IB生物 细胞呼吸与光合作用 核心考点

1. 酶的调控与代谢控制 / Enzyme Regulation and Metabolic Control

酶是生物催化剂，通过降低活化能加速生化反应。在IB Biology中，你需要理解两种核心调控机制：竞争性抑制与非竞争性抑制。竞争性抑制剂与底物结构相似，占据酶的活性位点，这种抑制可以通过增加底物浓度来克服。而非竞争性抑制剂则结合在酶的变构位点上，改变活性位点的三维构象，即使提高底物浓度也无法逆转其抑制效果。这一区别在分析Lineweaver-Burk双倒数图时尤为关键：竞争性抑制使Km增大而Vmax不变，非竞争性抑制则使Vmax降低而Km不变。

Enzymes lower the activation energy of biochemical reactions without being consumed. IB examiners expect you to distinguish between competitive inhibition, where the inhibitor resembles the substrate and binds the active site reversibly, and non-competitive inhibition, where the inhibitor binds an allosteric site and alters the conformation of the active site irreversibly with respect to substrate concentration. On Lineweaver-Burk plots, competitive inhibition increases the apparent Km (x-intercept shifts right) while Vmax remains unchanged, whereas non-competitive inhibition decreases Vmax (y-intercept shifts up) while Km stays the same. Make sure you can sketch these graphs from memory — Paper 2 frequently includes a 4-mark drawing question on enzyme kinetics.

反馈抑制是代谢调控的经典范例。当异亮氨酸的终产物积累到一定浓度时，它会结合到合成通路第一个酶—-苏氨酸脱氨酶的变构位点上，抑制其活性，从而关闭整条合成链。这种末端产物抑制机制在IB考试中反复出现，因为它同时涉及变构调控、非竞争性抑制和代谢通路整合三个知识点。苏氨酸到异亮氨酸的转化路径是理解反馈抑制的最佳模型：苏氨酸经过五步酶促反应生成异亮氨酸，而最后一步的产物反过来抑制第一步的酶。

End-product inhibition, also known as feedback inhibition, is a cornerstone of metabolic regulation. In the isoleucine synthesis pathway, threonine deaminase catalyzes the first committed step. When isoleucine accumulates, it binds the allosteric site of threonine deaminase, causing a conformational change that prevents substrate binding. This shuts down the entire five-step pathway. The beauty of this mechanism is its efficiency — the cell conserves both energy and raw materials by only producing isoleucine when levels are low. In IB exam answers, always link feedback inhibition to non-competitive inhibition and mention the concept of metabolic pathway integration for top marks.

2. 细胞呼吸：从糖酵解到电子传递链 / Cell Respiration: Glycolysis to the ETC

细胞呼吸分为四个阶段：糖酵解、连接反应、克雷布斯循环和氧化磷酸化。糖酵解发生在细胞质中，将一分子葡萄糖(六碳)分解为两个丙酮酸分子(三碳)，净产生2个ATP和2个NADH。这个过程不需要氧气，是所有生物体共有的能量获取方式。值得注意的是，糖酵解中的磷酸化属于底物水平磷酸化—-磷酸基团直接从磷酸化的中间产物转移到ADP上，这与氧化磷酸化中通过ATP合酶的机制完全不同。

Glycolysis occurs in the cytoplasm and converts one molecule of glucose (6C) into two molecules of pyruvate (3C), yielding a net gain of 2 ATP and 2 NADH. The phosphorylation of glucose by hexokinase is the first committed step and requires an investment of 2 ATP. In the payoff phase, four ATP molecules are produced via substrate-level phosphorylation, giving the net yield of 2 ATP per glucose. IB candidates must be able to state the precise locations: glycolysis in the cytoplasm, link reaction in the mitochondrial matrix, Krebs cycle in the matrix, and oxidative phosphorylation across the inner mitochondrial membrane. Location questions are easy marks — do not lose them.

在有氧条件下，丙酮酸进入线粒体基质，经历连接反应：脱羧并氧化，与辅酶A结合生成乙酰辅酶A，同时释放一分子CO2并产生一分子NADH。乙酰辅酶A随后进入克雷布斯循环—-一个八步循环反应，每轮氧化一个乙酰基(二碳)，产生3个NADH、1个FADH2和1个ATP（通过底物水平磷酸化），并释放2个CO2。因为每个葡萄糖产生两个乙酰辅酶A，所以克雷布斯循环每分子葡萄糖总计贡献6个NADH、2个FADH2和2个ATP。记住：CO2中的碳原子并非直接来自吸入的氧气，而是来自葡萄糖碳骨架的逐步氧化。

The link reaction in the mitochondrial matrix converts each pyruvate into acetyl-CoA through oxidative decarboxylation. Pyruvate loses one carbon as CO2, and the remaining two-carbon acetyl group is transferred to coenzyme A. One NAD+ is reduced to NADH per pyruvate. The Krebs cycle then oxidizes each acetyl group completely: for every turn, three NADH, one FADH2, and one ATP (via substrate-level phosphorylation of GDP to GTP, then to ATP) are produced, along with two CO2 molecules. Per glucose molecule, the Krebs cycle runs twice, doubling these yields. A common misconception is that the oxygen atoms in CO2 come from inhaled O2 — they actually come from the carbon skeleton of glucose and from water molecules participating in hydrolysis reactions within the cycle.

氧化磷酸化是ATP产量最高的阶段，发生在线粒体内膜上。NADH和FADH2将高能电子传递给电子传递链中的一系列蛋白复合体（I-IV），电子在传递过程中释放能量，驱动质子从线粒体基质泵入膜间隙。这建立了跨内膜的质子浓度梯度和电化学梯度。质子通过ATP合酶回流到基质时，驱动ATP合成—-这就是化学渗透理论的核心。一分子NADH氧化约产生2.5个ATP，一分子FADH2约产生1.5个ATP。总计，一分子葡萄糖经完全有氧氧化可产生约30-32个ATP。

Oxidative phosphorylation is the major ATP-producing stage, occurring on the inner mitochondrial membrane. Electrons from NADH and FADH2 are passed through a series of protein complexes (I through IV), each with progressively higher electronegativity. The energy released pumps protons from the matrix into the intermembrane space, establishing a proton motive force — an electrochemical gradient combining both concentration difference and electrical potential. Protons flow back through ATP synthase (Complex V), driving the rotational catalysis that phosphorylates ADP. This chemiosmotic mechanism, proposed by Peter Mitchell, unifies the logic of ATP production across respiration and photosynthesis. Each NADH yields approximately 2.5 ATP, each FADH2 yields approximately 1.5 ATP. Accounting for the cost of transporting cytosolic NADH into the matrix, one glucose molecule produces roughly 30-32 ATP under aerobic conditions.

3. 光合作用：光反应与卡尔文循环 / Photosynthesis: Light Reactions and the Calvin Cycle

光合作用分为光依赖反应和光独立反应(卡尔文循环)。光反应发生在类囊体薄膜上，利用光能裂解水分子(光解)，释放氧气、产生ATP和NADPH。光系统II (PSII)吸收680nm波长的光，激发电子经电子传递链传递至光系统I (PSI)。PSI吸收700nm波长的光，再次激发电子，最终将NADP+还原为NADPH。电子传递过程中，质子在类囊体腔内积累，建立质子梯度，驱动ATP合酶合成ATP—-这一过程称为光合磷酸化，与线粒体中的化学渗透机制异曲同工。

The light-dependent reactions take place in the thylakoid membranes of chloroplasts. Photosystem II (P680) absorbs light energy, exciting electrons that are passed through an electron transport chain — plastoquinone, cytochrome b6f complex, and plastocyanin — to Photosystem I (P700). PSI re-excites the electrons, which ultimately reduce NADP+ to NADPH via ferredoxin and NADP reductase. Meanwhile, the photolysis of water at PSII replenishes the lost electrons, releasing O2 and H+ into the thylakoid lumen. The proton gradient across the thylakoid membrane drives ATP synthase to produce ATP in a process called photophosphorylation. IB students should note the elegant parallel with oxidative phosphorylation: both use chemiosmosis, both rely on membrane-bound electron carriers, and both produce ATP via proton gradients. This comparative understanding is gold for Paper 2 essays.

卡尔文循环发生在叶绿体基质中，利用光反应产生的ATP和NADPH将CO2固定为甘油酸-3-磷酸(G3P，三碳糖磷酸)。循环分为三个阶段：羧化(CO2固定)、还原和RuBP再生。Rubisco酶催化CO2与核酮糖-1,5-二磷酸(RuBP)结合，生成不稳定的六碳中间体，随即裂解为两个三碳的磷酸甘油酸(PGA)分子。PGA被ATP磷酸化后被NADPH还原为G3P。每6分子G3P中，5分子用于再生RuBP，1分子输出用于合成葡萄糖、淀粉或其他有机物。因此，每合成一分子葡萄糖需要固定6个CO2，消耗18个ATP和12个NADPH。

The Calvin cycle operates in the chloroplast stroma, using ATP and NADPH from the light reactions to fix CO2 into glycerate-3-phosphate (G3P). The cycle has three phases: carboxylation, reduction, and regeneration of RuBP. Rubisco catalyzes the addition of CO2 to ribulose-1,5-bisphosphate (RuBP), producing an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (PGA). PGA is then phosphorylated by ATP and reduced by NADPH to form G3P. For every six G3P molecules produced, five are recycled to regenerate three RuBP molecules, and one G3P exits the cycle for carbohydrate synthesis. To produce one glucose molecule, the cycle must fix six CO2 molecules, consuming 18 ATP and 12 NADPH. Understanding this stoichiometry is essential: IB often asks you to calculate ATP and NADPH requirements given a certain carbohydrate output.

4. 化学渗透理论的统一性 / The Unity of Chemiosmosis

化学渗透理论是IB Biology HL中最优雅的统一概念之一。无论是线粒体内膜上的氧化磷酸化，还是叶绿体类囊体膜上的光合磷酸化，核心机制完全一致：高能电子沿电子传递链传递时释放的能量将质子从低浓度侧泵到高浓度侧，建立质子动力势；质子通过ATP合酶回流时，其势能被转化为ATP中的化学能。两者的关键区别在于质子泵送方向：线粒体中质子从基质泵入膜间隙，叶绿体中质子从基质泵入类囊体腔；因此线粒体的ATP在基质中合成，而叶绿体的ATP在基质(Stroma)中合成。IB考试特别喜欢比较这两种系统，要求学生绘制标注清晰的膜结构图，展示电子传递链组分和ATP合酶的位置。

Chemiosmosis is the unifying principle behind ATP synthesis in both respiration and photosynthesis. In mitochondria, electrons from NADH and FADH2 travel through Complexes I-IV, pumping protons from the matrix into the intermembrane space. In chloroplasts, electrons excited by light travel through PSII, the cytochrome b6f complex, and PSI, pumping protons from the stroma into the thylakoid lumen. In both cases, the resulting proton gradient drives ATP synthase. The structural orientation differs: mitochondrial ATP synthase protrudes into the matrix, while chloroplast ATP synthase faces the stroma. IB Paper 2 frequently includes a 7-mark question asking you to compare and contrast these two systems. Prepare a table in your revision notes with columns for location, electron source, final electron acceptor, proton pumping direction, and ATP synthesis location — this structured comparison will earn you full marks every time.