📚 The Light Independent Reactions of Photosynthesis | 光合作用的光不依赖反应（暗反应）
The light-independent reactions, often called the Calvin cycle, are a series of biochemical processes that occur in the stroma of chloroplasts. Although they do not directly require light, they depend on the products of the light-dependent reactions (ATP and reduced NADP) to fix carbon dioxide into organic molecules. In this article, we explore the stages, key molecules, and regulation of the Calvin cycle, essential for A-Level Biology.
光不依赖反应，常被称为卡尔文循环，是在叶绿体基质中进行的一系列生化过程。虽然它们不需要光直接参与，但依赖于光反应产生的ATP和还原型NADP，将二氧化碳固定为有机物。本文将探讨卡尔文循环的各个阶段、关键分子及其调控，这些内容对A-Level生物学至关重要。
1. Location and Overview | 反应场所与概述
The light-independent reactions take place in the stroma of chloroplasts, the fluid-filled space surrounding the thylakoid membranes. This compartment contains all the enzymes necessary for the Calvin cycle, including the crucial enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO). The stroma also holds the substrates RuBP, ATP, and reduced NADP generated by the light-dependent reactions on the thylakoid membranes.
光不依赖反应发生在叶绿体的基质中，即围绕类囊体膜的充满液体的空间。这个区域含有卡尔文循环所需的所有酶，包括关键酶二磷酸核酮糖羧化酶/加氧酶（RuBisCO）。基质中还含有底物RuBP，以及由类囊体膜上的光反应产生的ATP和还原型NADP。
2. Key Molecules and Components | 关键分子与组分
• Ribulose bisphosphate (RuBP) – a 5-carbon sugar phosphate that acts as the primary CO₂ acceptor.
• 二磷酸核酮糖（RuBP）——一种五碳糖磷酸，作为CO₂的初始受体。
• Carbon dioxide (CO₂) – the inorganic carbon source that enters the stroma from the atmosphere via stomata.
• 二氧化碳（CO₂）——无机碳源，通过气孔从大气进入基质。
• Glycerate 3-phosphate (GP) – a 3-carbon compound formed immediately after carbon fixation.
• 3-磷酸甘油酸（GP）——碳固定后立即生成的三碳化合物。
• Triose phosphate (TP) – the 3-carbon sugar phosphate produced upon reduction of GP; also called glyceraldehyde 3-phosphate (GALP).
• 磷酸丙糖（TP）——GP还原后生成的三碳糖磷酸，也称3-磷酸甘油醛（GALP）。
• ATP – provides the energy for phosphorylation reactions.
• ATP——为磷酸化反应提供能量。
• Reduced NADP – supplies the reducing power (hydrogen atoms) to convert GP into TP.
• 还原型NADP——提供将GP转化为TP所需的还原力（氢原子）。
3. Carbon Fixation: The Carboxylation Step | 碳固定：羧化步骤
In the first major stage, CO₂ combines with RuBP (a 5-carbon compound) in a reaction catalysed by the enzyme RuBisCO. The unstable 6-carbon intermediate immediately splits into two molecules of glycerate 3-phosphate (GP), each containing three carbon atoms. This is the point at which inorganic carbon enters the organic world of the cell.
在第一主要阶段，CO₂在RuBisCO酶的催化下与RuBP（五碳化合物）结合。不稳定的六碳中间体随即分裂为两分子3-磷酸甘油酸（GP），每分子含三个碳原子。这是无机碳进入细胞有机世界的节点。
RuBP (5C) + CO₂ → 2 × glycerate 3-phosphate (GP) (3C)
RuBP (5C) + CO₂ → 2 × 3-磷酸甘油酸 (GP) (3C)
4. Reduction of GP to Triose Phosphate | GP还原为磷酸丙糖
Each GP molecule is first phosphorylated by ATP, and then reduced by reduced NADP. The energy from ATP and the reducing power of reduced NADP convert the carboxyl group of GP into an aldehyde group, yielding triose phosphate (TP). For every CO₂ fixed, 2 ATP and 2 reduced NADP are consumed at this stage (because 2 GP are formed per CO₂).
每个GP分子首先被ATP磷酸化，然后被还原型NADP还原。ATP的能量和还原型NADP的还原力将GP的羧基转化为醛基，生成磷酸丙糖（TP）。每固定一分子CO₂，此阶段消耗2 ATP和2还原型NADP（因为每分子CO₂产生2分子GP）。
GP + ATP + reduced NADP → TP + ADP + Pi + NADP
GP + ATP + 还原型NADP → TP + ADP + Pi + NADP
5. Regeneration of RuBP | RuBP的再生
To keep the cycle turning, the majority of TP molecules are used to regenerate RuBP. Through a series of carbon-skeleton rearrangements, five TP molecules (total 15 carbons) are reorganized into three RuBP molecules (total 15 carbons). This process requires additional ATP. Without regeneration, the CO₂ acceptor would quickly run out.
为了保持循环运转，大部分TP分子被用于再生RuBP。通过一系列的碳骨架重排，五分子TP（共15个碳）被重组成三分子RuBP（共15个碳）。此过程需要额外的ATP。没有再生，CO₂的受体将很快耗尽。
5 TP (3C) → 3 RuBP (5C)
5 磷酸丙糖 (3C) → 3 RuBP (5C)
The net effect is that for every 3 CO₂ molecules fixed, 6 TP are produced; 5 of these TP are recycled to regenerate 3 RuBP, leaving 1 TP available for synthesis of carbohydrates.
最终净结果是，每固定3分子CO₂会产生6个TP；其中5个TP用于再生3个RuBP，剩余1个TP可用于合成碳水化合物。
6. Fate of Triose Phosphate: Synthesis of Carbohydrates | 磷酸丙糖的去向：合成碳水化合物
The TP that exits the cycle can be used in several ways. Two TP molecules combine to form a hexose sugar, such as glucose or fructose, which can then be converted into sucrose for transport, starch for short-term storage in the chloroplast, or cellulose for cell wall construction. Additionally, TP can feed into fatty acid and amino acid synthesis.
离开循环的TP可有多种去向。两个TP分子结合形成己糖，如葡萄糖或果糖，这些己糖随后可转化为蔗糖用于运输，转化为淀粉在叶绿体中短期储存，或转化为纤维素用于构建细胞壁。此外，TP还可进入脂肪酸和氨基酸的合成途径。
7. Energy and Reducing Power Requirements | 能量与还原力需求
The Calvin cycle is energy-expensive. For the net synthesis of one hexose sugar (6 carbon atoms), 6 CO₂ must be fixed. The overall consumption is shown below:
卡尔文循环耗能巨大。净合成一分子己糖（6个碳）需要固定6分子CO₂。总消耗如下表所示：
| per CO₂ fixed | per glucose (6 CO₂) |
| 3 ATP | 18 ATP |
| 2 reduced NADP | 12 reduced NADP |
3 CO₂ + 9 ATP + 6 reduced NADP → 1 TP (exported) + 9 ADP + 8 Pi + 6 NADP
3 CO₂ + 9 ATP + 6 还原型NADP → 1 TP (输出) + 9 ADP + 8 Pi + 6 NADP
These large quantities of ATP and reduced NADP must be supplied continuously by the light-dependent reactions, highlighting the intimate link between the two sets of processes.
如此大量的ATP和还原型NADP必须由光反应持续提供，这突显了两组过程之间的紧密联系。
8. Regulation of the Calvin Cycle | 卡尔文循环的调控
The Calvin cycle is finely tuned to the availability of light. In the light, the pH of the stroma rises (becomes more alkaline) as H⁺ are pumped into the thylakoid space, and Mg²⁺ ions move into the stroma. These changes activate RuBisCO and other Calvin cycle enzymes. Furthermore, a protein called RuBisCO activase requires ATP from light reactions to remove inhibitory sugar phosphates from the active site of RuBisCO. In the dark, conditions reverse and the cycle slows down, preventing wasteful consumption of ATP and reduced NADP.
卡尔文循环会精细地响应光的可用性。在光照下，由于H⁺被泵入类囊体腔，基质pH升高（变得更碱），同时Mg²⁺进入基质。这些变化激活RuBisCO及其他卡尔文循环酶。此外，一种名为RuBisCO活化酶的蛋白质需要光反应产生的ATP来移除RuBisCO活性位点上的抑制性糖磷酸。在黑暗中，条件逆转，循环减慢，从而避免无效消耗ATP和还原型NADP。
9. The ‘Dark’ Reactions and Their Dependence on Light | “暗反应”及其对光的依赖
The term “light-independent reactions” can be misleading because, in healthy plants, the Calvin cycle operates mainly during daylight. If plants are kept in continuous darkness, the Calvin cycle soon stops because the supply of ATP and reduced NADP runs out, and the stroma reverts to conditions that inactivate key enzymes. Therefore, the reactions do not require light directly, but they are absolutely dependent on the products of the light-dependent reactions.
“光不依赖反应”这一术语可能产生误导，因为在健康植物中，卡尔文循环主要在白天运行。如果将植物置于持续黑暗中，卡尔文循环很快停止，因为ATP和还原型NADP的供应中断，基质环境也回复到使关键酶失活的状态。因此，这些反应不直接需要光，但绝对依赖于光反应的产物。
10. Common Misconceptions and Summary | 常见误区与总结
A frequent misconception is that the Calvin cycle can function indefinitely in the dark, or that it is entirely separate from the light reactions. In reality, the two stages are coupled through the energy carriers ATP and reduced NADP. Another error is believing that TP is exclusively used to make glucose; TP acts as a versatile intermediate for many biosynthetic pathways. In summary, the light-independent reactions fix CO₂, reduce the fixed carbon to triose phosphate using ATP and reduced NADP, and regenerate the CO₂ acceptor RuBP. This elegant cycle is the foundation of carbohydrate production in plants and ultimately sustains most life on Earth.
一个常见误区是认为卡尔文循环可在黑暗中无限进行，或者它与光反应完全分离。事实上，这两个阶段通过能量载体ATP和还原型NADP紧密耦合。另一个错误是认为TP仅用于制造葡萄糖；TP实际上是多种生物合成途径的多功能中间体。总结来说，光不依赖反应固定CO₂，利用ATP和还原型NADP将固定的碳还原为磷酸丙糖，并再生CO₂受体RuBP。这个精妙的循环是植物合成碳水化合物的基础，并最终支撑了地球上大多数生命。
Published by TutorHao | Biology Revision Series | aleveler.com
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