Respiration for IGCSE WJEC Biology | IGCSE WJEC 生物：呼吸作用考点精讲

📚 Respiration for IGCSE WJEC Biology | IGCSE WJEC 生物：呼吸作用考点精讲

Respiration is a fundamental process that releases energy from food molecules, such as glucose, to fuel every activity in living cells. It is not simply ‘breathing’ – that is ventilation. In this article, we break down the key facts you need for the IGCSE WJEC Biology exam: from the structure of ATP and mitochondria to the differences between aerobic and anaerobic pathways, the critical experiments, and how your body deals with oxygen debt. Each explanation is given in both English and Chinese to support bilingual learners and help you master the topic fully.

呼吸作用是从食物分子（如葡萄糖）中释放能量以驱动活细胞内一切活动的根本过程。它不仅仅是“呼吸”——那叫作通气。在这篇文章中，我们将逐一拆解 IGCSE WJEC 生物考试的关键知识点：从 ATP 和线粒体的结构，到有氧与无氧途径的区别、重要的实验，以及身体如何处理氧债。每个解释都提供了中英双语版，帮助双语学习者全面掌握该主题。

1. What is Respiration? | 什么是呼吸作用？

Respiration is the chemical process by which energy is released from organic compounds, primarily glucose, inside living cells. It is an exothermic reaction because it transfers energy to the surroundings. Every living cell carries out respiration continuously to stay alive. The energy released is used to drive metabolic reactions such as active transport, protein synthesis, muscle contraction, nerve impulse transmission, and maintaining a constant body temperature in mammals.

呼吸作用是在活细胞内、从有机化合物（主要是葡萄糖）中释放能量的化学过程。它是一个放热反应，因为它将能量传递给周围环境。每个活细胞都持续进行呼吸作用以维持生命。释放的能量被用于驱动新陈代谢反应，例如主动运输、蛋白质合成、肌肉收缩、神经冲动的传递以及哺乳动物维持恒定体温。

The overall word equation for aerobic respiration is:
Overall word equation for aerobic respiration:

Glucose + Oxygen → Carbon dioxide + Water (+ Energy)

A more detailed balanced chemical equation is:
更为详细的配平化学方程式是：

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ Energy as ATP)

Respiration is controlled by enzymes, so factors like temperature and pH affect its rate. It is not the same as gaseous exchange (ventilation) – gaseous exchange simply supplies the oxygen needed for respiration and removes the carbon dioxide produced.

呼吸作用受酶控制，因此温度和 pH 等因素会影响其速率。呼吸作用与气体交换（通气）不同——气体交换只是提供呼吸作用所需的氧气，并排除产生的二氧化碳。

2. ATP – The Energy Currency of Cells | ATP——细胞的能量货币

The energy released during respiration is not used directly. Instead, it is stored in the high-energy bonds of a molecule called adenosine triphosphate (ATP). ATP is often called the ‘energy currency’ of cells because it can be transported around the cell and broken down easily to release energy exactly where and when it is needed.

呼吸作用释放的能量并不会被直接利用，而是储存在一个称为三磷酸腺苷（ATP）的分子中。ATP 常被称为细胞的“能量货币”，因为它可以在细胞内运输，并且很容易被分解，在需要的时间和地点精准释放能量。

ATP consists of a nitrogenous base (adenine), a sugar (ribose), and three phosphate groups. The bond between the second and third phosphate group is particularly high in energy. When a cell needs energy, an enzyme breaks this bond in a hydrolysis reaction:

ATP + H₂O → ADP + Pᵢ (+ Energy released)

ATP 由一个含氮碱基（腺嘌呤）、一个糖（核糖）和三个磷酸基团组成。第二和第三个磷酸基团之间的键尤其富含能量。当细胞需要能量时，酶会在水解反应中断开这个键：

ATP + H₂O → ADP + 无机磷酸盐 (Pᵢ) (+ 释放能量)

The products are adenosine diphosphate (ADP) and an inorganic phosphate group. The energy released can be used instantly for processes like active transport against a concentration gradient or muscle fibre contraction. ATP is a small, soluble molecule that can diffuse to all parts of the cell.

产物是二磷酸腺苷（ADP）和一个无机磷酸基团。释放的能量可以立即用于诸如逆浓度梯度主动运输或肌纤维收缩等过程。ATP 是一种较小的可溶性分子，可以扩散到细胞的所有部位。

3. The Mitochondrion – Site of Aerobic Respiration | 线粒体——有氧呼吸的场所

While the first stage of respiration (glycolysis) occurs in the cytoplasm, the later, oxygen-dependent stages take place inside mitochondria. Mitochondria are rod-shaped organelles, 1–10 µm long, found in the cytoplasm of almost all eukaryotic cells. Cells with high energy demands – such as muscle cells, sperm cells, and nerve cells – contain many thousands of mitochondria.

尽管呼吸作用的第一阶段（糖酵解）发生在细胞质中，但后续需氧的阶段在线粒体内进行。线粒体是杆状细胞器，长 1–10 微米，存在于几乎所有真核细胞的细胞质中。能量需求高的细胞——如肌肉细胞、精子细胞和神经细胞——含有成千上万个线粒体。

A mitochondrion has two membranes: a smooth outer membrane and a heavily folded inner membrane. The folds are called cristae, and they greatly increase the surface area available for the chemical reactions of the electron transport chain and ATP synthesis. The space enclosed by the inner membrane is the matrix, which contains enzymes for the Krebs cycle, as well as mitochondrial DNA and ribosomes. The inner membrane is impermeable to most ions, which allows a proton gradient to build up – essential for oxidative phosphorylation.

线粒体有两层膜：一层光滑的外膜和一层高度折叠的内膜。这些折叠称为嵴，它们极大地增加了电子传递链和 ATP 合成化学反应所需的表面积。内膜包围的空间是基质，其中含有克雷布斯循环所需的酶，还含有线粒体 DNA 和核糖体。内膜对大多数离子不通透，这使得质子梯度得以建立——这对于氧化磷酸化至关重要。

Adaptation of mitochondria to their function (key for WJEC):

线粒体对其功能的适应（WJEC 考试重点）：

The folded inner membrane (cristae) provides a large surface area for electron carrier proteins and ATP synthase enzymes. / 折叠的内膜（嵴）为电子载体蛋白和 ATP 合酶提供了巨大的表面积。
The matrix contains all the enzymes needed for the link reaction and Krebs cycle. / 基质含有连接反应和克雷布斯循环所需的所有酶。
A small intermembrane space allows a steep proton gradient to be established quickly. / 狭小的膜间隙使得能迅速建立陡峭的质子梯度。
Mitochondrial DNA means the organelle can make some of its own proteins rapidly. / 线粒体 DNA 意味着该细胞器可以快速合成自身的部分蛋白质。

4. Aerobic Respiration – The Complete Breakdown | 有氧呼吸——葡萄糖的彻底分解

Aerobic respiration uses oxygen to break down glucose completely into carbon dioxide and water. It releases a large amount of energy: theoretically, up to 38 molecules of ATP per glucose molecule. The process can be divided into four main stages: glycolysis, the link reaction, the Krebs cycle, and the electron transport chain. For IGCSE WJEC, you are not required to learn every intermediate compound, but you must understand the overall outcomes and the role of oxygen.

有氧呼吸利用氧气将葡萄糖彻底分解为二氧化碳和水，并释放大量能量：理论上，每个葡萄糖分子最多可产生 38 个 ATP。该过程可分为四个主要阶段：糖酵解、连接反应、克雷布斯循环和电子传递链。对于 IGCSE WJEC，你不需要记住每一个中间产物，但必须理解总体产物和氧气的作用。

Key summary of aerobic respiration stages:

有氧呼吸各阶段要点总结：

Stage 阶段	Location 位置	Inputs 输入	Outputs 输出
Glycolysis 糖酵解	Cytoplasm 细胞质	Glucose (6C) 葡萄糖（6C）	2 Pyruvate (3C), 2 ATP (net), 2 NADH
Link Reaction 连接反应	Mitochondrial matrix 线粒体基质	2 Pyruvate (3C) 2 丙酮酸（3C）	2 Acetyl CoA (2C), 2 CO₂, 2 NADH
Krebs Cycle 克雷布斯循环	Mitochondrial matrix 线粒体基质	2 Acetyl CoA (2C) 2 乙酰辅酶 A（2C）	4 CO₂, 2 ATP, 6 NADH, 2 FADH₂
Electron Transport Chain 电子传递链	Inner mitochondrial membrane 线粒体内膜	NADH, FADH₂, O₂	H₂O, ~34 ATP

Oxygen acts as the final electron acceptor in the electron transport chain. Without oxygen, the electrons would back up, NADH and FADH₂ would not be oxidised back to NAD⁺ and FAD, and the Krebs cycle would halt. This explains why oxygen is essential for aerobic respiration.

氧气在电子传递链中充当最终的电子受体。没有氧气，电子就会积聚，NADH 和 FADH₂ 无法被氧化回 NAD⁺ 和 FAD，克雷布斯循环就会停止。这解释了为什么氧气对有氧呼吸不可或缺。

5. Anaerobic Respiration in Animals – Lactic Acid Fermentation | 动物细胞的无氧呼吸——乳酸发酵

When oxygen is scarce, for example during vigorous exercise, animal cells can produce ATP by anaerobic respiration. Only glycolysis continues – it does not require oxygen. The pyruvate produced is converted into lactate (lactic acid) to regenerate NAD⁺, which allows glycolysis to keep running and keep producing a small amount of ATP.

当氧气稀缺时，例如剧烈运动期间，动物细胞可以通过无氧呼吸产生 ATP。只有糖酵解继续进行——它不需要氧气。产生的丙酮酸被转化为乳酸（乳酸盐）以再生 NAD⁺，从而使糖酵解得以持续进行，并产生少量 ATP。

The word equation for anaerobic respiration in animals is:
动物无氧呼吸的文字方程式：

Glucose → Lactic acid (+ small amount of ATP)

Chemical equation:
化学方程式：

C₆H₁₂O₆ → 2C₃H₆O₃ (+ ~2 ATP)

Lactic acid builds up in the muscles and causes muscle fatigue and cramping. It lowers the pH, which can interfere with enzyme function and muscle contraction. After exercise, the lactic acid must be removed. It is transported in the blood to the liver, where it is oxidised back to pyruvate and then either to carbon dioxide and water via aerobic respiration, or used to synthesise glucose. This recovery process requires extra oxygen – the oxygen debt.

乳酸在肌肉中积累，导致肌肉疲劳和痉挛。它会降低 pH 值，从而干扰酶的功能和肌肉收缩。运动后，乳酸必须被清除。它通过血液运输到肝脏，在肝中被氧化回丙酮酸，然后或通过有氧呼吸转化为二氧化碳和水，或用于合成葡萄糖。这个恢复过程需要额外的氧气——这就是氧债。

6. Anaerobic Respiration in Plants and Yeast – Alcoholic Fermentation | 植物和酵母的无氧呼吸——酒精发酵

Plants and microorganisms such as yeast carry out a different form of anaerobic respiration. Instead of producing lactic acid, they convert pyruvate to ethanol (alcohol) and carbon dioxide. This process is called alcoholic fermentation and is widely used in the production of bread, beer, and wine. Again, its primary purpose is to regenerate NAD⁺ so that glycolysis can continue supplying ATP under anaerobic conditions.

植物和酵母等微生物进行另一种形式的无氧呼吸。它们不产生乳酸，而是将丙酮酸转化为乙醇（酒精）和二氧化碳。这一过程称为酒精发酵，被广泛用于面包、啤酒和葡萄酒的生产。同样，其主要目的是再生 NAD⁺，以使糖酵解在无氧条件下能继续提供 ATP。

Word equation:
文字方程式：

Glucose → Ethanol + Carbon dioxide (+ small amount of ATP)

Chemical equation:
化学方程式：

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (+ ~2 ATP)

Alcoholic fermentation is economically important. In baking, the carbon dioxide produced causes dough to rise; the ethanol evaporates during cooking. In brewing, yeast ferments sugars to produce ethanol, which is the alcohol in drinks. For WJEC, you should be able to compare animal and plant/yeast anaerobic pathways and identify the different end products.

酒精发酵具有重要的经济意义。在烘焙中，产生的二氧化碳使面团膨松；乙醇在烘烤过程中蒸发。在酿造中，酵母发酵糖类产生乙醇，即饮品中的酒精。对于 WJEC 考试，你应能比较动物和植物/酵母的无氧途径，并识别不同的最终产物。

7. Comparing Aerobic and Anaerobic Respiration | 比较有氧呼吸与无氧呼吸

This comparison is a classic WJEC exam question. You must be able to state the key differences clearly.

这个比较是 WJEC 考试的经典题型。你必须能够清晰地陈述关键区别。

Feature 特征	Aerobic 有氧呼吸	Anaerobic 无氧呼吸
Oxygen requirement 需氧情况	Requires oxygen 需要氧气	Does not require oxygen 不需要氧气
Location in cell 细胞内场所	Cytoplasm and mitochondria 细胞质和线粒体	Cytoplasm only 仅细胞质
Breakdown of glucose 葡萄糖分解	Complete to CO₂ and H₂O 彻底分解为 CO₂ 和 H₂O	Incomplete 不彻底：lactic acid or ethanol + CO₂
ATP yield per glucose 每个葡萄糖的 ATP 产量	High (~36–38 ATP) 高（约 36–38 ATP）	Low (2 ATP) 低（2 ATP）
End products 终产物	CO₂ and H₂O	Animals: lactic acid 动物：乳酸 \| Plants/yeast: ethanol + CO₂
Speed of ATP production ATP 产生速度	Slower 较慢	Faster 较快 (provides immediate ATP 提供即时 ATP)

Note that anaerobic respiration produces ATP more quickly because it doesn’t rely on the many-step mitochondrial stages, but it is far less efficient in terms of energy extracted per glucose molecule. This is why organisms use aerobic respiration whenever possible.

请注意，无氧呼吸产生 ATP 的速度更快，因为它不依赖于多步的线粒体阶段，但从每个葡萄糖分子中提取能量的角度来说，效率要低得多。这就是为什么生物体尽可能进行有氧呼吸。

8. Oxygen Debt – Recovery After Exercise | 氧债——运动后的恢复

During vigorous exercise, the demand for ATP exceeds the rate at which oxygen can be supplied to the muscles. Muscle cells switch to anaerobic respiration, producing lactic acid. The accumulation of lactic acid creates an oxygen debt. This is the amount of extra oxygen needed after exercise to remove the lactic acid and restore the body to its resting state.

在剧烈运动期间，ATP 的需求超过了向肌肉供氧的速率。肌肉细胞转而进行无氧呼吸，产生乳酸。乳酸的积累就产生了氧债，即运动后清除乳酸并将身体恢复到静息状态所需的额外氧气量。

The oxygen debt is used to:

氧债被用于：

Convert lactic acid back to pyruvate in the liver. / 在肝脏中将乳酸转化回丙酮酸。
Oxidise the pyruvate through aerobic respiration, producing CO₂ and H₂O, or use it to resynthesise glucose (gluconeogenesis). / 通过有氧呼吸氧化丙酮酸，产生 CO₂ 和 H₂O，或将其用于再合成葡萄糖（糖异生）。
Replenish oxygen stores in myoglobin of muscle cells and in blood haemoglobin. / 补充肌肉细胞中肌红蛋白和血液中血红蛋白的氧储备。
Resynthesise ATP and phosphocreatine stores in muscles. / 重新合成肌肉中的 ATP 和磷酸肌酸储备。

This explains why you continue to breathe heavily and your heart rate remains elevated for some time after vigorous exercise – your body is repaying the oxygen debt.

这解释了为什么剧烈运动后你会持续大口呼吸、心率在一段时间内保持升高——你的身体正在偿还氧债。

9. Practical Investigation – Demonstrating Respiration | 实验探究——演示呼吸作用

WJEC expects you to know simple experiments that show living organisms respire and produce carbon dioxide. A classic setup uses germinating seeds or live invertebrates (like woodlice or maggots) and a hydrogencarbonate indicator or limewater.

WJEC 要求你了解显示生物体进行呼吸并产生二氧化碳的简单实验。一个经典的装置使用萌发种子或活的无脊椎动物（如潮虫或蛆）以及碳酸氢盐指示剂或石灰水。

Experiment outline:

实验概要：

Place germinating seeds on a wire mesh platform in a test tube, suspended above hydrogencarbonate indicator solution (or limewater). / 将萌发种子放在试管中的金属网平台上，悬挂在碳酸氢盐指示剂溶液（或石灰水）上方。
Seal the tube with a bung. Set up a control using boiled (dead) seeds. / 用塞子密封试管。设立对照，使用煮过的（死亡的）种子。
Observe the colour change of the indicator: red → yellow, indicating an increase in carbon dioxide concentration (due to respiration). Limewater turns milky/cloudy. / 观察指示剂的颜色变化：红 → 黄，表明二氧化碳浓度升高（由于呼吸作用）。石灰水变浑浊。

Another experiment measures the rate of respiration using a respirometer. This device measures oxygen uptake by an organism. Soda lime or potassium hydroxide solution is used to absorb the carbon dioxide produced, so any change in the volume of air inside the respirometer is due to oxygen consumption. The decrease in volume can be observed as a coloured liquid moving along a capillary tube.

另一个实验使用呼吸计测量呼吸速率。该装置测量生物体的氧气吸收量。使用钠石灰或氢氧化钾溶液吸收产生的二氧化碳，这样呼吸计内空气体积的任何变化都是由氧气消耗引起的。体积的减少可观察到有色液体沿毛细管移动。

Key variables to control: temperature (use a water bath), mass of organisms, and time. This is a common exam question on controlled variables and reliability.

需要控制的关键变量：温度（使用水浴）、生物体质量和时间。这是关于控制变量和可靠性的常见考题。

10. Gas Exchange and Its Link to Respiration | 气体交换及其与呼吸作用的关联

Respiration should not be confused with gaseous exchange, but the two are closely linked. Gas exchange is the physical process of obtaining oxygen from the environment and removing carbon dioxide. In mammals, this occurs in the alveoli of the lungs; in fish, it occurs across the gill filaments; in plants, it occurs through stomata and lenticels. Respiration, on the other hand, is the intracellular, enzyme-controlled process that uses oxygen and produces carbon dioxide.

呼吸作用不应与气体交换混淆，但两者密切相关。气体交换是从环境中获取氧气并排除二氧化碳的物理过程。在哺乳动物中，这发生在肺泡中；在鱼类中，发生在鳃丝上；在植物中，通过气孔和皮孔进行。而呼吸作用是细胞内受酶控制的过程，消耗氧气并产生二氧化碳。

The rate of gas exchange must match the rate of respiration to maintain efficient ATP production. Features such as a large surface area (alveoli, gills, spongy mesophyll), thin exchange surfaces (one cell thick), a steep concentration gradient maintained by ventilation and blood flow, and a moist surface are all adaptations to maximise gas exchange and thus support respiration.

气体交换的速率必须与呼吸作用的速率相匹配，以维持高效的 ATP 生产。诸如大的表面积（肺泡、鳃、海绵叶肉）、薄的交换表面（仅一个细胞厚）、通过通气和血液流动维持的陡峭浓度梯度，以及潮湿的表面，这些都是最大化气体交换从而支持呼吸作用的适应特征。

For WJEC, you may be asked to link the structure of the respiratory system to the needs of aerobic respiration, especially in the context of exercise and oxygen debt.

对于 WJEC 考试，你可能会被要求将呼吸系统的结构联系到有氧呼吸的需求，特别是在运动和氧债的背景下。

11. Quick Summary of Key Equations and Facts | 关键方程式和知识点速览

Here is a concise summary for last-minute revision. Remember these must-know equations:

以下是供考前冲刺的简洁总结。请记住这些必背方程式：

Aerobic: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Anaerobic (animals): C₆H₁₂O₆ → 2C₃H₆O₃

Anaerobic (plants/yeast): C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Important concepts:

重要概念：

Respiration is exothermic and releases energy for life processes. / 呼吸作用是放热反应，为生命过程释放能量。
ATP is the immediate energy carrier; it provides energy when the terminal phosphate bond is hydrolysed. / ATP 是即时能量载体；在其末端磷酸键水解时提供能量。
Aerobic respiration yields up to 38 ATP per glucose; anaerobic yields only 2 ATP per glucose. / 有氧呼吸每个葡萄糖最多产生 38 个 ATP；无氧呼吸每个葡萄糖仅产生 2 个 ATP。
Anaerobic respiration regenerates NAD⁺ to keep glycolysis going. / 无氧呼吸再生 NAD⁺ 以维持糖酵解。
Oxygen debt is the extra oxygen required to remove lactic acid after exercise. / 氧债是运动后清除乳酸所需的额外氧气。
Mitochondria are adapted by having folded cristae, matrix enzymes, and their own DNA. / 线粒体因具有折叠的嵴、基质酶和自身 DNA 而适应呼吸作用。

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Respiration for IGCSE WJEC Biology | IGCSE WJEC 生物：呼吸作用 考点精讲