Respiration: ATP Production and Pathways | 呼吸作用：ATP 的产生与代谢途径

📚 Respiration: ATP Production and Pathways | 呼吸作用：ATP 的产生与代谢途径

Respiration is the set of metabolic reactions that release chemical energy from organic molecules such as glucose and transfer it into ATP. It occurs continuously in all living cells, providing the energy needed for active transport, muscle contraction, cell division, and synthesis of macromolecules.

呼吸作用是一系列代谢反应，它从葡萄糖等有机分子中释放化学能，并将能量转移到 ATP 中。所有活细胞都在持续进行呼吸作用，为主动运输、肌肉收缩、细胞分裂和大分子合成提供能量。

In aerobic respiration, glucose is fully oxidised to carbon dioxide and water. The overall equation is:

在有氧呼吸中，葡萄糖被完全氧化成二氧化碳和水。总反应式为：

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

ATP is the universal energy currency of the cell. Its hydrolysis to ADP and inorganic phosphate releases about 30.5 kJ mol⁻¹, which can be coupled to energy-requiring processes:

ATP 是细胞中通用的能量货币。ATP 水解为 ADP 和无机磷酸时约释放 30.5 kJ mol⁻¹ 的能量，这些能量可与需要能量的过程偶联：

ATP + H₂O ⇌ ADP + Pᵢ + energy

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1. Overview of Respiration and ATP | 呼吸作用与 ATP 概述

Aerobic respiration can be divided into four main stages: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. Glycolysis occurs in the cytoplasm, while the other three stages occur in the mitochondria.

有氧呼吸可分为四个主要阶段：糖酵解、连接反应、克雷布斯循环和氧化磷酸化。糖酵解发生在细胞质中，而其余三个阶段发生在线粒体中。

Anaerobic respiration only includes glycolysis followed by a fermentation pathway to regenerate NAD⁺. It does not require oxygen and produces much less ATP.

无氧呼吸只包括糖酵解，以及随后用于再生 NAD⁺ 的发酵途径。无氧呼吸不需要氧气，产生的 ATP 要少得多。

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2. Glycolysis: Splitting Glucose | 糖酵解：葡萄糖的分解

Glycolysis takes place in the cytoplasm and does not require oxygen. It converts one molecule of glucose, a six-carbon sugar, into two molecules of pyruvate, a three-carbon compound.

糖酵解发生在细胞质中，不需要氧气。它将一分子六碳糖葡萄糖转化为两分子三碳化合物丙酮酸。

The process begins with phosphorylation of glucose using two ATP molecules. This makes glucose more reactive and prevents it from leaving the cell. The phosphorylated six-carbon molecule is then split into two triose phosphate molecules.

该过程首先使用两分子 ATP 对葡萄糖进行磷酸化。这使葡萄糖更具反应性，并防止其离开细胞。磷酸化的六碳分子随后被分解为两个三碳糖磷酸分子。

Each triose phosphate is oxidised by removing hydrogen, which is accepted by NAD to form reduced NAD (NADH). Finally, phosphate groups are transferred to ADP to produce ATP by substrate-level phosphorylation.

每个三碳糖磷酸通过脱氢被氧化，氢由 NAD 接受，形成还原型 NAD（NADH）。最后，磷酸基团通过底物水平磷酸化转移到 ADP 上，生成 ATP。

The net products of glycolysis per glucose molecule are:

每分子葡萄糖经糖酵解的净产物为：

glucose → 2 pyruvate + 2 ATP + 2 reduced NAD (NADH)

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3. Link Reaction: From Pyruvate to Acetyl-CoA | 连接反应：从丙酮酸到乙酰辅酶 A

In the presence of oxygen, pyruvate enters the mitochondrial matrix by active transport. Here, the link reaction converts each pyruvate into acetyl coenzyme A (acetyl-CoA).

在有氧条件下，丙酮酸通过主动运输进入线粒体基质。在这里，连接反应将每个丙酮酸转化为乙酰辅酶 A。

During the link reaction, pyruvate undergoes decarboxylation, releasing one molecule of carbon dioxide. It is also oxidised by NAD, producing reduced NAD. The remaining two-carbon acetyl group combines with coenzyme A to form acetyl-CoA.

在连接反应中，丙酮酸发生脱羧反应，释放一分子二氧化碳。它同时被 NAD 氧化，生成还原型 NAD。剩余的二碳乙酰基与辅酶 A 结合，形成乙酰辅酶 A。

For each pyruvate molecule, the reaction is:

每个丙酮酸分子的反应为：

pyruvate + CoA + NAD⁺ → acetyl-CoA + CO₂ + reduced NAD

Since each glucose produces two pyruvate molecules, the link reaction yields two acetyl-CoA, two carbon dioxide molecules, and two reduced NAD per glucose.

由于每个葡萄糖产生两个丙酮酸分子，因此连接反应每个葡萄糖产生两个乙酰辅酶 A、两分子二氧化碳和两个还原型 NAD。

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4. Krebs Cycle: Oxidation of Acetyl Groups | 克雷布斯循环：乙酰基的氧化

The Krebs cycle occurs in the mitochondrial matrix. Acetyl-CoA delivers its two-carbon acetyl group to a four-carbon compound called oxaloacetate, forming a six-carbon compound called citrate.

克雷布斯循环发生在线粒体基质中。乙酰辅酶 A 将其二碳乙酰基交给四碳化合物草酰乙酸，形成六碳化合物柠檬酸。

Through a series of decarboxylation and dehydrogenation reactions, citrate is converted back to oxaloacetate. In one turn of the cycle, two carbon dioxide molecules are released, and hydrogen atoms are transferred to three NAD and one FAD, producing three reduced NAD and one reduced FAD. One ATP is also formed directly by substrate-level phosphorylation.

通过一系列脱羧和脱氢反应，柠檬酸又被转化为草酰乙酸。循环每进行一圈，释放两分子二氧化碳，氢原子分别转移到三个 NAD 和一个 FAD 上，生成三个还原型 NAD 和一个还原型 FAD。此外还通过底物水平磷酸化直接生成一分子 ATP。

For each turn of the Krebs cycle:

克雷布斯循环每圈：

acetyl-CoA + 3NAD⁺ + FAD + ADP + Pᵢ + 2H₂O → 2CO₂ + 3 reduced NAD + reduced FAD + ATP + CoA

Since two acetyl-CoA molecules enter the cycle per glucose, the total products per glucose are four CO₂, six reduced NAD, two reduced FAD, and two ATP.

由于每个葡萄糖有两个乙酰辅酶 A 进入循环，因此每个葡萄糖的总产物为四分子 CO₂、六个还原型 NAD、两个还原型 FAD 和两个 ATP。

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5. Oxidative Phosphorylation and Chemiosmosis | 氧化磷酸化与化学渗透

Oxidative phosphorylation takes place on the inner mitochondrial membrane. Reduced NAD and reduced FAD donate electrons to the electron transport chain, a series of protein complexes and mobile carriers.

氧化磷酸化发生在线粒体内膜上。还原型 NAD 和还原型 FAD 将电子传递给电子传递链，即一系列蛋白质复合体和移动载体。

As electrons pass along the chain, energy is released and used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space. This creates an electrochemical gradient, also called a proton motive force.

当电子沿传递链传递时，释放的能量被用来将质子（H⁺）从线粒体基质泵入膜间隙。这形成了电化学梯度，也称为质子动力。

Protons then flow back into the matrix through ATP synthase, a channel protein. This flow drives the synthesis of ATP from ADP and Pᵢ in a process called chemiosmosis.

质子随后通过 ATP 合酶流回基质。ATP 合酶是一种通道蛋白。质子的流动驱动 ADP 与 Pᵢ 合成 ATP，这一过程称为化学渗透。

Oxygen acts as the final electron acceptor at the end of the chain. It combines with electrons and protons to form water:

氧气是传递链末端的最终电子受体。它与电子和质子结合生成水：

½O₂ + 2H⁺ + 2e⁻ → H₂O

Each reduced NAD typically leads to the production of about 2.5 ATP, while each reduced FAD leads to about 1.5 ATP. Inhibitors such as cyanide block electron transport, and uncouplers such as DNP allow protons to leak across the membrane without ATP synthesis.

每个还原型 NAD 通常可产生约 2.5 个 ATP，每个还原型 FAD 可产生约 1.5 个 ATP。氰化物等抑制剂会阻断电子传递，而 DNP 等解偶联剂则使质子泄漏过膜而不合成 ATP。

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6. Anaerobic Respiration in Mammals and Yeast | 哺乳动物和酵母的无氧呼吸

Without oxygen, the electron transport chain cannot accept electrons, so reduced NAD cannot be reoxidised and oxidative phosphorylation stops. To keep glycolysis running, cells must regenerate NAD⁺ using fermentation pathways.

没有氧气时，电子传递链无法接受电子，因此还原型 NAD 不能被重新氧化，氧化磷酸化停止。为了使糖酵解继续进行，细胞必须通过发酵途径再生 NAD⁺。

In mammals and some bacteria, pyruvate is reduced to lactate by reduced NAD. This regenerates NAD⁺ and allows glycolysis to continue:

在哺乳动物和某些细菌中，丙酮酸被还原型 NAD 还原为乳酸。这再生了 NAD⁺，使糖酵解得以继续：

pyruvate + reduced NAD → lactate + NAD⁺

In yeast and some plants, pyruvate is first decarboxylated to ethanal, releasing carbon dioxide. Ethanal is then reduced to ethanol by reduced NAD:

在酵母和一些植物中，丙酮酸首先脱羧生成乙醛，并释放二氧化碳。乙醛再被还原型 NAD 还原为乙醇：

pyruvate → ethanal + CO₂ → ethanol + NAD⁺

Anaerobic respiration produces only the two ATP molecules from glycolysis because substrate-level phosphorylation in the Krebs cycle and oxidative phosphorylation do not occur.

无氧呼吸只产生糖酵解阶段的两个 ATP，因为克雷布斯循环中的底物水平磷酸化和氧化磷酸化都不会发生。

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7. Energy Yields and Efficiency | 能量产量与效率

The total ATP yield from aerobic respiration can be calculated by combining substrate-level phosphorylation and oxidative phosphorylation. The table below summarises the products per glucose.

有氧呼吸的总 ATP 产量可通过合并底物水平磷酸化和氧化磷酸化来计算。下表总结了每个葡萄糖的产物。

Stage	Reduced NAD	Reduced FAD	ATP by substrate-level phosphorylation
Glycolysis	2	0	2
Link reaction	2	0	0
Krebs cycle	6	2	2
Total	10	2	4

The theoretical maximum is often quoted as 38 ATP per glucose, but in many eukaryotic cells the actual yield is closer to 30-32 ATP. This is because reduced NAD produced in glycolysis must be shuttled into mitochondria at an energy cost, and some protons leak across the inner membrane.

理论上，每个葡萄糖最多可产生 38 个 ATP，但在许多真核细胞中，实际产量约为 30-32 个 ATP。这是因为糖酵解产生的还原型 NAD 需要耗能运输进入线粒体，而且部分质子会渗漏过线粒体内膜。

The efficiency of aerobic respiration is about 34% of the energy available in glucose, with the rest released as heat. This is calculated as:

有氧呼吸的效率约为葡萄糖中可用能量的 34%，其余能量以热的形式

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