A-Level Biology: Respiration – Key Exam Points | A-Level 生物：呼吸作用考点精讲

📚 A-Level Biology: Respiration – Key Exam Points | A-Level 生物：呼吸作用考点精讲

Cellular respiration is one of the most fundamental and heavily examined topics in A-Level Biology. It covers the elegant ways in which cells harvest energy from organic molecules, either in the presence or absence of oxygen. A clear understanding of the four stages of aerobic respiration – glycolysis, link reaction, Krebs cycle and oxidative phosphorylation – together with anaerobic pathways, ATP yields and experimental techniques, is essential for top marks. This guide breaks down every key point into bite‑sized, paired English and Chinese explanations to help you master the content.

细胞呼吸是 A-Level 生物学中最基础也最常考的主题之一。它展示了细胞如何在有氧或无氧条件下优雅地从有机分子中获取能量。透彻理解有氧呼吸的四个阶段——糖酵解、连接反应、克雷布斯循环和氧化磷酸化，以及无氧途径、ATP 产率和实验技术，是取得高分的关键。本文把每个考点拆解为简洁的英文与中文对照讲解，助你完全掌握。

1. Overview of Respiration | 呼吸作用概述

Cellular respiration is the controlled release of energy from organic compounds to produce ATP. It is not the same as breathing (ventilation). Breathing is the physical movement of air, whereas respiration is a series of enzyme‑catalysed reactions inside cells. ATP acts as the universal energy currency, coupling energy‑releasing catabolic reactions to energy‑requiring anabolic processes.

细胞呼吸是从有机化合物中受控释放能量以产生 ATP 的过程。它与呼吸运动（通气）不同。呼吸运动是空气的物理进出，而细胞呼吸是细胞内一系列酶促反应。ATP 是通用的能量货币，将放能分解反应与需能合成过程耦联在一起。

Aerobic respiration requires oxygen and fully oxidises substrates to CO₂ and H₂O, yielding around 32–38 molecules of ATP per glucose. Anaerobic respiration occurs in the absence of oxygen, only partially oxidises glucose and produces just 2 ATP per glucose molecule. The overall equation for aerobic respiration of glucose is:

有氧呼吸需要氧气，将底物彻底氧化为 CO₂ 和 H₂O，每个葡萄糖分子约产生 32–38 个 ATP。无氧呼吸在没有氧气的情况下进行，仅使葡萄糖部分氧化，每个葡萄糖只能产生 2 个 ATP。葡萄糖有氧呼吸的总方程式为：

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

2. Glycolysis | 糖酵解

Glycolysis takes place in the cytoplasm of the cell and does not require oxygen. It splits one molecule of glucose (6‑carbon) into two molecules of pyruvate (3‑carbon). The process can be divided into four main stages: phosphorylation, lysis, oxidation and ATP formation.

糖酵解发生在细胞质中，不需要氧气。它将一分子葡萄糖（6碳）分解为两分子丙酮酸（3碳）。该过程可分为四个主要阶段：磷酸化、裂解、氧化和 ATP 生成。

Phosphorylation: Two ATP molecules are used to add phosphate groups to glucose, making it more reactive. This produces fructose‑1,6‑bisphosphate.

磷酸化：消耗两分子 ATP 将磷酸基团加到葡萄糖上，使其更具反应活性，生成果糖‑1,6‑二磷酸。

Lysis: The 6‑carbon sugar is split into two molecules of triose phosphate (3‑carbon each).

裂解：6碳糖被分解为两分子磷酸丙糖（各含3碳）。

Oxidation: Hydrogen atoms are removed from triose phosphate by the coenzyme NAD⁺, forming reduced NAD (NADH). At the same time, two molecules of ATP are produced per triose phosphate by substrate‑level phosphorylation.

氧化：辅酶 NAD⁺ 从磷酸丙糖上移走氢原子，形成还原态 NAD（NADH）。同时，每分子磷酸丙糖通过底物水平磷酸化产生两分子 ATP。

Net products per glucose: 2 ATP (4 produced minus 2 used), 2 reduced NAD and 2 pyruvate molecules. Glycolysis is the sole source of ATP in cells that lack mitochondria, such as red blood cells, and it provides the substrates for the link reaction when oxygen is present.

葡萄糖的净产物：2 ATP（产生4个、消耗2个）、2 还原态 NAD 和 2 分子丙酮酸。糖酵解是红细胞等没有线粒体的细胞唯一的 ATP 来源；在有氧时，它为连接反应提供底物。

3. The Link Reaction | 连接反应

The link reaction occurs in the mitochondrial matrix. Each pyruvate (3C) is transported into the mitochondrion, where it undergoes oxidative decarboxylation. The enzyme pyruvate dehydrogenase removes a carbon atom in the form of CO₂ and oxidises the remaining 2‑carbon fragment, which then combines with coenzyme A to form acetyl CoA.

连接反应发生在线粒体基质中。每个丙酮酸（3C）被转运至线粒体，在那里经历氧化脱羧。丙酮酸脱氢酶以 CO₂ 的形式移除一个碳原子，氧化剩下的2碳片段，然后此片段与辅酶 A 结合形成乙酰辅酶 A。

During this reaction, one molecule of CO₂ is released and one molecule of reduced NAD is produced per pyruvate. Since one glucose yields two pyruvates, the link reaction occurs twice per glucose molecule, giving: 2 acetyl CoA, 2 CO₂ and 2 reduced NAD. No ATP is made directly in this stage.

在此反应中，每分子丙酮酸释放一分子 CO₂ 并产生一分子还原态 NAD。因为一个葡萄糖产生两分子丙酮酸，连接反应每个葡萄糖进行两次，生成：2 乙酰辅酶 A、2 CO₂ 和 2 还原态 NAD。该阶段不直接产生 ATP。

4. The Krebs Cycle | 克雷布斯循环

The Krebs cycle (citric acid cycle) also takes place in the mitochondrial matrix. Each acetyl CoA (2C) combines with a 4‑carbon molecule, oxaloacetate, to form a 6‑carbon citrate. Citrate is then progressively oxidised and decarboxylated through a cycle of reactions that regenerates oxaloacetate, ready to combine with another acetyl CoA.

克雷布斯循环（柠檬酸循环）同样发生在线粒体基质中。每分子乙酰辅酶 A（2C）与 4 碳分子草酰乙酸结合，形成 6 碳的柠檬酸。柠檬酸经过一系列氧化和脱羧反应，重新生成草酰乙酸，准备与下一个乙酰辅酶 A 结合。

For every turn of the cycle, the following are produced: 2 CO₂ molecules, 1 ATP (as GTP by substrate‑level phosphorylation), 3 reduced NAD and 1 reduced FAD. Because two acetyl CoA molecules enter the cycle per glucose, the total yield per glucose molecule from the Krebs cycle is: 4 CO₂, 2 ATP, 6 reduced NAD and 2 reduced FAD.

每个循环产生：2 CO₂、1 ATP（以 GTP 形式通过底物水平磷酸化）、3 还原态 NAD 和 1 还原态 FAD。由于每分子葡萄糖提供两分子乙酰辅酶 A，因而每个葡萄糖在克雷布斯循环中的总产量为：4 CO₂、2 ATP、6 还原态 NAD 和 2 还原态 FAD。

The reduced coenzymes (NADH and FADH₂) carry high‑energy electrons to the electron transport chain, where the bulk of ATP is produced. The CO₂ released is the waste product exhaled from the body.

还原态辅酶（NADH 和 FADH₂）将高能电子携带至电子传递链，绝大部分 ATP 在那里生成。释放的 CO₂ 就是呼出的代谢废物。

5. Oxidative Phosphorylation | 氧化磷酸化

Oxidative phosphorylation is the final stage of aerobic respiration and takes place on the inner mitochondrial membrane. It involves the electron transport chain (ETC) and chemiosmosis. Reduced NAD and reduced FAD donate electrons to the chain, which are then passed through a series of protein complexes (I, II, III, IV) and mobile carriers.

氧化磷酸化是有氧呼吸的最后阶段，发生在线粒体内膜上。它包括电子传递链（ETC）和化学渗透。还原态 NAD 和还原态 FAD 把电子传递给电子传递链，电子随后经过一系列蛋白质复合体（I、II、III、IV）和可移动载体依次传递。

As electrons move down the chain, energy is released and used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space. This creates an electrochemical gradient (a proton motive force) across the inner membrane. Protons then flow back into the matrix through ATP synthase, a protein channel that uses the energy of this flow to synthesise ATP from ADP + Pi. This coupling of electron transport to ATP synthesis is called chemiosmosis.

电子沿链传递时释放能量，用于将质子（H⁺）从线粒体基质泵入膜间腔。这就在内膜两侧建立起电化学梯度（质子动力势）。质子随后通过 ATP 合酶流回基质，该蛋白质通道利用质子流中的能量催化 ADP + Pi 合成 ATP。这种电子传递与 ATP 合成相耦联的机制称为化学渗透。

Oxygen acts as the final electron acceptor, combining with electrons and protons to form water: 4e⁻ + 4H⁺ + O₂ → 2H₂O. Without oxygen, electrons would back up and the ETC would halt, stopping ATP synthesis via oxidative phosphorylation.

氧是最终的电子受体，与电子和质子结合生成水：4e⁻ + 4H⁺ + O₂ → 2H₂O。没有氧气，电子将堵塞，电子传递链停止，通过氧化磷酸化合成 ATP 的过程便会中断。

On average, each reduced NAD yields about 2.5 ATP, and each reduced FAD yields about 1.5 ATP because FADH₂ donates electrons at a later point in the chain, resulting in fewer protons being pumped.

平均而言，每分子还原态 NAD 约产生 2.5 个 ATP，每分子还原态 FAD 约产生 1.5 个 ATP，因为 FADH₂ 在电子传递链更下游处提供电子，导致泵出的质子数量较少。

6. Anaerobic Respiration in Animals and Plants | 动物与植物的无氧呼吸

When oxygen is unavailable, oxidative phosphorylation cannot occur. Cells rely solely on glycolysis for ATP, and they must regenerate NAD⁺ from reduced NAD so that glycolysis can continue. This is achieved by anaerobic pathways that differ between animals and yeast/plants.

当没有氧气时，氧化磷酸化无法进行。细胞只能依靠糖酵解产生 ATP，并且必须将还原态 NAD 重新氧化为 NAD⁺，以便糖酵解持续进行。这一目的通过在动物与酵母/植物中不同的无氧途径实现。

In animals (lactate fermentation): Pyruvate (3C) is reduced by reduced NAD to form lactate (3C). The enzyme lactate dehydrogenase catalyses this reaction, regenerating NAD⁺. The overall ATP yield is just 2 ATP per glucose. Lactate can be transported to the liver and converted back to glucose when oxygen becomes available (Cori cycle), but its accumulation in muscles can cause temporary fatigue.

在动物中（乳酸发酵）：丙酮酸（3C）被还原态 NAD 还原为乳酸（3C）。乳酸脱氢酶催化该反应，再生 NAD⁺。每个葡萄糖仅产生 2 ATP。当氧气恢复时，乳酸可被运至肝脏并重新转变为葡萄糖（科里循环），但乳酸在肌肉中的堆积会引起暂时性疲劳。

In plants and yeast (alcoholic fermentation): Pyruvate is first decarboxylated to form ethanal (2C), releasing CO₂, and then ethanal is reduced by reduced NAD to ethanol (2C). This also regenerates NAD⁺. The net ATP yield is again 2 per glucose. The CO₂ produced causes dough to rise and gives beer its fizz.

在植物和酵母中（酒精发酵）：丙酮酸首先脱羧形成乙醛（2C），释放 CO₂，然后乙醛被还原态 NAD 还原为乙醇（2C）。这同样再生了 NAD⁺。每个葡萄糖净产 2 ATP。产生的 CO₂ 使面团膨胀，也使啤酒产生气泡。

Animals: C₃H₄O₃ + reduced NAD → C₃H₆O₃ + NAD⁺

Plants/yeast: C₃H₄O₃ → C₂H₄O + CO₂ ; C₂H₄O + reduced NAD → C₂H₅OH + NAD⁺

7. ATP Yield – Total Energy Balance | ATP 产率——总能量账

The theoretical maximum ATP yield from the complete oxidation of one glucose molecule differs between prokaryotes and eukaryotes. In prokaryotes, which have no mitochondrial transport costs, the maximum is 38 ATP. In eukaryotic cells, the transport of cytoplasmic NADH into the mitochondrion uses energy, giving a typical net yield of about 30–36 ATP depending on the shuttle used.

一个葡萄糖被彻底氧化产生的理论最大 ATP 量在原核生物和真核生物中不同。原核生物没有线粒体穿梭能耗，最多可产生 38 个 ATP。在真核细胞中，将细胞质 NADH 运入线粒体需要消耗能量，根据所用的穿梭系统，典型净得约 30–36 ATP。

The table below summarises the ATP contributed by each stage (based on 2.5 ATP per reduced NAD and 1.5 ATP per reduced FAD, with a glycerol‑phosphate shuttle giving 1.5 ATP per cytoplasmic NADH).

下表汇总了各阶段贡献的 ATP（基于每还原态 NAD 产生 2.5 ATP、每还原态 FAD 产生 1.5 ATP，并采用磷酸甘油穿梭使每分子胞质 NADH 产生 1.5 ATP）。

Stage	Direct ATP / GTP	Reduced NAD	Reduced FAD	Total ATP equivalent
Glycolysis (cytoplasm)	2 (net)	2	0	2 + (2 × 1.5) = 5
Link reaction (matrix)	0	2	0	2 × 2.5 = 5
Krebs cycle (matrix)	2 (GTP)	6	2	2 + (6 × 2.5) + (2 × 1.5) = 20
Total (eukaryote)	4	10	2	~30 – 32 ATP

Note that the malate‑aspartate shuttle can yield 2.5 ATP per cytoplasmic NADH, raising the total closer to 36 ATP. Exam boards often quote a range of 30–38; be sure to learn the figure expected in your specification.

注意，苹果酸‑天冬氨酸穿梭可使每分子胞质 NADH 产生 2.5 ATP，使总数接近 36 ATP。考试局通常给出 30–38 的范围；务必记住你考试大纲要求的数值。

8. Respiratory Substrates and Respiratory Quotient (RQ) | 呼吸底物与呼吸商 (RQ)

Not all respiration uses glucose alone. Cells can also oxidise lipids, amino acids and other carbohydrates. The respiratory quotient (RQ) is a useful indicator of the type of substrate being respired. It is defined as:

呼吸作用并非只利用葡萄糖。细胞还可以氧化脂质、氨基酸和其他碳水化合物。呼吸

Published by TutorHao | A-Level Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

A-Level Biology: Respiration – Key Exam Points | A-Level 生物：呼吸作用 考点精讲