📚 Chloroplast structure and function | 叶绿体结构与功能
Chloroplasts are the organelles in plant cells and algal cells where photosynthesis occurs. They capture light energy and convert it into chemical energy stored in glucose and other carbohydrates. A detailed understanding of chloroplast structure helps explain how the light-dependent and light-independent reactions of photosynthesis are organised and regulated.
叶绿体是植物细胞和藻类细胞中进行光合作用的细胞器。它们捕获光能，并将其转化为储存在葡萄糖和其他碳水化合物中的化学能。深入理解叶绿体结构有助于解释光合作用的光依赖反应和光独立反应是如何组织与调控的。
1. Overview of Chloroplast Structure | 叶绿体结构概述
Chloroplasts are lens-shaped or discoid organelles, typically 2-10 μm in length and 1-3 μm in width. A single mesophyll cell in a green leaf usually contains 20-100 chloroplasts, depending on light conditions and plant species.
叶绿体是透镜状或圆盘状细胞器，通常长 2-10 微米，宽 1-3 微米。绿色叶片中的一个叶肉细胞通常含有 20-100 个叶绿体，具体数量取决于光照条件和植物种类。
The chloroplast belongs to the plastid family and is enclosed by an envelope of two membranes. Its internal structure is divided into three main regions: the outer and inner membranes, the stroma, and the thylakoid system, which includes grana and intergranal lamellae.
叶绿体属于质体家族，由双层膜组成的被膜包围。其内部结构主要分为三个区域：外膜和内膜、基质以及类囊体系统，类囊体系统包括基粒和基粒间片层。
2. Double Membrane Envelope | 双层膜被膜
The chloroplast envelope consists of an outer membrane and an inner membrane separated by a narrow intermembrane space. The outer membrane is relatively permeable to small molecules and ions because it contains channel-forming proteins called porins.
叶绿体被膜由外膜和内膜组成，两层膜之间有一个狭窄的膜间隙。外膜对小分子和离子相对通透，因为它含有称为孔蛋白的通道形成蛋白。
The inner membrane is selectively permeable and contains specific transport proteins. These proteins control the movement of metabolites such as phosphate, triose phosphate, and ATP between the cytosol and the stroma. Unlike the inner membrane of mitochondria, the chloroplast inner membrane is not folded into cristae.
内膜具有选择通透性，含有特异性的转运蛋白。这些蛋白控制磷酸、磷酸三碳糖和 ATP 等代谢物在细胞质与基质之间的移动。与线粒体内膜不同，叶绿体内膜并不折叠形成嵴。
3. Stroma | 基质
The stroma is the fluid-filled compartment inside the inner membrane and surrounding the thylakoid system. It contains the enzymes required for the Calvin cycle, including ribulose bisphosphate carboxylase/oxygenase, commonly called RuBisCO.
基质是内膜内部、类囊体系统周围的液体区室。它含有卡尔文循环所需的酶，包括核酮糖二磷酸羧化酶/加氧酶，通常称为 RuBisCO。
The stroma also contains chloroplast DNA, 70S ribosomes, starch grains, and lipid droplets. During photosynthesis, the pH and concentrations of Mg²⁺ and H⁺ in the stroma change. These changes help activate key Calvin cycle enzymes when light is available.
基质还含有叶绿体 DNA、70S 核糖体、淀粉粒和脂滴。在光合作用过程中，基质的 pH 以及 Mg²⁺ 和 H⁺ 浓度会发生变化。这些变化有助于在有光条件下激活关键的卡尔文循环酶。
4. Thylakoid Membrane and Grana | 类囊体膜与基粒
Thylakoids are flattened, membrane-bound sacs. Stacks of thylakoids are called grana (singular: granum). The grana are connected by stroma thylakoids, also known as intergranal lamellae, which allow the lumen of different thylakoids to be continuous.
类囊体是扁平、由膜包围的囊。类囊体的堆叠称为基粒（单数：granum）。基粒通过基质类囊体（也称为基粒间片层）相互连接，使不同类囊体的腔彼此连通。
The thylakoid membrane contains photosynthetic pigments, electron carriers, and ATP synthase. Stacking thylakoids into grana greatly increases the membrane surface area, so large numbers of photosystems and electron transport chains can be packed into a small volume.
类囊体膜含有光合色素、电子载体和 ATP 合酶。将类囊体堆叠成基粒大大增加了膜表面积，因此大量光系统和电子传递链可以被密集地装进很小的空间内。
The space enclosed by the thylakoid membrane is the thylakoid lumen. During the light-dependent reactions, protons are pumped into the lumen, creating a proton concentration gradient across the thylakoid membrane. This gradient drives ATP synthesis.
类囊体膜包围的空间是类囊体腔。在光依赖反应中，质子被泵入类囊体腔，在类囊体膜两侧形成质子浓度梯度。这一梯度驱动 ATP 的合成。
5. Photosynthetic Pigments | 光合色素
Chlorophyll a is the primary photosynthetic pigment, while chlorophyll b and carotenoids are accessory pigments. These pigments are arranged with proteins into photosystems embedded in the thylakoid membrane.
叶绿素 a 是主要的光合色素，而叶绿素 b 和类胡萝卜素是辅助色素。这些色素与蛋白质一起排列成光系统，嵌入类囊体膜中。
Each photosystem consists of a light-harvesting antenna complex and a reaction centre. The antenna pigments absorb photons and pass excitation energy to the reaction centre. Photosystem II (PSII) has a reaction centre that absorbs maximally at 680 nm, while Photosystem I (PSI) absorbs maximally at 700 nm.
每个光系统由一个捕光天线复合物和一个反应中心组成。天线色素吸收光子，并将激发能传递到反应中心。光系统 II（PSII）的反应中心在 680 nm 处吸收最强，而光系统 I（PSI）在 700 nm 处吸收最强。
- Chlorophyll a absorbs mainly red and blue-violet light | 叶绿素 a 主要吸收红光和蓝紫光
- Chlorophyll b absorbs blue and orange-red light | 叶绿素 b 吸收蓝光和橙红光
- Carotenoids absorb blue-green light and protect chlorophyll from photo-oxidation | 类胡萝卜素吸收蓝绿光并保护叶绿素免于光氧化
Because pigments absorb different wavelengths, the combined action of all pigments allows the chloroplast to use a wider range of the visible spectrum.
由于色素吸收不同波长的光，所有色素的共同作用使叶绿体能够利用更广范围的可见光谱。
6. Light-Dependent Reactions | 光依赖反应
The light-dependent reactions take place on the thylakoid membrane. They begin when photons are absorbed by PSII. This excites electrons, which are passed to an electron transport chain. To replace these electrons, PSII catalyses the photolysis of water.
光依赖反应发生在类囊体膜上。当 PSII 吸收光子时反应开始。光子激发电子，电子被传递到电子传递链。为了补充这些电子，PSII 催化水的光解。
2H₂O → 4H⁺ + 4e⁻ + O₂
Excited electrons pass from PSII through plastoquinone, the cytochrome b₆f complex, and plastocyanin to PSI. The energy released during electron transfer is used to pump protons from the stroma into the thylakoid lumen.
激发电子从 PSII 依次经过质体醌、细胞色素 b₆f 复合物和质体蓝素传递到 PSI。电子传递过程中释放的能量用于将质子从基质泵入类囊体腔。
At PSI, electrons are re-energised by light. They are then passed to ferredoxin and finally to the enzyme NADP⁺ reductase, which reduces NADP⁺ to NADPH in the stroma.
在 PSI 中，电子再次被光能激发。随后电子传递到铁氧还蛋白，最终到达 NADP⁺ 还原酶，该酶在基质中将 NADP⁺ 还原为 NADPH。
Protons return to the stroma by flowing through ATP synthase. This movement is called chemiosmosis and drives the phosphorylation of ADP to ATP. The overall process is called non-cyclic photophosphorylation.
质子通过 ATP 合酶流回基质。这种质子流动称为化学渗透，驱动 ADP 磷酸化生成 ATP。整个电子传递过程称为非环式光合磷酸化。
In some conditions, cyclic photophosphorylation occurs: electrons from PSI return to the cytochrome complex and then back to PSI. This produces ATP but not NADPH or oxygen.
在某些条件下会发生环式光合磷酸化：电子从 PSI 返回细胞色素复合物，再回到 PSI。该过程产生 ATP，但不产生 NADPH 或氧气。
7. Light-Independent Reactions: The Calvin Cycle | 光独立反应：卡尔文循环
The Calvin cycle takes place in the stroma and uses the ATP and NADPH produced by the light-dependent reactions to fix carbon dioxide into organic molecules. The cycle occurs in three main stages: carbon fixation, reduction, and regeneration of RuBP.
卡尔文循环发生在基质中，利用光依赖反应产生的 ATP 和 NADPH 将二氧化碳固定为有机分子。该循环包括三个主要阶段：碳固定、还原和 RuBP 的再生。
During carbon fixation, RuBisCO catalyses the reaction between CO₂ and ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP), a three-carbon compound.
在碳固定阶段，RuBisCO 催化 CO₂ 与核酮糖二磷酸（RuBP）反应，生成两分子甘油酸-3-磷酸（GP），一种三碳化合物。
During reduction, GP is phosphorylated by ATP and then reduced by NADPH to form triose phosphate (TP). This stage consumes the ATP and NADPH generated in the light-dependent reactions.
在还原阶段，GP 先被 ATP 磷酸化，然后被 NADPH 还原，生成磷酸三碳糖（TP）。此阶段消耗光依赖反应中产生的 ATP 和 NADPH。
Most TP is used to regenerate RuBP so that carbon fixation can continue. Some TP leaves the cycle to synthesise glucose, starch, sucrose, amino acids, and lipids.
大部分 TP 用于再生 RuBP，使碳固定得以持续进行。一部分 TP 离开循环，用于合成葡萄糖、淀粉、蔗糖、氨基酸和脂质。
8. Chloroplast DNA and Ribosomes | 叶绿体 DNA 与核糖体
Chloroplasts contain their own circular DNA and 70S ribosomes, which are similar in size and structure to those of prokaryotes. This is strong evidence for the endosymbiotic theory, which proposes that chloroplasts evolved from free-living cyanobacteria engulfed by an ancestral eukaryotic cell.
叶绿体含有自身的环状 DNA 和 70S 核糖体，其大小和结构与原核生物类似。这是内共生理论的有力证据。该理论认为叶绿体是由祖先真核细胞吞噬的自由生活的蓝细菌进化而来。
Chloroplast DNA encodes some chloroplast proteins and RNAs, but most chloroplast proteins are encoded by nuclear DNA and imported from the cytosol. This shows that the chloroplast is a semi-autonomous organelle.
叶绿体 DNA 编码部分叶绿体蛋白和 RNA，但大多数叶绿体蛋白由核 DNA 编码并从细胞质输入。这说明叶绿体是一种半自主性细胞器。
9. Starch Grains and Lipid Droplets | 淀粉粒与脂滴
Starch grains are temporary storage structures found in the stroma. When the rate of glucose production exceeds the rate of sucrose export, glucose is polymerised into insoluble starch, which accumulates as grains.
淀粉粒是基质中的临时储存结构。当葡萄糖生成速率超过蔗糖输出速率时，葡萄糖会聚合成不溶性淀粉，以淀粉粒形式积累。
Starch can be broken down during the night to release sugars for respiration and growth. Lipid droplets are also present in the stroma; they supply material for membrane synthesis and act as energy reserves.
淀粉可在夜间被分解，释放糖类用于呼吸和生长。脂滴也存在于基质中；它们为膜合成提供原料，并作为能量储备。
10. Adaptations for Photosynthesis | 适应光合作用的结构特征
Every part of the chloroplast is adapted to make photosynthesis efficient. The thylakoid membranes provide a large surface area for the proteins and pigments involved in light absorption and electron transport.
叶绿体的每个部分都适应于提高光合作用效率。类囊体膜提供了巨大的表面积，用于容纳参与光吸收和电子传递的蛋白质与色素。
The stroma has a suitable pH and contains the enzymes for the Calvin cycle. Because the stroma surrounds the thylakoids, ATP and NADPH can diffuse quickly from the thylakoid membrane to the enzymes that need them.
基质具有合适的 pH，并含有卡尔文循环所需的酶。由于基质包围着类囊体，ATP 和 NADPH 可以从类囊体膜迅速扩散到需要它们的酶附近。
| Structural feature | 结构特征 | Adaptation | 适应意义 |
|---|---|
| Many grana stacks | 大量基粒堆叠 | Increase membrane area for light absorption and electron transport | 增加光吸收和电子传递的膜面积 |
| Thylakoid lumen | 类囊体腔 | Allows proton accumulation to form a chemiosm
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