📚 Core Processes of Energy Transfer in Living Organisms | 生物体内能量转移的核心过程
Energy flow is fundamental to all life. Organisms require a continuous supply of energy to maintain order, drive chemical reactions, and perform mechanical work. This article explores the central pathways by which energy is captured, stored, transferred, and released within biological systems, with a focus on ATP, respiration, and photosynthesis.
能量流动是生命的基础。生物体需要持续的能量供应来维持有序状态、驱动化学反应并完成机械功。本文聚焦ATP、呼吸作用和光合作用，探讨生物系统内能量被捕获、储存、转移和释放的核心途径。
1. ATP: The Universal Energy Currency | ATP：通用能量货币
Adenosine triphosphate (ATP) is the immediate energy donor in cells. It consists of adenine, ribose, and three phosphate groups. The bonds between the phosphate groups, especially the terminal one, are high-energy bonds. Hydrolysis of ATP to ADP and inorganic phosphate (Pi) releases approximately 30.5 kJ/mol under standard conditions.
三磷酸腺苷（ATP）是细胞中的直接供能物质。它由腺嘌呤、核糖和三个磷酸基团组成。磷酸基团之间的键，尤其是末端磷酸键，属于高能键。在标准条件下，ATP水解为ADP和无机磷酸（Pi）释放约30.5 kJ/mol的能量。
ATP is not a long-term energy store; rather, it acts as a shuttle, transferring energy from exergonic reactions (e.g., respiration) to endergonic reactions (e.g., protein synthesis, muscle contraction).
ATP并非长期能量储存物质，而是作为能量穿梭体，将放能反应（如呼吸作用）释放的能量转移至吸能反应（如蛋白质合成、肌肉收缩）。
| Process | Energy Input / Output |
| ATP hydrolysis | Output ≈ 30.5 kJ/mol |
| ATP synthesis | Input ≈ 30.5 kJ/mol (from respiration/photosynthesis) |
2. Redox Reactions and Electron Carriers | 氧化还原反应与电子载体
Energy transfer in cells is largely achieved through oxidation-reduction (redox) reactions. In these reactions, electrons are transferred from a reducing agent to an oxidising agent. The movement of electrons is accompanied by the transfer of protons (H⁺) in many biological systems.
细胞内的能量转移主要通过氧化还原反应实现。在这些反应中，电子从还原剂转移到氧化剂。在许多生物体系中，电子的移动伴随着质子（H⁺）的转移。
Key electron carriers include NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide). They accept electrons and hydrogen atoms to become reduced (NADH, FADH₂), and deliver them to the electron transport chain, where energy is harnessed to synthesise ATP.
关键的电子载体包括NAD（烟酰胺腺嘌呤二核苷酸）和FAD（黄素腺嘌呤二核苷酸）。它们接受电子和氢原子而被还原（NADH、FADH₂），并将电子传递至电子传递链，在那里能量被用于合成ATP。
NAD⁺ + 2H⁺ + 2e⁻ → NADH + H⁺
FAD + 2H⁺ + 2e⁻ → FADH₂
3. Overview of Respiration | 呼吸作用概述
Cellular respiration is the process by which organic compounds are broken down to release ATP. Glucose is the typical substrate, although fats and proteins can also be used. The overall equation for aerobic respiration is:
细胞呼吸是分解有机化合物以释放ATP的过程。葡萄糖是典型底物，脂肪和蛋白质也可被利用。有氧呼吸的总方程式为：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP + heat)
Respiration occurs in four main stages: glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. Each stage occupies a specific cellular location and yields distinct products.
呼吸作用主要分为四个阶段：糖酵解、连接反应、克雷布斯循环和氧化磷酸化。每个阶段位于特定细胞部位，并产生不同的产物。
4. Glycolysis | 糖酵解
Glycolysis takes place in the cytoplasm and does not require oxygen. Glucose (6C) is phosphorylated using two ATP molecules and then split into two triose phosphate molecules. Each triose phosphate is oxidised, producing NADH and ATP via substrate-level phosphorylation. The net yield per glucose is 2 ATP, 2 NADH, and 2 pyruvate (3C).
糖酵解发生在细胞质基质中，不需要氧气。葡萄糖（6C）消耗2个ATP被磷酸化，然后裂解为两个三碳糖磷酸。每个三碳糖磷酸被氧化，通过底物水平磷酸化产生NADH和ATP。每分子葡萄糖净产生2个ATP、2个NADH和2个丙酮酸（3C）。
Glucose + 2NAD⁺ + 2ADP + 2Pi → 2 Pyruvate + 2NADH + 2H⁺ + 2ATP + 2H₂O
Substrate-level phosphorylation is the direct transfer of a phosphate group from a substrate to ADP, catalysed by an enzyme. This is distinct from oxidative phosphorylation, which uses the proton gradient.
底物水平磷酸化是酶催化下磷酸基团从底物直接转移至ADP的过程。这与利用质子梯度的氧化磷酸化不同。
5. Link Reaction and Krebs Cycle | 连接反应与克雷布斯循环
In aerobic conditions, pyruvate enters the mitochondrial matrix. The link reaction converts pyruvate to acetyl-CoA, producing one NADH and one CO₂ per pyruvate. Acetyl-CoA (2C) then combines with oxaloacetate (4C) to form citrate (6C), initiating the Krebs cycle.
Per turn of the Krebs cycle (per acetyl-CoA), the products are: 3 NADH, 1 FADH₂, 1 ATP (or GTP), and 2 CO₂. Since each glucose produces two pyruvate and thus two acetyl-CoA, the cycle runs twice per glucose.
每轮克雷布斯循环（每分子乙酰辅酶A）的产物为：3个NADH、1个FADH₂、1个ATP（或GTP）和2个CO₂。由于每分子葡萄糖产生两分子丙酮酸，进而生成两分子乙酰辅酶A，因此每个葡萄糖需要运行两轮循环。
6. Oxidative Phosphorylation and the Electron Transport Chain | 氧化磷酸化与电子传递链
The final stage of aerobic respiration occurs on the inner mitochondrial membrane. NADH and FADH₂ donate electrons to the electron transport chain, a series of protein complexes (I, II, III, IV). As electrons pass along the chain, energy is released and used to pump protons (H⁺) from the matrix into the intermembrane space, creating an electrochemical gradient.
有氧呼吸的最后阶段发生在线粒体内膜上。NADH和FADH₂将电子捐赠给电子传递链——一系列蛋白质复合体（I、II、III、IV）。当电子沿链传递时，释放的能量用于将质子（H⁺）从基质泵入膜间隙，形成电化学梯度。
Protons then flow back into the matrix through ATP synthase, a process called chemiosmosis. This flow drives the phosphorylation of ADP to ATP. Finally, oxygen acts as the terminal electron acceptor, combining with electrons and protons to form water.
随后质子通过ATP合酶流回基质，这一过程称为化学渗透。质子流驱动ADP磷酸化生成ATP。最后，氧作为最终电子受体，与电子和质子结合生成水。
½O₂ + 2e⁻ + 2H⁺ → H₂O
One NADH yields approximately 2.5 ATP, and one FADH₂ yields approximately 1.5 ATP in the electron transport chain.
在电子传递链中，每分子NADH约产生2.5个ATP，每分子FADH₂约产生1.5个ATP。
7. Anaerobic Respiration | 无氧呼吸
When oxygen is limiting, pyruvate cannot enter the Krebs cycle. Instead, it is reduced by NADH to regenerate NAD⁺, allowing glycolysis to continue. In animals, pyruvate is converted to lactate; in yeast, it is converted to ethanol and CO₂.
当氧气不足时，丙酮酸无法进入克雷布斯循环。相反，丙酮酸被NADH还原以再生NAD⁺，从而使糖酵解得以继续。在动物细胞中，丙酮酸转化为乳酸；在酵母中，丙酮酸转化为乙醇和CO₂。
Pyruvate + NADH → Lactate + NAD⁺ (animals)
Pyruvate → Acetaldehyde + CO₂ → Ethanol + NAD⁺ (yeast)
Anaerobic respiration yields only 2 ATP per glucose, far less than aerobic respiration, and may lead to oxygen debt in muscles.
无氧呼吸每分子葡萄糖仅产生2个ATP，远低于有氧呼吸，并可能导致肌肉中的氧债。
8. Photosynthesis: Capturing Light Energy | 光合作用：捕获光能
Photosynthesis converts light energy into chemical energy. It occurs in chloroplasts, with the overall equation:
光合作用将光能转化为化学能，发生在叶绿体中，总方程式为：
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
The process is divided into two stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). The former produce ATP and reduced NADP (NADPH), while the latter uses these products to fix CO₂ into carbohydrates.
光合作用分为两个阶段：光依赖反应和光不依赖反应（卡尔文循环）。前者产生ATP和还原型NADP（NADPH），后者利用这些产物将CO₂固定为碳水化合物。
9. Light-Dependent Reactions | 光依赖反应
Thylakoid membranes contain photosystems II and I. When light strikes chlorophyll, electrons become excited and leave the reaction centre. The electron passes through an electron transport chain, releasing energy to pump protons into the thylakoid lumen. This creates a proton gradient used by ATP synthase to produce ATP (photophosphorylation).
类囊体膜含有光系统II和光系统I。当光照射叶绿素时，电子被激发并离开反应中心。电子通过电子传递链，释放能量用于将质子泵入类囊体腔，形成质子梯度，ATP合酶利用该梯度合成ATP（光合磷酸化）。
Water is split in the oxygen-evolving complex, providing electrons to replace those lost from photosystem II, releasing O₂ and protons. In non-cyclic photophosphorylation, electrons ultimately reduce NADP⁺ to NADPH.
水在放氧复合体中被裂解，提供电子以补充光系统II失去的电子，同时释放O₂和质子。在非循环光合磷酸化中，电子最终将NADP⁺还原为NADPH。
10. Calvin Cycle | 卡尔文循环
The Calvin cycle occurs in the stroma and is light-independent. It has three phases: carboxylation, reduction, and regeneration. In carboxylation, CO₂ is fixed by ribulose bisphosphate (RuBP, 5C) with the enzyme RuBisCO, producing two molecules of glycerate 3-phosphate (GP, 3C).
卡尔文循环发生在叶绿体基质中，不直接依赖光。它分为三个阶段：羧化、还原和再生。在羧化阶段，CO₂在RuBisCO酶催化下被核酮糖二磷酸（RuBP，5C）固定，产生两分子三磷酸甘油酸（GP，3C）。
In reduction, GP is phosphorylated by ATP and reduced by NADPH to form glyceraldehyde 3-phosphate (G3P). Most G3P is used to regenerate RuBP, while a net output of one molecule of G3P per three turns exits the cycle to synthesise glucose and other organic compounds.
在还原阶段，GP被ATP磷酸化并被NADPH还原，形成三磷酸甘油醛（G3P）。大部分G3P用于再生RuBP，每三轮循环净输出一分子G3P，用于合成葡萄糖及其他有机化合物。
11. Comparison of Respiration and Photosynthesis | 呼吸作用与光合作用的比较
Both processes involve electron transport chains, ATP synthesis via chemiosmosis, and redox reactions. However, respiration is catabolic, oxidises glucose and releases energy, while photosynthesis is anabolic, reduces CO₂ and stores energy in carbohydrates.
两个过程都涉及电子传递链、化学渗透合成ATP以及氧化还原反应。然而，呼吸作用是分解代谢，氧化葡萄糖并释放能量；光合作用是合成代谢，还原CO₂并将能量储存于碳水化合物中。
| Feature | Respiration | Photosynthesis |
| Overall reaction direction | C₆H₁₂O₆ → CO₂ + H₂O | CO₂ + H₂O → C₆H₁₂O₆ |
| Energy requirement | Releases energy | Requires light energy |
| Location in cell | Cytoplasm + mitochondria | Chloroplasts |
| Key electron carrier | NAD⁺ / FAD | NADP⁺ |
12. Energy Transfer Efficiency and Metabolic Integration | 能量转移效率与代谢整合
The efficiency of aerobic respiration is about 34–40%, with the remaining energy released as heat. This heat is important for endothermic organisms. The ATP produced is used for biosynthesis, active transport, movement, and cell signalling.
有氧呼吸的效率约为34%–40%，其余能量以热的形式释放。这对恒温动物非常重要。产生的ATP用于生物合成、主动运输、运动和细胞信号传导。
Energy transfer in organisms is tightly regulated by feedback mechanisms. High ATP levels inhibit glycolysis and the Krebs cycle, while high ADP levels stimulate respiration. Similarly, light intensity and CO₂ concentration regulate photosynthesis. Together, these pathways form a dynamic network that maintains cellular energy homeostasis.
生物体内的能量转移受反馈机制严格调控。高ATP水平抑制糖酵解和克雷布斯循环，而高ADP水平则刺激呼吸作用。同样，光强度和CO₂浓度调节光合作用。这些途径共同构成一个动态网络，维持细胞能量稳态。
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