Respiration in OCR A-Level Biology: Key Points Revision | A-Level OCR 生物：呼吸作用考点精讲

📚 Respiration in OCR A-Level Biology: Key Points Revision | A-Level OCR 生物：呼吸作用考点精讲

Respiration is a fundamental biological process that releases energy from organic molecules in a series of enzyme-controlled reactions. In the OCR A-Level Biology specification, understanding the stages of aerobic and anaerobic respiration, the roles of coenzymes, the structure of the mitochondrion, and the concept of the respiratory quotient is essential. This article covers every key point you need to succeed in the exam.

呼吸作用是通过一系列酶控反应从有机分子中释放能量的基本生物学过程。在 OCR A-Level 生物学大纲中，理解有氧与无氧呼吸的各阶段、辅酶的作用、线粒体的结构以及呼吸商的概念是考试的关键。本文涵盖了你在考试中取得好成绩所需的每个要点。

1. Overview of Respiration | 呼吸作用概述

Respiration is the process by which organic molecules, typically glucose, are broken down to release energy in the form of ATP (adenosine triphosphate). Aerobic respiration requires oxygen and yields a large amount of ATP, while anaerobic respiration occurs in the absence of oxygen and yields much less ATP. The overall equation for aerobic respiration is:

呼吸作用是将有机分子（通常是葡萄糖）分解以 ATP（腺苷三磷酸）形式释放能量的过程。有氧呼吸需要氧气并产生大量 ATP，而无氧呼吸在缺氧时发生，产生的 ATP 少得多。有氧呼吸的总方程式为：

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP + heat)

This process is not a single reaction but a metabolic pathway involving four main stages: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. Each stage occurs in a specific location within the cell or mitochondrion.

这一过程不是单一反应，而是包括四个主要阶段的代谢途径：糖酵解、连接反应、克雷布斯循环和氧化磷酸化。每个阶段都发生在细胞或线粒体的特定部位。

2. Glycolysis | 糖酵解

Glycolysis takes place in the cytoplasm of the cell and does not require oxygen. It involves the splitting of one molecule of glucose (a 6-carbon sugar) into two molecules of pyruvate (a 3-carbon compound). The process consists of phosphorylation and oxidation reactions.

糖酵解发生在细胞质中，不需要氧气。它涉及将一个葡萄糖分子（六碳糖）分解为两个丙酮酸分子（三碳化合物）。该过程包括磷酸化和氧化反应。

First, glucose is phosphorylated using 2 molecules of ATP to form hexose bisphosphate. This makes the molecule more reactive and prevents it from leaving the cell. Hexose bisphosphate then splits into two molecules of triose phosphate. Each triose phosphate is oxidised to form pyruvate. During this oxidation, hydrogen atoms are transferred to the coenzyme NAD⁺, forming 2 molecules of reduced NAD (NADH). Additionally, 4 molecules of ATP are produced by substrate-level phosphorylation, giving a net yield of 2 ATP molecules since 2 ATP were used initially.

首先，葡萄糖利用 2 分子 ATP 磷酸化形成己糖二磷酸。这使得分子更加活泼并防止其离开细胞。然后己糖二磷酸分解为两分子磷酸丙糖。每个磷酸丙糖被氧化形成丙酮酸。在此氧化过程中，氢原子转移到辅酶 NAD⁺，形成 2 分子还原型 NAD（NADH）。此外，通过底物水平磷酸化产生 4 分子 ATP，由于最初消耗了 2 分子 ATP，净得 2 分子 ATP。

The overall products from glycolysis per glucose molecule are: 2 pyruvate, 2 ATP (net), and 2 NADH. These pyruvate molecules are then actively transported into the mitochondrial matrix for the next stage.

每个葡萄糖分子在糖酵解中的总产物为：2 丙酮酸、2 ATP（净）、2 NADH。这些丙酮酸分子随后被主动运输到线粒体基质中进行下一步反应。

3. Link Reaction | 连接反应

The link reaction occurs in the mitochondrial matrix. Its name reflects its role in linking glycolysis to the Krebs cycle. In this step, each pyruvate (3C) is converted into acetyl coenzyme A (acetyl CoA, a 2-carbon molecule).

连接反应发生在线粒体基质中。其名称反映了它连接糖酵解和克雷布斯循环的作用。在此步骤中，每个丙酮酸（3C）转化为乙酰辅酶 A（乙酰 CoA，一个二碳分子）。

Pyruvate enters the mitochondrial matrix via active transport. It undergoes decarboxylation, meaning one carbon atom is removed as a molecule of carbon dioxide (CO₂). At the same time, it is oxidised, and the removed hydrogen atoms are accepted by NAD⁺, forming one molecule of reduced NAD per pyruvate. The remaining 2-carbon acetyl group combines with coenzyme A to form acetyl CoA. Since two pyruvate molecules enter from one glucose molecule, the link reaction occurs twice per glucose.

丙酮酸通过主动运输进入线粒体基质。它发生脱羧反应，即一个碳原子以二氧化碳（CO₂）分子形式被移除。同时，它被氧化，脱下的氢原子由 NAD⁺ 接受，每个丙酮酸形成一分子还原型 NAD。剩余的 2-碳乙酰基团与辅酶 A 结合形成乙酰 CoA。由于一个葡萄糖分子产生两个丙酮酸，连接反应每个葡萄糖发生两次。

The products of the link reaction per glucose are: 2 acetyl CoA, 2 CO₂, and 2 NADH. No ATP is produced directly.

每个葡萄糖在连接反应中的产物为：2 乙酰 CoA、2 CO₂、2 NADH。没有直接产生 ATP。

4. Krebs Cycle | 克雷布斯循环

The Krebs cycle (also known as the citric acid cycle) takes place in the mitochondrial matrix. It is a closed cycle of enzyme-controlled reactions that further oxidises the acetyl group from acetyl CoA, producing energy carriers and CO₂.

克雷布斯循环（也称柠檬酸循环）发生在线粒体基质中。它是一个酶控反应的闭环，进一步氧化来自乙酰 CoA 的乙酰基团，产生能量载体和 CO₂。

Acetyl CoA (2C) combines with a 4-carbon compound, oxaloacetate, to form a 6-carbon compound, citrate. Coenzyme A is released and can be reused. Citrate is then converted through a series of reactions back into oxaloacetate. During the cycle, two decarboxylations occur, releasing 2 molecules of CO₂. Hydrogen atoms are removed by dehydrogenase enzymes and transferred to coenzymes: NAD⁺ is reduced to NADH (three times), and FAD is reduced to FADH₂ (once). One molecule of ATP is produced directly by substrate-level phosphorylation (from GTP, which can be converted to ATP).

乙酰 CoA（2C）与一个四碳化合物草酰乙酸结合，形成六碳化合物柠檬酸。辅酶 A 被释放并可再利用。柠檬酸随后通过一系列反应重新转化为草酰乙酸。在循环中发生两次脱羧，释放 2 分子 CO₂。氢原子由脱氢酶移除并转移到辅酶：NAD⁺ 三次被还原为 NADH，FAD 一次被还原为 FADH₂。通过底物水平磷酸化直接产生一分子 ATP（来自 GTP，可转化为 ATP）。

Since the link reaction yields two acetyl CoA per glucose, the Krebs cycle turns twice for each glucose molecule. The total products per glucose are: 4 CO₂, 6 NADH, 2 FADH₂, and 2 ATP (by substrate-level phosphorylation).

由于每个葡萄糖的连接反应产生两个乙酰 CoA，克雷布斯循环每个葡萄糖转动两次。每葡萄糖总产物为：4 CO₂、6 NADH、2 FADH₂、2 ATP（底物水平磷酸化）。

5. Oxidative Phosphorylation: Electron Transport Chain | 氧化磷酸化：电子传递链

Oxidative phosphorylation is the final stage of aerobic respiration and occurs on the inner mitochondrial membrane (cristae). It involves two linked processes: the electron transport chain and chemiosmosis. This stage produces the vast majority of ATP.

氧化磷酸化是有氧呼吸的最后阶段，发生在线粒体内膜（嵴）上。它涉及两个相互关联的过程：电子传递链和化学渗透。这一阶段产生了绝大部分的 ATP。

The reduced coenzymes NADH and FADH₂, produced in earlier stages, donate their hydrogen atoms. These hydrogen atoms are split into protons (H⁺) and electrons (e⁻). The electrons are passed along a series of protein complexes and carriers in the inner membrane, known as the electron transport chain. As electrons move from one carrier to the next, they lose energy. This energy is used by the protein complexes to pump protons from the mitochondrial matrix into the intermembrane space.

在前几个阶段产生的还原型辅酶 NADH 和 FADH₂ 提供它们的氢原子。这些氢原子分裂为质子（H⁺）和电子（e⁻）。电子沿内膜上一系列蛋白质复合体和载体传递，称为电子传递链。当电子从一个载体移动到下一个载体时，它们失去能量。蛋白质复合体利用此能量将质子从线粒体基质泵入膜间隙。

This creates an electrochemical gradient (a proton motive force) across the inner membrane, with a higher concentration of protons in the intermembrane space than in the matrix. The electrons are finally accepted by oxygen, the terminal electron acceptor, which combines with protons to form water: ½O₂ + 2e⁻ + 2H⁺ → H₂O. Without oxygen, electrons would back up along the chain, halting oxidative phosphorylation.

这在内膜上形成了电化学梯度（质子驱动力），膜间隙中的质子浓度高于基质。电子最终被末端电子受体氧接受，氧与质子结合形成水：½O₂ + 2e⁻ + 2H⁺ → H₂O。如果没有氧，电子会在链上堆积，使氧化磷酸化停止。

6. Chemiosmosis and ATP Synthesis | 化学渗透与ATP合成

Chemiosmosis is the process by which the proton gradient across the inner mitochondrial membrane drives ATP synthesis. The membrane is impermeable to protons, so they can only cross back into the matrix through specialised channel proteins called ATP synthase (stalked particles).

化学渗透是指线粒体内膜两侧的质子梯度驱动 ATP 合成的过程。内膜对质子不通透，因此质子只能通过称为 ATP 合酶（柄颗粒）的特殊通道蛋白流回基质。

As protons flow down their concentration gradient through ATP synthase, the enzyme rotates and catalyses the phosphorylation of ADP to ATP: ADP + Pᵢ → ATP. This is known as oxidative phosphorylation because the energy comes from the oxidation of reduced coenzymes.

当质子顺浓度梯度流过 ATP 合酶时，该酶旋转并催化 ADP 磷酸化生成 ATP：ADP + Pᵢ → ATP。这被称为氧化磷酸化，因为能量来自还原型辅酶的氧化。

Each NADH that enters the electron transport chain yields approximately 2.5 ATP molecules, and each FADH₂ yields about 1.5 ATP (because FADH₂ donates electrons further down the chain, resulting in fewer protons pumped). The theoretical maximum ATP yield from one molecule of glucose in aerobic respiration is around 30–32 ATP, though values vary slightly depending on the precise mechanism.

进入电子传递链的每个 NADH 大约产生 2.5 分子 ATP，每个 FADH₂ 大约产生 1.5 分子 ATP（因为 FADH₂ 在链的更下游提供电子，导致泵出的质子较少）。一分子葡萄糖在有氧呼吸中的理论最大 ATP 产量约为 30-32 ATP，尽管数值根据精确机制略有变化。

Stage	Location	ATP directly made	Reduced coenzymes
Glycolysis	Cytoplasm	2 (net)	2 NADH
Link reaction	Matrix	0	2 NADH
Krebs cycle	Matrix	2	6 NADH, 2 FADH₂
Oxidative phosphorylation	Inner membrane	~26–28	NADH and FADH₂ oxidised

7. Anaerobic Respiration in Animals: Lactate Fermentation | 动物中的无氧呼吸：乳酸发酵

When oxygen is absent or in short supply, cells cannot carry out the link reaction, Krebs cycle, or oxidative phosphorylation. Instead, only glycolysis continues, as it does not require oxygen. However, glycolysis needs a supply of NAD⁺ to accept hydrogen atoms. In aerobic conditions, reduced NAD is reoxidised via the electron transport chain, but in anaerobic conditions, an alternative pathway must regenerate NAD⁺.

当缺乏氧气或供氧不足时，细胞无法进行连接反应、克雷布斯循环或氧化磷酸化。此时只有糖酵解继续进行，因为它不需要氧气。然而，糖酵解需要 NAD⁺ 来接受氢原子。在有氧条件下，还原型 NAD 通过电子传递链重新氧化，但在无氧条件下，必须通过另一途径再生 NAD⁺。

In animal cells, the pyruvate produced in glycolysis is reduced to lactate (lactic acid) by the enzyme lactate dehydrogenase. This reaction oxidises NADH back to NAD⁺, allowing glycolysis to continue producing a small amount of ATP. The overall equation is: pyruvate + NADH → lactate + NAD⁺. The build-up of lactate in muscles causes a fall in pH and may lead to muscle fatigue. After exercise, lactate can be converted back to pyruvate in the liver when oxygen is available (oxygen debt).

在动物细胞中，糖酵解产生的丙酮酸在乳酸脱氢酶作用下还原为乳酸。此反应将 NADH 重新氧化为 NAD⁺，使糖酵解能够继续产生少量 ATP。总方程式为：丙酮酸 + NADH → 乳酸 + NAD⁺。肌肉中乳酸的积累会导致 pH 下降，可能引起肌肉疲劳。运动后，当氧气充足时，乳酸可以在肝脏中被重新转化为丙酮酸（氧债）。

8. Anaerobic Respiration in Yeast: Ethanol Fermentation | 酵母中的无氧呼吸：酒精发酵

In some organisms such as yeast and certain plants, anaerobic respiration follows an ethanol fermentation pathway. Like in animals, glycolysis continues, but the regeneration of NAD⁺ occurs differently.

在某些生物（如酵母和某些植物）中，无氧呼吸遵循酒精发酵途径。与动物一样，糖酵解继续进行，但 NAD⁺ 的再生方式不同。

Pyruvate is first decarboxylated to form ethanal (acetaldehyde), releasing CO₂. Ethanal is then reduced by the enzyme alcohol dehydrogenase, using reduced NAD to form ethanol, which regenerates NAD⁺. The overall equation is: glucose → 2 ethanol + 2 CO₂ + 2 ATP. Ethanol is the end product characteristic of alcoholic fermentation, and the CO₂ released causes bread to rise or gives sparkling wine its bubbles.

丙酮酸首先脱羧形成乙醛，释放 CO₂。然后乙醛在醇脱氢酶作用下，利用还原型 NAD 被还原为乙醇，同时再生 NAD⁺。总方程式为：葡萄糖 → 2 乙醇 + 2 CO₂ + 2 ATP。乙醇是酒精发酵的特征性终产物，释放的 CO₂ 使面团膨胀或使起泡酒产生气泡。

A key exam point is to compare lactate and ethanol fermentation: both regenerate NAD⁺, but only ethanol fermentation releases CO₂, and the end products differ.

一个关键考点是比较乳酸发酵与酒精发酵：两者都再生 NAD⁺，但只有酒精发酵释放 CO₂，且终产物不同。

9. Respiratory Substrates and Respiratory Quotient | 呼吸底物与呼吸商

A respiratory substrate is any organic molecule that can be respired to release energy. Although glucose is the primary substrate, cells can also use lipids and amino acids. Lipids, when hydrolysed to fatty acids and glycerol, generate more ATP per gram than carbohydrates because they are more reduced and contain less oxygen.

呼吸底物是任何可通过呼吸作用释放能量的有机分子。虽然葡萄糖是主要底物，但细胞也可以利用脂质和氨基酸。脂质水解为脂肪酸和甘油后，每克产生的 ATP 比碳水化合物更多，因为它们还原度更高且含氧量更少。

The respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed in a given period: RQ = CO₂ produced / O₂ consumed. The RQ value gives an indication of the respiratory substrate being used. For carbohydrates, RQ = 1.0; for lipids, RQ ≈ 0.7; for proteins, RQ ≈ 0.9. Measurements of RQ can be made using a respirometer.

呼吸商（RQ）是在一定时间内产生的二氧化碳与消耗的氧气的比值：RQ = 产生的 CO₂ / 消耗的 O₂。RQ 值可以指示正在使用的呼吸底物。对于碳水化合物，RQ = 1.0；对于脂质，RQ ≈ 0.7；对于蛋白质，RQ ≈ 0.9。RQ 的测量可以使用呼吸计进行。

10. Factors Affecting Respiration Rate | 影响呼吸速率的因素

Several factors influence the rate of respiration in living organisms. Understanding these helps interpret experimental data and physiological responses.

有几个因素会影响生物体的呼吸速率。理解这些有助于解释实验数据和生理反应。

Temperature: Respiration is controlled by enzymes, so it increases with temperature up to an optimum, then denaturation occurs and the rate declines sharply.

温度：呼吸作用由酶控制，因此随温度升高而加快，直至最适温度，随后酶变性，速率急剧下降。
Oxygen concentration: For aerobic respiration, oxygen is the final electron acceptor. Limiting oxygen reduces the rate of oxidative phosphorylation.

氧气浓度：对于有氧呼吸，氧是末端电子受体。限制氧气会降低氧化磷酸化的速率。
Glucose availability: Substrate concentration affects the rate of glycolysis and the entire pathway.

葡萄糖可用性：底物浓度影响糖酵解和整个途径的速率。
Carbon dioxide concentration: High CO₂ levels can lower pH and inhibit certain enzymes, potentially slowing respiration.

二氧化碳浓度：高 CO₂ 水平可降低 pH 并抑制某些酶，可能减慢呼吸作用。

11. Methods to Measure Respiration: Respirometers | 测量呼吸的方法：呼吸计

A respirometer is a device used to measure the rate of respiration by monitoring either oxygen consumption or carbon dioxide production. A typical simple respirometer consists of a sealed chamber containing the organism, connected to a capillary tube containing a drop of coloured liquid. As the organism respires, it absorbs oxygen and produces CO₂. If a chemical such as soda lime (or potassium hydroxide solution) is added to absorb CO₂, the change in volume of gas is due solely to oxygen consumption. The movement of the liquid droplet along the capillary can be timed to calculate the rate of oxygen uptake.

呼吸计是用于通过监测氧气消耗量或二氧化碳产生量来测量呼吸速率的装置。典型的简单呼吸计由一个容纳生物体的密封室组成，连接到装有有色液滴的毛细管。当生物体呼吸时，吸收氧气并产生 CO₂。如果加入苏打石灰（或氢氧化钾溶液）等化学物质来吸收 CO₂，则气体体积的变化完全由氧气消耗引起。液滴沿毛细管的移动可以计时，以计算氧气吸收速率。

Variables to control include temperature (using a water bath), organism mass, and equilibration time. Respirometers can also compare RQ values by measuring volume changes with and without CO₂ absorption.

需要控制的变量包括温度（使用水浴）、生物体质量和平衡时间。呼吸计还可以通过测量有/无 CO₂ 吸收时的体积变化来比较 RQ 值。

12. Importance of Respiration in Living Organisms | 呼吸作用在生物体中的重要性

Respiration is essential because it provides ATP, the universal energy currency of cells. ATP is used for anabolic reactions (such as protein synthesis, DNA replication), active transport across membranes, muscle contraction, nerve impulse transmission, and maintenance of body temperature in endotherms. Without respiration, these processes would cease, and life could not be sustained.

呼吸作用至关重要，因为它提供 ATP，即细胞的通用能量货币。ATP 用于合成代谢反应（如蛋白质合成、DNA 复制）、跨膜主动运输、肌肉收缩、神经冲动传递以及恒温动物的体温维持。没有呼吸作用，这些过程将停止，生命无法维持。

Additionally, the intermediates of the Krebs cycle are used as building blocks for other biomolecules, linking respiration to biosynthesis. For example, oxaloacetate can be converted to amino acids, and acetyl CoA can be used in fatty acid synthesis.

此外，克雷布斯循环的中间产物被用作其他生物分子的构件，将呼吸作用与生物合成联系起来。例如，草酰乙酸可转化为氨基酸，乙酰 CoA 可用于脂肪酸合成。

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Respiration in OCR A-Level Biology: Key Points Revision | A-Level OCR 生物：呼吸作用 考点精讲