📚 Cellular Energy Metabolism Key Points | 细胞能量代谢重点梳理
This article provides a structured revision of cellular energy metabolism, covering glycolysis, the Krebs cycle, the electron transport chain, oxidative phosphorylation, and anaerobic respiration. These are core topics in A-level Biology and are frequently tested in exams.
本文系统梳理细胞能量代谢的核心考点，包括糖酵解、三羧酸循环、电子传递链、氧化磷酸化以及无氧呼吸。这些内容在A-level生物考试中属于核心考点，需要牢固掌握。
1. Overview of Energy Metabolism | 能量代谢概览
Respiration is a series of enzyme-controlled reactions that break down organic molecules, typically glucose, to release energy in the form of ATP. The energy is captured via both substrate-level phosphorylation and oxidative phosphorylation.
呼吸作用是一系列由酶控制的反应，将有机物（通常是葡萄糖）分解，释放能量并合成ATP。能量通过底物水平磷酸化和氧化磷酸化两种方式被捕获。
Glycolysis occurs in the cytoplasm and does not require oxygen.
糖酵解发生在细胞质中，不需要氧气。
Aerobic respiration includes glycolysis, the link reaction, the Krebs cycle, and the electron transport chain.
有氧呼吸包括糖酵解、连接反应、三羧酸循环和电子传递链。
Anaerobic respiration in mammals produces lactate; in yeast it produces ethanol and CO₂.
哺乳动物无氧呼吸产生乳酸；酵母无氧呼吸产生乙醇和CO₂。
NADH and FADH₂ act as electron carriers, delivering high-energy electrons to the electron transport chain.
NADH和FADH₂作为电子载体，将高能电子传递给电子传递链。
2. ATP Structure and Hydrolysis | ATP结构与水解
Adenosine triphosphate (ATP) consists of adenine, ribose, and three phosphate groups. The bonds between phosphate groups are high-energy; hydrolysis removes one phosphate group to form ADP and inorganic phosphate (Pi), releasing energy.
三磷酸腺苷（ATP）由腺嘌呤、核糖和三个磷酸基团组成。磷酸基团之间的键是高能键；水解时脱去一个磷酸基团，形成ADP和无机磷酸（Pi），同时释放能量。
ATP + H₂O → ADP + Pi + Energy
In cells, ATP is continuously regenerated from ADP and Pi using energy released during respiration. ATP is a universal energy currency because it is small, water-soluble, and can release energy quickly in small amounts.
在细胞中，ATP不断利用呼吸作用释放的能量，从ADP和Pi重新合成。ATP是通用的能量货币，因为它体积小、可溶于水，并能快速释放少量能量。
3. Glycolysis | 糖酵解
Glycolysis is the first stage of respiration, occurring in the cytoplasm. One molecule of glucose (C₆H₁₂O₆) is converted into two molecules of pyruvate (C₃H₄O₃).
糖酵解是呼吸作用的第一阶段，发生在细胞质中。一分子葡萄糖（C₆H₁₂O₆）转化为两分子丙酮酸（C₃H₄O₃）。
C₆H₁₂O₆ + 2 NAD⁺ + 2 ADP + 2 Pi → 2 C₃H₄O₃ + 2 ATP + 2 NADH + 2 H⁺
The process can be divided into two phases: an energy-investment phase (2 ATP used) and an energy-payoff phase (4 ATP produced, net gain of 2 ATP).
整个过程可分为两个阶段：能量投入阶段（消耗2个ATP）和能量回收阶段（产生4个ATP，净增加2个ATP）。
Glucose is phosphorylated using ATP to form glucose-6-phosphate, which traps glucose inside the cell.
葡萄糖被ATP磷酸化，形成6-磷酸葡萄糖，从而将葡萄糖“困”在细胞内。
Eventually, triose phosphate is converted to pyruvate, and NAD⁺ is reduced to NADH.
最终，磷酸丙糖转化为丙酮酸，同时NAD⁺被还原为NADH。
No CO₂ is released during glycolysis, and no oxygen is required.
糖酵解过程中不释放CO₂，也不需要氧气。
4. Link Reaction and Krebs Cycle | 连接反应和三羧酸循环
In aerobic respiration in eukaryotes, pyruvate enters the mitochondrial matrix. The link reaction occurs here, converting pyruvate to acetyl coenzyme A (acetyl-CoA), releasing CO₂ and producing reduced NAD.
在真核生物有氧呼吸中，丙酮酸进入线粒体基质。连接反应在此发生，将丙酮酸转化为乙酰辅酶A（acetyl-CoA），同时释放CO₂并产生还原型NAD。
Pyruvate + NAD⁺ + CoA → Acetyl-CoA + CO₂ + NADH + H⁺
Acetyl-CoA (2 carbons) enters the Krebs cycle, combining with a 4-carbon compound to form citrate (6 carbons). A series of enzyme-controlled reactions regenerates the 4-carbon compound.
乙酰辅酶A（含2个碳）进入三羧酸循环，与一个四碳化合物结合形成柠檬酸（含6个碳）。随后经过一系列酶促反应，重新生成四碳化合物。
| Per pyruvate (per turn of Krebs) | Per glucose (two turns) |
| 1 ATP (via substrate-level phosphorylation) | 2 ATP |
| 3 NADH | 6 NADH |
| 1 FADH₂ | 2 FADH₂ |
| 2 CO₂ | 4 CO₂ |
Note that the CO₂ released during aerobic respiration comes from the link reaction and the Krebs cycle, not from glycolysis.
注意：有氧呼吸释放的CO₂来自连接反应和三羧酸循环，而不是糖酵解。
5. Electron Transport Chain and Oxidative Phosphorylation | 电子传递链和氧化磷酸化
The electron transport chain is located in the inner mitochondrial membrane, embedded in a series of protein complexes. NADH and FADH₂ donate electrons to the chain, and the electrons pass along carriers with decreasing energy levels.
电子传递链位于线粒体内膜上，由一系列蛋白质复合物组成。NADH和FADH₂将电子传递给传递链，电子沿载体依次传递，能量逐渐降低。
Energy released as electrons pass down the chain is used to pump H⁺ ions from the matrix into the intermembrane space, creating an electrochemical gradient.
电子沿链传递时释放的能量用于将H⁺从基质泵入膜间隙，形成电化学梯度。
H⁺ ions flow back into the matrix through ATP synthase, driving the synthesis of ATP from ADP and Pi. This process is called chemiosmosis.
H⁺通过ATP合酶回流到基质，驱动ADP和Pi合成ATP。这一过程称为化学渗透。
electron acceptor. It combines with electrons and H⁺ to form water: ½O₂ + 2e⁻ + 2H⁺ → H₂O.
氧气是最终电子受体。它与电子和H⁺结合形成水：½O₂ + 2e⁻ + 2H⁺ → H₂O。
Oxidative phosphorylation describes the coupling of ATP synthesis to the oxidation of NADH and FADH₂. Each NADH yields approximately 2.5 ATP, and each FADH₂ yields approximately 1.5 ATP, depending on the syllabus.
氧化磷酸化是指将ATP合成与NADH和FADH₂的氧化偶联起来。每个NADH约产生2.5个ATP，每个FADH₂约产生1.5个ATP（具体数值依课程大纲而定）。
6. Anaerobic Respiration | 无氧呼吸
In the absence of oxygen, the electron transport chain cannot operate. NADH cannot be reoxidised by the chain, so NAD⁺ becomes unavailable for glycolysis. Anaerobic pathways regenerate NAD⁺ to allow glycolysis to continue.
在没有氧气的情况下，电子传递链无法运作，NADH无法通过传递链再氧化，导致NAD⁺供应不足，糖酵解无法继续。无氧途径通过再生NAD⁺来维持糖酵解。
In mammals, pyruvate is reduced to lactate by NADH, with the enzyme lactate dehydrogenase. No CO₂ is released.
在哺乳动物中，丙酮酸被NADH还原为乳酸，催化该反应的酶是乳酸脱氢酶。此过程不释放CO₂。
Pyruvate + NADH + H⁺ → Lactate + NAD⁺
In yeast, pyruvate is first decarboxylated to acetaldehyde (releasing CO₂), then reduced to ethanol by NADH.
在酵母中，丙酮酸先脱羧生成乙醛（释放CO₂），然后被NADH还原为乙醇。
Pyruvate → CO₂ + Acetaldehyde; Acetaldehyde + NADH + H⁺ → Ethanol + NAD⁺
Anaerobic respiration yields only 2 ATP per glucose (from glycolysis), much less than aerobic respiration because the Krebs cycle and oxidative phosphorylation do not occur.
无氧呼吸每分子葡萄糖仅净产生2个ATP（来自糖酵解），远小于有氧呼吸，因为三羧酸循环和氧化磷酸化无法进行。
7. Substrate-level vs Oxidative Phosphorylation | 底物水平磷酸化与氧化磷酸化
ATP can be made by two distinct mechanisms. Substrate-level phosphorylation directly transfers phosphate from a phosphorylated intermediate to ADP, without using the electron transport chain. It occurs in glycolysis and the Krebs cycle.
ATP有两种不同的生成方式。底物水平磷酸化是在酶催化下，直接将磷酸基团从高能中间产物转移给ADP，不经过电子传递链。此过程发生在糖酵解和三羧酸循环中。
Oxidative phosphorylation uses the electron transport chain and chemiosmosis, and it requires oxygen as the final electron acceptor. It produces the vast majority of ATP under aerobic conditions.
氧化磷酸化依赖电子传递链和化学渗透，需要氧气作为最终电子受体。在有氧条件下，绝大多数ATP由这种方式产生。
| Feature | Substrate-level | Oxidative |
| Location | Cytoplasm / matrix | Inner mitochondrial membrane |
| Requires O₂ | No | Yes |
| Uses electron transport chain | No | Yes |
| Main ATP yield per glucose | 4 (net 2 in glycolysis + 2 in Krebs) | ~26–28 |
8. Respiratory Substrates and Energy Values | 呼吸底物与能量值
Although glucose is the classic substrate, other molecules can also be respired. Proteins and lipids can be broken down and enter the respiratory pathways at different points.
虽然葡萄糖是经典呼吸底物，其他分子也可以被呼吸利用。蛋白质和脂肪可以被分解，并从不同节点进入呼吸途径。
Lipids release more energy per gram than carbohydrates because they are more highly reduced (contain many C–H bonds).
每克脂质释放的能量比碳水化合物更多，因为脂质还原程度更高（含有大量C–H键）。
Proteins are usually used only in starvation; amino acids are deaminated before the carbon skeleton enters respiration.
蛋白质通常仅在饥饿时被使用；氨基酸先脱去氨基，其碳骨架再进入呼吸作用。
Respiratory quotient (RQ) = CO₂ produced ÷ O₂ consumed. RQ = 1.0 for carbohydrates, ~0.7 for fats.
呼吸商（RQ）= 产生的CO₂ ÷ 消耗的O₂。碳水化合物的RQ为1.0，脂肪约为0.7。
9. Summary Table of Aerobic Respiration | 有氧呼吸总表
The table below summarises the main stages and their net outputs for one glucose molecule under aerobic conditions.
下表总结了在有氧条件下，一分子葡萄糖经各阶段的主要净产物。
| Stage | Location | ATP (net) | NADH | FADH₂ | CO₂ |
| Glycolysis | Cytoplasm | 2 | 2 | 0 | 0 |
| Link reaction (×2) | Mitochondrial matrix | 0 | 2 | 0 | 2 |
| Krebs cycle (×2) | Mitochondrial matrix | 2 | 6 | 2 | 4 |
| Oxidative phosphorylation | Inner mitochondrial membrane | ~26–28 | — | — | 0 |
Total: approximately 30–32 ATP per glucose in aerobic conditions, depending on the efficiency of the electron transport chain and the shuttle system used.
总计：有氧条件下每分子葡萄糖约产生30–32个ATP，具体取决于电子传递链效率以及穿梭系统的类型。
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