📚 A-Level CIE Biology: Respiration – Key Concepts | A-Level CIE 生物：呼吸作用 考点精讲
Respiration is the process by which living cells release energy from organic molecules such as glucose. It is a series of enzyme-controlled reactions that produce ATP, the universal energy currency. This article covers every essential topic in the CIE A-Level Biology syllabus for respiration, including aerobic and anaerobic pathways, mitochondrial structure, the link reaction, the Krebs cycle, oxidative phosphorylation, and the concept of respiratory substrates and the respiratory quotient.
呼吸作用是活细胞从葡萄糖等有机分子中释放能量的过程。它是一系列由酶控制的反应，产生通用能量货币 ATP。本文涵盖了 CIE A-Level 生物教学大纲中呼吸作用的所有核心考点，包括有氧和无氧途径、线粒体结构、连接反应、克雷布斯循环、氧化磷酸化，以及呼吸底物和呼吸商的概念。
1. Overview of Respiration | 呼吸作用总览
Respiration can be divided into aerobic respiration, which requires oxygen, and anaerobic respiration, which occurs in the absence of oxygen. Aerobic respiration fully oxidises glucose to carbon dioxide and water, releasing a large amount of energy. Anaerobic respiration yields only a small amount of ATP and produces either lactate (in animals) or ethanol and carbon dioxide (in yeast and plants).
呼吸作用可分为需要氧气参与的有氧呼吸，以及在无氧条件下发生的无氧呼吸。有氧呼吸将葡萄糖完全氧化为二氧化碳和水，释放大量能量。无氧呼吸仅产生少量 ATP，并在动物中生成乳酸，在酵母和植物中生成乙醇和二氧化碳。
The overall equation for aerobic respiration is:
有氧呼吸的总方程式为：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP)
Respiration involves four main stages: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. Glycolysis occurs in the cytoplasm, while the remaining stages take place inside the mitochondria.
呼吸作用包括四个主要阶段：糖酵解、连接反应、克雷布斯循环和氧化磷酸化。糖酵解发生在细胞质中，其余阶段均在线粒体内进行。
2. Structure of Mitochondria | 线粒体结构
Mitochondria are the sites of aerobic respiration. They are surrounded by a double membrane. The outer membrane is smooth and permeable to small molecules, while the inner membrane is highly folded into cristae, which greatly increase the surface area for oxidative phosphorylation. The inner membrane encloses the matrix, which contains enzymes for the link reaction and the Krebs cycle, as well as mitochondrial DNA and ribosomes.
线粒体是有氧呼吸的场所。它们由双层膜包裹。外膜光滑，对小分子具有通透性；内膜则高度折叠形成嵴，大大增加了氧化磷酸化的表面积。内膜包裹着基质，基质中含有连接反应和克雷布斯循环所需的酶，以及线粒体 DNA 和核糖体。
The space between the two membranes is called the intermembrane space. During oxidative phosphorylation, protons are pumped into this space, creating an electrochemical gradient that drives ATP synthesis.
两层膜之间的空间称为膜间隙。在氧化磷酸化过程中，质子被泵入膜间隙，形成驱动 ATP 合成的电化学梯度。
| Structure | 结构 | Function | 功能 |
| Outer membrane | 外膜 | Permeable to small solutes | 允许小分子溶质通过 |
| Inner membrane (cristae) | 内膜（嵴） | Contains electron transport chain & ATP synthase; folded to increase surface area | 含有电子传递链和 ATP 合酶；折叠以增加表面积 |
| Intermembrane space | 膜间隙 | Accumulates protons (H⁺) to form proton gradient | 积累质子形成质子梯度 |
| Matrix | 基质 | Contains enzymes for link reaction & Krebs cycle, mitochondrial DNA, ribosomes | 含连接反应和克雷布斯循环的酶、线粒体DNA、核糖体 |
3. Glycolysis | 糖酵解
Glycolysis is the first stage of both aerobic and anaerobic respiration. It 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 then oxidised to form two molecules of pyruvate (3C).
糖酵解是有氧呼吸和无氧呼吸共有的第一阶段。它发生在细胞质中，不需要氧气。一个葡萄糖分子（6C）先使用 2 个 ATP 进行磷酸化，然后裂解成两个丙糖磷酸分子（3C）。随后，它们被氧化生成两个丙酮酸分子（3C）。
During glycolysis, 4 ATP are produced by substrate-level phosphorylation, resulting in a net gain of 2 ATP per glucose. Additionally, two molecules of the coenzyme NAD⁺ are reduced to NADH.
在糖酵解过程中，通过底物水平磷酸化产生 4 个 ATP，因此每分子葡萄糖净得 2 个 ATP。此外，两个辅酶 NAD⁺ 被还原为 NADH。
The key points are:
要点如下：
- Location: cytoplasm | 场所：细胞质
- Input: glucose (6C), 2 ATP, 2 NAD⁺ | 投入：葡萄糖 (6C)、2 ATP、2 NAD⁺
- Output: 2 pyruvate (3C), net 2 ATP, 2 NADH | 产出：2 丙酮酸 (3C)、净得 2 ATP、2 NADH
- Oxygen requirement: none | 需氧情况：无
4. The Link Reaction | 连接反应
When oxygen is present, pyruvate enters the mitochondrial matrix via active transport. In the matrix, each pyruvate is decarboxylated (CO₂ removed) and dehydrogenated (hydrogen removed) by the enzyme complex pyruvate dehydrogenase. The removed hydrogen is accepted by NAD⁺, forming reduced NAD (NADH).
当有氧气存在时，丙酮酸通过主动运输进入线粒体基质。在基质中，每个丙酮酸在丙酮酸脱氢酶复合体的催化下发生脱羧（移除 CO₂）和脱氢（移除氢）。脱下的氢被 NAD⁺ 接受，形成还原型 NAD (NADH)。
The remaining 2-carbon acetyl group combines with coenzyme A (CoA) to form acetyl CoA. Thus, for each glucose molecule, two pyruvate molecules produce two acetyl CoA, two CO₂, and two NADH. No ATP is produced directly in this step.
剩下的 2 碳乙酰基与辅酶 A (CoA) 结合，形成乙酰辅酶 A。因此，每分子葡萄糖的两个丙酮酸共产生两分子乙酰辅酶 A、两分子 CO₂ 和两分子 NADH。此步骤不直接产生 ATP。
The overall equation per pyruvate is:
每个丙酮酸的总方程式为：
Pyruvate + NAD⁺ + CoA → Acetyl CoA + CO₂ + NADH
5. The 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 (2C) combines with a 4-carbon compound, oxaloacetate, to form a 6-carbon citrate. In a series of reactions, the citrate is decarboxylated (2 CO₂ released) and dehydrogenated, regenerating oxaloacetate. The cycle turns twice per glucose molecule (once per acetyl CoA).
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 (2C) 与四碳化合物草酰乙酸结合，生成六碳柠檬酸。经过一系列反应，柠檬酸发生脱羧（释放 2 个 CO₂）和脱氢，最终再生草酰乙酸。每分子葡萄糖对应两次循环（每个乙酰辅酶 A 一次）。
For each turn of the cycle, the products are:
每循环一圈的产物为：
- 2 CO₂ (the remaining carbons from the original glucose are now all released) | 2 CO₂（原始葡萄糖的碳至此全部释放）
- 3 NADH | 3 NADH
- 1 reduced FAD (FADH₂) | 1 还原型 FAD (FADH₂)
- 1 ATP by substrate-level phosphorylation (GTP in some organisms) | 1 ATP 通过底物水平磷酸化产生（某些生物中是 GTP）
Thus, per glucose, the Krebs cycle yields 4 CO₂, 6 NADH, 2 FADH₂, and 2 ATP (directly). Together with the link reaction and glycolysis, all carbon atoms from glucose are released as CO₂.
因此，每分子葡萄糖在克雷布斯循环中产生 4 CO₂、6 NADH、2 FADH₂ 和 2 ATP（直接）。连接反应和糖酵解脱碳后，葡萄糖的所有碳原子都以 CO₂ 形式释放。
6. Oxidative Phosphorylation & the Electron Transport Chain | 氧化磷酸化与电子传递链
Oxidative phosphorylation occurs across the inner mitochondrial membrane and consists of the electron transport chain (ETC) and chemiosmosis. NADH and FADH₂, produced in previous stages, deliver hydrogen atoms to the ETC. The hydrogen atoms split into protons (H⁺) and electrons (e⁻).
氧化磷酸化发生在线粒体内膜上，由电子传递链和化学渗透两个部分组成。前几个阶段产生的 NADH 和 FADH₂ 将氢原子传递给电子传递链。氢原子分解为质子 (H⁺) 和电子 (e⁻)。
The electrons pass along a series of electron carriers (protein complexes I, II, III, IV) embedded in the inner membrane. As electrons move, energy is released, which is used to pump protons from the matrix into the intermembrane space. This creates a high concentration of protons in the intermembrane space, forming an electrochemical gradient (proton-motive force).
电子沿内膜上的一系列电子载体（蛋白质复合体 I, II, III, IV）传递。电子移动时释放的能量用于将质子从基质泵入膜间隙，使膜间隙中质子浓度升高，形成电化学梯度（质子动力）。
At the end of the chain, electrons combine with molecular oxygen (the final electron acceptor) and protons to form water. This is why oxygen is essential for aerobic respiration.
在链的末端，电子与最终电子接受体——氧气——以及质子结合，生成水。这就是有氧呼吸必需氧气的原因。
Protons flow back from the intermembrane space into the matrix through ATP synthase, a channel protein with a rotating subunit. This flow drives the synthesis of ATP from ADP and inorganic phosphate (Pi). This process is called chemiosmosis.
质子通过 ATP 合酶（一种具有旋转亚基的通道蛋白）从膜间隙流回基质。这一流动驱动 ADP 和无机磷酸 (Pi) 合成 ATP。此过程称为化学渗透。
The ATP yield from oxidative phosphorylation is approximately 28 ATP per glucose (although yields vary slightly depending on the shuttle system used for NADH from glycolysis).
氧化磷酸化每分子葡萄糖可产生约 28 个 ATP（但因糖酵解产生的 NADH 使用的穿梭系统不同，产量会略有差异）。
7. Anaerobic Respiration in Animals | 动物的无氧呼吸
When oxygen is scarce, animal cells cannot sustain oxidative phosphorylation. However, glycolysis still occurs, generating 2 ATP per glucose. To regenerate NAD⁺ (which is consumed during glycolysis), pyruvate is reduced to lactate by the enzyme lactate dehydrogenase. This process is known as lactic acid fermentation.
当氧气不足时，动物细胞无法维持氧化磷酸化。但糖酵解仍可进行，每分子葡萄糖产生 2 个 ATP。为再生糖酵解过程中消耗的 NAD⁺，丙酮酸在乳酸脱氢酶催化下被还原为乳酸。此过程称为乳酸发酵。
The reduction of pyruvate to lactate uses the hydrogen from NADH, converting it back to NAD⁺. This allows glycolysis to continue producing small amounts of ATP in the absence of oxygen. However, lactate accumulation lowers pH and may cause muscle fatigue and cramping. Lactate can later be converted back to pyruvate in the liver and used for aerobic respiration when oxygen is available again (the Cori cycle).
丙酮酸还原为乳酸消耗了 NADH 中的氢，使 NAD⁺ 得以再生，从而让糖酵解在无氧条件下能继续产生少量 ATP。但乳酸积累会降低 pH，可能导致肌肉疲劳和痉挛。当氧气重新充足时，乳酸可在肝脏中重新转化为丙酮酸，并用于有氧呼吸（科里循环）。
8. Anaerobic Respiration in Yeast & Plants | 酵母与植物的无氧呼吸
In yeast and some plant cells (e.g. root tips under waterlogged conditions), anaerobic respiration takes the form of alcoholic fermentation. Pyruvate is decarboxylated to ethanal (acetaldehyde) by pyruvate decarboxylase, releasing CO₂. Ethanal is then reduced to ethanol by alcohol dehydrogenase, using hydrogen from NADH, regenerating NAD⁺.
在酵母和一些植物细胞中（如淹水条件下的根尖），无氧呼吸采用酒精发酵的形式。丙酮酸在丙酮酸脱羧酶的催化下脱羧生成乙醛，并释放 CO₂。乙醛随后在乙醇脱氢酶作用下，利用 NADH 中的氢被还原为乙醇，同时再生 NAD⁺。
This process also yields only 2 ATP per glucose from glycolysis, but it allows glycolysis to continue. The production of ethanol is irreversible in these organisms, and ethanol can be toxic at high concentrations.
该过程每分子葡萄糖也只通过糖酵解产生 2 个 ATP，但使糖酵解得以持续。在这些生物中，乙醇的生成是不可逆的，高浓度乙醇具有毒性。
The key comparison between the two anaerobic pathways:
两种无氧途径的主要对比如下：
| Feature | 特征 | Lactic acid fermentation (animals) | 乳酸发酵 (动物) | Alcoholic fermentation (yeast/plants) | 酒精发酵 (酵母/植物) |
| End product | 终产物 | Lactate | 乳酸 | Ethanol + CO₂ | 乙醇 + CO₂ |
| CO₂ released? | 是否释放 CO₂ | No | 否 | Yes | 是 |
| Reversibility | 可逆性 | Reversible (in liver) | 可逆（肝脏中） | Irreversible | 不可逆 |
| ATP yield per glucose | 每分子葡萄糖 ATP 产量 | 2 | 2 | 2 | 2 |
9. Respiratory Substrates & Respiratory Quotient (RQ) | 呼吸底物与呼吸商
Although glucose is the most common respiratory substrate, cells can also respire lipids and proteins. The energy yield varies: lipids provide more than twice the energy per gram compared to carbohydrates because they are more reduced (contain more hydrogen atoms relative to oxygen). Proteins release a similar amount of energy per gram as carbohydrates, but they must first be deaminated.
虽然葡萄糖是最常见的呼吸底物，但细胞也可氧化脂质和蛋白质。能量产量各不相同：脂质每克提供的能量是碳水化合物的两倍以上，因为其还原程度更高（相对氧原子含有更多的氢原子）。蛋白质每克释放的能量与碳水化合物相似，但必须先经过脱氨基作用。
The respiratory quotient (RQ) is a useful indicator of which substrate is being respired. RQ is defined as:
呼吸商 (RQ) 是判断正在被氧化的底物类型的有用指标。RQ 定义如下：
RQ = CO₂ produced / O₂ consumed
Typical RQ values:
典型的 RQ 值：
- Carbohydrate: about 1.0 (e.g. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, RQ = 6/6 = 1.0) | 碳水化合物：约 1.0
- Lipid: about 0.7 (lipids are more reduced and require more O₂ relative to CO₂ released) | 脂质：约 0.7（脂质还原程度高，相对 CO₂ 释放需要更多 O₂）
- Protein: around 0.8–0.9 | 蛋白质：约 0.8–0.9
Anaerobic respiration can distort RQ measurements because CO₂ production sometimes occurs without O₂ consumption. For example, in yeast alcoholic fermentation, RQ would theoretically be infinite (since O₂ consumption is zero).
无氧呼吸会干扰 RQ 的测量，因为有时 CO₂ 的产生并不伴随 O₂ 的消耗。例如，在酵母酒精发酵中，理论上 RQ 为无穷大（O₂ 消耗为零）。
10. ATP Yield & Efficiency | ATP 产量与效率
The theoretical maximum ATP yield from the complete aerobic oxidation of one glucose molecule is approximately 30–32 ATP. This depends on the shuttle used to transfer NADH from glycolysis into the mitochondria (the malate-aspartate shuttle yields about 32 ATP, while the glycerol-3-phosphate shuttle yields slightly less).
一分子葡萄糖完全有氧氧化的理论最大 ATP 产量约为 30–32 ATP。具体数值取决于将糖酵解产生的 NADH 运入线粒体的穿梭系统（苹果酸-天冬氨酸穿梭产生约 32 ATP，磷酸甘油穿梭产生略少）。
A typical breakdown is:
典型分解如下：
- Glycolysis: 2 ATP + 2 NADH (equivalent to 3–5 ATP, depending on shuttle) | 糖酵解：2 ATP + 2 NADH（视穿梭系统相当于 3–5 ATP）
- Link reaction: 2 NADH (about 5 ATP) | 连接反应：2 NADH（约 5 ATP）
- Krebs cycle: 2 ATP + 6 NADH (about 15 ATP) + 2 FADH₂ (about 3 ATP) | 克雷布斯循环：2 ATP + 6 NADH（约 15 ATP）+ 2 FADH₂（约 3 ATP）
- Total: about 30–32 ATP per glucose | 总计：每分子葡萄糖约 30–32 ATP
The efficiency of aerobic respiration is roughly 32%, meaning about 32% of the energy in glucose is converted to ATP; the rest is lost as heat. Compared to anaerobic respiration (which captures only about 2% of the energy), aerobic respiration is far more efficient.
有氧呼吸的效率大约为 32%，即葡萄糖中约 32% 的能量转化为 ATP，其余以热的形式散失。与无氧呼吸（仅捕获约 2% 的能量）相比，有氧呼吸效率高得多。
The role of NAD and FAD as hydrogen carriers is critical throughout respiration. They link the early stages (glycolysis, link reaction, Krebs cycle) to the ETC, ensuring the stepwise release of energy and its coupling to ATP synthesis.
NAD 和 FAD 作为氢载体在整个呼吸过程中起着关键作用。它们将前期阶段（糖酵解、连接反应、克雷布斯循环）与电子传递链连接起来，确保能量逐步释放并与 ATP 合成相耦联。
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