IB生物 酶动力学 糖解 呼吸链 卡尔文循环
IB 生物从 2023 年 8 月开始教授新大纲，2025 年 5 月首次评估，这一点在 IBO 官网上写得很清楚。新大纲把内容压成四个主题，酶与代谢、细胞呼吸、光合作用这三节都归在 Theme C「Interaction and interdependence」下面，编号分别是 C1.1、C1.2、C1.3。外部考试占 80%，科学探究（scientific investigation）占 20%。Paper 1A 只考选择题，Paper 1B 与 Paper 2A 都是数据题，Paper 2B 才是长答。换句话说，这三节考的不只是背定义，还要会读图、会算、会把酶和能量的逻辑讲成一句完整的话。考纲要求写明的不是某个数字，而是每一步的因果。
IB Biology moved to a new guide in August 2023, with first assessment in May 2025. The three sections on enzymes, respiration and photosynthesis sit inside Theme C, Interaction and interdependence, coded C1.1 to C1.3. External papers carry 80 per cent of the marks and the scientific investigation another 20, and where Paper 1A is multiple choice, 1B and 2A are built on unfamiliar data. What examiners reward is not a definition alone but the ability to read a graph, run a calculation and explain where the energy and the atoms actually go.
C1.1 酶与代谢：活化能、活性位点、诱导契合
酶是球状蛋白，靠活性位点与底物结合，把反应的活化能压低。压低活化能的意思是让同样一批分子在体温下就能越过能垒，反应速率因此上升。酶自己不提供能量，也不改变反应的净放能，它只是换了一条更省力的路径。诱导契合模型讲的是活性位点先松后紧，底物靠近时位点发生轻微形变，把底物压到接近过渡态的构象。很多学生以为酶在反应结束后被消耗掉，实际上酶是可重复使用的催化剂，真正被消耗的是底物。代谢通路就是一连串这样的酶促反应，前一步的产物是后一步的底物。通路的总速率由最慢的那一步决定，这一步叫限速步。
Enzymes are globular proteins whose active site binds a substrate and lowers the activation energy of the reaction. Lowering that barrier lets a population of molecules cross it at body temperature, so the rate rises without any energy being supplied by the enzyme. The induced fit model describes an active site that is not a rigid lock but tightens around the substrate, distorting it towards the transition state. A common misconception is that enzymes are used up in the reaction, when in fact they emerge unchanged while the substrate is consumed; in a pathway the slowest step, the rate-limiting step, sets the pace for the whole chain.
温度与 pH：最适点、变性、可逆与不可逆
温度升高让分子运动加快，单位时间内有效碰撞变多，速率上升，酶促反应的 Q10 通常接近 2。越过最适温度之后，维持三级结构的氢键与离子键开始断裂，活性位点形状改变，速率骤降，这个过程叫变性。变性在多数情况下不可逆，因为三级结构一旦散开，重新折叠的条件并不存在于细胞里。人的酶最适温度在 37 摄氏度上下，嗜热细菌的酶可以到 70 摄氏度以上，这是氨基酸序列与结构差异带来的结果。pH 改变的是活性位点附近氨基酸侧链的带电状态，胃蛋白酶最适 pH 约为 2，胰蛋白酶约为 8。常见的错误是只写温度升高使酶失活，却不区分低温的可逆抑制与高温的不可逆变性。答题时把这两句话分开写，分数就稳了。
Raising temperature speeds up molecular motion, so more collisions carry enough energy and the rate climbs, with a Q10 close to 2 for enzyme-catalysed reactions. Past the optimum, hydrogen bonds and ionic bonds holding the tertiary structure begin to break, the active site changes shape and the rate collapses; this denaturation is usually irreversible because a cell offers no conditions for refolding. Human enzymes peak near 37 degrees Celsius while enzymes from thermophilic bacteria keep working above 70, a difference carried by amino acid sequence and structure. pH alters the charge on side chains around the active site, which is why pepsin works near pH 2 and trypsin near pH 8; cold slows an enzyme reversibly while heat destroys it.
底物浓度、竞争性与非竞争性抑制剂
底物浓度上升时初速先线性增加，酶活性位点逐渐被占满后曲线走平。这一段平台说明酶已经饱和，再加底物也不会更快，剩下的只有等酶把产物放出来。竞争性抑制剂与底物争夺同一个活性位点，分子结构往往与底物相似，提高底物浓度可以把抑制顶回去。所以竞争性抑制下表观 Km 变大，最大速率不变。非竞争性抑制剂结合在活性位点以外的位置，改变酶的构象，加多少底物都救不回来，最大速率因此下降。重症肌无力用的新斯的明抑制乙酰胆碱酯酶，就是竞争性抑制的常见例子。阅卷时，画抑制曲线只画一条线而不标出有抑制剂与无抑制剂两组，是丢分的常见原因。两条线加清楚标注，比任何形容词都直接。
As substrate concentration rises the initial rate climbs almost linearly, then flattens once the active sites are saturated and extra substrate changes nothing. A competitive inhibitor resembles the substrate and competes for the same active site, so a higher substrate concentration can outcompete it; apparent Km rises while maximum rate is unchanged. A non-competitive inhibitor binds elsewhere on the enzyme and distorts its shape, which no amount of substrate can reverse, so maximum rate falls. Neostigmine blocking acetylcholinesterase in myasthenia gravis is a familiar competitive case, and a single unlabelled curve instead of two labelled lines is one of the fastest ways to lose marks.
终点产物抑制与代谢通路的调控
终点产物抑制是通路最后一步的产物回头抑制前面某一步的酶，通常抑制第一步或分支点的酶。苏氨酸合成异亮氨酸要经过五个酶，终产物异亮氨酸抑制第一个酶，这条负反馈让中间产物不至于堆积。细胞呼吸里 ATP 抑制磷酸果糖激酶，柠檬酸也有抑制作用，糖解速率因此跟着细胞的能量状态走。很多学生把终点产物抑制与竞争性抑制混为一谈，前者的抑制剂是通路自己的产物，结合位点往往是变构位点而不是活性位点。变构结合改变的是酶的构象，不占据底物的位置。回答长答题时把通路画出来，标清抑制箭头指向哪一个酶，比写一段形容词更有用。
End-product inhibition means the final product of a pathway feeds back to inhibit an earlier enzyme, usually the first one or the enzyme at a branch point. Converting threonine to isoleucine takes five enzymes, and isoleucine inhibits the first, a feedback loop that stops intermediates piling up. In respiration ATP inhibits phosphofructokinase and citrate reinforces the brake, so glycolytic flux tracks the energy state of the cell. This is not competitive inhibition, because the inhibitor is the pathway's own product and it usually binds an allosteric site rather than the active site; a labelled diagram with the arrow pointing at the right enzyme earns marks that prose never will.
糖解：细胞质、净 2 ATP、2 NADH
糖解发生在细胞质，不需要氧，也不需要线粒体。一个葡萄糖先被磷酸化，再裂成两分子三碳糖，随后氧化成两分子丙酮酸。整个过程净得 2 个 ATP 和 2 个 NADH。所谓净得 2 个，是因为前期消耗了 2 个 ATP 用于磷酸化，后期每分子三碳糖产 2 个 ATP，总计 4 个减 2 个。这一步的 ATP 来自底物水平磷酸化，磷酸基团直接从底物转到 ADP 上，不经过 ATP 合酶。红细胞没有线粒体，只能靠糖解与乳酸发酵供能，这是理解糖解独立性最直接的例子。糖解也是唯一在无氧条件下仍能持续产 ATP 的阶段。
Glycolysis happens in the cytoplasm and needs neither oxygen nor a mitochondrion. One glucose is phosphorylated, split into two three-carbon sugars and oxidised to two pyruvates, with a net gain of 2 ATP and 2 NADH. The word net matters because two ATP are spent during phosphorylation while four are produced later, two per triose phosphate. These ATP come from substrate-level phosphorylation, where a phosphate group is handed straight from a substrate to ADP rather than through ATP synthase. Red blood cells, which have no mitochondria, run on glycolysis and lactate fermentation alone, and glycolysis is the only stage that keeps making ATP when oxygen runs out.
丙酮酸氧化与克雷布斯循环
丙酮酸先被运进线粒体基质，脱羧去掉一个碳变成乙酰辅酶 A。这一步放出二氧化碳，同时把 NAD+ 还原成 NADH。乙酰辅酶 A 与草酰乙酸结合成柠檬酸，进入克雷布斯循环，转一圈又回到草酰乙酸。每一轮循环都有脱羧与脱氢，产生 NADH、FADH2 与少量 ATP，氢原子被带到内膜交给电子传递链。葡萄糖里六个碳最终全部变成二氧化碳，但二氧化碳不是一次性放出的，前两个碳在丙酮酸氧化阶段就离开了。很多学生算不清每一轮循环放出的二氧化碳数量，记住每个乙酰辅酶 A 放两个就够了。把每一轮的碳数写清楚，数据题的推断部分就不会乱。
Pyruvate enters the mitochondrial matrix, loses a carbon as carbon dioxide and becomes acetyl coenzyme A, generating NADH on the way. Acetyl coenzyme A joins oxaloacetate to form citrate, and the cycle turns back to oxaloacetate while carbons leave as carbon dioxide. Each turn runs decarboxylations and dehydrogenations that yield NADH, FADH2 and a little ATP, with the hydrogen passed to the electron transport chain on the inner membrane. All six carbons of glucose end up as carbon dioxide, but not at once, since the first two leave during pyruvate oxidation; two per acetyl coenzyme A is the number worth remembering.
氧化磷酸化与化学渗透
电子沿内膜上的电子传递链逐级传递，每传一步释放的能量被用来把质子从基质泵到膜间隙。质子在内膜两侧形成浓度差与电荷差，两者合起来叫质子动力势。质子经 ATP 合酶回流到基质，带动 ATP 合酶转动，把 ADP 与磷酸合成 ATP，这个过程叫化学渗透。氧是末端电子受体，接受电子与质子生成水。没有氧电子链就堵住，NADH 无法再生，糖解也会跟着停。氰化物抑制细胞色素氧化酶，电子传递在末端被切断，细胞即使周围有氧也用不上。这也是呼吸计实验中用氢氧化钠吸收二氧化碳时要小心通气的原因。
Electrons pass down the electron transport chain on the inner membrane, and the energy released at each step pumps protons from the matrix into the intermembrane space. That creates both a concentration and a charge difference across the membrane, and together they form the proton motive force. Protons return through ATP synthase, driving the enzyme to join ADP and phosphate, a process called chemiosmosis. Oxygen is the final electron acceptor and combines with electrons and protons to form water, so without it the chain jams, NADH cannot be regenerated and glycolysis stalls; cyanide blocks cytochrome oxidase, which is why poisoned cells cannot use oxygen even when it is present.
无氧呼吸：乳酸发酵与乙醇发酵
无氧条件下电子传递链停摆，NADH 无法通过氧化磷酸化变回 NAD+，糖解就会因为缺少 NAD+ 而停下。人体细胞用乳酸脱氢酶把丙酮酸还原成乳酸，同时把 NADH 氧化成 NAD+，让糖解继续运转。酵母走另一条路，丙酮酸先脱羧成乙醛，再被还原成乙醇，同时放出二氧化碳。发面与酿酒用的就是这条路径，气体让面团膨胀，乙醇在烘烤时挥发。每分子葡萄糖只净得 2 个 ATP，比有氧呼吸少得多，所以剧烈运动时肌肉要靠大量分解葡萄糖来补。乳酸后来会被运到肝脏重新利用，氧债说的就是这一段恢复过程。
Without oxygen the electron transport chain stops, NADH cannot be reoxidised by oxidative phosphorylation and glycolysis would halt for want of NAD+. Human cells reduce pyruvate to lactate using lactate dehydrogenase, regenerating NAD+ so glycolysis can carry on. Yeast takes a different route, decarboxylating pyruvate to ethanal and reducing that to ethanol while releasing carbon dioxide, which is the chemistry behind bread and brewing. The yield is only 2 ATP per glucose, far below aerobic respiration, which is why hard-working muscle burns through glucose so fast; lactate is later shipped to the liver, and the oxygen debt describes that recovery.
ATP 产量与呼吸商
有氧呼吸的 ATP 产量常写成每分子葡萄糖约 30 个，教材给出的区间一般是 30 到 32。这个差别来自穿梭系统与质子漏，所以它是一个近似值而不是恒定值。IBO 的口径只要求说明产量是近似值，不需要背到一个确定数字。呼吸商 RQ 等于释放的二氧化碳体积除以消耗的氧气体积，用呼吸计在恒温下测体积变化。糖类底物的 RQ 接近 1.0，脂类约 0.7，蛋白质在 0.8 到 0.9 之间，混合底物落在中间。大部分学生会在这里丢分，因为忘了先校正温度与气压，或者把体积变化的正负号弄反。数据题里先写公式再代数字，读数就不会错。
Aerobic respiration is usually quoted as about 30 ATP per glucose, with textbook ranges of 30 to 32. That spread comes from the shuttle used and from proton leak, so it is an approximation rather than a fixed constant, which is how the IBO guide treats it. The respiratory quotient is carbon dioxide released divided by oxygen consumed, measured with a respirometer at constant temperature. Carbohydrate gives an RQ near 1.0, lipid around 0.7 and protein between 0.8 and 0.9, so mixed substrates sit in between; forgetting to correct for temperature and pressure, or reversing the sign of the volume change, is where most candidates drop the mark.
C1.3 光反应：类囊体膜与水的光解
光反应在类囊体膜上进行，色素分子吸收光子后把电子激发到更高能级。光系统 II 的电子被激发后经电子传递链流向光系统 I，留下的电子空缺由光解水补上。水被分解成氧气、质子与电子，氧气就这样从水里来，而不是从二氧化碳来。电子沿链传递时释放的能量用于把质子泵进类囊体腔，质子经 ATP 合酶回流到基质，生成 ATP。光系统 I 的电子最终传给 NADP+，与质子结合生成 NADPH，进入卡尔文循环。循环光磷酸化只产 ATP 不产 NADPH，这一点在长答题里常被拿来区分两条路径。同位素标记实验正是用氧 18 证明氧气来自水。
The light-dependent reactions take place on the thylakoid membranes, where pigment molecules absorb photons and raise electrons to a higher energy level. Electrons excited in photosystem II travel down an electron transport chain to photosystem I, and the gap they leave is filled by photolysis of water, which splits into oxygen, protons and electrons. Energy released along the chain pumps protons into the thylakoid space, and their return through ATP synthase makes ATP. Electrons from photosystem I finally reduce NADP+ to NADPH, which feeds the Calvin cycle; cyclic photophosphorylation makes ATP alone, and oxygen-18 labelling is the classic evidence that the oxygen comes from water.
卡尔文循环：rubisco、RuBP、GP 与 TP
卡尔文循环在基质里进行，第一步是 rubisco 把二氧化碳固定到五碳的 RuBP 上。一个二氧化碳加一个 RuBP 生成两个三碳的 GP，所以六个二氧化碳才够合成一个葡萄糖。GP 用光反应送来的 ATP 与 NADPH 还原成 TP。TP 有两条去处，大部分用来再生 RuBP 维持循环，少部分用来合成糖与氨基酸。rubisco 同时也催化氧气与 RuBP 结合，温度高时加氧反应比例上升，光呼吸因此变强。这是高温下净光合速率下降的一个原因，也是 C4 植物在热带占优势的部分解释。常见的错误是把光反应与卡尔文循环的位置写反，类囊体膜与基质不能对调。
The Calvin cycle runs in the stroma, starting with rubisco fixing carbon dioxide onto the five-carbon RuBP. One carbon dioxide plus one RuBP gives two molecules of the three-carbon GP, so six turns are needed for one glucose. ATP and NADPH from the light reactions reduce GP to TP, most of which regenerates RuBP while a small share becomes sugar and amino acids. Rubisco also accepts oxygen, and at high temperature that oxygenase activity rises, so photorespiration increases and net photosynthesis falls; swapping the thylakoid membrane with the stroma is one of the most common errors in long answers.
限制因子、补偿点与色素层析
限制因子是当前最缺、一动就能改变速率的那一个因素。光强、二氧化碳浓度与温度都可能成为限制因子，关键看曲线当前停在哪一段。光强增加时速率上升，到某一点后曲线走平，说明光不再是限制因子，二氧化碳浓度或温度接管。光补偿点是光合速率等于呼吸速率时的光强，低于这个点植物净释放二氧化碳。测色素用纸层析，Rf 值等于色素移动距离除以溶剂前沿距离，Rf 越大说明色素在固定相里吸附越弱。数据题常把吸收光谱与作用光谱放在一起，红光与蓝紫光区域吸收高，作用光谱的峰值通常也落在这两段。把两条曲线的峰值位置对上，解释才算完整。
A limiting factor is whichever variable is currently in shortest supply, so raising it changes the rate; light intensity, carbon dioxide concentration and temperature can all take that role, and the curve tells you which one is active. As light intensity increases the rate rises, then flattens once something else becomes limiting. The compensation point is the light intensity at which photosynthesis equals respiration, and below it a plant releases carbon dioxide on balance. Pigments are separated by paper chromatography, where Rf is the distance moved by the pigment divided by the distance moved by the solvent front, so a larger Rf means weaker adsorption; data questions pair absorption and action spectra, and the peaks line up in the red and blue-violet regions.
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