—lactate is the ion that accumulates in cells.
回答无氧呼吸相关的试题时，切勿说它“不产生ATP”或“代替糖酵解”。相反，应强调ATP仅在糖酵解阶段生成，而发酵通过再生NAD⁺来保证糖酵解的持续进行。一个常见错误是将乳酸（lactate）与乳酸酸（lactic acid）混淆——在细胞内积累的是乳酸根离子。
Another misconception is that lactate is a waste product with no further use. In reality, lactate can be reconverted to pyruvate and used as an energy source by the heart and slow‑twitch muscle fibres. Also, note that anaerobic respiration in the context of prokaryotes using nitrate or sulfate is a distinct process from fermentation, as it involves an electron transport chain and more ATP production.
另一个误解是认为乳酸是无用废产物。事实上，乳酸可重新转化为丙酮酸，并被心脏和慢肌纤维用作能源。同时注意，原核生物利用硝酸盐或硫酸盐进行的无氧呼吸与发酵截然不同，因为它涉及电子传递链并产生更多ATP。
Finally, always be precise with terminology: 'anaerobic respiration' in eukaryotic cells often refers to fermentation, whereas in some A‑Level mark schemes, 'respiration' may be defined more strictly. Use the terms 'lactate fermentation' or 'alcoholic fermentation' when describing the pathways to demonstrate precise knowledge.
最后，术语务必准确：真核细胞中的“无氧呼吸”通常指发酵，而在某些A‑Level评分方案中，“呼吸”一词定义可能更严格。描述途径时使用“乳酸发酵”或“乙醇发酵”等术语，以展示精确认知。
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