📚 IB Biology Cellular Respiration: Glycolysis and the Krebs Cycle Explained | IB生物学 细胞呼吸 糖酵解 克雷布斯循环
Cellular respiration is the set of controlled, enzyme-catalysed reactions through which cells release the chemical energy locked inside organic molecules and transfer it to ATP. For IB Biology students, glycolysis and the Krebs cycle are not simply two names to memorise: they sit at the heart of Topic 8 (Metabolism, Cell Respiration and Photosynthesis), they underpin the respiration practicals in the internal assessment programme, and they appear in exam questions that ask you to outline, explain, compare or deduce. This article follows the pathway in the exact order in which the chemistry happens, from a single glucose molecule to the ATP that powers the cell.
细胞呼吸是细胞内一系列受到精密调控、由酶催化的反应，把有机物中储存的化学能释放出来并转移到 ATP 中。对 IB 生物学的学生来说，糖酵解和克雷布斯循环不只是两个需要背诵的名词：它们是 Topic 8（代谢、细胞呼吸与光合作用）的核心，是实验课程中呼吸测量实验的理论基础，也是考题中要求你 outline、explain、compare 或 deduce 的常客。本文按照化学反应真实发生的顺序，把从一个葡萄糖分子到驱动细胞的 ATP 的整条通路讲清楚。
1. Why Cellular Respiration Matters | 细胞呼吸为何重要
Every living process - active transport, muscle contraction, DNA replication, protein synthesis - depends on ATP. Respiration is therefore the metabolic hub of the cell, and it is the mirror image of photosynthesis: photosynthesis builds glucose using light energy, while respiration oxidises glucose and captures the released energy in a usable form. IB examiners frequently test the link between the two, so you should be able to state that the products of photosynthesis are the substrates of respiration and vice versa.
每一个生命活动——主动运输、肌肉收缩、DNA 复制、蛋白质合成——都依赖 ATP。因此呼吸作用是细胞代谢的枢纽，而且它是光合作用的镜像：光合作用利用光能合成葡萄糖，呼吸作用则氧化葡萄糖并把释放的能量捕获为可利用的形式。IB 考官经常考查两者之间的联系，所以你必须能够指出：光合作用的产物正是呼吸作用的底物，反之亦然。
The overall equation for aerobic respiration summarises a very large number of individual steps. The energy value shown is the enthalpy change for complete oxidation, but only part of it is captured as ATP - the rest is released as heat, which is why the reaction is exergonic but not perfectly efficient.
有氧呼吸的总反应式概括了大量分步反应。下面给出的能量值是彻底氧化的焓变，但其中只有一部分被捕获为 ATP，其余以热能形式散失——这正是该反应放能却并非百分之百高效的原因。
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ΔH ≈ −2870 kJ mol⁻¹)
A useful study habit is to treat the equation as a summary, never as a mechanism. No single enzyme converts glucose straight into carbon dioxide and water; the conversion happens in four sequential stages, each with its own location, enzymes and products. Understanding why the stages exist - and why they are separated in space - is worth far more marks than memorising a list.
一个有用的学习习惯是：把总反应式当作总结，而绝不要当作机理。没有任何一种酶能把葡萄糖一步变成二氧化碳和水；这一转化分为四个先后相连的阶段，每个阶段都有自己的场所、酶和产物。理解这些阶段为何存在、为何在空间上被分隔，比死记一份清单更值分。
2. The Big Picture: Four Stages of Aerobic Respiration | 全局图景：有氧呼吸的四个阶段
Aerobic respiration proceeds through glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. The first three are substrate-level pathways that strip hydrogen atoms and carbon atoms from the fuel molecule; the fourth uses the electrons carried by those hydrogen atoms to drive ATP synthesis. Only the last stage requires oxygen directly.
有氧呼吸依次经历糖酵解、连接反应、克雷布斯循环和氧化磷酸化四个阶段。前三个阶段属于底物水平的途径，从燃料分子上剥离氢原子和碳原子；第四阶段则利用这些氢原子携带的电子来驱动 ATP 合成。只有最后一个阶段直接需要氧气。
| Stage 阶段 | Location 场所 | Key products 主要产物 |
| Glycolysis 糖酵解 | Cytoplasm 细胞质基质 | 2 pyruvate, 2 ATP (net), 2 NADH |
| Link reaction 连接反应 | Mitochondrial matrix 线粒体基质 | 2 acetyl-CoA, 2 CO₂, 2 NADH |
| Krebs cycle 克雷布斯循环 | Mitochondrial matrix 线粒体基质 | 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP |
| Oxidative phosphorylation 氧化磷酸化 | Inner mitochondrial membrane 线粒体内膜 | ≈28 ATP, H₂O |
Notice the logic: the carbon atoms leave as CO₂ in stages two and three, while the hydrogen atoms are handed to electron carriers in every stage except the very last. Oxidative phosphorylation then reunites those electrons with oxygen at the end of the chain. If you can draw this table from memory, most outline questions become straightforward.
注意这里的逻辑：碳原子在第二、第三阶段以 CO₂ 的形式离开，而氢原子除最后阶段外，每一步都被交给电子载体。氧化磷酸化随后让这些电子在电子传递链末端与氧重新结合。如果你能凭记忆默写出这张表，大多数 outline 类题目就变得很简单了。
3. ATP and NAD⁺: Energy Currency and Electron Carriers | ATP 与 NAD⁺：能量货币与电子载体
ATP is a nucleotide derivative consisting of adenine, ribose and three phosphate groups. Its usefulness comes from the repulsion between the negatively charged phosphate groups: removing the terminal phosphate relieves that strain and releases a useful quantity of energy. This is why ATP is described as the universal energy currency - it is portable, stable enough to store, and easily hydrolysed where energy is needed.
ATP 是核苷酸衍生物，由腺嘌呤、核糖和三个磷酸基团组成。它的价值来自带负电的磷酸基团之间的排斥：移除末端磷酸基团能缓解这种张力，并释放出适量可利用的能量。正因如此，ATP 被称为通用能量货币——便于运输、足够稳定以储存能量，又能在需要能量处被轻易水解。
ATP + H₂O → ADP + Pi + energy (≈30.5 kJ mol⁻¹)
ADP + Pi + energy → ATP + H₂O (phosphorylation)
NAD⁺ and FAD are coenzymes that act as hydrogen and electron carriers. When a substrate is oxidised, the hydrogen it loses is transferred to NAD⁺, producing reduced NAD (NADH). The reduced coenzyme can then deliver electrons to the electron transport chain, where its oxidation releases energy for ATP synthesis.
NAD⁺ 和 FAD 是作为氢载体和电子载体发挥作用的辅酶。当底物被氧化时，它失去的氢被转移给 NAD⁺，生成还原型 NAD（NADH）。还原型辅酶随后可以把电子交给电子传递链，其被氧化的过程释放能量用于合成 ATP。
NAD⁺ + 2e⁻ + H⁺ → NADH (oxidation-reduction)
FAD + 2e⁻ + 2H⁺ → FADH₂
H⁺. Both versions describe the same event: two electrons and one proton are accepted by the coenzyme, leaving one proton in solution. Being able to switch between the two conventions without confusion is a small but real exam advantage.
在 IB 的简化写法中，你常常看到 NAD⁺ + 2H → NADH + H⁺。两种写法描述的是同一件事：辅酶接受了两个电子和一个质子，剩下一个质子留在溶液中。能在两种表达之间自如切换而不混淆，是一个虽小却真实的考试优势。
4. Glycolysis Part 1: Investment and Splitting | 糖酵解（上）：能量投资与裂解
Glycolysis means "splitting of sugar" and occurs in the cytoplasm of every cell, from bacteria to human neurones. It does not require oxygen and does not require a mitochondrion, which is why anaerobic organisms and red blood cells can still obtain ATP from glucose. The pathway converts one six-carbon glucose molecule into two three-carbon pyruvate molecules through a sequence of about ten enzyme-catalysed steps.
糖酵解的字面意思是“糖的裂解”，发生在从细菌到人类神经元的每一个细胞的细胞质中。它既不需要氧，也不需要线粒体，因此厌氧生物和红细胞仍然能从葡萄糖中获得 ATP。该途径通过约十步酶促反应，把一个六碳葡萄糖分子转化为两个三碳丙酮酸分子。
The first half of glycolysis is an energy investment phase. Glucose is first phosphorylated to glucose-6-phosphate, then rearranged to fructose-6-phosphate, and finally phosphorylated again to fructose-1,6-bisphosphate. Two ATP molecules are consumed in this phase, and the double phosphorylation makes the molecule symmetrical and reactive enough to be split.
糖酵解的前半段是能量投资阶段。葡萄糖先被磷酸化为葡萄糖-6-磷酸，再重排为果糖-6-磷酸，最后再次磷酸化生成果糖-1,6-二磷酸。这一阶段消耗两分子 ATP，而两次磷酸化使分子既对称又有足够活性，从而能够被裂解。
Glucose + 2ATP → Fructose-1,6-bisphosphate + 2ADP
The lysis step then splits the six-carbon sugar phosphate into two three-carbon triose phosphates, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, which are interconverted. From this point onwards everything happens in duplicate, which explains why the yields of all later steps are even numbers.
随后裂解步骤把六碳糖磷酸切成两个三碳丙糖磷酸：甘油醛-3-磷酸和磷酸二羟丙酮，两者可以相互转化。从这一步起，所有反应都成对进行，这也解释了为什么后面各步的产物数量都是偶数。
A common exam trap is to forget that two ATP are invested. Students who remember only the payoff phase report a gross yield of four ATP and forget to subtract the two already spent, so always state the net figure explicitly.
常见的考试陷阱是忘记投入了两分子 ATP。只记住回报阶段的学生会报出 4 个 ATP 的总产量，却忘了扣除已经花掉的 2 个，所以一定要明确写出净产量。
5. Glycolysis Part 2: Oxidation and ATP Payoff | 糖酵解（下）：氧化与 ATP 净收益
Each triose phosphate is oxidised by the removal of hydrogen, which is transferred to NAD⁺, and simultaneously phosphorylated using inorganic phosphate rather than ATP. The resulting 1,3-bisphosphoglycerate is a high-energy intermediate, and its conversion to 3-phosphoglycerate transfers a phosphate group directly to ADP. This direct transfer is called substrate-level phosphorylation.
每个丙糖磷酸通过脱氢被氧化，氢转移给 NAD⁺，同时利用无机磷酸（而非 ATP）被磷酸化。生成的 1,3-二磷酸甘油酸是高能中间产物，它转化为 3-磷酸甘油酸时把磷酸基团直接交给 ADP。这种直接转移称为底物水平磷酸化。
2 × Triose phosphate + 2NAD⁺ + 2Pi → 2 × 1,3-bisphosphoglycerate + 2NADH + 2H⁺
The remaining steps convert 3-phosphoglycerate to 2-phosphoglycerate, then by dehydration to phosphoenolpyruvate (PEP), which is the second high-energy intermediate. PEP donates its phosphate to ADP to form pyruvate and ATP. Again substrate-level phosphorylation, again two molecules per glucose.
余下的步骤把 3-磷酸甘油酸转变为 2-磷酸甘油酸，再通过脱水生成磷酸烯醇式丙酮酸（PEP），这是第二个高能中间产物。PEP 把磷酸交给 ADP，生成丙酮酸和 ATP。这同样是底物水平磷酸化，同样是每分子葡萄糖产生两份。
2 × Phosphoenolpyruvate + 2ADP → 2 × Pyruvate + 2ATP
Net: Glucose + 2NAD⁺ + 2ADP + 2Pi → 2 Pyruvate + 2NADH + 2H⁺ + 2ATP + 2H₂O
So glycolysis delivers four ATP per glucose but invests two, giving a net gain of two. It also produces two reduced NAD. The pyruvate formed still contains most of the original chemical energy, which is precisely why the pathway cannot stop here in aerobic conditions.
因此糖酵解每分子葡萄糖产生 4 个 ATP，但投入 2 个，净收益为 2 个，同时生成 2 个还原型 NAD。生成的丙酮酸仍然保留了原始化学能的大部分——这正是有氧条件下该途径不能就此停止的原因。
6. The Link Reaction: Pyruvate to Acetyl-CoA | 连接反应：丙酮酸到乙酰辅酶 A
In eukaryotes, pyruvate is actively transported from the cytoplasm into the mitochondrial matrix. There it undergoes oxidative decarboxylation: a carbon atom is removed as CO₂ while hydrogen is removed and transferred to NAD⁺. The remaining two-carbon acetyl group is bound to coenzyme A to form acetyl-CoA.
在真核细胞中，丙酮酸被主动运输从细胞质送入线粒体基质。在那里它经历氧化脱羧：一个碳原子以 CO₂ 形式被移除，同时脱氢并把氢交给 NAD⁺。剩下的二碳乙酰基与辅酶 A 结合，生成乙酰辅酶 A。
Pyruvate + CoA + NAD⁺ → Acetyl-CoA + CO₂ + NADH + H⁺
The link reaction happens twice per glucose molecule because glycolysis produced two pyruvate. Therefore, per glucose, the link reaction yields two acetyl-CoA, two CO₂ and two NADH, and no ATP at all. Students who expect an ATP payoff here lose marks.
由于糖酵解产生了两个丙酮酸，连接反应每分子葡萄糖要进行两次。因此每分子葡萄糖经连接反应产生 2 个乙酰辅酶 A、2 个 CO₂ 和 2 个 NADH，完全不产生 ATP。指望在这一步收获 ATP 的学生会丢分。
The name "link reaction" is descriptive rather than chemical: it is the bridge that connects the cytoplasmic, oxygen-independent pathway to the mitochondrial, oxygen-dependent pathways. It also explains why the first carbon dioxide you can measure from a respiring cell appears only after pyruvate enters the mitochondrion.
“连接反应”这个名字是描述性的而非化学命名：它把细胞质中不依赖氧的途径，与线粒体中依赖氧的途径连接起来。它也解释了为什么从呼吸中的细胞里能测到的第一批二氧化碳，只有在丙酮酸进入线粒体之后才出现。
7. The Krebs Cycle: Step by Step | 克雷布斯循环逐步解析
The Krebs cycle, also called the citric acid cycle or tricarboxylic acid (TCA) cycle, takes place in the mitochondrial matrix. Its defining feature is that the starting compound, a four-carbon acid, is regenerated at the end of each turn, so a small amount of oxaloacetate can process an unlimited number of acetyl groups.
克雷布斯循环又称柠檬酸循环或三羧酸（TCA）循环，发生在线粒体基质中。它的决定性特征是：起始化合物（一种四碳酸）在每一轮结束时被重新生成，因此少量草酰乙酸就能处理数量不限的乙酰基。
In the first turn, the two-carbon acetyl group from acetyl-CoA combines with four-carbon oxaloacetate to form six-carbon citrate, and coenzyme A is released to be reused. Citrate is then rearranged to isocitrate before the cycle proper begins.
在第一轮中，来自乙酰辅酶 A 的二碳乙酰基与四碳草酰乙酸结合，生成六碳柠檬酸，辅酶 A 被释放并可重复使用。随后柠檬酸重排为异柠檬酸，循环才真正开始。
| Reaction 反应 | Carbon change 碳数变化 | Product 产物 |
| Citrate → isocitrate 柠檬酸 → 异柠檬酸 | 6C → 6C | Isomerisation only 仅异构化 |
| Isocitrate → α-ketoglutarate | 6C → 5C | NADH + CO₂ |
| α-ketoglutarate → succinyl-CoA | 5C → 4C | NADH + CO₂ |
| Succinyl-CoA → succinate | 4C → 4C | ATP (substrate-level) |
| Succinate → fumarate |
