The Light Independent Reactions of Photosynthesis | 光合作用的光独立反应

📚 The Light Independent Reactions of Photosynthesis | 光合作用的光独立反应

The light independent reactions of photosynthesis, also known as the Calvin cycle, use the products of the light dependent reactions to fix carbon dioxide into organic molecules. They are essential for building sugars and other biological macromolecules in plants, algae, and cyanobacteria.

光合作用的光独立反应，也称为卡尔文循环，利用光依赖反应的产物将二氧化碳固定为有机分子。它们对于植物、藻类和蓝细菌中糖类及其他生物大分子的合成至关重要。

Understanding the Calvin cycle is a core part of the Cambridge A-Level Biology syllabus. You need to be able to describe the location, the main stages, the enzymes involved, the energy requirements, and the role of the products.

理解卡尔文循环是剑桥 A-Level 生物课程的核心内容。你需要能够描述其发生部位、主要阶段、涉及的酶、能量需求以及产物的作用。

1. Introduction | 引言

The light independent reactions are the second major stage of photosynthesis. They do not absorb light directly; instead, they use ATP and reduced NADP produced by the light dependent reactions to reduce carbon dioxide to carbohydrate.

光独立反应是光合作用的第二个主要阶段。它们不直接吸收光，而是利用光依赖反应产生的 ATP 和还原型 NADP，将二氧化碳还原为碳水化合物。

Although these reactions are sometimes called the ‘dark reactions’, this name is misleading because they do not require darkness. They simply do not require light as a direct energy source.

尽管这些反应有时被称为 ‘暗反应’，这一名称具有误导性，因为它们并不需要黑暗环境。它们只是不直接以光作为能量来源。

2. Location and Conditions | 发生部位与条件

The Calvin cycle takes place in the stroma of the chloroplast. The stroma is the colourless fluid matrix surrounding the thylakoid membranes, and it contains the enzymes, substrates, and coenzymes needed for carbon fixation.

卡尔文循环发生在叶绿体基质中。基质是围绕类囊体膜的无色液体基质，含有碳固定所需的酶、底物和辅酶。

The stroma provides a suitable pH for rubisco activity and holds dissolved CO₂, RuBP, ATP, and reduced NADP. This allows the cycle to operate efficiently without being directly exposed to light.

基质为 Rubisco 的活性提供适宜的 pH 值，并含有溶解的 CO₂、RuBP、ATP 和还原型 NADP。这使得循环能够在不直接接触光的情况下高效运行。

Stroma location: rubisco and other Calvin cycle enzymes are found here.
基质定位： Rubisco 和卡尔文循环的其他酶位于此处。
Light not directly required: the cycle can continue in the dark for a short time if ATP and reduced NADP are still available.
不直接需要光： 如果仍有 ATP 和还原型 NADP，循环可在黑暗中短暂继续。

3. Overview of the Calvin Cycle | 卡尔文循环概述

The Calvin cycle can be divided into three main phases: carbon fixation, reduction, and regeneration of RuBP. Each turn of the cycle fixes one molecule of CO₂.

卡尔文循环可分为三个主要阶段：碳固定、还原和 RuBP 再生。循环每运转一次固定一分子 CO₂。

For every three CO₂ molecules fixed, the net product is one molecule of triose phosphate. This triose phosphate can then be used to synthesise glucose, sucrose, starch, amino acids, and fatty acids.

每固定三分子 CO₂，净产物是一分子磷酸丙糖。这些磷酸丙糖可用于合成葡萄糖、蔗糖、淀粉、氨基酸和脂肪酸。

3 CO₂ + 3 RuBP → 6 GP → 6 TP → 5 TP regenerate 3 RuBP + 1 TP net

3 CO₂ + 3 RuBP → 6 GP → 6 TP → 5 TP 再生 3 RuBP + 1 TP 净产物

4. Carbon Fixation by Rubisco | Rubisco 催化的碳固定

In the first phase, each CO₂ molecule combines with a 5-carbon acceptor called ribulose bisphosphate, or RuBP. This reaction is catalysed by the enzyme rubisco, which is one of the most abundant proteins on Earth.

在第一阶段，每个 CO₂ 分子与一个称为核酮糖二磷酸（RuBP）的 5 碳受体结合。该反应由 Rubisco 酶催化，Rubisco 是地球上含量最丰富的蛋白质之一。

The product is an unstable 6-carbon intermediate that immediately breaks down into two molecules of glycerate 3-phosphate, also known as GP or PGA. Each GP molecule contains three carbon atoms.

产物是一个不稳定的 6 碳中间体，它立即分解为两分子甘油酸-3-磷酸，也称为 GP 或 PGA。每个 GP 分子含三个碳原子。

RuBP (C₅) + CO₂ → 2 × GP (C₃)

RuBP (5 碳) + CO₂ → 2 × GP (3 碳)

This is called carboxylation, and it is the stage at which inorganic carbon enters the organic cycle. Rubisco has a low affinity for CO₂, so a high concentration of the enzyme is needed to maintain an adequate rate.

这一过程称为羧化，是无机碳进入有机循环的阶段。Rubisco 对 CO₂ 的亲和力较低，因此需要高浓度的酶来维持足够的反应速率。

5. Reduction of GP to TP | GP 还原为磷酸丙糖

In the second phase, each GP molecule is phosphorylated by ATP and then reduced by reduced NADP. This converts glycerate 3-phosphate into triose phosphate, also called TP or G3P.

在第二阶段，每个 GP 分子被 ATP 磷酸化，然后被还原型 NADP 还原。这将甘油酸-3-磷酸转化为磷酸丙糖，也称为 TP 或 G3P。

The energy and hydrogen for this reduction come from ATP and reduced NADP generated in the light dependent reactions. This is the key point at which light energy, stored chemically, drives the synthesis of carbohydrate.

该还原所需的能量和氢来自光依赖反应中产生的 ATP 和还原型 NADP。这是光能以化学形式储存并驱动碳水化合物合成的关键环节。

GP (C₃) + ATP + NADPH → TP (C₃) + ADP + Pᵢ + NADP⁺

GP (3 碳) + ATP + NADPH → TP (3 碳) + ADP + Pᵢ + NADP⁺

For every six GP molecules produced from three CO₂, six ATP and six reduced NADP molecules are used in this reduction step.

由三个 CO₂ 产生的每六分子 GP，需要消耗六分子 ATP 和六分子还原型 NADP 进行还原。

6. Regeneration of RuBP | RuBP 的再生

In the third phase, five of the six triose phosphate molecules are used to regenerate three molecules of RuBP. This requires a series of enzyme-catalysed reactions and consumes ATP.

在第三阶段，六个磷酸丙糖分子中有五个用于再生三个 RuBP 分子。这需要一系列酶促反应并消耗 ATP。

Regeneration is essential because RuBP is the CO₂ acceptor. Without it, the cycle would stop even if CO₂ and energy were still available.

再生至关重要，因为 RuBP 是 CO₂ 受体。没有 RuBP，即使 CO₂ 和能量仍然存在，循环也会停止。

The remaining one TP molecule is the net gain from three CO₂ molecules. It can be exported from the chloroplast for the synthesis of other organic compounds.

剩余的一分子 TP 是三个 CO₂ 分子的净收益。它可以被运出叶绿体，用于合成其他有机化合物。

7. Energy and Reducing Power: ATP and NADPH | 能量与还原力：ATP 和 NADPH

The Calvin cycle is an endergonic process overall. It is driven by the chemical energy released when ATP is hydrolysed and by the reducing power of reduced NADP.

卡尔文循环总体上是一个吸能过程。它由 ATP 水解时释放的化学能和还原型 NADP 的还原力驱动。

For every three CO₂ molecules fixed, the cycle uses nine ATP molecules and six reduced NADP molecules. Six ATP and six reduced NADP are used in the reduction of GP, and three ATP are used in the regeneration of RuBP.

每固定三分子 CO₂，循环消耗九分子 ATP 和六分子还原型 NADP。其中六分子 ATP 和六分子还原型 NADP 用于 GP 的还原，三分子 ATP 用于 RuBP 的再生。

Input per 3 CO₂	Used in
6 ATP	Reduction of 6 GP to 6 TP
6 reduced NADP	Reduction of 6 GP to 6 TP
3 ATP	Regeneration of 3 RuBP

These coenzymes and ATP are produced in the thylakoid membrane during the light dependent reactions. This is why the light independent reactions ultimately depend on light energy.

这些辅酶和 ATP 是在光依赖反应期间由类囊体膜产生的。这就是光独立反应最终依赖光能的原因。

8. Products and Their Fate | 产物及其去向

The immediate carbohydrate product of the Calvin cycle is triose phosphate. Two TP molecules can be combined to form one hexose sugar, such as glucose, but TP itself is also a key intermediate in chloroplast metabolism.

卡尔文循环的直接碳水化合物产物是磷酸丙糖。两个 TP 分子可以结合形成一个己糖，如葡萄糖，但 TP 本身也是叶绿体代谢中的关键中间体。

TP can be used to synthesise glucose and fructose, which can then form sucrose for transport. It can also be converted to starch for storage, or used to make cellulose for cell walls.

TP 可用于合成葡萄糖和果糖，进而形成蔗糖用于运输。它也可以转化为淀粉用于储存，或用于制造细胞壁的纤维素。

Some TP is also used to produce amino acids and fatty acids, provided that nitrogen and other mineral elements are available. Therefore the Calvin cycle is central to plant biosynthesis.

一些 TP 还用于合成氨基酸和脂肪酸，前提是有氮和其他矿质元素可利用。因此，卡尔文循环是植物生物合成的核心。

9. Limiting Factors and Control | 限制因素与调控

The rate of the Calvin cycle is affected by CO₂ concentration, temperature, and the availability of ATP and reduced NADP. At low CO₂ concentration, rubisco cannot fix carbon efficiently, so the cycle slows.

卡尔文循环的速率受 CO₂ 浓度、温度以及 ATP 和还原型 NADP 的可利用性影响。在低 CO₂ 浓度下，Rubisco 无法高效固定碳，因此循环减慢。

Temperature affects the rate because rubisco and other enzymes have an optimum temperature. Below the optimum, reactions slow; above it, enzymes may denature, and the rate falls sharply.

温度影响速率，因为 Rubisco 和其他酶有最适温度。低于最适温度时反应变慢；高于最适温度时酶可能变性，速率急剧下降。

Light intensity indirectly limits the Calvin cycle because it controls the production of ATP and reduced NADP in the light dependent reactions. In darkness, the cycle stops once these supplies are exhausted.

光照强度间接限制卡尔文循环，因为它控制光依赖反应中 ATP 和还原型 NADP 的生成。在黑暗中，一旦这些供应耗尽，循环就会停止。

CO₂ concentration: low CO₂ limits carbon fixation.
CO₂ 浓度： 低 CO₂ 限制碳固定。
Temperature: affects enzyme activity, especially rubisco.
温度： 影响酶活性，尤其是 Rubisco。
Light intensity: affects ATP and reduced NADP supply.
光照强度： 影响 ATP 和还原型 NADP 的供应。

10. Experimental Evidence: Calvin’s Lollipop Experiment | 实验证据：卡尔文 ‘棒棒糖’ 实验

Melvin Calvin and his colleagues investigated the pathway of carbon fixation using the green alga Chlorella. They supplied the algae with radioactive carbon dioxide containing carbon-14.

梅尔文·卡尔文及其同事利用绿藻小球藻研究了碳固定途径。他们向藻类提供含有碳-14 的放射性二氧化碳。

The algae were illuminated so that photosynthesis could proceed. At different time intervals, samples were rapidly killed in hot alcohol, and the radioactive compounds were separated by two-dimensional chromatography.

藻类被光照，使光合作用得以进行。在不同时间间隔，样品在热酒精中迅速杀死，并通过二维色谱法分离放射性化合物。

After about five seconds, the first labelled organic compound was glycerate 3-phosphate. This showed that GP is an early product of carbon fixation. Later, label appeared in triose phosphate, sugars, and amino acids.

大约五秒后，第一个被标记的有机化合物是甘油酸-3-磷酸。这表明 GP 是碳固定的早期产物。随后，标记出现在磷酸丙糖、糖类和氨基酸中。

This experiment provided strong evidence for the sequence of the Calvin cycle and earned Calvin the Nobel Prize in Chemistry in 1961.

该实验为卡尔文循环的反应顺序提供了有力证据，卡尔文因此于 1961 年获得诺贝尔化学奖。

11. Comparison with Light-Dependent Reactions | 与光依赖反应的比较

The light dependent and light independent reactions are closely linked. The light dependent reactions convert light energy into chemical energy in the form of ATP and reduced NADP, while the light independent reactions use this chemical energy to fix CO₂.

光依赖反应和光独立反应密切相关。光依赖反应将光能转化为 ATP 和还原型 NADP 形式的化学能，而光独立反应利用这些化学能固定 CO₂。

Feature	Light Dependent Reactions	Light Independent Reactions
Location	Thylakoid membranes	Stroma
Direct light	Required	Not required directly
Inputs	Light, H₂O, ADP, NADP⁺	CO₂, ATP, reduced NADP, RuBP
Products	ATP, reduced NADP, O₂	TP, ADP, NADP⁺, RuBP

In the light dependent reactions, water is split by photolysis to release electrons, protons, and oxygen. In the light independent reactions, water is not a reactant; instead, CO₂ is reduced to carbohydrate.

在光依赖反应中，水通过光解作用释放电子、质子和氧气。在光独立反应中，水不是反应物；相反，CO₂ 被还原为碳水化合物。

12. Exam Tips and Common Misconceptions | 考试提示与常见误区

A common misconception is that the Calvin cycle occurs at night because it is called the ‘dark reaction’. In fact, it runs during the day when ATP and reduced NADP are being produced.

一个常见的误区是，由于卡尔文循环被称为 ‘暗反应’，因此认为它发生在夜间。实际上，它在白天 ATP 和还原型 NADP 产生时运行。

Another common error is to think that ATP and reduced NADP are produced in the stroma. They are actually produced in the thylakoid membrane and then diffuse into the stroma for use in the Calvin cycle.

另一个常见错误是认为 ATP 和还原型 NADP 在基质中产生。实际上它们是在类囊体膜中产生，然后扩散到基质中供卡尔文循环使用。

When asked to explain why the Calvin cycle slows in darkness, you should state that the supply of ATP and reduced NADP from the light dependent reactions decreases, so GP cannot be reduced to TP and RuBP cannot be regenerated.

当被要求解释为何卡尔文循环在黑暗中减慢时，你应该说明来自光依赖反应的 ATP 和还原型 NADP 供应减少，因此 GP 无法还原为 TP，RuBP 也无法再生。

Remember that rubisco catalyses the fixation of CO₂, not the reduction of GP. The reduction of GP is catalysed by other enzymes and requires ATP and reduced NADP.

记住 Rubisco 催化 CO₂ 的固定，而不是 GP 的还原。GP 的还原由其他酶催化，并需要 ATP 和还原型 NADP。

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The Light Independent Reactions of Photosynthesis | 光合作用的光独立反应