📚 Trapping Light Energy | 捕获光能
Photosynthesis begins with the capture of light energy by photosynthetic pigments inside chloroplasts. This seemingly simple event – a photon interacting with a pigment molecule – is the entry point for almost all energy in living systems. In A-Level Biology, ‘trapping light energy’ means understanding where light is absorbed, how pigment-protein complexes transfer excitation energy, and how that energy is converted into chemical forms such as ATP and reduced NADP.
光合作用始于叶绿体内光合色素对光能的捕获。这个看似简单的事件——光子与色素分子相互作用——是几乎所有生命系统能量的入口。在 A-Level 生物课程中，“捕获光能”意味着理解光在哪里被吸收、色素-蛋白质复合体如何传递激发能，以及这些能量如何转化为 ATP 和还原型 NADP 等化学形式。
1. The importance of trapping light energy | 捕获光能的重要性
Living cells need a continuous input of free energy to maintain their highly ordered structure, transport substances and synthesise polymers. Photoautotrophs such as green plants, algae and cyanobacteria trap light energy and convert it into chemical potential energy in organic molecules. This primary productivity supports heterotrophs and therefore almost every food chain on Earth.
活细胞需要持续输入自由能来维持高度有序的结构、运输物质并合成聚合物。绿色植物、藻类和蓝细菌等光合自养生物能够捕获光能，并将其转化为有机分子中的化学势能。这种初级生产力支撑着异养生物，因此也支撑着地球上几乎所有的食物链。
In photosynthesis, light energy is not used directly to make glucose. Instead, it is first trapped by pigments and used to generate two energy carriers: ATP and reduced NADP. These then drive the Calvin cycle, which fixes carbon dioxide into sugars. Thus ‘trapping’ means converting electromagnetic energy into a form that enzymes can use.
在光合作用中，光能并不会直接用于制造葡萄糖。光能首先被色素捕获，用于生成两种能量载体：ATP 和还原型 NADP。随后它们驱动卡尔文循环，将二氧化碳固定为糖类。因此，“捕获”意味着将电磁能转化为酶可以使用的形式。
2. Chloroplast structure and sites of light capture | 叶绿体结构与光捕获场所
The chloroplast is the organelle of photosynthesis. It is bounded by a double membrane. Inside, a third membrane system forms flattened sacs called thylakoids. A stack of thylakoids is a granum, and the fluid matrix surrounding the thylakoids is the stroma.
叶绿体是进行光合作用的细胞器。它由双层膜包围。内部第三套膜系统形成扁平的囊状结构，称为类囊体。一堆类囊体称为基粒，类囊体周围的液体基质称为叶绿体基质。
The light-dependent reactions are located in the thylakoid membrane because this is where photosynthetic pigments, electron carriers and ATP synthase are embedded. The stroma contains enzymes of the Calvin cycle. This compartmentalisation separates light trapping from carbon fixation, allowing different conditions in each compartment.
光反应发生在类囊体膜上，因为光合色素、电子载体和 ATP 合酶都嵌入在该膜中。叶绿体基质含有卡尔文循环的酶。这种区室化将光能捕获与碳固定分开，使每个区域可以维持不同的条件。
3. Photosynthetic pigments and absorption spectra | 光合色素与吸收光谱
Pigments are molecules that absorb specific wavelengths of visible light and reflect or transmit others. The main pigments in plants are chlorophyll a, chlorophyll b and carotenoids. Chlorophyll a is the primary pigment; it is located in the reaction centre and also acts as an antenna pigment. Chlorophyll b and carotenoids are accessory pigments.
色素是吸收特定波长可见光并反射或透射其他波长光的分子。植物中的主要色素是叶绿素 a、叶绿素 b 和类胡萝卜素。叶绿素 a 是主要色素；它位于反应中心，同时也作为天线色素发挥作用。叶绿素 b 和类胡萝卜素是辅助色素。
Each pigment has a characteristic absorption spectrum showing which wavelengths it absorbs most strongly. Chlorophyll a absorbs mainly blue-violet light at about 430 nm and red light at about 662 nm, while reflecting green light. Carotenoids absorb blue and green light and appear yellow, orange or red. Accessory pigments widen the range of wavelengths that can be trapped.
每种色素都有特征吸收光谱，显示它吸收最强的波长。叶绿素 a 主要吸收约 430 nm 的蓝紫光和约 662 nm 的红光，同时反射绿光。类胡萝卜素吸收蓝光和绿光，呈现黄色、橙色或红色。辅助色素扩大了可被捕获的波长范围。
| Pigment (色素) | Colour (颜色) | Main absorption (主要吸收) | Role (作用) |
|---|---|---|---|
| Chlorophyll a (叶绿素 a) | Green (绿色) | Blue-violet and red (蓝紫光和红光) | Primary pigment, reaction centre and antenna (主要色素，反应中心和天线) |
| Chlorophyll b (叶绿素 b) | Yellow-green (黄绿色) | Blue and orange-red (蓝光和橙红光) | Accessory pigment (辅助色素) |
| Carotenoids (类胡萝卜素) | Yellow, orange, red (黄色、橙色、红色) | Blue and green (蓝光和绿光) | Accessory pigments, photoprotection (辅助色素，光保护) |
4. Action spectrum and the role of accessory pigments | 作用光谱与辅助色素的作用
An absorption spectrum is measured using a spectrophotometer. An action spectrum shows the rate of photosynthesis at different wavelengths of light. The action spectrum is often slightly broader than the absorption spectrum of chlorophyll a because accessory pigments transfer absorbed energy to chlorophyll a.
吸收光谱使用分光光度计测定。作用光谱显示光合作用在不同波长光下的速率。作用光谱通常比叶绿素 a 的吸收光谱略宽，因为辅助色素将吸收的能量传递给叶绿素 a。
The red and blue ends of the spectrum support the highest rates of photosynthesis. Green light is least effective because it is reflected and transmitted rather than absorbed. This explains why leaves appear green.
光谱的蓝端和红端支持最高的光合作用速率。绿光效果最差，因为它被反射和透射，而不是被吸收。这解释了为什么叶片呈现绿色。
5. Antenna complexes and resonance energy transfer | 天线复合体与共振能量传递
Within the thylakoid membrane, pigment molecules are bound to proteins in large assemblies called photosystems. Each photosystem has many antenna chlorophyll and carotenoid molecules surrounding a reaction centre. The antenna pigments do not pass electrons; they harvest photons and pass excitation energy.
在类囊体膜内，色素分子与蛋白质结合形成大型复合体，称为光系统。每个光系统都有许多天线叶绿素和类胡萝卜素分子围绕着一个反应中心。天线色素不传递电子；它们捕获光子并传递激发能。
When a photon is absorbed, an electron in the pigment is raised to a higher energy level. The excitation energy is then passed from one pigment molecule to the next by resonance energy transfer. This process is rapid and does not involve movement of an electron from molecule to molecule. It funnels energy towards a specialised chlorophyll a pair in the reaction centre.
当光子被吸收时，色素中的一个电子被提升到更高的能级。随后激发能通过共振能量传递从一个色素分子传递到下一个色素分子。这一过程非常迅速，不涉及电子在分子之间的移动。它将能量汇集到反应中心的一对特殊叶绿素 a 上。
6. Reaction centres and photoactivation | 反应中心与光活化
The reaction centre contains a special pair of chlorophyll a molecules. In Photosystem II this pair is called P680 because it absorbs light most strongly at 680 nm. In Photosystem I the pair is called P700 because it absorbs best at 700 nm. When excitation energy reaches the reaction centre, the special chlorophyll a donates an excited electron to a primary electron acceptor.
反应中心含有一对特殊的叶绿素 a 分子。在光系统 II 中，这对分子称为 P680，因为它对 680 nm 的光吸收最强。在光系统 I 中，这对分子称为 P700，因为它在 700 nm 处吸收最好。当激发能到达反应中心时，特殊叶绿素 a 将一个激发电子传递给初级电子受体。
This is the first redox event of the light-dependent reactions. The reaction centre has been photoactivated: light energy has caused the loss of a high-energy electron. The oxidised chlorophyll then becomes a strong oxidising agent and must be re-reduced by an electron donor, which in Photosystem II is water.
这是光反应中的第一个氧化还原事件。反应中心发生了光活化：光能使其失去一个高能电子。被氧化的叶绿素随后成为强氧化剂，必须由电子供体重新还原。在光系统 II 中，这个电子供体是水。
P680 + photon energy → P680⁺ + e⁻
P700 + photon energy → P700⁺ + e⁻
7. Photosystem II and the splitting of water | 光系统 II 与水的裂解
Photosystem II supplies electrons to the electron transport chain. After P680 loses an electron, it is a powerful oxidant. It withdraws electrons from water molecules in the thylakoid lumen. The water-splitting complex, also called the oxygen-evolving complex, catalyses this reaction.
光系统 II 为电子传递链提供电子。P680 失去
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