📚 IB Biology Cell Respiration, Electron Transport Chain and Photophosphorylation | IB生物 细胞呼吸、电子传递链与光合磷酸化
Cellular respiration and photosynthesis are the two great energy-transducing pathways of life, and IB Biology examiners return to them year after year because they link almost every core topic: membrane structure, enzymes, redox chemistry, transport and metabolic regulation. In this revision article we follow the electrons step by step, from glucose in the cytoplasm to oxygen at the end of the mitochondrial electron transport chain, and then from water inside the thylakoid lumen to NADPH in the stroma, explaining chemiosmosis, oxidative phosphorylation and photophosphorylation in the precise language the mark scheme expects.
细胞呼吸与光合作用是生命世界中两条最重要的能量转换途径，IB 生物考官多年反复考查它们，正是因为这两条途径几乎串起了所有核心主题：膜结构、酶、氧化还原化学、物质运输与代谢调控。在本篇复习文章中，我们将沿着电子一步步追踪：从细胞质中的葡萄糖，到线粒体电子传递链末端被氧接收；再从类囊体腔内的水，到基质中的 NADPH，并用评分标准期望的精确语言讲清化学渗透、氧化磷酸化与光合磷酸化。
1. Overview of Cellular Energy Metabolism | 细胞能量代谢总览
All living cells must obtain energy in a usable form, and that form is ATP. ATP is described as the universal energy currency because hydrolysis of its terminal phosphoanhydride bond releases a useful quantity of free energy, roughly 30.5 kJ mol⁻¹ under standard conditions, and because the same molecule can be synthesised and hydrolysed by enzymes in every cell type. ATP is not a long-term store; it is a short-lived intermediate that couples exergonic reactions to endergonic ones.
所有活细胞都必须获得可用形式的能量，而这一形式就是 ATP。ATP 被称为通用能量货币，原因有二：其一，其末端磷酸酐键水解可释放适量自由能，标准条件下约为 30.5 kJ mol⁻¹；其二，同一分子可在所有细胞类型中被酶合成与水解。ATP 并非长期储能物质，而是一种短寿命的中间体，把放能反应与吸能反应偶联起来。
Metabolism is conventionally divided into catabolism, the oxidative breakdown of complex molecules that releases energy, and anabolism, the reductive synthesis of complex molecules that consumes energy. Respiration is catabolic and exergonic overall; photosynthesis is anabolic and endergonic, driven by light energy. Both pathways depend on reversible redox reactions in which electrons are transferred between carriers, and both use electron transport chains embedded in membranes to convert electron flow into a proton gradient.
代谢通常分为分解代谢与合成代谢：前者指复杂分子被氧化分解并释放能量，后者指复杂分子被还原合成并消耗能量。细胞呼吸总体属于放能的分解代谢；光合作用总体属于吸能的合成代谢，由光能驱动。两条途径都依赖可逆的氧化还原反应，电子在载体之间传递，并且都利用镶嵌于膜上的电子传递链，把电子流转化为质子梯度。
substrate, whereas the light-dependent reactions of photosynthesis reduce NADP⁺ to NADPH by oxidising water. In both cases the reduced carrier is later re-oxidised at a membrane-bound chain, and that re-oxidation is what pumps protons.
考试中的关键区分是：细胞呼吸通过氧化底物来还原电子载体（NAD⁺ → NADH，FAD → FADH₂）；而光合作用的光反应则通过氧化水来把 NADP⁺ 还原为 NADPH。两种情况下，还原型载体随后都在膜结合链上被重新氧化，而正是这一再氧化过程驱动质子泵送。
2. Glycolysis and the Link Reaction | 糖酵解与连接反应
Glycolysis occurs in the cytoplasm and requires no membrane and no oxygen, which is why it is found in every living organism. Glucose, a six-carbon sugar, is phosphorylated by two ATP molecules in the investment phase, then split into two triose phosphate molecules, and finally oxidised and dephosphorylated in the payoff phase. The net equation is:
糖酵解发生在细胞质中，既不需要膜，也不需要氧，这正是它存在于所有生物中的原因。六碳糖葡萄糖在投资阶段被两分子 ATP 磷酸化，随后裂解为两分子磷酸丙糖，最后在回收阶段被氧化并脱磷酸。净反应方程式为：
C₆H₁₂O₆ + 2NAD⁺ + 2ADP + 2Pᵢ → 2C₃H₄O₃ + 2NADH + 2H⁺ + 2ATP + 2H₂O
The net yield per glucose is therefore 2 ATP by substrate-level phosphorylation and 2 reduced NAD. Because the enzyme phosphofructokinase is inhibited by ATP and citrate and activated by AMP and fructose-2,6-bisphosphate, glycolysis is tightly regulated according to the cell's energy charge. The word "net" matters: four ATP are produced but two are consumed.
因此每分子葡萄糖净产 2 分子 ATP（底物水平磷酸化）和 2 分子还原型 NAD。由于磷酸果糖激酶受 ATP 与柠檬酸抑制、受 AMP 与果糖-2,6-二磷酸激活，糖酵解依细胞的能荷被严格调控。"净"字很关键：共产生 4 分子 ATP，但消耗 2 分子。
If oxygen is available, pyruvate enters the mitochondrial matrix by active transport. There the link reaction (oxidative decarboxylation) removes one carbon as CO₂ and one hydrogen pair as NADH, attaching coenzyme A to the remaining two-carbon fragment:
若有氧存在，丙酮酸经主动运输进入线粒体基质。在那里，连接反应（氧化脱羧）以 CO₂ 形式脱去一个碳，以 NADH 形式脱去一对氢，并把辅酶 A 接到剩余的二碳片段上：
2C₃H₄O₃ + 2NAD⁺ + 2CoA → 2CH₃CO·CoA + 2CO₂ + 2NADH + 2H⁺
Acetyl coenzyme A is the universal entry point to the Krebs cycle, and the link reaction is irreversible, which is one reason animals cannot convert fat or acetyl-CoA back into glucose. No ATP is generated in this step, but two CO₂ and two NADH are produced per glucose.
乙酰辅酶 A 是进入克雷布斯循环的通用入口，而连接反应不可逆，这也是动物无法把脂肪或乙酰辅酶 A 反向转化为葡萄糖的原因之一。该步骤不产生 ATP，但每分子葡萄糖产生 2 分子 CO₂ 和 2 分子 NADH。
3. The Krebs Cycle | 克雷布斯循环
The Krebs cycle, also called the citric acid cycle or tricarboxylic acid cycle, takes place in the mitochondrial matrix. Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C), which is then progressively decarboxylated and dehydrogenated over eight enzymatic steps until oxaloacetate is regenerated, ready to accept the next acetyl group.
克雷布斯循环又称柠檬酸循环或三羧酸循环，发生在线粒体基质中。乙酰辅酶 A（2C）与草酰乙酸（4C）结合生成柠檬酸（6C），随后经八步酶促反应逐步脱羧、脱氢，直至草酰乙酸再生，准备接受下一个乙酰基。
Per turn of the cycle, the following outputs are produced from one acetyl group:
每转一圈循环，一个乙酰基会产生以下产物：
- 2 × CO₂ released by decarboxylation | 脱羧释放 2 × CO₂
- 3 × reduced NAD (NADH + H⁺) | 3 × 还原型 NAD（NADH + H⁺）
- 1 × reduced FAD (FADH₂) | 1 × 还原型 FAD（FADH₂）
- 1 × ATP (or GTP) by substrate-level phosphorylation | 1 × ATP（或 GTP），经底物水平磷酸化
acetyl-CoA + 3NAD⁺ + FAD + GDP + Pᵢ + 2H₂O → 2CO₂ + 3NADH + FADH₂ + GTP + CoA
Because glucose yields two pyruvate, the cycle turns twice per glucose, giving a total of 4 CO₂, 6 NADH, 2 FADH₂ and 2 ATP. Note that the cycle itself releases only a small fraction of the ATP eventually available; the bulk of the energy remains bound in the reduced coenzymes, which must be re-oxidised by the electron transport chain. The cycle is amphibolic, meaning it serves both catabolic and anabolic roles, since intermediates such as α-ketoglutarate and oxaloacetate are precursors for amino acids and haem groups.
由于一分子葡萄糖产生两分子丙酮酸，循环转运两圈，共产生 4 CO₂、6 NADH、2 FADH₂ 和 2 ATP。注意循环本身只释放了最终可得 ATP 的一小部分；大部分能量仍保存在还原型辅酶中，必须由电子传递链重新氧化。该循环具有两栖性，即兼具分解与合成功能，因为 α-酮戊二酸、草酰乙酸等中间产物是氨基酸与血红素基团的前体。
4. Structure and Role of the Electron Transport Chain | 电子传递链的结构与作用
The electron transport chain (ETC) is a series of protein complexes and mobile carriers located in the inner mitochondrial membrane. The membrane is folded into cristae, greatly increasing surface area and therefore the number of ETCs and ATP synthase molecules per mitochondrion. Muscle cells and active neurones have densely packed cristae, which is a structural adaptation for high ATP demand.
电子传递链（ETC）由一系列蛋白复合体与可移动载体组成，位于线粒体内膜上。内膜折叠形成嵴，大大增加表面积，从而增加每个线粒体的 ETC 与 ATP 合酶数量。肌细胞和活跃神经元具有密集的嵴，这是对高 ATP 需求的结构适应。
Electrons enter at two points. NADH is oxidised by complex I (NADH dehydrogenase), which pumps protons; FADH₂ is oxidised by complex II (succinate dehydrogenase), which does not pump protons. Electrons then travel via the lipid-soluble carrier ubiquinone (coenzyme Q) to complex III, and via the peripheral protein cytochrome c to complex IV (cytochrome oxidase). Complexes I, III and IV each pump protons from the matrix into the intermembrane space.
电子从两个位点进入。NADH 被复合体 I（NADH 脱氢酶）氧化，该复合体泵送质子；FADH₂ 被复合体 II（琥珀酸脱氢酶）氧化，该复合体不泵送质子。电子随后经脂溶性载体泛醌（辅酶 Q）传至复合体 III，再经外周蛋白细胞色素 c 传至复合体 IV（细胞色素氧化酶）。复合体 I、III、IV 均把质子从基质泵入膜间隙。
Oxygen is the final electron acceptor. At complex IV, four electrons, four protons and one oxygen molecule combine to form two water molecules:
氧是最终电子受体。在复合体 IV 处，四个电子、四个质子与一个氧分子结合，生成两分子水：
4e⁻ + 4H⁺ + O₂ → 2H₂O
This step is essential for two reasons: it removes electrons from the chain so that electron flow can continue, and it consumes protons from the matrix, helping to maintain the gradient. Without oxygen the entire chain becomes reduced, electron flow stops, NADH cannot be re-oxidised, and the Krebs cycle halts. This is why cyanide, which inhibits cytochrome oxidase, is lethal within minutes.
该步骤至关重要，原因有二：其一，它把电子从链上移走，使电子流得以继续；其二，它消耗基质中的质子，有助于维持梯度。没有氧，整条链被还原，电子流停止，NADH 无法再氧化，克雷布斯循环随之停止。这就是氰化物（抑制细胞色素氧化酶）可在数分钟内致命的原因。
5. Chemiosmosis and ATP Synthesis | 化学渗透与ATP合成
Chemiosmosis is the mechanism that converts the proton gradient into ATP, and it was proposed by Peter Mitchell in 1961. Electron transport pumps protons across the inner mitochondrial membrane, producing a higher H⁺ concentration in the intermembrane space than in the matrix. This difference creates both a chemical gradient (a pH difference of about 1 unit) and an electrical gradient (the membrane potential), and the two together constitute the proton-motive force.
化学渗透是把质子梯度转化为 ATP 的机制，由 Peter Mitchell 于 1961 年提出。电子传递把质子泵过线粒体内膜，使膜间隙的 H⁺ 浓度高于基质。这一差异同时形成化学梯度（约 1 个 pH 单位的差值）与电位梯度（膜电位），二者共同构成质子动力势。
Protons return to the matrix only through ATP synthase, a large enzyme with a hydrophobic channel (F₀) spanning the membrane and a catalytic head (F₁) projecting into the matrix. As protons flow down the gradient through F₀, the enzyme rotates, and the F₁ head catalyses the condensation of ADP with inorganic phosphate:
质子只能通过 ATP 合酶返回基质。该酶体积庞大，含跨膜疏水通道（F₀）和伸入基质的催化头部（F₁）。当质子顺梯度流过 F₀ 时，酶发生旋转，F₁ 头部催化 ADP 与无机磷酸缩合：
ADP + Pᵢ → ATP + H₂O
This process is called oxidative phosphorylation because it depends on the oxidation of NADH and FADH₂. A useful approximation is that around 3 protons are required for each ATP synthesised. Because ATP synthesis is driven by the gradient rather than by electron transport directly, the two processes are separable: an uncoupling agent such as 2,4-dinitrophenol (DNP) carries protons across the membrane, so electron transport continues at maximum speed and oxygen is consumed rapidly, but no ATP is made and energy is released as heat.
该过程称为氧化磷酸化，因为它依赖 NADH 与 FADH₂ 的氧化。一个有用的近似是：每合成 1 分子 ATP 约需 3 个质子。由于 ATP 合成由梯度而非电子传递直接驱动，两个过程可以彼此分离：解偶联剂如 2,4-二硝基苯酚（DNP）可携带质子穿过膜，于是电子传递仍以最高速度进行、氧被迅速消耗，但不产生 ATP，能量以热的形式释放。
6. ATP Yield, Anaerobic Respiration and Inhibitors | ATP产量、无氧呼吸与抑制剂
Because the reduced coenzymes generated in the cytoplasm during glycolysis cannot cross the inner mitochondrial membrane, their electrons must be shuttled by carrier systems. The glycerol phosphate shuttle delivers electrons to FAD, yielding about 1.5 ATP per NADH, while the malate-aspartate shuttle delivers them to NAD in the matrix, yielding about 2.5 ATP. This is why textbook totals differ between tissues.
由于糖酵解在细胞质中生成的还原型辅酶无法穿过线粒体内膜，其电子必须由穿梭系统转运。磷酸甘油穿梭把电子交给 FAD，每分子 NADH 约得 1.5 ATP；苹果酸-天冬氨酸穿梭把电子交给基质中的 NAD，每分子 NADH 约得 2.5 ATP。这就是不同组织理论总 ATP 产量不同的原因。
A commonly quoted modern total is about 32 ATP per glucose, though 30–34 should be accepted as a legitimate range in IB answers. Older textbooks give 36 or 38 ATP because they assume 3 ATP per NADH; examiners now expect candidates to acknowledge uncertainty rather than quote a single figure with false confidence.
现代常引用的总产量约为每分子葡萄糖 32 ATP，但在 IB 作答中，30–34 都应视为合理范围。较早的教材给出 36 或 38 ATP，是因为假设每个 NADH 产 3 ATP；如今的考官希望考生承认这种不确定性，而不是自信地给出单一数值。
When oxygen is absent, the ETC cannot operate and NADH accumulates. To keep glycolysis running, cells regenerate NAD⁺ by reducing pyruvate. In mammals this is lactic acid fermentation, catalysed by lactate dehydrogenase, producing lactate. In yeast and plants it is alcoholic fermentation, in which pyruvate is decarboxylated to ethanal and then reduced to ethanol by alcohol dehydrogenase. Both pathways yield only 2 ATP per glucose, which is why intense exercise cannot be sustained indefinitely.
缺氧时 ETC 无法运转，NADH 累积。为维持糖酵解，细胞通过还原丙酮酸再生 NAD⁺。哺乳动物进行乳酸发酵，由乳酸脱氢酶催化生成乳酸；酵母和植物进行酒精发酵，丙酮酸先脱羧生成乙醛，再由乙醇脱氢酶还原为乙醇。两条途径每分子葡萄糖仅产 2 ATP，这就是剧烈运动无法无限持续的原因。
Respiratory inhibitors are frequently examined. Cyanide and carbon monoxide inhibit cytochrome oxidase; rotenone inhibits complex I; antimycin inhibits complex III; oligomycin blocks the proton channel of ATP synthase so that protons cannot return and the gradient becomes too steep for further pumping; DNP uncouples the gradient. Note that if the gradient cannot be maintained, electron transport also slows, so inhibitors of ATP synthase indirectly inhibit respiration.
呼吸抑制剂是高频考点。氰化物与一氧化碳抑制细胞色素氧化酶；鱼藤酮抑制复合体 I；抗霉素抑制复合体 III；寡霉素阻断 ATP 合酶的质子通道，使质子无法返回、梯度过陡而进一步泵送受阻；DNP 则解偶联梯度。注意：若梯度无法维持，电子传递也会减慢，因此 ATP 合酶的抑制剂会间接抑制呼吸。
7. Photosynthesis: An Overview of the Light-Dependent Reactions | 光合作用：光反应总览
Photosynthesis has two stages. The light-dependent reactions occur on the thylakoid membranes of the chloroplast and convert light energy into chemical energy as ATP and NADPH, releasing oxygen from water. The light-independent reactions (the Calvin cycle) occur in the stroma and use that ATP and NADPH to reduce CO₂ into carbohydrate. The overall summary equation is often written as:
光合作用分两个阶段。光反应发生在叶绿体的类囊体膜上，把光能转化为化学能，生成 ATP 与 NADPH，并从水中释放氧气。光不依赖反应（卡尔文循环）发生在基质中，利用这些 ATP 与 NADPH 把 CO₂ 还原为糖类。总体概括方程式常写为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The thylakoid membrane contains four multiprotein complexes: photosystem II (PSII), the cytochrome b₆f complex, photosystem I (PSI) and ATP synthase, together with the mobile carriers plastoquinone (PQ), plastocyanin (PC) and ferredoxin. Each photosystem consists of an antenna complex of accessory pigments that funnel energy to a reaction centre containing a special pair of chlorophyll a molecules. Accessory pigments such as chlorophyll b and carotenoids absorb wavelengths that chlorophyll a absorbs poorly, broadening the action spectrum.
类囊体膜上含四种多蛋白复合体：光系统 II（PSII）、细胞色素 b₆f 复合体、光系统 I（PSI）和 ATP 合酶，以及可移动载体质体醌（PQ）、质体蓝素（PC）和铁氧还蛋白。每个光系统由辅助色素组成的捕光天线复合体构成，把能量汇聚到含特殊叶绿素 a 分子对（反应中心色素对）的反应中心。叶绿素 b 与类胡萝卜素等辅助色素可吸收叶绿素 a 吸收较弱的波长，从而拓宽作用光谱。
When a photon is absorbed by the reaction centre of PSII (P680), an electron is excited to a higher energy level and passed to the primary electron acceptor. The resulting electron hole is filled by an electron extracted from water in the oxygen-evolving complex, a reaction that also releases protons into the lumen and produces molecular oxygen:
当光子被 PSII 反应中心（P680）吸收时，一个电子被激发到更高能级并传给初级电子受体。由此产生的电子空穴由氧释放复合体从水中夺取的电子填补，该反应同时把质子释放到类囊体腔并生成分子氧：
2H₂O → 4H⁺ + 4e⁻ + O₂
This is the origin of the oxygen we breathe, a fact confirmed experimentally by Robert Hill in 1937 and by ¹⁸O labelling experiments showing that all the oxygen released comes from water rather than from carbon dioxide.
这是我们所呼吸氧气的来源。Robert Hill 于 1937 年的实验以及 ¹⁸O 同位素标记实验均证实：释放的全部氧气都来自水，而非二氧化碳。
8. Non-Cyclic Photophosphorylation | 非循环光合磷酸化
Non-cyclic photophosphorylation uses both photosystems in series and produces ATP, NADPH and oxygen. The pathway is often drawn as a Z-scheme because the energy of the electron rises twice and falls twice. Electrons flow in one direction only, from water to NADP⁺, so the process is non-cyclic and requires a continuous supply of water.
非循环光合磷酸化串联使用两个光系统，产生 ATP、NADPH 和氧气。该途径常被画成 Z 型方案，因为电子的能量两次升高、两次下降。电子只沿一个方向流动，从水到 NADP⁺，因此该过程为非循环的，需要持续供水。
(P700). At the cytochrome b₆f complex, protons are pumped from the stroma into the thylakoid lumen. At PSI, light of a slightly longer wavelength re-excites the electrons, raising them to an even more negative redox potential, and they are passed via ferredoxin to NADP⁺ reductase, where NADP⁺ is reduced:
离开 PSII 后，电子沿链传递：PSII → 质体醌 → 细胞色素 b₆f → 质体蓝素 → PSI（P700）。在细胞色素 b₆f 复合体处，质子从基质被泵入类囊体腔。在 PSI 处，波长稍长的光再次激发电子，使其氧化还原电位更负，随后经铁氧还蛋白传给 NADP⁺ 还原酶，NADP⁺ 被还原：
NADP⁺ + 2e⁻ + H⁺ → NADPH
Chemiosmosis then couples the proton gradient to ATP synthesis, and the overall stoichiometry for non-cyclic photophosphorylation can be summarised as:
随后化学渗透把质子梯度与 ATP 合成偶联起来，非循环光合磷酸化的总体化学计量可概括为：
2H₂O + 2NADP⁺ + 3ADP + 3Pᵢ → O₂ + 2NADPH + 2H⁺ + 3ATP
A common misconception is that the electron itself is a "product" of photosynthesis; in fact the electron is recycled within the membrane, and the stable products are ATP, NADPH and O₂. Another is that ATP is made by the photosystems directly; ATP is only made by ATP synthase, downstream of the gradient.
常见误解之一是认为电子本身是光合作用的"产物"；实际上电子在膜内被循环使用，稳定产物是 ATP、NADPH 和 O₂。另一个误解是认为光系统直接产生 ATP；ATP 只由 ATP 合酶在梯度下游生成。
9. Cyclic Photophosphorylation | 循环光合磷酸化
Cyclic photophosphorylation involves only photosystem I. Light excites P700, the electron is passed to ferredoxin, and instead of reducing NADP⁺ it is returned via the cytochrome b₆f complex and plastocyanin back to PSI. The electron therefore travels in a closed loop and no water is split, no oxygen is evolved and no NADPH is produced. However, protons are still pumped at the b₆f complex, so ATP is generated.
循环光合磷酸化只涉及光系统 I。光激发 P700 后，电子传给铁氧还蛋白，但它不再还原 NADP⁺，而是经细胞色素 b₆f 复合体和质体蓝素返回 PSI。因此电子沿闭合回路运行，水不被裂解，不释放氧气，也不产生 NADPH；但 b₆f 复合体仍泵送质子，故可生成 ATP。
ADP + Pᵢ → ATP (light-driven, PSI only)
Cyclic photophosphorylation is important because the Calvin cycle consumes ATP and NADPH in a ratio of roughly 3:2, which is more ATP than non-cyclic photophosphorylation alone supplies. Cyclic flow tops up the ATP deficit. It is the predominant or only mode of photophosphorylation in some photosynthetic bacteria that lack PSII and cannot split water, and it is thought to operate in guard cells and bundle sheath cells where additional ATP is needed.
循环光合磷酸化很重要，因为卡尔文循环消耗 ATP 与 NADPH 的比例约为 3:2，比仅靠非循环光合磷酸化提供的 ATP 更多，循环电子流可补足 ATP 缺口。在某些缺少 PSII、无法裂解水的光合细菌中，它是主要甚至唯一的光合磷酸化方式；在需要额外 ATP 的保卫细胞和维管束鞘细胞中，据认为也可进行。
| Feature | 特征 | Non-cyclic | 非循环 | Cyclic | 循环 |
| Photosystems | 光系统 | PSII and PSI | PSII 与 PSI | PSI only | 仅 PSI |
| Water split | 是否裂解水 | Yes | 是 | No | 否 |
| O₂ released | 是否释放 O₂ | Yes | 是 | No | 否 |
| NADPH produced
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