Respiration | 呼吸作用

📚 Respiration | 呼吸作用

Respiration is the sequence of enzyme‑controlled reactions that break down respiratory substrates, such as glucose, to release energy in the form of ATP. This fundamental process powers every active living cell, from bacterial metabolism to human muscle contraction. In IB and WJEC Biology, understanding the precise stages, locations, and regulatory mechanisms of both aerobic and anaerobic pathways is essential for linking biochemistry to whole‑organ physiology.

呼吸作用是酶控反应序列，将葡萄糖等呼吸底物分解，释放 ATP 形式的能量。这个基本过程为一切活跃的活细胞供能，从细菌代谢到人类肌肉收缩皆如此。在 IB 和 WJEC 生物课程中，透彻理解有氧与无氧途径的精确阶段、发生部位和调控机制，是连接生物化学与整体生理学的关键。

1. The Necessity of Respiration | 呼吸作用的必要性

All living cells require a continuous supply of energy to perform essential functions such as active transport, muscle contraction, nerve impulse transmission, and biosynthesis of macromolecules. The immediate source of this energy is adenosine triphosphate (ATP). The hydrolysis of the terminal phosphate bond in ATP releases about 30.5 kJ mol⁻¹ of free energy, which can be directly coupled to endergonic cellular processes.

所有活细胞都需要持续的能量供应来进行主动运输、肌肉收缩、神经冲动传导和大分子的生物合成等重要功能。这种能量的直接来源是三磷酸腺苷（ATP）。ATP 末端磷酸键的水解释放约 30.5 kJ mol⁻¹ 的自由能，可直接耦合到吸能的细胞过程。

Respiration is not synonymous with breathing. In biology, ‘respiration’ refers to the intracellular, metabolic pathways that convert the chemical energy stored in organic fuels into ATP. Breathing, on the other hand, is the physical process of ventilating the lungs to facilitate gas exchange, supplying oxygen for aerobic respiration and removing carbon dioxide. The link between the two is that oxygen is the terminal electron acceptor in the electron transport chain, allowing aerobic respiration to proceed efficiently.

呼吸作用不等同于呼吸运动。在生物学中，“呼吸作用”指的是将储存于有机燃料中的化学能转化为 ATP 的细胞内的代谢途径。而呼吸运动是通气换气的物理过程，为有氧呼吸提供氧气并排出二氧化碳。二者的联系在于，氧气是电子传递链的末端电子受体，使有氧呼吸得以高效进行。

2. Overview of Aerobic and Anaerobic Pathways | 有氧与无氧途径概述

Aerobic respiration requires the presence of oxygen and can be summarized by the balanced equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP). It involves four main stages: glycolysis in the cytoplasm, the link reaction and Krebs cycle in the mitochondrial matrix, and oxidative phosphorylation on the inner mitochondrial membrane. Up to about 32 molecules of ATP can be produced per molecule of glucose under ideal conditions.

有氧呼吸需要氧气存在，其总方程式为：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量（ATP）。它包含四个主要阶段：细胞质中的糖酵解，线粒体基质中的链接反应和克雷布斯循环，以及线粒体内膜上的氧化磷酸化。每分子葡萄糖在理想条件下可产生多达约 32 分子 ATP。

Anaerobic respiration occurs in the absence of oxygen. Only glycolysis proceeds, followed by a fermentation pathway that regenerates NAD⁺ from NADH so that glycolysis can continue to produce a small amount of ATP. In animal cells, pyruvate is reduced to lactate; in yeast and plants, pyruvate is decarboxylated and then reduced to ethanol. Anaerobic respiration yields only 2 ATP per glucose, but it is vital for rapid energy supply during strenuous exercise or in waterlogged soils.

无氧呼吸在缺氧时发生。只有糖酵解继续进行，随后通过发酵途径从 NADH 再生 NAD⁺，使糖酵解能持续产生少量 ATP。在动物细胞中，丙酮酸被还原为乳酸；在酵母和植物中，丙酮酸脱羧后被还原为乙醇。无氧呼吸每分子葡萄糖仅产生 2 ATP，但对剧烈运动或缺水稻田中的快速能量供应至关重要。

3. Glycolysis | 糖酵解

Glycolysis is a ten‑step, enzyme‑catalyzed sequence that occurs in the cytoplasm. One molecule of glucose (6C) is phosphorylated using 2 ATP and then split into two molecules of triose phosphate (3C). Each triose phosphate is subsequently oxidized to pyruvate (3C) in a series of steps that generate ATP and reduce NAD⁺ to NADH.

糖酵解是发生在细胞质中的十步酶促反应序列。一分子葡萄糖（6C）先经磷酸化消耗 2 ATP，然后裂解为两分子磷酸丙糖（3C）。每分子磷酸丙糖随后通过一系列反应被氧化成丙酮酸（3C），此过程产生 ATP 并将 NAD⁺ 还原为 NADH。

The net yield of glycolysis per glucose molecule is 2 ATP (substrate‑level phosphorylation), 2 NADH, and 2 pyruvate molecules. All the carbon from glucose is conserved in pyruvate. Glycolysis does not require oxygen, making it the universal first stage of both aerobic and anaerobic respiration. The key regulatory enzyme, phosphofructokinase, is allosterically inhibited by ATP and citrate, ensuring that the pathway slows when the cell’s energy charge is high.

每分子葡萄糖糖酵解的净产量为 2 ATP（底物水平磷酸化）、2 NADH 和 2 分子丙酮酸。葡萄糖中的碳全部保留在丙酮酸中。糖酵解不依赖氧气，因此它是有氧和无氧呼吸的共同起始阶段。关键调节酶磷酸果糖激酶受 ATP 和柠檬酸的别构抑制，确保当细胞能量充足时途径减慢。

4. Link Reaction | 链接反应

If oxygen is available, each pyruvate molecule enters the mitochondrial matrix via active transport. Here the link reaction takes place: pyruvate is decarboxylated (CO₂ is removed) and oxidized, with the remaining 2‑carbon acetyl group transferred onto coenzyme A (CoA) to form acetyl‑CoA. One molecule of NAD⁺ is reduced to NADH per pyruvate.

若有氧气，每分子丙酮酸通过主动运输进入线粒体基质。在此发生链接反应：丙酮酸脱羧（移除 CO₂）并被氧化，剩余的二碳乙酰基转移到辅酶 A（CoA）上形成乙酰辅酶 A。每分子丙酮酸还原一分子 NAD⁺ 为 NADH。

Since one glucose molecule yields two pyruvates, the link reaction overall produces 2 acetyl‑CoA, 2 CO₂, and 2 NADH. No ATP is made directly. The acetyl‑CoA then enters the Krebs cycle, while the CO₂ diffuses out of the mitochondrion and cell, ultimately being exhaled. The link reaction is irreversible and is the point at which carbohydrate carbon enters the central metabolic furnace of the cell.

由于一分子葡萄糖产生两分子丙酮酸，链接反应总共产生 2 乙酰辅酶 A、2 CO₂ 和 2 NADH，不直接生成 ATP。乙酰辅酶 A 随后进入克雷布斯循环，而 CO₂ 扩散出线粒体和细胞，最终被呼出。链接反应不可逆，是碳水化合物碳进入细胞核心代谢熔炉的入口。

5. Krebs Cycle | 克雷布斯循环

The Krebs cycle (also called the citric acid cycle or TCA cycle) is a closed loop of enzyme‑controlled reactions in the mitochondrial matrix. Each acetyl‑CoA (2C) combines with a 4‑carbon oxaloacetate to form a 6‑carbon citrate. Over a series of decarboxylations, oxidations, and rearrangements, the citrate is progressively broken back down to oxaloacetate, releasing 2 CO₂ molecules, reducing 3 NAD⁺ to NADH and 1 FAD to FADH₂, and generating 1 ATP by substrate‑level phosphorylation.

克雷布斯循环（也称柠檬酸循环或 TCA 循环）是线粒体基质中的闭环酶控反应。每个乙酰辅酶 A（2C）与四碳的草酰乙酸结合，形成六碳柠檬酸。通过一系列脱羧、氧化和重排反应，柠檬酸逐步被降解回草酰乙酸，释放出 2 分子 CO₂，将 3 NAD⁺ 还原为 NADH、1 FAD 还原为 FADH₂，并以底物水平磷酸化产生 1 ATP。

Per glucose molecule, the cycle turns twice, yielding 4 CO₂, 6 NADH, 2 FADH₂, and 2 ATP. All the original carbon from glucose is now released as CO₂. Most of the energy extracted is stored temporarily in the reduced coenzymes NADH and FADH₂, which then donate electrons to the electron transport chain. The Krebs cycle is also amphibolic, providing precursors for amino acid and lipid synthesis.

每分子葡萄糖使循环运转两圈，共产生 4 CO₂、6 NADH、2 FADH₂ 和 2 ATP。葡萄糖原有碳现在全部以 CO₂ 释放。提取的大部分能量暂时储存在还原辅酶 NADH 和 FADH₂ 中，后者随后将电子提供给电子传递链。克雷布斯循环还具有两用代谢功能，为氨基酸和脂质合成提供前体。

6. Electron Transport Chain and Chemiosmosis | 电子传递链与化学渗透

The electron transport chain (ETC) is located on the inner mitochondrial membrane. Reduced coenzymes (NADH and FADH₂) from glycolysis, the link reaction, and the Krebs cycle donate electrons to a series of protein complexes and mobile carriers. Electrons pass from complex I or II through ubiquinone to complex III, then through cytochrome c to complex IV, where oxygen acts as the final electron acceptor, forming water: ½O₂ + 2e⁻ + 2H⁺ → H₂O.

电子传递链（ETC）位于线粒体内膜。来自糖酵解、链接反应和克雷布斯循环的还原辅酶（NADH 和 FADH₂）将电子提供给一系列蛋白复合体和移动载体。电子从复合体 I 或 II 经泛醌传递至复合体 III，再经细胞色素 c 到达复合体 IV，在此氧气作为最终电子受体生成水：½O₂ + 2e⁻ + 2H⁺ → H₂O。

As electrons move through the chain, their energy is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient (proton motive force). This gradient stores potential energy. Protons can only flow back into the matrix through the enzyme ATP synthase, a process called chemiosmosis. The flow of protons drives the rotation of ATP synthase, which catalyzes the phosphorylation of ADP to ATP. This mechanism, the chemiosmotic theory, is central to oxidative phosphorylation.

电子传递过程中释放的能量用于将质子（H⁺）从线粒体基质泵入膜间空间，形成电化学梯度（质子动力）。该梯度储存势能。质子只能通过 ATP 合酶流回基质，此过程称为化学渗透。质子流驱动 ATP 合酶旋转，催化 ADP 磷酸化为 ATP。化学渗透理论是氧化磷酸化的核心机制。

7. Energy Yield of Aerobic Respiration | 有氧呼吸的能量产量

The theoretical maximum ATP yield from one molecule of glucose is often quoted as 38 ATP, but this is rarely achieved in situ. The oxidation of each cytosolic NADH may yield either 2.5 or 1.5 ATP depending on the shuttle system used to transfer electrons into the mitochondrion. A realistic modern consensus gives about 30–32 ATP per glucose.

一分子葡萄糖的理论最大 ATP 产量常被引用为 38 ATP，但在体内罕能达到。每个胞质 NADH 氧化所产生的 ATP 可能为 2.5 或 1.5，取决于将电子转运进线粒体的穿梭系统。现代实际估计为每葡萄糖约 30–32 ATP。

A typical breakdown is: 2 ATP from glycolysis, 2 ATP from the Krebs cycle (as GTP), and about 25 ATP from oxidative phosphorylation via NADH and FADH₂. The energy released by the complete oxidation of glucose totals approximately 2880 kJ mol⁻¹; the conservation of about 960 kJ in ATP represents an efficiency of roughly 33%, with the remainder lost as heat to maintain body temperature.

典型分解为：糖酵解 2 ATP，克雷布斯循环 2 ATP（以 GTP 形式），以及通过 NADH 和 FADH₂ 经氧化磷酸化产生的约 25 ATP。葡萄糖完全氧化释放的能量约 2880 kJ mol⁻¹；以 ATP 保存约 960 kJ，效率约 33%，其余以热的形式散失，用于维持体温。

8. Anaerobic Respiration: Lactate Fermentation | 无氧呼吸：乳酸发酵

In animal skeletal muscle during vigorous exercise, oxygen supply may be insufficient to meet the demands of the electron transport chain. Under these conditions, the pyruvate produced by glycolysis is reduced to lactate by the enzyme lactate dehydrogenase, using NADH as the reductant. This regenerates NAD⁺, which is essential to keep glycolysis running at a high rate.

在剧烈运动中，动物骨骼肌的氧气供应可能不足以满足电子传递链的需求。此时，糖酵解产生的丙酮酸被乳酸脱氢酶还原为乳酸，以 NADH 为还原剂。这一过程使 NAD⁺ 再生，后者对维持高糖酵解速率至关重要。

The overall equation is: C₆H₁₂O₆ → 2 lactate + 2 ATP. Although it yields just 2 ATP per glucose, lactate fermentation allows rapid ATP production without oxygen. Lactate can be transported to the liver via the bloodstream, where it is converted back to glucose through gluconeogenesis (the Cori cycle). Accumulation of lactate lowers pH in muscle fibres, contributing to fatigue, and is eventually oxidized when oxygen becomes available again.

总方程式为：C₆H₁₂O₆ → 2 乳酸 + 2 ATP。虽然每分子葡萄糖仅生成 2 ATP，但乳酸发酵实现了无氧下的快速 ATP 生产。乳酸可通过血液运至肝脏，经糖异生重新转化为葡萄糖（Cori 循环）。乳酸积累使肌纤维 pH 降低，导致疲劳，并在氧气恢复后被氧化。

9. Anaerobic Respiration: Alcoholic Fermentation | 无氧呼吸：酒精发酵

Yeast and some plant tissues, when deprived of oxygen, convert pyruvate first to ethanal (acetaldehyde) by pyruvate decarboxylase, releasing CO₂. Ethanal is then reduced to ethanol by alcohol dehydrogenase, using NADH and regenerating NAD⁺. The net process produces ethanol, CO₂, and 2 ATP per glucose.

酵母和某些植物组织在缺氧时，首先由丙酮酸脱羧酶将丙酮酸转化为乙醛，释放 CO₂。随后乙醛被醇脱氢酶以 NADH 为还原剂还原为乙醇，同时再生 NAD⁺。整个过程每分子葡萄糖产生乙醇、CO₂ 和 2 ATP。

The equation is: C₆H₁₂O₆ → 2 ethanol + 2 CO₂ + 2 ATP. This type of fermentation is exploited in baking, where CO₂ causes dough to rise, and in brewing, where ethanol is the desired product. Because ethanol can accumulate to toxic levels that kill yeast cells, the alcohol concentration in wines and beers is self‑limiting unless distillation is used.

方程式为：C₆H₁₂O₆ → 2 乙醇 + 2 CO₂ + 2 ATP。这类发酵被用于烘焙（CO₂ 使面团膨胀）和酿造（乙醇为目标产物）。由于乙醇可累积至毒死酵母细胞的水平，酒类饮品中的酒精浓度具有自我限制性，除非采用蒸馏。

10. Respiratory Quotient (RQ) | 呼吸商

The respiratory quotient (RQ), also called respiratory exchange ratio, is the volume ratio of CO₂ produced to O₂ consumed in a given period: RQ = CO₂ evolved ÷ O₂ consumed. Its value depends on the nature of the respiratory substrate being oxidized. For carbohydrates, RQ = 1.0, because equal volumes of CO₂ and O₂ are involved: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.

呼吸商（RQ）又称呼吸交换比，是给定时间内产生 CO₂ 体积与消耗 O₂ 体积的比值：RQ = CO₂ 释放量 ÷ O₂ 消耗量。其值取决于被氧化的呼吸底物性质。碳水化合物完全氧化时 RQ = 1.0，因为消耗的 O₂ 与产生的 CO₂ 体积相等。

For lipids, RQ is typically about 0.7, because lipids are more reduced and require more oxygen for complete oxidation relative to the CO₂ released. For proteins, RQ is around 0.8–0.9. Measuring RQ in a respirometer allows biologists to infer which fuel is being used by an organism at a given time, providing insight into metabolic states, such as during fasting, exercise, or hibernation.

脂质的 RQ 通常约为 0.7，因为脂质还原度更高，完全氧化时相对于 CO₂ 释放量需要更多氧气。蛋白质的 RQ 约为 0.8–0.9。通过呼吸计测定 RQ，生物学家可推断生物在特定时刻使用哪种燃料，从而了解禁食、运动或冬眠等代谢状态。

11. Factors Affecting the Rate of Respiration | 影响呼吸速率的因素

Respiration is an enzyme‑driven process, so temperature strongly influences reaction rates. Within physiological limits, the rate increases with a rise in temperature due to increased kinetic energy of enzymes and substrates. However, above the optimum (around 40–45 °C in human cells), enzymes denature, causing a sharp decline. In plants and poikilotherms, Q₁₀ values for respiration are often between 2 and 3, meaning the rate doubles or triples with every 10 °C rise.

呼吸作用受酶驱动，因此温度强烈影响反应速率。在生理范围内，温度升高因酶和底物动能增加而使速率加快。但超过最适温度（人体细胞约 40–45 °C），酶变性，速率急剧下降。在植物和变温动物中，呼吸作用的 Q₁₀ 值常在 2–3 之间，即每升高 10 °C，速率翻倍或增至三倍。

Oxygen concentration is a limiting factor for aerobic respiration. At low oxygen tensions, the rate of electron transport chain activity decreases, and eventually cells switch to anaerobic pathways. In fruits and flooded roots, oxygen diffusion can be severely restricted. For fresh produce storage, controlled atmosphere conditions with reduced O₂ and elevated CO₂ slow respiration and delay ripening.

氧气浓度对有氧呼吸是一限制因子。低氧分压下，电子传递链活性下降，最终细胞转向无氧途径。在果实和淹水根系中，氧气扩散可能严重受限。对于生鲜贮存，降低 O₂、提高 CO₂ 的气调条件可减缓呼吸作用，延迟成熟。

Water availability and respiratory substrate concentration also modulate respiration. Dormant seeds have extremely low water content and metabolism; imbibition triggers rapid respiratory resurgence. Glucose availability from glycogen or starch breakdown can upregulate glycolysis through insulin or hormonal signals.

水分和呼吸底物浓度也调节呼吸作用。休眠种子含水量极低，代谢停滞；吸胀引起呼吸作用迅速回升。通过糖原或淀粉分解产生的葡萄糖，可经由胰岛素或激素信号上调糖酵解。

12. Practical Investigations of Respiration | 呼吸作用的实验探究

The rate of respiration can be measured by using a simple respirometer. One common setup consists of a sealed chamber containing living organisms (germinating seeds, woodlice, or yeast in glucose solution), connected to a manometer or a graduated capillary tube. A soda lime or KOH solution is placed in the chamber to absorb the CO₂ produced, so any change in gas volume represents O₂ consumption.

呼吸速率可用简单呼吸计测量。一个常见装置由装有活生物体（萌发种子、潮虫或葡萄糖溶液中的酵母）的密封小室构成，连接压力计或刻度毛细管。小室内放置苏打石灰或 KOH 溶液以吸收产生的 CO₂，因此任何气体体积变化代表耗氧量。

A typical procedure: allow the apparatus to equilibrate, then record the movement of a coloured liquid drop in the capillary over a set time. The rate of oxygen consumption is calculated as volume per unit time. Variables such as temperature (using water baths), substrate type, or pH can be manipulated. For yeast, methylene blue can be used as a redox indicator: it decolourises as respiration proceeds under anaerobic conditions, providing a qualitative measure of dehydrogenase activity.

典型操作：让装置平衡，随后记录设定时间内毛细管中有色液滴的移动距离。耗氧速率以单位时间体积计算。温度（使用水浴）、底物类型或 pH 等变量可被操控。对于酵母，亚甲基蓝可用作氧化还原指示剂：在无氧条件下随着呼吸进行而褪色，提供脱氢酶活性的定性测量。

In IB Biology, students are expected to design and interpret such experiments, considering control runs (e.g., with inactive boiled seeds) to account for changes in ambient temperature and pressure. The use of data loggers with oxygen and CO₂ sensors can provide continuous, precise measurements that allow calculation of RQ and evaluation of substrate use over time.

在 IB 生物中，学生应能设计并解释这类实验，考虑对照运行（如灭活煮沸种子）以扣除环境温度和压力变化。使用带 O₂ 和 CO₂ 传感器的数据记录仪可连续精确测量，从而计算 RQ 并评估底物随时间的使用。

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Respiration | 呼吸作用