📚 Photosynthesis | 光合作用
Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy stored in organic molecules such as glucose. In A-level biology, it is studied as a two-stage energy transfer process: the light-dependent stage on the thylakoid membranes and the light-independent stage in the stroma.
光合作用是绿色植物、藻类和某些细菌将光能转化为储存在葡萄糖等有机分子中的化学能的过程。在 A-level 生物学中，它被研究为一个两阶段的能量传递过程：发生在类囊体膜上的光反应阶段和发生在基质中的暗反应阶段。
1. Overview: energy conversion and overall equation | 概述：能量转换与总方程式
Photosynthesis is an endergonic process, meaning it requires an input of energy. Light energy absorbed by chlorophyll is used to split water, release oxygen and generate ATP and reduced NADP, which then drive the formation of carbohydrate from carbon dioxide in the Calvin cycle.
光合作用是一个吸能过程，也就是说它需要能量输入。叶绿素吸收的光能用于分解水、释放氧气，并产生 ATP 和还原型 NADP，这些产物随后在卡尔文循环中驱动二氧化碳形成碳水化合物。
The overall equation is often simplified as:
总方程式通常简化为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This summary hides many intermediate steps, but it shows that carbon dioxide is reduced to glucose and water is oxidised to oxygen. In reality, the oxygen produced comes from the photolysis of water, not from carbon dioxide.
这个总方程式掩盖了许多中间步骤，但它表明二氧化碳被还原为葡萄糖，水被氧化为氧气。实际上，产生的氧气来自水的光解，而不是来自二氧化碳。
Energy-wise, photosynthesis traps light energy in chemical bonds. The energy is released later during respiration to synthesise ATP, so photosynthesis can be seen as the primary energy input for most ecosystems.
从能量角度看，光合作用把光能储存在化学键中。这些能量之后在呼吸作用中被释放出来用于合成 ATP，因此光合作用可以被视为大多数生态系统的主要能量输入方式。
2. Chloroplast structure and adaptations | 叶绿体结构与适应
Photosynthesis in eukaryotes takes place inside chloroplasts. A chloroplast is surrounded by a double membrane, or envelope, which controls the exchange of substances between the cytoplasm and the chloroplast interior.
真核生物的光合作用发生在叶绿体内。叶绿体由双层膜（即被膜）包围，这层膜控制细胞质与叶绿体内部之间的物质交换。
Inside the chloroplast, the stroma is a fluid matrix containing enzymes, ribosomes, starch grains and circular DNA. Suspended in the stroma are flattened membrane sacs called thylakoids, which stack to form grana. Thylakoids contain chlorophyll and other photosynthetic pigments, electron carriers and ATP synthase.
在叶绿体内部，基质是一种液态基质，含有酶、核糖体、淀粉粒和环状 DNA。基质中悬浮着扁平的膜囊，称为类囊体；类囊体堆叠形成基粒。类囊体含有叶绿素和其他光合色素、电子载体以及 ATP 合酶。
This arrangement is highly adapted for photosynthesis. The grana provide a large surface area for pigment molecules and electron carriers, the thylakoid membranes maintain a proton gradient for chemiosmosis, and the stroma contains the enzymes needed for the Calvin cycle.
这种结构高度适应光合作用的需要。基粒为色素分子和电子载体提供了巨大的表面积，类囊体膜维持用于化学渗透的质子梯度，而基质含有卡尔文循环所需的酶。
3. Photosynthetic pigments and absorption spectra | 光合色素与吸收光谱
Photosynthetic pigments absorb certain wavelengths of visible light and reflect others. The main pigments in green plants are chlorophyll a, chlorophyll b and carotenoids. Chlorophyll a is the primary pigment directly involved in transferring light energy to chemical energy; the other pigments are accessory pigments.
光合色素吸收可见光中特定波长的光，并反射其他波长的光。绿色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 是直接参与将光能转化为化学能的主要色素；其他色素为辅助色素。
The absorption spectrum shows how much light a pigment absorbs at each wavelength. Chlorophyll a and b absorb strongly in the blue-violet and red regions, while carotenoids absorb blue-green light. Green light is reflected, which is why leaves appear green.
吸收光谱显示色素在各个波长下吸收光的能力。叶绿素 a 和 b 在蓝紫光和红光区域吸收强烈，而类胡萝卜素吸收蓝绿光。绿光被反射，因此叶片呈现绿色。
| Pigment | Colour | Main absorption |
| Chlorophyll a | Blue-green | Red and blue-violet light |
| Chlorophyll b | Yellow-green | Blue and orange-red light |
| Carotenoids | Yellow, orange or red | Blue-green light |
The action spectrum shows the overall rate of photosynthesis at different wavelengths. It usually matches the combined absorption spectrum of all pigments, confirming that absorbed light energy drives photosynthesis.
作用光谱显示不同波长下光合作用的总体速率。它通常与所有色素的联合吸收光谱相匹配，这证实了吸收的光能驱动光合作用。
4. Light-dependent stage: photoactivation and photosystems | 光反应阶段：光活化与光系统
The light-dependent stage occurs on the thylakoid membranes. Its products are ATP, reduced NADP and oxygen. This stage requires light energy to excite electrons in chlorophyll molecules.
光反应阶段发生在类囊体膜上。其产物是 ATP、还原型 NADP 和氧气。这一阶段需要光能来激发叶绿素分子中的电子。
Chlorophyll and accessory pigments are arranged in light-harvesting complexes called photosystems. There are two types: Photosystem II (PSII) and Photosystem I (PSI). PSII absorbs light best at a wavelength of 680 nm, so its reaction centre is called P680. PSI absorbs best at 700 nm and is called P700.
叶绿素和辅助色素排列在称为光系统的捕光复合体中。光系统有两种类型：光系统 II（PSII）和光系统 I（PSI）。PSII 在 680 nm 波长处吸收最强，因此其反应中心称为 P680；PSI 在 700 nm 处吸收最强，称为 P700。
When a photon is absorbed, energy is passed from pigment to pigment by resonance transfer until it reaches the reaction centre. There, an electron is raised to a higher energy level and is captured by an electron acceptor. This is called photoactivation.
当一个光子被吸收时，能量通过共振传递从一个色素传递到另一个色素，直到到达反应中心。在反应中心，一个电子被提升到更高的能级并被电子受体捕获。这个过程称为光活化。
5. Non-cyclic photophosphorylation | 非循环光合磷酸化
Non-cyclic photophosphorylation involves both photosystems and produces ATP, reduced NADP and oxygen. It is the main pathway of the light-dependent stage.
非循环光合磷酸化涉及两个光系统，产生 ATP、还原型 NADP 和氧气。它是光反应阶段的主要途径。
In PSII, light energy excites an electron in P680. The energised electron is passed along an electron transport chain to PSI. P680 becomes oxidised, and it receives replacement electrons from the photolysis of water. Water splitting also releases hydrogen ions into the thylakoid lumen and produces oxygen gas.
在 PSII 中，光能激发 P680 中的一个电子。高能电子沿电子传递链传递到 PSI。P680 被氧化，并从水的光解中获得替代电子。水的分解还会向类囊体腔释放氢离子并产生氧气。
At PSI, light energy excites an electron in P700. This electron is transferred to a final electron acceptor, which also accepts hydrogen ions, leading to the reduction of NADP⁺ to reduced NADP (NADPH).
在 PSI 中，光能激发 P700 中的一个电子。这个电子被传递给最终电子受体，该受体同时接受氢离子，从而使 NADP⁺ 还原为还原型 NADP（NADPH）。
The flow of electrons from water to NADP is unidirectional, so it is described as non-cyclic. The overall products are ATP, NADPH and O₂.
电子从水到 NADP 的流动是单向的，因此被称为非循环。总的产物是 ATP、NADPH 和 O₂。
6. Cyclic photophosphorylation | 循环光合磷酸化
Cyclic photophosphorylation involves only Photosystem I. The excited electron from P700 is passed along electron carriers and then returns to the same photosystem instead of reducing NADP.
循环光合磷酸化只涉及光系统 I。来自 P700 的激发电子沿电子载体传递，然后返回同一个光系统，而不是还原 NADP。
As electrons move through the electron transport chain, hydrogen ions are pumped into the thylakoid lumen, creating a proton gradient. This gradient is used to produce ATP by chemiosmosis.
当电子沿电子传递链移动时，氢离子被泵入类囊体腔，形成质子梯度。该梯度通过化学渗透用于产生 ATP。
Cyclic photophosphorylation does not produce reduced NADP and does not release oxygen. It provides additional ATP when the Calvin cycle needs more ATP than reduced NADP.
循环光合磷酸化不产生还原型 NADP，也不释放氧气。当卡尔文循环需要的 ATP 多于还原型 NADP 时，它可以提供额外的 ATP。
7. Chemiosmosis and ATP synthesis | 化学渗透与 ATP 合成
During the light-dependent stage, hydrogen ions accumulate inside the thylakoid lumen. This happens because photolysis produces H⁺ and because electron transport chains actively pump H⁺ from the stroma into the lumen.
在光反应阶段，氢离子在类囊体腔内积累。这是因为光解产生 H⁺，并且电子传递链主动将 H⁺ 从基质泵入类囊体腔。
The result is a higher H⁺ concentration inside the thylakoid than in the stroma, generating an electrochemical gradient and a proton motive force. Hydrogen ions flow back into the stroma through ATP synthase, and the energy of this flow is used to form ATP from ADP and inorganic phosphate.
结果是类囊体内部的 H⁺ 浓度高于基质，从而产生电化学梯度和质子动力势。氢离子通过 ATP 合酶流回基质，这一流动的能量用于将 ADP 和无机磷酸合成 ATP。
This mechanism of ATP production is called photophosphorylation and is very similar to oxidative phosphorylation in mitochondria, except that the proton gradient is established across the thylakoid membrane using light energy.
这种生成 ATP 的机制称为光合磷酸化，与线粒体中的氧化磷酸化非常相似，只不过质子梯度是利用光能跨类囊体膜建立的。
8. The Calvin cycle: carbon fixation and reduction | 卡尔文循环：碳固定与还原
The light-independent stage, or Calvin cycle, takes place in the stroma. It uses ATP and reduced NADP from the light-dependent stage to convert carbon dioxide into carbohydrate.
光反应阶段之后的暗反应阶段，即卡尔文循环，发生在基质中。它利用光反应阶段产生的 ATP 和还原型 NADP 将二氧化碳转化为碳水化合物。
Carbon dioxide combines with a five-carbon acceptor called ribulose bisphosphate (RuBP). This reaction is catalysed by the enzyme ribulose bisphosphate carboxylase/oxygenase, usually called RuBisCO. The immediate product is two molecules of the three-carbon compound glycerate 3-phosphate (GP).
二氧化碳与一种称为核酮糖二磷酸（RuBP）的五碳受体结合。该反应由核酮糖二磷酸羧化酶/加氧酶（通常称为 RuBisCO）催化。直接产物是两分子三碳化合物甘油酸-3-磷酸（GP）。
Each GP molecule is then reduced and phosphorylated to triose phosphate (TP). This step uses ATP and reduced NADP. Energy and hydrogen are transferred from the light-dependent stage to the Calvin cycle.
每分子 GP 随后被还原并磷酸化为磷酸丙糖（TP）。这一步骤使用 ATP 和还原型 NADP。能量和氢从光反应阶段传递到卡尔文循环。
9. Regeneration of RuBP and product fate | RuBP 再生与产物去向
The Calvin cycle must regenerate RuBP to continue fixing carbon dioxide. For every six molecules of TP produced, five are used to regenerate three molecules of RuBP, a process that requires ATP. One TP molecule remains as a net gain.
卡尔文循环必须再生 RuBP 才能继续固定二氧化碳。每产生六分子 TP，就有五分子用于再生三分子 RuBP，这个过程需要 ATP。剩下一分子 TP 作为净增益。
The TP that is not recycled can be used to synthesise glucose, sucrose, starch, cellulose, lipids and amino acids. Glucose and starch are common storage products, while cellulose contributes to cell wall structure.
没有参与循环的 TP 可用于合成葡萄糖、蔗糖
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