📚 C4 Plants: The Hatch-Slack Pathway and Kranz Anatomy | C4植物：哈奇-斯莱克途径与花环结构
C4 plants are a group of species that have evolved a special photosynthetic adaptation to reduce photorespiration and maintain high rates of photosynthesis in hot, bright, and often dry environments. They first fix carbon dioxide into a four-carbon compound before the Calvin cycle, and they show a distinctive leaf structure called Kranz anatomy. This article explains the C4 pathway step by step, compares C4 with C3 photosynthesis, and highlights the key points needed for Cambridge A-Level Biology.
C4植物是一类进化出特殊光合适应机制的植物，能够在高温、强光和干旱环境中减少光呼吸并保持较高的光合速率。它们先将二氧化碳固定为一种四碳化合物，再进入卡尔文循环，并且具有称为花环结构的特殊叶片结构。本文将逐步解释C4途径，比较C4与C3光合作用，并突出剑桥A-Level生物考试所需的关键要点。
The route is named after its discoverers, Marshall Hatch and C. R. Slack, who worked out the sequence of reactions in the 1960s, which is why it is also called the Hatch-Slack pathway. Two extra ideas run through the whole topic: a biochemical shuttle that pumps CO₂, and an anatomical arrangement that keeps the pumped CO₂ in place.
这条途径以其发现者马歇尔·哈奇（Marshall Hatch）和C. R. 斯莱克（C. R. Slack）命名，他们在20世纪60年代阐明了这一系列反应，因此它也被称为哈奇-斯莱克途径。贯穿整个主题的有两个核心思想：一个把CO₂"泵"起来的生化穿梭机制，以及一种把泵来的CO₂"锁"在原地的解剖结构。
1. What Are C4 Plants? | 什么是C4植物？
C4 plants are species that initially fix carbon dioxide into a four-carbon compound, oxaloacetate, before the Calvin cycle operates. The name C4 comes from this first stable four-carbon product. Well-known examples include maize, sugarcane, sorghum, and many tropical grasses. In these plants, photosynthesis is spatially separated between two different cell types: mesophyll cells and bundle sheath cells.
C4植物是指二氧化碳首先被固定为四碳化合物草酰乙酸，然后才进入卡尔文循环的植物。C4这一名称来自最初的稳定四碳产物。常见例子包括玉米、甘蔗、高粱和许多热带禾本科植物。在这些植物中，光合作用在两种不同的细胞类型之间实现空间分离：叶肉细胞和维管束鞘细胞。
The C4 route is therefore best described as a CO₂-concentrating mechanism rather than a replacement for the Calvin cycle. It spends extra ATP to build up a high concentration of CO₂ around rubisco, so that the enzyme works as a carboxylase and hardly ever as an oxygenase.
因此，C4途径最准确的描述是一种CO₂浓缩机制，而不是卡尔文循环的替代品。它额外消耗ATP，在rubisco周围建立起高浓度的CO₂，使该酶几乎只进行羧化反应，而极少进行加氧反应。
Only about 3% of flowering plant species use the C4 pathway, yet these species account for roughly a quarter of global terrestrial primary productivity, because they dominate many hot grasslands, savannas and tropical farming systems.
只有大约3%的被子植物物种使用C4途径，但这些物种却贡献了全球陆地初级生产力的大约四分之一，因为它们主导了许多炎热的草原、稀树草原和热带农业系统。
2. The Problem: Photorespiration in C3 Plants | 问题：C3植物的光呼吸
In C3 plants, rubisco fixes CO₂ directly into 3-phosphoglycerate in the Calvin cycle. However, rubisco can also use O₂ as a substrate when the leaf temperature is high and the CO₂ concentration inside the leaf is low. This oxygenase reaction produces phosphoglycolate, which must be recycled through a process called photorespiration.
在C3植物中，rubisco（核酮糖二磷酸羧化酶-加氧酶）在卡尔文循环中直接把CO₂固定为3-磷酸甘油酸。然而，当叶片温度高、叶内CO₂浓度低时，rubisco也能以O₂为底物。这一加氧反应产生磷酸乙醇酸，必须通过称为光呼吸的过程进行回收。
RuBP + CO₂ → 2 × 3-phosphoglycerate
RuBP + O₂ → 3-phosphoglycerate + phosphoglycolate
Photorespiration consumes ATP and NADPH, releases previously fixed CO₂, and produces hydrogen peroxide, so it lowers the net yield of photosynthesis. It becomes far more likely when stomata are partly closed to save water, because the internal CO₂ concentration then falls while O₂ continues to be produced by the light-dependent reactions.
光呼吸消耗ATP和NADPH，释放已经固定的CO₂，并产生过氧化氢，因此降低光合作用的净产量。当植物为了保水而部分关闭气孔时，这一过程更容易发生，因为此时叶内CO₂浓度下降，而光反应仍在不断产生O₂。
Rubisco has a relatively low affinity for CO₂ and cannot reliably tell CO₂ from O₂. In a hot, bright C3 leaf, photorespiration can waste 20-25% of the carbon that has just been fixed, and in extreme conditions the loss is even greater.
rubisco对CO₂的亲和力相对较低，并且无法可靠地区分CO₂与O₂。在高温强光的C3叶片中，光呼吸可以浪费掉刚刚固定下来的20-25%的碳，在极端条件下损失更大。
Any adaptation that raises the CO₂:O₂ ratio at the active site of rubisco will therefore improve photosynthetic efficiency. C4 plants achieve exactly this, and they do it with a biochemical pump powered by extra ATP.
因此，任何能够提高rubisco活性位点处CO₂:O₂比值的适应，都会改善光合效率。C4植物正是做到了这一点，而且是用一个由额外ATP驱动的生化泵来实现的。
3. Kranz Anatomy: Structure of the C4 Leaf | 花环结构：C4叶片的结构
Kranz is German for wreath, and Kranz anatomy describes the ring of bundle sheath cells that surrounds each leaf vein, with a second, tighter ring of mesophyll cells around it. The two concentric rings look like a wreath in cross-section, and this arrangement is the structural signature of a C4 leaf.
Kranz在德语中意为"花环"，花环结构描述的是围绕每条叶脉的一圈维管束鞘细胞，以及其外又包围一圈排列紧密的叶肉细胞。这两个同心圆环在横切面上看起来像花环，这一排列正是C4叶片的结构标志。
- Bundle sheath cells are large, have thick walls, contain many chloroplasts and ribosomes, and hold a very high concentration of rubisco. | 维管束鞘细胞体积大、细胞壁厚、含有大量叶绿体和核糖体，并且rubisco浓度很高。
- Mesophyll cells form a tight ring outside, with PEP carboxylase in the cytosol and chloroplasts that possess well-developed grana. | 外侧的叶肉细胞紧密排成环状，其细胞质中含PEP羧化酶，叶绿体具有发育良好的基粒。
- There are very few intercellular air spaces around the bundle sheath cells, so CO₂ released inside them cannot easily diffuse away. | 维管束鞘细胞周围几乎没有细胞间隙，因此在其内部释放的CO₂不容易扩散逃逸。
- Numerous plasmodesmata connect mesophyll and bundle sheath cells, allowing rapid transport of malate, pyruvate and other metabolites. | 丰富的胞间连丝连接叶肉细胞与维管束鞘细胞，使苹果酸、丙酮酸等代谢物能够快速运输。
- Veins are densely packed, so that no mesophyll cell is far from a bundle sheath cell. | 叶脉排列密集，使任何叶肉细胞距离维管束鞘细胞都不远。
Chloroplast dimorphism is an important extra detail. In the classic NADP-ME subtype such as maize and sugarcane, mesophyll chloroplasts have well-developed grana but lack rubisco, while bundle sheath chloroplasts contain rubisco but are largely agranal. Having little or no photosystem II in the bundle sheath reduces O₂ production in the very compartment where CO₂ must be kept concentrated.
叶绿体二型性是一个重要的补充细节。在玉米、甘蔗等经典的NADP-ME亚型中，叶肉细胞的叶绿体具有发育良好的基粒但缺乏rubisco；而维管束鞘细胞的叶绿体含有rubisco，却基本上没有基粒。维管束鞘中几乎没有光系统II，可以减少在必须维持高CO₂浓度的区室中产生O₂。
This division of labour is the essence of C4 photosynthesis: the two stages of carbon fixation are physically separated in space rather than being carried out in the same chloroplast at the same time.
这种分工正是
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