A-Level生物光合作用光反应卡尔文循环

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. This A-Level Biology topic covers the two main stages: the light-dependent reactions that occur in the thylakoid membranes, and the light-independent reactions (Calvin cycle) that take place in the stroma of chloroplasts.
光合作用是植物、藻类和某些细菌将光能转化为储存在葡萄糖中的化学能的过程。这个A-Level生物主题涵盖两个主要阶段：发生在类囊体膜上的光反应，以及在叶绿体基质中进行的暗反应（卡尔文循环）。

Chloroplast Structure / 叶绿体结构

The chloroplast is a double-membrane organelle containing an internal membrane system of flattened sacs called thylakoids. A stack of thylakoids is called a granum (plural: grana), and the fluid surrounding the thylakoids is the stroma. The thylakoid membrane houses photosystems I and II, electron transport chains, and ATP synthase. The stroma contains the enzymes for the Calvin cycle, including RuBisCO, the most abundant protein on Earth.
叶绿体是双层膜细胞器，内部含有称为类囊体的扁平囊状膜系统。一叠类囊体称为基粒，围绕类囊体的液体是基质。类囊体膜上分布着光系统I和II、电子传递链和ATP合酶。基质中含有卡尔文循环的酶，包括地球上最丰富的蛋白质RuBisCO。

Photosynthetic Pigments / 光合色素

Chlorophyll a is the primary pigment, absorbing maximally at around 430 nm (blue) and 662 nm (red). Chlorophyll b is an accessory pigment extending the absorption range to capture more light energy. Carotenoids and xanthophylls are additional accessory pigments that protect chlorophyll from photo-oxidation by absorbing excess light energy and dissipating it as heat. These pigments are arranged in light-harvesting complexes (antenna complexes) that funnel energy to the reaction centres of the photosystems.
叶绿素a是主要色素，最大吸收波长约为430纳米（蓝光）和662纳米（红光）。叶绿素b是辅助色素，扩展了吸收范围以捕获更多光能。类胡萝卜素和叶黄素是额外的辅助色素，通过吸收多余光能并以热量形式耗散来保护叶绿素免受光氧化。这些色素排列在捕光复合体（天线复合体）中，将能量传递到光系统的反应中心。

Light-Dependent Reactions / 光反应

The light-dependent reactions take place on the thylakoid membranes and require light energy directly. There are two main pathways: non-cyclic photophosphorylation, which produces ATP, reduced NADP, and oxygen, and cyclic photophosphorylation, which produces only ATP. Both pathways involve the flow of electrons through an electron transport chain and the chemiosmotic synthesis of ATP.
光反应发生在类囊体膜上，直接需要光能。主要有两条途径：非循环光合磷酸化，产生ATP、还原型NADP和氧气；以及循环光合磷酸化，仅产生ATP。两条途径都涉及电子通过电子传递链的流动和ATP的化学渗透合成。

Non-Cyclic Photophosphorylation / 非循环光合磷酸化

In Photosystem II (PSII), light energy excites electrons in the reaction centre chlorophyll (P680), causing them to be emitted and captured by the primary electron acceptor. The oxidised P680 is reduced by electrons from the photolysis of water: 2H₂O = 4H⁺ + 4e^– + O₂. The excited electrons pass along an electron transport chain (plastoquinone, cytochrome b6f complex, plastocyanin) to Photosystem I (PSI), generating a proton gradient across the thylakoid membrane that drives ATP synthesis via chemiosmosis.
在光系统II（PSII）中，光能激发反应中心叶绿素（P680）中的电子，使其被释放并被初级电子受体捕获。氧化的P680通过水的光解获得电子被还原：2H₂O = 4H⁺ + 4e^– + O₂。激发的电子沿电子传递链（质体醌、细胞色素b6f复合体、质体蓝素）传递到光系统I（PSI），在类囊体膜两侧产生质子梯度，通过化学渗透驱动ATP合成。

In PSI, light energy excites electrons in P700. These electrons are passed to ferredoxin and then to the enzyme NADP reductase, which reduces NADP to reduced NADP (NADPH): NADP + 2e^– + H⁺ = reduced NADP. The oxidised P700 is reduced by electrons arriving from PSII via the electron transport chain. The products of non-cyclic photophosphorylation are ATP, reduced NADP, and oxygen (released as a waste product).
在PSI中，光能激发P700中的电子。这些电子传递到铁氧还蛋白，然后传递到NADP还原酶，该酶将NADP还原为还原型NADP（NADPH）：NADP + 2e^– + H⁺ = 还原型NADP。氧化的P700由来自PSII经电子传递链到达的电子还原。非循环光合磷酸化的产物是ATP、还原型NADP和氧气（作为废物释放）。

Cyclic Photophosphorylation / 循环光合磷酸化

Cyclic photophosphorylation involves only PSI. Excited electrons from P700 pass to ferredoxin, then back to the cytochrome b6f complex and ultimately return to the oxidised P700 via plastocyanin. This cyclic electron flow generates a proton gradient for ATP synthesis but produces no reduced NADP and no oxygen. The Calvin cycle requires more ATP than reduced NADP (3 ATP per 2 reduced NADP), so cyclic photophosphorylation provides the additional ATP needed.
循环光合磷酸化仅涉及PSI。来自P700的激发电子传递到铁氧还蛋白，然后返回细胞色素b6f复合体，最终通过质体蓝素回到氧化的P700。这种循环电子流产生质子梯度用于ATP合成，但不产生还原型NADP和氧气。卡尔文循环需要的ATP多于还原型NADP（每2个还原型NADP需要3个ATP），因此循环光合磷酸化提供所需的额外ATP。

Chemiosmosis in Photosynthesis / 光合作用中的化学渗透

As electrons move through the electron transport chain, protons (H⁺) are actively transported from the stroma into the thylakoid lumen. This occurs at the water photolysis step (releasing H⁺ into the lumen) and at the cytochrome b6f complex (pumping H⁺ from stroma to lumen). The resulting proton gradient means the thylakoid lumen has a higher H⁺ concentration (lower pH) than the stroma. Protons diffuse back into the stroma through ATP synthase (chemiosmosis), and this flow of protons drives the synthesis of ATP from ADP and inorganic phosphate (P_i). This is called photophosphorylation.
当电子沿电子传递链移动时，质子（H⁺）从基质被主动转运到类囊体腔中。这发生在水的光解步骤（将H⁺释放到腔中）和细胞色素b6f复合体处（将H⁺从基质泵入腔中）。产生的质子梯度意味着类囊体腔比基质具有更高的H⁺浓度（更低的pH值）。质子通过ATP合酶扩散回基质（化学渗透），这个质子流动驱动从ADP和磷酸盐（P_i）合成ATP。这称为光合磷酸化。

Light-Independent Reactions: The Calvin Cycle / 暗反应：卡尔文循环

The Calvin cycle takes place in the stroma and does not require light directly, though it depends on the products of the light-dependent reactions (ATP and reduced NADP). The cycle has three main stages: carbon fixation, reduction, and regeneration of the carbon dioxide acceptor, ribulose bisphosphate (RuBP). The enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase) catalyses the first step. Six turns of the Calvin cycle are needed to produce one glucose molecule.
卡尔文循环在基质中进行，不直接需要光，但依赖于光反应的产物（ATP和还原型NADP）。该循环有三个主要阶段：碳固定、还原和二氧化碳受体核酮糖二磷酸（RuBP）的再生。RuBisCO酶催化第一步。生产一个葡萄糖分子需要卡尔文循环运行六次。

Stage 1: Carbon Fixation / 第一阶段：碳固定

Carbon dioxide (CO₂) from the atmosphere combines with RuBP (a 5-carbon sugar with two phosphate groups) in a reaction catalysed by RuBisCO. The product is an unstable 6-carbon intermediate that immediately splits into two molecules of glycerate 3-phosphate (GP), a 3-carbon compound. This is why the Calvin cycle is sometimes called the C3 pathway. For each CO₂ fixed, two GP molecules are produced.
来自大气的二氧化碳（CO₂）与RuBP（带有两个磷酸基团的五碳糖）在RuBisCO催化的反应中结合。产物是一个不稳定的六碳中间体，立即分裂成两个甘油酸-3-磷酸（GP）分子，一种三碳化合物。这就是为什么卡尔文循环有时被称为C3途径。每固定一个CO₂，产生两个GP分子。

Stage 2: Reduction / 第二阶段：还原

Each GP molecule is reduced to triose phosphate (TP), also called glyceraldehyde 3-phosphate (GALP), using ATP and reduced NADP from the light-dependent reactions. The ATP provides energy (GP is phosphorylated) and reduced NADP provides the reducing power (hydrogen atoms). For each GP reduced, one ATP and one reduced NADP are consumed. Of the two TP molecules produced per CO₂ fixed, one sixth is used to synthesise glucose and other organic molecules, while the remaining five sixths are used to regenerate RuBP.
每个GP分子使用来自光反应的ATP和还原型NADP被还原为磷酸丙糖（TP），也称为甘油醛-3-磷酸（GALP）。ATP提供能量（GP被磷酸化），还原型NADP提供还原力（氢原子）。每还原一个GP，消耗一个ATP和一个还原型NADP。每固定一个CO₂产生的两个TP分子中，六分之一用于合成葡萄糖和其他有机分子，其余六分之五用于再生RuBP。

Stage 3: Regeneration of RuBP / 第三阶段：RuBP的再生

Five of every six TP molecules produced are used to regenerate the three RuBP molecules needed for the cycle to continue. This regeneration process requires ATP. Five TP molecules (each 3C, total 15C) are rearranged through a series of reactions to form three RuBP molecules (each 5C, total 15C). This step ensures that the cycle can continue fixing more CO₂. The remaining one TP molecule (3C) from each six produced is available for the synthesis of glucose, sucrose, starch, amino acids, and other organic compounds needed by the plant.
每六个产生的TP分子中有五个用于再生循环继续所需的三个RuBP分子。这个再生过程需要ATP。五个TP分子（每个3C，总共15C）通过一系列反应重新排列形成三个RuBP分子（每个5C，总共15C）。这一步确保循环能够继续固定更多的CO₂。每六个产生的TP分子中剩下的一个（3C）可用于合成植物所需的葡萄糖、蔗糖、淀粉、氨基酸和其他有机化合物。

Overall Stoichiometry of the Calvin Cycle / 卡尔文循环的总化学计量

To produce one molecule of glucose (C₆H₁₂O₆), six turns of the Calvin cycle are required, fixing six CO₂ molecules. This consumes 18 ATP and 12 reduced NADP. The 18 ATP come from both non-cyclic and cyclic photophosphorylation. The overall balanced equation for photosynthesis is: 6CO₂ + 6H₂O + light energy = C₆H₁₂O₆ + 6O₂. However, it is important to note that the oxygen released comes from water, not from carbon dioxide.
生产一个葡萄糖分子（C₆H₁₂O₆）需要卡尔文循环运行六次，固定六个CO₂分子。这消耗18个ATP和12个还原型NADP。这18个ATP来自非循环和循环光合磷酸化。光合作用的总体平衡方程式是：6CO₂ + 6H₂O + 光能 = C₆H₁₂O₆ + 6O₂。然而，需要注意的是，释放的氧气来自水，而不是二氧化碳。

Limiting Factors in Photosynthesis / 光合作用的限制因素

The rate of photosynthesis is affected by several limiting factors: light intensity, carbon dioxide concentration, and temperature. At low light intensity, the light-dependent reactions are rate-limiting. As light intensity increases, the rate rises until another factor (such as CO₂ concentration) becomes limiting. At the light compensation point, the rates of photosynthesis and respiration are equal (net gas exchange is zero). Temperature affects the rate mainly through enzyme activity, particularly RuBisCO. Above an optimum temperature (typically 25-30C for C3 plants), the rate decreases as enzymes denature.
光合作用速率受多个限制因素影响：光强度、二氧化碳浓度和温度。在低光强下，光反应是速率限制步骤。随着光强增加，速率上升，直到另一个因素（如CO₂浓度）成为限制因素。在光补偿点，光合作用和呼吸作用速率相等（净气体交换为零）。温度主要通过酶活性影响速率，特别是RuBisCO。超过最适温度（C3植物通常为25-30摄氏度），随着酶变性，速率下降。

Measuring Photosynthesis Rate / 测量光合作用速率

The rate of photosynthesis can be measured in several ways: by measuring the volume of oxygen produced per unit time using a photosynthometer (with aquatic plants like Elodea), by measuring the rate of CO₂ uptake using a pH indicator (hydrogencarbonate indicator changes from orange-red to purple as CO₂ is absorbed), or by measuring the increase in dry mass of plant tissue over time. In experiments, it is important to control variables such as light wavelength, CO₂ concentration (using sodium hydrogencarbonate solution), and temperature (using a water bath).
光合作用速率可以通过多种方法测量：使用光合作用计测量单位时间产生的氧气体积（使用伊乐藻等水生植物），使用pH指示剂测量CO₂吸收速率（碳酸氢盐指示剂随CO₂被吸收从橙红色变为紫色），或测量植物组织干重随时间的增加。在实验中，控制变量很重要，例如光波长、CO₂浓度（使用碳酸氢钠溶液）和温度（使用水浴）。

Absorption and Action Spectra / 吸收光谱和作用光谱

The absorption spectrum shows the relative amount of light absorbed by photosynthetic pigments at different wavelengths. Chlorophyll a absorbs most strongly in the blue-violet (around 430 nm) and red (around 662 nm) regions, and least in the green region (around 550 nm), which it reflects, giving leaves their green colour. The action spectrum shows the rate of photosynthesis at different wavelengths. The two spectra correspond closely, confirming that the pigments absorbing light are responsible for driving photosynthesis. The highest rates of photosynthesis occur in the blue-violet and red regions of the visible spectrum.
吸收光谱显示光合色素在不同波长下吸收的相对光量。叶绿素a在蓝紫色（约430纳米）和红色（约662纳米）区域吸收最强，在绿色区域（约550纳米）吸收最少，它反射绿光，使叶子呈现绿色。作用光谱显示不同波长下的光合作用速率。两个光谱紧密对应，证实吸收光的色素负责驱动光合作用。光合作用速率最高发生在可见光谱的蓝紫色和红色区域。

Photorespiration / 光呼吸

RuBisCO can act as both a carboxylase (fixing CO₂) and an oxygenase (fixing O₂). When the concentration of CO₂ is low and O₂ is high, RuBisCO binds to oxygen instead of carbon dioxide in a process called photorespiration. This produces a 2-carbon compound (phosphoglycolate) instead of two GP molecules, reducing the efficiency of photosynthesis by up to 25%. Photorespiration occurs more frequently at high temperatures because stomata close to conserve water, reducing CO₂ uptake. C4 plants and CAM plants have evolved adaptations to minimise photorespiration.
RuBisCO既可以作为羧化酶（固定CO₂）也可以作为加氧酶（固定O₂）。当CO₂浓度低而O₂浓度高时，RuBisCO与氧气结合而不是与二氧化碳结合，这个过程称为光呼吸。这产生一个二碳化合物（磷酸乙醇酸）而不是两个GP分子，使光合作用效率降低多达25%。光呼吸在高温下更频繁发生，因为气孔关闭以保存水分，减少了CO₂的吸收。C4植物和CAM植物已经进化出适应机制以最小化光呼吸。

Key Bilingual Terms / 关键双语术语

Understanding photosynthesis is fundamental to A-Level Biology, linking concepts from biochemistry, cell biology, and plant physiology. Students should be able to describe the structure of chloroplasts, explain the light-dependent and light-independent reactions in detail, interpret absorption and action spectra, and discuss limiting factors. This knowledge provides the foundation for further study in plant science, ecology, and biotechnology.
理解光合作用是A-Level生物学的基础，它将生物化学、细胞生物学和植物生理学的概念联系起来。学生应该能够描述叶绿体的结构，详细解释光反应和暗反应，解读吸收光谱和作用光谱，并讨论限制因素。这些知识为植物科学、生态学和生物技术的进一步学习奠定了基础。

A-Level生物 光合作用 光反应 卡尔文循环