A-Level Biology: Cellular Respiration — Glycolysis to ATP | 细胞呼吸完整指南

Cellular respiration is one of the most fundamental and frequently examined topics in A-Level Biology. This comprehensive guide covers every stage of aerobic respiration — from glycolysis to oxidative phosphorylation — with detailed explanations, key equations, and examiner tips.

What is Cellular Respiration?

Cellular respiration is the metabolic process by which cells break down organic molecules (primarily glucose) to release energy in the form of ATP (adenosine triphosphate). ATP serves as the universal energy currency of the cell, powering processes such as active transport, muscle contraction, and biosynthesis.

The overall equation for aerobic respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (≈38 ATP)

Aerobic respiration occurs in four main stages, each taking place in a specific cellular location:

1. Glycolysis — cytoplasm
2. Link Reaction — mitochondrial matrix
3. Krebs Cycle (Citric Acid Cycle) — mitochondrial matrix
4. Oxidative Phosphorylation (Electron Transport Chain + Chemiosmosis) — inner mitochondrial membrane (cristae)

Stage 1: Glycolysis

Glycolysis (from Greek: glykys = “sweet”, lysis = “splitting”) is the first stage of both aerobic and anaerobic respiration. It occurs in the cytoplasm of the cell and does not require oxygen.

Key Steps of Glycolysis

Glycolysis converts one molecule of glucose (6C) into two molecules of pyruvate (3C each). The process can be divided into two phases:

Energy Investment Phase (Phosphorylation)

• Glucose is phosphorylated to glucose phosphate using 1 ATP → ADP
• Glucose phosphate is phosphorylated again to hexose bisphosphate using another ATP → ADP
• Hexose bisphosphate splits into two molecules of triose phosphate (3C each)
• 2 ATP molecules have been consumed so far

Energy Payoff Phase (Oxidation)

• Each triose phosphate is oxidised (loses hydrogen) to form pyruvate
• The hydrogen atoms are transferred to NAD⁺, forming reduced NAD (NADH)
• 4 ATP molecules are produced by substrate-level phosphorylation

Net Products of Glycolysis (per glucose)

Stage 2: The Link Reaction

Also known as the pyruvate oxidation or bridge reaction, this stage links glycolysis to the Krebs Cycle. It occurs in the mitochondrial matrix.

For each pyruvate molecule entering the mitochondrion:

1. Pyruvate is decarboxylated (CO₂ is removed) by the enzyme pyruvate dehydrogenase
2. Pyruvate is oxidised (hydrogen is removed) — the hydrogen is accepted by NAD⁺ to form NADH
3. The resulting 2-carbon acetyl group combines with coenzyme A (CoA) to form acetyl CoA

Pyruvate (3C) + CoA + NAD⁺ → Acetyl CoA (2C) + CO₂ + NADH

Products per glucose (×2 pyruvate): 2 Acetyl CoA, 2 CO₂, 2 NADH

Stage 3: The Krebs Cycle

The Krebs Cycle (also called the Citric Acid Cycle or TCA Cycle), discovered by Sir Hans Krebs in 1937, takes place in the mitochondrial matrix. It is a cyclical series of oxidation-reduction reactions.

Key Steps (per turn — one acetyl CoA)

1. Formation of citrate (6C): Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C). CoA is released and recycled.
2. Decarboxylation and dehydrogenation: Citrate is converted to a 5C compound, releasing CO₂. NAD⁺ is reduced to NADH.
3. Second decarboxylation and dehydrogenation: The 5C compound is converted to a 4C compound, releasing another CO₂. Another NAD⁺ is reduced to NADH.
4. Substrate-level phosphorylation: ATP is produced directly from ADP + Pi.
5. FAD reduction: FAD is reduced to FADH₂.
6. Final NAD⁺ reduction: Another NAD⁺ is reduced to NADH. Oxaloacetate (4C) is regenerated, ready to accept the next acetyl CoA.

Products per turn of the Krebs Cycle

Stage 4: Oxidative Phosphorylation

Oxidative phosphorylation is the final and most ATP-productive stage of aerobic respiration. It occurs on the inner mitochondrial membrane (cristae) and consists of two linked processes:

4a. Electron Transport Chain (ETC)

The ETC is a series of electron carriers (protein complexes I–IV) embedded in the inner mitochondrial membrane. These carriers have progressively higher electron affinity (more electronegative), allowing electrons to flow downhill energetically.

1. Reduced NAD (NADH) donates electrons to Complex I. Reduced FAD (FADH₂) donates electrons to Complex II.
2. Electrons pass from one carrier to the next in a series of redox reactions
3. As electrons move, the energy released is used to pump protons (H⁺) from the matrix into the intermembrane space
4. This creates an electrochemical proton gradient (higher H⁺ concentration in the intermembrane space)
5. Oxygen acts as the final electron acceptor at Complex IV, combining with electrons and H⁺ to form water: ½O₂ + 2H⁺ + 2e⁻ → H₂O

4b. Chemiosmosis

Chemiosmosis is the process by which the proton gradient drives ATP synthesis, proposed by Peter Mitchell (Nobel Prize, 1978):

• Protons accumulate in the intermembrane space, creating a proton motive force
• H⁺ ions flow back into the matrix through ATP synthase (a transmembrane protein complex)
• The flow of protons through ATP synthase causes it to rotate, catalysing the reaction: ADP + Pi → ATP
• This is called oxidative phosphorylation because it couples oxidation (electron transport) with phosphorylation (ATP synthesis)

ATP Yield from Oxidative Phosphorylation

Total ATP Balance Sheet (per glucose)

Anaerobic Respiration (When Oxygen is Limited)

When oxygen is unavailable, the electron transport chain cannot operate. Organisms must regenerate NAD⁺ from NADH to keep glycolysis running. Two main pathways exist:

Anaerobic Respiration in Animals (Lactate Fermentation)

In mammalian muscle cells during intense exercise:

• Pyruvate is reduced to lactate by the enzyme lactate dehydrogenase
• NADH is oxidised back to NAD⁺, allowing glycolysis to continue
• Only 2 ATP are produced per glucose (from glycolysis only)

Pyruvate + NADH → Lactate + NAD⁺

Anaerobic Respiration in Yeast and Plants (Alcoholic Fermentation)

• Pyruvate is decarboxylated to ethanal (2C), releasing CO₂
• Ethanal is reduced to ethanol by alcohol dehydrogenase, using NADH
• NAD⁺ is regenerated, allowing glycolysis to continue
• Only 2 ATP are produced per glucose

Pyruvate → Ethanal + CO₂ → Ethanol + NAD⁺

Respirometers: Measuring Respiration Rate

A respirometer is used to measure the rate of respiration by monitoring oxygen consumption or carbon dioxide production. A typical setup includes:

• A sealed chamber containing the organism (e.g., germinating seeds or invertebrates)
• A manometer (U-tube containing coloured liquid) to measure pressure changes
• A CO₂ absorbent (e.g., soda lime or KOH solution) to remove CO₂, so pressure change reflects O₂ consumption only
• A control tube (with inert material of equal volume) to compensate for temperature/pressure fluctuations

Key Terminology Glossary

Common Exam Questions

1. Describe the process of glycolysis. (4–6 marks) — Cover location, phosphorylation, lysis, oxidation, net products.
2. Explain the role of NAD and FAD in respiration. (3–4 marks) — They are hydrogen carriers; they accept H atoms to become reduced (NADH/FADH₂); they transport H to the ETC for oxidative phosphorylation.
3. Explain why oxygen is essential for aerobic respiration. (2–3 marks) — Oxygen is the final electron acceptor in the ETC; without it, electrons cannot pass through the chain, ETC stops, NADH cannot be reoxidised, Krebs Cycle and link reaction cannot continue.
4. Compare aerobic and anaerobic respiration. (4–6 marks) — Include: oxygen requirement, location of stages, ATP yield, end products.

细胞呼吸是A-Level生物课程中最基础且最常考的主题之一。这份全面指南涵盖有氧呼吸的每个阶段——从糖酵解到氧化磷酸化——包含详细解释、关键方程式和考官提示。

什么是细胞呼吸？

细胞呼吸是细胞分解有机分子（主要是葡萄糖）以释放能量并生成ATP（三磷酸腺苷）的代谢过程。ATP是细胞的通用能量货币，为主动运输、肌肉收缩和生物合成等过程提供动力。

有氧呼吸的总方程式为：

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量（≈38 ATP）

有氧呼吸分为四个主要阶段，每个阶段发生在特定的细胞位置：

1. 糖酵解 — 细胞质
2. 连接反应 — 线粒体基质
3. 克雷布斯循环（柠檬酸循环） — 线粒体基质
4. 氧化磷酸化（电子传递链 + 化学渗透） — 线粒体内膜（嵴）

第一阶段：糖酵解

糖酵解（源自希腊语：glykys = “甜”，lysis = “分裂”）是有氧和无氧呼吸的共同第一阶段。它发生在细胞的细胞质中，不需要氧气。

糖酵解的关键步骤

糖酵解将一分子葡萄糖（6C）转化为两分子丙酮酸（各3C）。该过程可分为两个阶段：

能量投入阶段（磷酸化）

• 葡萄糖被磷酸化为磷酸葡萄糖，消耗1 ATP → ADP
• 磷酸葡萄糖再次被磷酸化为己糖二磷酸，再消耗1 ATP → ADP
• 己糖二磷酸分裂为两分子磷酸丙糖（各3C）
• 至此已消耗2分子ATP

能量回报阶段（氧化）

• 每个磷酸丙糖被氧化（失去氢）形成丙酮酸
• 氢原子传递给NAD⁺，形成还原型NAD（NADH）
• 通过底物水平磷酸化产生4分子ATP

糖酵解的净产物（每分子葡萄糖）

第二阶段：连接反应

也称为丙酮酸氧化或桥梁反应，此阶段将糖酵解与克雷布斯循环连接起来。它发生在线粒体基质中。

对于每个进入线粒体的丙酮酸分子：

1. 丙酮酸在丙酮酸脱氢酶的作用下脱羧（移除CO₂）
2. 丙酮酸被氧化（移除氢）——氢由NAD⁺接收形成NADH
3. 生成的2碳乙酰基与辅酶A（CoA）结合形成乙酰辅酶A

丙酮酸（3C）+ CoA + NAD⁺ → 乙酰辅酶A（2C）+ CO₂ + NADH

每分子葡萄糖的产物（×2丙酮酸）：2乙酰辅酶A，2CO₂，2NADH

第三阶段：克雷布斯循环

克雷布斯循环（也称柠檬酸循环或TCA循环），由汉斯·克雷布斯爵士于1937年发现，发生在线粒体基质中。这是一系列循环的氧化还原反应。

关键步骤（每轮——一个乙酰辅酶A）

1. 柠檬酸（6C）的生成：乙酰辅酶A（2C）与草酰乙酸（4C）结合形成柠檬酸（6C）。CoA被释放并循环使用。
2. 脱羧和脱氢：柠檬酸转化为5C化合物，释放CO₂。NAD⁺被还原为NADH。
3. 第二次脱羧和脱氢：5C化合物转化为4C化合物，释放另一个CO₂。另一个NAD⁺被还原为NADH。
4. 底物水平磷酸化：直接从ADP + Pi产生ATP。
5. FAD还原：FAD被还原为FADH₂。
6. 最终NAD⁺还原：另一个NAD⁺被还原为NADH。草酰乙酸（4C）再生，准备接收下一个乙酰辅酶A。

克雷布斯循环每轮的产物

第四阶段：氧化磷酸化

氧化磷酸化是有氧呼吸的最终阶段，也是ATP产量最高的阶段。它发生在线粒体内膜（嵴）上，由两个相互关联的过程组成：

4a. 电子传递链（ETC）

ETC是嵌入线粒体内膜的一系列电子载体（蛋白复合体I–IV）。这些载体具有逐渐增高的电子亲和力（更电负性），使电子能够沿能量梯度向下流动。

1. 还原型NAD（NADH）向复合体I提供电子。还原型FAD（FADH₂）向复合体II提供电子。
2. 电子通过一系列氧化还原反应从一个载体传递到下一个载体
3. 当电子移动时，释放的能量用于将质子（H⁺）从基质泵入膜间隙
4. 这创造了一个电化学质子梯度（膜间隙中H⁺浓度更高）
5. 氧气在复合体IV处作为最终电子受体，与电子和H⁺结合生成水：½O₂ + 2H⁺ + 2e⁻ → H₂O

4b. 化学渗透

化学渗透是质子梯度驱动ATP合成的过程，由彼得·米切尔提出（1978年诺贝尔奖）：

• 质子在膜间隙积累，产生质子动力势
• H⁺离子通过ATP合酶（一种跨膜蛋白复合体）流回基质
• 质子流过ATP合酶使其旋转，催化反应：ADP + Pi → ATP
• 这被称为氧化磷酸化，因为它将氧化（电子传递）与磷酸化（ATP合成）耦合在一起

总ATP平衡表（每分子葡萄糖）

无氧呼吸（氧气受限时）

当没有氧气时，电子传递链无法运作。生物体必须从NADH再生NAD⁺以维持糖酵解运行。存在两种主要途径：

动物中的无氧呼吸（乳酸发酵）

在剧烈运动期间的哺乳动物肌肉细胞中：

• 丙酮酸被乳酸脱氢酶还原为乳酸
• NADH被氧化回NAD⁺，使糖酵解得以继续
• 每分子葡萄糖仅产生2 ATP（仅来自糖酵解）

丙酮酸 + NADH → 乳酸 + NAD⁺

酵母和植物中的无氧呼吸（酒精发酵）

• 丙酮酸脱羧为乙醛（2C），释放CO₂
• 乙醛被乙醇脱氢酶还原为乙醇，使用NADH
• NAD⁺再生，使糖酵解得以继续
• 每分子葡萄糖仅产生2 ATP

丙酮酸 → 乙醛 + CO₂ → 乙醇 + NAD⁺

呼吸计：测量呼吸速率

呼吸计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。典型设置包括：

• 一个密封室，含有生物体（如萌发中的种子或无脊椎动物）
• 一个压力计（装有有色液体的U型管）用于测量压力变化
• CO₂吸收剂（如碱石灰或KOH溶液）用于移除CO₂，因此压力变化仅反映O₂消耗
• 对照管（装有等体积的惰性材料）用于补偿温度/压力波动

常见考试问题

1. 描述糖酵解的过程。（4–6分）——涵盖位置、磷酸化、裂解、氧化、净产物。
2. 解释NAD和FAD在呼吸中的作用。（3–4分）——它们是氢载体；接受氢原子变成还原型（NADH/FADH₂）；将氢运送到ETC进行氧化磷酸化。
3. 解释为什么氧气对于有氧呼吸至关重要。（2–3分）——氧气是ETC中的最终电子受体；没有它，电子无法通过链，ETC停止，NADH无法重新氧化，克雷布斯循环和连接反应无法继续。
4. 比较有氧呼吸和无氧呼吸。（4–6分）——包括：氧气需求、各阶段的位置、ATP产量、终产物。

This bilingual guide covers all key concepts in A-Level Biology Cellular Respiration. Practice applying these concepts to past paper questions for exam success. | 这份双语指南涵盖了A-Level生物细胞呼吸的所有关键概念。结合历年真题练习这些概念，助你考试成功。