📚 Respiration | 呼吸作用 考点精讲
Respiration is the enzyme‑controlled release of energy from organic compounds in living cells. It occurs in a series of stepwise reactions, allowing the energy to be captured as ATP and used for cellular work. For WJEC A‑Level Biology, you must be able to describe the four main stages of aerobic respiration, compare anaerobic pathways, calculate respiratory quotients, and interpret respirometer data.
呼吸作用是活细胞中有机化合物在酶控制下释放能量的过程。它通过一系列逐步反应进行，使能量能够以 ATP 的形式被捕获并用于细胞活动。对于 WJEC A‑Level 生物考试，你需要能够描述需氧呼吸的四个主要阶段、比较无氧呼吸途径、计算呼吸商以及解读呼吸计数据。
1. Overview of Respiration | 呼吸作用概述
Respiration is not just 'breathing' – + 6O₂ → 6CO₂ + 6H₂O + energy (ATP). However, this hides the complexity of the pathways involved, which are tightly regulated and compartmentalised in eukaryotic cells.
呼吸作用不仅仅是“呼吸”——它是一个将营养物质中的化学能转移到 ATP 的代谢过程。葡萄糖需氧呼吸的总方程式为 C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量（ATP）。然而，这掩盖了所涉及途径的复杂性，这些途径在真核细胞中受到严格调控并分区进行。
Aerobic respiration consists of four stages: glycolysis (cytoplasm), the link reaction (mitochondrial matrix), the Krebs cycle (mitochondrial matrix), and oxidative phosphorylation (inner mitochondrial membrane). Anaerobic respiration only uses glycolysis followed by a fermentation step to regenerate NAD⁺.
需氧呼吸包括四个阶段：糖酵解（细胞质）、连接反应（线粒体基质）、克雷布斯循环（线粒体基质）和氧化磷酸化（线粒体内膜）。无氧呼吸仅使用糖酵解，随后通过一个发酵步骤来再生 NAD⁺。
2. Glycolysis | 糖酵解
Glycolysis occurs in the cytoplasm and does not require oxygen. One molecule of glucose (6C) is phosphorylated using 2 ATP and then split into two molecules of triose phosphate (3C). These are oxidised to form two molecules of pyruvate (CH₃COCOOH), yielding a net gain of 2 ATP (substrate‑level phosphorylation) and 2 reduced NAD (NADH + H⁺).
糖酵解发生在细胞质中，不需要氧气。一分子葡萄糖 (6C) 利用 2 个 ATP 进行磷酸化，然后分裂成两分子磷酸丙糖 (3C)。它们被氧化形成两分子丙酮酸 (CH₃COCOOH)，净生成 2 个 ATP（底物水平磷酸化）和 2 个还原型 NAD (NADH + H⁺)。
The key steps are phosphorylation of glucose using hexokinase and phosphofructokinase, lysis, and then substrate‑level phosphorylation where phosphate groups are transferred to ADP. The NADH produced will be used later in oxidative phosphorylation or recycled during anaerobic respiration.
关键步骤是使用己糖激酶和磷酸果糖激酶对葡萄糖进行磷酸化、裂解，然后进行底物水平磷酸化，其中磷酸基团被转移至 ADP。产生的 NADH 稍后将用于氧化磷酸化，或在无氧呼吸过程中被循环利用。
3. Link Reaction | 连接反应
The link reaction occurs in the mitochondrial matrix when oxygen is present. Pyruvate enters the mitochondrion via active transport and is decarboxylated (CO₂ removed) and dehydrogenated (hydrogen removed). It combines with coenzyme A to form acetyl coenzyme A (acetyl‑CoA, CH₃COSCoA). One molecule of NAD⁺ is reduced to NADH + H⁺ per pyruvate.
连接反应在线粒体基质中进行，需有氧气存在。丙酮酸通过主动运输进入线粒体，并经历脱羧（去除 CO₂）和脱氢（去除氢）。它与辅酶 A 结合形成乙酰辅酶 A (acetyl‑CoA, CH₃COSCoA)。每分子丙酮酸使一分子 NAD⁺ 还原为 NADH + H⁺。
This irreversible reaction links glycolysis to the Krebs cycle. Because two pyruvates are produced per glucose, the link reaction yields 2 acetyl‑CoA, 2 CO₂, and 2 NADH + H⁺ per glucose molecule.
这个不可逆反应将糖酵解与克雷布斯循环连接起来。由于每个葡萄糖产生两个丙酮酸，连接反应每分子葡萄糖产生 2 个乙酰辅酶 A、2 个 CO₂ 和 2 个 NADH + H⁺。
4. The Krebs Cycle | 克雷布斯循环
The Krebs cycle takes place in the mitochondrial matrix. Acetyl‑CoA (2C) combines with a 4‑carbon compound (oxaloacetate) to form a 6‑carbon compound (citrate). Through a series of enzyme‑controlled reactions, the citrate is decarboxylated and dehydrogenated, regenerating the 4‑carbon acceptor. Per turn of the cycle, 3 NADH, 1 FADH₂, 1 GTP (converted to ATP), and 2 CO₂ are produced.
克雷布斯循环发生在线粒体基质中。乙酰辅酶 A (2C) 与一个 4 碳化合物（草酰乙酸）结合，形成一个 6 碳化合物（柠檬酸）。通过一系列酶控反应，柠檬酸经历脱羧和脱氢，再生 4 碳受体。循环每转一圈产生 3 个 NADH、1 个 FADH₂、1 个 GTP（转化为 ATP）和 2 个 CO₂。
Since each glucose yields two acetyl‑CoA, the cycle turns twice. Therefore, the total products from the Krebs cycle per glucose are 6 NADH, 2 FADH₂, 2 ATP (via GTP), and 4 CO₂. The reduced coenzymes carry high‑energy electrons to the electron transport chain.
由于每个葡萄糖产生两个乙酰辅酶 A，循环进行两圈。因此，每个葡萄糖从克雷布斯循环获得的总产物是 6 个 NADH、2 个 FADH₂、2 个 ATP（通过 GTP）和 4 个 CO₂。还原型辅酶将高能电子携带至电子传递链。
5. Oxidative Phosphorylation | 氧化磷酸化
Oxidative phosphorylation occurs on the inner mitochondrial membrane. Reduced NAD and FAD from earlier stages donate electrons to the electron transport chain (ETC). As electrons pass along the chain, energy is released, which pumps protons (H⁺) from the matrix into the intermembrane space, creating an electrochemical gradient.
氧化磷酸化发生在线粒体内膜上。来自早期阶段的还原型 NAD 和 FAD 将电子提供给电子传递链 (ETC)。当电子沿链传递时，释放出能量，将质子 (H⁺) 从基质泵入膜间腔，从而产生电化学梯度。
Protons flow back into the matrix through ATP synthase, a process called chemiosmosis, driving the synthesis of ATP from ADP and Pᵢ. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. The theoretical maximum ATP yield from oxidation of one NADH is ~2.5 ATP, and from FADH₂ is ~1.5 ATP, giving a total of about 32 ATP per glucose in eukaryotes.
质子通过 ATP 合酶流回基质，这一过程称为化学渗透，驱动由 ADP 和 Pᵢ 合成 ATP。氧气作为最终电子受体，与电子和质子结合形成水。理论上，氧化一个 NADH 最多产生约 2.5 个 ATP，氧化一个 FADH₂ 产生约 1.5 个 ATP，因此在真核生物中每分子葡萄糖总共产生约 32 个 ATP。
6. Anaerobic Respiration: Lactate Fermentation | 无氧呼吸：乳酸发酵
In animal cells and some bacteria, when oxygen is insufficient, glycolysis continues but pyruvate is reduced to lactate (CH₃CH(OH)COOH) by lactate dehydrogenase. This reaction oxidises NADH back to NAD⁺, allowing glycolysis to proceed and produce a small amount of ATP (2 per glucose) via substrate‑level phosphorylation.
在动物细胞和某些细菌中，当氧气不足时，糖酵解继续进行，但丙酮酸被乳酸脱氢酶还原为乳酸 (CH₃CH(OH)COOH)。该反应将 NADH 氧化回 NAD⁺，使糖酵解得以继续，并通过底物水平磷酸化产生少量 ATP（每葡萄糖 2 个）。
Lactate can be transported to the liver and converted back to glucose (the Cori cycle) or oxidised further when oxygen becomes available. Accumulation of lactate in muscles can lower pH and contribute to fatigue.
乳酸可被转运至肝脏并转化回葡萄糖（科里循环），或在氧气可用时被进一步氧化。肌肉中乳酸的积累会降低 pH，导致疲劳。
7. Alcoholic Fermentation | 酒精发酵
In plants and fungi such as yeast, anaerobic respiration produces ethanol and CO₂. Pyruvate is first decarboxylated to ethanal (CH₃CHO) by pyruvate decarboxylase, releasing CO₂. Ethanal is then reduced to ethanol (C₂H₅OH) by alcohol dehydrogenase, using NADH. This regenerates NAD⁺, maintaining glycolysis.
在植物和真菌（如酵母）中，无氧呼吸产生乙醇和 CO₂。丙酮酸首先被丙酮酸脱羧酶脱羧生成乙醛 (CH₃CHO)，释放 CO₂。然后乙醛被乙醇脱氢酶利用 NADH 还原为乙醇 (C₂H₅OH)。这就再生了 NAD⁺，维持了糖酵解。
₁₂O₆ → 2 C₂H₅OH + 2 CO₂ + energy (2 ATP). This process is exploited in baking and brewing.
酒精发酵的总方程式为 C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ + 能量（2 ATP）。该工艺被用于烘焙和酿造。
8. Respiratory Quotient (RQ) | 呼吸商
CO₂ evolved ÷ O₂ consumed. It can be used to deduce the type of substrate being respired.
呼吸商是呼吸作用过程中产生的 CO₂ 体积与消耗的 O₂ 体积之比。RQ = 产生的 CO₂ ÷ 消耗的 O₂。它可用于推断被呼吸的底物类型。
in ≈ 0.9. However, value can be affected by simultaneous use of multiple substrates and by anaerobic respiration.
典型的 RQ 值：碳水化合物 = 1.0（C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O，比率为 6/6）；脂质 ≈ 0.7（分子内 O₂ 较少，燃烧需要更多 O₂）；蛋白质 ≈ 0.9。然而，该值可能受多种底物同时使用以及无氧呼吸的影响。
Calculating RQ from respirometer data involves measuring the distance a fluid bubble moves with and without a CO₂ absorbent (e.g., KOH). If KOH is present, CO₂ is absorbed, so volume change represents O₂ consumption. Without KOH, the change represents O₂ consumed minus CO₂ produced.
通过呼吸计数据计算 RQ 涉及测量在有、无 CO₂ 吸收剂（如 KOH）的情况下液体气泡移动的距离。如果存在 KOH，CO₂ 被吸收，因此体积变化代表 O₂ 消耗量。若无 KOH，变化则代表 O₂ 消耗量减去 CO₂ 产生量。
9. Investigating Respiration Using a Respirometer | 用呼吸计研究呼吸作用
A respirometer is used to measure the rate of aerobic respiration of organisms or tissues. It typically consists of a sealed chamber containing the respiring material, connected to a capillary U‑tube containing a manometer fluid. A CO₂‑absorbing chemical (KOH or soda lime) is placed in the chamber to remove CO₂, so that the volume change of gas directly corresponds to O₂ uptake.
呼吸计用于测量生物体或组织的有氧呼吸速率。它通常由一个装有呼吸材料的密封室组成，连接到一个含有测压液体的毛细 U 形管。室内放置 CO₂ 吸收化学物质（KOH 或钠石灰）以去除 CO₂，从而使气体体积变化直接对应于 O₂ 吸收量。
The rate is calculated by measuring the movement of the fluid per unit time, taking into account the cross‑sectional area of the capillary tube. Controls should account for changes in temperature and atmospheric pressure. Valid respirometer experiments control temperature with a water bath, ensure an airtight set‑up, and allow time for equilibration.
速率通过测量单位时间内液体的移动距离来计算，需考虑到毛细管的横截面积。对照组应考虑到温度和大气压的变化。有效的呼吸计实验应用水浴控制温度、确保装置气密，并留出时间进行平衡。
10. Key Comparisons and Exam Tips | 关键比较与考试技巧
Be prepared to compare the ATP yield, location, and products of the different stages. Glycolysis produces ATP via substrate‑level phosphorylation, while the greatest ATP yield comes from oxidative phosphorylation. Remember that the Krebs cycle and link reaction occur only under aerobic conditions, whereas glycolysis occurs in both aerobic and anaerobic respiration.
准备好比较不同阶段的 ATP 产量、发生部位和产物。糖酵解通过底物水平磷酸化产生 ATP，而最大量的 ATP 来自氧化磷酸化。请记住，克雷布斯循环和连接反应仅在需氧条件下发生，而糖酵解在需氧和无氧呼吸中均会发生。
A common mistake is confusing the NAD⁺ regeneration strategies: lactate fermentation and alcoholic fermentation both oxidise NADH to NAD⁺, but the end products differ. Also, practise RQ calculations carefully: if a table gives O₂ consumption and CO₂ production, RQ is simply the ratio; if using a respirometer, consider whether KOH is present or not.
一个常见的错误是混淆 NAD⁺ 再生策略：乳酸发酵和酒精发酵均将 NADH 氧化为 NAD⁺，但终产物不同。另外，仔细练习 RQ 计算：如果表格给出了 O₂ 消耗量和 CO₂ 产生量，RQ 就是它们的比值；如果使用呼吸计，请考虑是否放置了 KOH。
Finally, when drawing the ETC and chemiosmosis, label the inner membrane, ATP synthase, flow of protons, and final formation of water clearly. Use precise terminology: 'decarboxylation', 'dehydrogenation', 'substrate‑level phosphorylation', 'chemiosmosis'.
最后，在绘制电子传递链和化学渗透图时，要清楚地标注内膜、ATP 合酶、质子流以及水的最终形成。使用精确术语：“脱羧”、“脱氢”、“底物水平磷酸化”、“化学渗透”。
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