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

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

Respiration is the process by which cells break down organic molecules, primarily glucose, to release energy in the form of ATP. In AQA A-Level Biology, you must understand both aerobic and anaerobic respiration, including the four main stages: glycolysis, the link reaction, the Krebs cycle, and the electron transport chain. You are expected to know the precise locations within the cell, the key inputs and outputs, the role of coenzymes (NAD, FAD, coenzyme A), and how ATP is synthesised by substrate-level phosphorylation and oxidative phosphorylation.

呼吸作用是细胞分解有机分子（主要是葡萄糖）以释放ATP能量的过程。在AQA A-Level生物课程中，你需要掌握有氧呼吸和无氧呼吸，包括四个主要阶段：糖酵解、链接反应、克雷布斯循环和电子传递链。你需要准确知道各阶段在细胞内的发生位置、关键的输入和输出、辅酶（NAD、FAD、辅酶A）的作用，以及ATP如何通过底物水平磷酸化和氧化磷酸化合成。

1. Overview of Respiration | 呼吸作用概述

Respiration is not simply ‘breathing’; it is a series of enzyme-controlled reactions that release chemical energy from organic molecules. In eukaryotes, 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 C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy as ATP). However, this equation hides the complexity of the multi-step pathways.

呼吸作用不仅仅是“呼吸”；它是一系列酶控反应，从有机分子中释放化学能。在真核生物中，有氧呼吸需要氧气并产生大量ATP，而无氧呼吸在无氧条件下发生，产生的ATP少得多。有氧呼吸的总方程式为：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O（+ 以ATP形式存在的能量）。然而，这个方程式隐藏了多步骤途径的复杂性。

2. Glycolysis | 糖酵解

Glycolysis takes place in the cytoplasm of the cell. It is the first stage of both aerobic and anaerobic respiration. Glucose (a 6-carbon sugar) is phosphorylated by 2 ATP to make it more reactive, then it splits into two 3-carbon molecules called triose phosphate (TP). Each TP is oxidised by removing hydrogen, which is transferred to NAD to form 2 reduced NAD (NADH). During this oxidation, 4 ATP are produced directly by substrate-level phosphorylation, giving a net yield of 2 ATP (since 2 were used) and 2 NADH per glucose. The final product is two molecules of pyruvate.

糖酵解发生在细胞质中。它是有氧呼吸和无氧呼吸的第一阶段。葡萄糖（一种6碳糖）被2个ATP磷酸化，使其更具反应性，然后分裂成两个3碳分子，称为磷酸丙糖（TP）。每个TP通过脱氢氧化，氢被传递给NAD形成2个还原型NAD（NADH）。在此氧化过程中，通过底物水平磷酸化直接产生4个ATP，净产2个ATP（因为使用了2个），每分子葡萄糖产生2个NADH。最终产物是两分子丙酮酸。

The key points to remember: NAD is reduced to NADH; substrate-level phosphorylation occurs when a phosphate group is transferred from a donor molecule directly to ADP; no oxygen is required here.

需要记住的关键点：NAD被还原为NADH；底物水平磷酸化是指磷酸基团从供体分子直接转移给ADP的过程；此阶段不需要氧气。

3. The Link Reaction | 链接反应

Pyruvate enters the mitochondrial matrix via active transport. Once inside, each pyruvate (3C) is decarboxylated (CO₂ is removed) and oxidised (hydrogen is removed). The hydrogen is accepted by NAD to form reduced NAD. The remaining 2-carbon acetyl group combines with coenzyme A (CoA) to form acetyl coenzyme A (acetyl CoA). Since two pyruvates are produced from one glucose, the link reaction occurs twice per glucose.

丙酮酸通过主动运输进入线粒体基质。进入后，每个丙酮酸（3C）发生脱羧（去除CO₂）和氧化（去除氢）。氢被NAD接受形成还原型NAD。剩下的2碳乙酰基与辅酶A（CoA）结合，形成乙酰辅酶A（乙酰CoA）。由于一分子葡萄糖产生两分子丙酮酸，每分子葡萄糖的链接反应发生两次。

Summary per glucose: 2 acetyl CoA, 2 CO₂, and 2 reduced NAD are produced. No ATP is made directly in this step.

每分子葡萄糖总结：产生2个乙酰CoA、2个CO₂和2个还原型NAD。此步骤不直接产生ATP。

4. The Krebs Cycle | 克雷布斯循环

The Krebs cycle occurs in the mitochondrial matrix. Acetyl CoA (2C) delivers its acetyl group to a 4-carbon acceptor molecule (oxaloacetate) to form a 6-carbon citrate. Citrate is then converted back to oxaloacetate in a series of oxidation-reduction and decarboxylation reactions. During one turn of the cycle, two CO₂ molecules are released, and the molecule is oxidised to regenerate oxaloacetate. The removed hydrogen atoms are accepted by NAD and FAD, producing 3 reduced NAD and 1 reduced FAD. Additionally, one ATP (or GTP) is produced by substrate-level phosphorylation. Since two acetyl CoA molecules enter per glucose, the cycle turns twice, doubling these products.

克雷布斯循环发生在线粒体基质中。乙酰CoA（2C）将其乙酰基传递给一个4碳受体分子（草酰乙酸），形成6碳柠檬酸。柠檬酸随后通过一系列氧化还原和脱羧反应转化回草酰乙酸。循环每转一圈，释放两个CO₂分子，分子被氧化以再生草酰乙酸。脱下的氢原子被NAD和FAD接受，产生3个还原型NAD和1个还原型FAD。此外，通过底物水平磷酸化产生1个ATP（或GTP）。由于每分子葡萄糖进入两个乙酰CoA，循环转两圈，这些产物加倍。

Per glucose: 4 CO₂, 6 reduced NAD, 2 reduced FAD, and 2 ATP are produced. The CO₂ you exhale comes from these decarboxylation steps.

每分子葡萄糖：产生4个CO₂、6个还原型NAD、2个还原型FAD和2个ATP。你呼出的CO₂就来自这些脱羧步骤。

5. The Electron Transport Chain and Oxidative Phosphorylation | 电子传递链与氧化磷酸化

All the reduced NAD and FAD molecules from the previous stages donate their electrons and protons to the electron transport chain (ETC), located on the inner mitochondrial membrane (cristae). The electrons pass through a series of protein complexes (I, II, III, IV) and mobile carriers, moving to successively lower energy levels. The energy released is used to pump protons (H⁺) from the matrix into the intermembrane space, creating a proton gradient (a high concentration of H⁺ in the intermembrane space).

前几个阶段产生的所有还原型NAD和FAD将其电子和质子提供给位于线粒体内膜（嵴）上的电子传递链（ETC）。电子通过一系列蛋白质复合物（I、II、III、IV）和移动载体传递，逐级下降至较低能级。释放的能量用于将质子（H⁺）从基质泵入膜间隙，形成质子梯度（膜间隙中H⁺浓度高）。

Oxygen acts as the final electron acceptor. It combines with electrons and protons to form water. If oxygen is absent, electrons cannot be passed on, the chain backs up, and no proton gradient is generated. Therefore, oxygen is essential for aerobic respiration.

氧气充当最终的电子受体。它与电子和质子结合形成水。如果没有氧气，电子无法传递，链会阻塞，就不会产生质子梯度。因此，氧气对有氧呼吸至关重要。

Chemiosmosis: The protons then diffuse back into the matrix through ATP synthase (a channel protein). This flow down the electrochemical gradient drives the synthesis of ATP from ADP and inorganic phosphate. This process is called oxidative phosphorylation because it relies on the energy from oxidation reactions.

化学渗透：质子随后通过ATP合酶（一种通道蛋白）流回基质。这种沿电化学梯度向下的流动驱动ADP和无机磷酸合成ATP。这一过程称为氧化磷酸化，因为它依赖氧化反应的能量。

6. ATP Yield in Aerobic Respiration | 有氧呼吸的ATP产量

The theoretical maximum yield of ATP from one glucose molecule is often quoted as 38 ATP, but in eukaryotes the actual yield is closer to 30–32 ATP. This discrepancy arises because the NADH produced during glycolysis must be shuttled into the mitochondria, which uses energy. You are not required to memorise exact numbers for AQA, but you should understand that the majority of ATP comes from oxidative phosphorylation, with a smaller contribution from substrate-level phosphorylation in glycolysis and the Krebs cycle.

一分子葡萄糖的理论最大ATP产量常被引述为38个ATP，但在真核生物中实际产量接近30–32个ATP。这种差异是因为糖酵解产生的NADH需要穿梭进入线粒体，这需要耗能。在AQA考试中你不必记忆确切数字，但应理解大部分ATP来自氧化磷酸化，糖酵解和克雷布斯循环中的底物水平磷酸化贡献较小。

Stage	Net ATP (per glucose)	Reduced coenzymes
Glycolysis	2	2 NADH
Link Reaction	0	2 NADH
Krebs Cycle	2	6 NADH, 2 FADH₂
Oxidative phosphorylation	~28	Uses above

Note: each NADH can produce about 2.5 ATP, and each FADH₂ about 1.5 ATP, due to the different entry points into the electron transport chain.

注意：由于进入电子传递链的位点不同，每个NADH大约可产生2.5个ATP，每个FADH₂大约1.5个ATP。

7. Anaerobic Respiration in Animals and Plants | 动物和植物的无氧呼吸

In the absence of oxygen, the electron transport chain cannot operate. However, glycolysis can still produce 2 ATP and 2 NADH per glucose. To keep glycolysis running, the NADH must be re-oxidised back to NAD. This is achieved by transferring the hydrogen from reduced NAD to pyruvate (or a derivative), a process called fermentation.

在无氧条件下，电子传递链无法运行。但糖酵解仍可每分子葡萄糖产生2个ATP和2个NADH。为使糖酵解继续进行，必须将NADH重新氧化为NAD。这通过将还原型NAD的氢转移给丙酮酸（或其衍生物）来实现，这一过程称为发酵。

In animals: Pyruvate accepts hydrogen from reduced NAD and is converted to lactic acid (lactate) by the enzyme lactate dehydrogenase. This occurs in the cytoplasm. Lactic acid can accumulate in muscles causing fatigue. After exercise, oxygen debt must be repaid to oxidise lactic acid back to pyruvate, which can then enter aerobic pathways.

在动物中：丙酮酸接受来自还原型NAD的氢，在乳酸脱氢酶作用下转化为乳酸（乳酸盐）。这发生在细胞质中。乳酸可在肌肉中积累导致疲劳。运动后，需偿还氧债将乳酸氧化回丙酮酸，然后进入有氧途径。

In plants and some fungi: Pyruvate is first decarboxylated to ethanal (acetaldehyde), which then accepts hydrogen from reduced NAD to form ethanol. This process is called alcoholic fermentation. NAD is regenerated, allowing glycolysis to continue.

在植物和某些真菌中：丙酮酸首先脱羧生成乙醛，然后乙醛接受来自还原型NAD的氢形成乙醇。这一过程称为酒精发酵。NAD得以再生，使糖酵解能继续进行。

Both types of anaerobic respiration yield only 2 ATP per glucose, far less than the ~30–32 ATP from aerobic respiration.

两种类型的无氧呼吸每分子葡萄糖仅产2个ATP，远少于有氧呼吸的约30–32个ATP。

8. Coenzymes and Their Roles | 辅酶及其作用

NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) are crucial hydrogen carriers. They accept hydrogen atoms (protons and electrons) and become reduced. They then transport these to the electron transport chain, where they are oxidised, releasing the protons and electrons. Coenzyme A (CoA) carries acetyl groups into the Krebs cycle. Without these coenzymes, respiration would halt because intermediates would not be oxidised effectively.

NAD（烟酰胺腺嘌呤二核苷酸）和FAD（黄素腺嘌呤二核苷酸）是关键的氢载体。它们接受氢原子（质子和电子）而被还原，然后将它们运至电子传递链，在那里被氧化，释放质子和电子。辅酶A（CoA）将乙酰基带入克雷布斯循环。没有这些辅酶，呼吸作用会停止，因为中间产物无法有效氧化。

9. Experimental Evidence: Using Respirometers | 实验证据：使用呼吸计

Respirometers measure the rate of respiration by monitoring oxygen consumption or carbon dioxide production. A simple respirometer consists of a sealed chamber containing the organism, connected to a manometer. Soda lime or potassium hydroxide is used to absorb CO₂, so any change in volume is due to O₂ uptake. The movement of a coloured liquid in the manometer can be recorded over time. Variables such as temperature, substrate type, or germination stage can be investigated. Data can be used to calculate respiration rates and compare aerobic and anaerobic contributions.

呼吸计通过监测耗氧量或二氧化碳产生量来测量呼吸速率。简单的呼吸计由一个装有生物的密封室和与之相连的液体压力计组成。用碱石灰或氢氧化钾吸收CO₂，因此体积的任何变化都是由于O₂的消耗。压力计中着色液体的移动可随时间记录。可以研究温度、底物类型或萌发阶段等变量。数据可用于计算呼吸速率并比较有氧和无氧的贡献。

10. Summary of Key Points for Exams | 考试要点总结

Glycolysis in cytoplasm; net 2 ATP, 2 NADH, 2 pyruvate.
Link reaction in matrix: pyruvate → acetyl CoA; CO₂ released, NAD reduced.
Krebs cycle in matrix: acetyl CoA → 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP per turn.
ETC on cristae; oxygen is final electron acceptor; proton gradient drives ATP synthase.
Anaerobic: animal cells produce lactic acid; plant cells/yeast produce ethanol and CO₂.
Coenzymes are vital: NAD/FAD carry hydrogen; CoA carries acetyl groups.
Substrate-level phosphorylation vs. oxidative phosphorylation.

糖酵解在细胞质中；净产2个ATP，2个NADH，2个丙酮酸。
链接反应在基质中：丙酮酸 → 乙酰CoA；释放CO₂，NAD被还原。
克雷布斯循环在基质中：乙酰CoA → 每圈2个CO₂，3个NADH，1个FADH₂，1个ATP。
电子传递链在嵴上；氧气是最终电子受体；质子梯度驱动ATP合酶。
无氧呼吸：动物细胞产生乳酸；植物细胞/酵母产生乙醇和CO₂。
辅酶至关重要：NAD/FAD 携带氢；辅酶A 携带乙酰基。
区分底物水平磷酸化与氧化磷酸化。

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

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