Photosynthesis: Light-Dependent and Light-Independent Reactions | 光合作用：光反应与暗反应详解

Introduction | 引言

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It is arguably the most important biochemical process on Earth — it produces the oxygen we breathe and forms the base of nearly every food chain. For A-Level Biology students, understanding photosynthesis in detail is essential, as it appears across all major exam boards including AQA, Edexcel, OCR, and CIE.

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中的化学能的过程。可以说，这是地球上最重要的生化过程——它产生我们呼吸的氧气，并构成几乎所有食物链的基础。对于A-Level生物学学生来说，详细了解光合作用至关重要，因为它出现在包括AQA、Edexcel、OCR和CIE在内的所有主要考试局中。

Photosynthesis occurs in two main stages: the light-dependent reactions (which require light and occur in the thylakoid membranes) and the light-independent reactions (also known as the Calvin cycle, which do not directly require light and occur in the stroma). This article will guide you through both stages in detail, covering the key molecules, processes, and exam tips you need to succeed.

光合作用分为两个主要阶段：光反应（需要光，发生在类囊体膜上）和暗反应（也称为卡尔文循环，不直接需要光，发生在基质中）。本文将详细介绍这两个阶段，涵盖你需要掌握的关键分子、过程和考试技巧。

Overview of the Chloroplast | 叶绿体概述

Before diving into the reactions, it is important to understand the structure of the chloroplast, as the location of each reaction is critical:

在深入反应之前，了解叶绿体的结构很重要，因为每个反应的位置至关重要：

• Thylakoid membranes (类囊体膜): A system of flattened, fluid-filled sacs. The membranes contain photosystems, electron carriers, and ATP synthase. The light-dependent reactions occur here.
• Grana (基粒): Stacks of thylakoids, which maximise the surface area for light absorption.
• Stroma (基质): The fluid-filled matrix surrounding the thylakoids. Contains enzymes for the Calvin cycle, including RuBisCO. The light-independent reactions occur here.
• Photosystems (光系统): Protein complexes containing photosynthetic pigments (chlorophyll a, chlorophyll b, carotenoids) that absorb light energy. Photosystem II (PSII) absorbs best at 680 nm; Photosystem I (PSI) absorbs best at 700 nm.

Photosynthetic Pigments | 光合色素

Photosynthetic pigments are molecules that absorb specific wavelengths of light. Different pigments absorb different wavelengths, allowing the plant to capture a broader spectrum of light energy. The main pigments include:

光合色素是吸收特定波长光的分子。不同色素吸收不同波长，使植物能够捕获更广泛的光能。主要色素包括：

• Chlorophyll a (叶绿素a): The primary pigment, located in the reaction centre of both photosystems. Absorbs mainly red (680-700 nm) and blue-violet light. Reflects green light, which is why plants appear green.
• Chlorophyll b (叶绿素b): An accessory pigment that absorbs blue light (450-500 nm) and transfers energy to chlorophyll a.
• Carotenoids (类胡萝卜素): Accessory pigments including beta-carotene and xanthophylls. Absorb blue-green light and protect chlorophyll from photo-oxidation.

A key practical skill for A-Level is chromatography — separating photosynthetic pigments and calculating their Rf values. The formula is:

A-Level的一个关键实验技能是色谱法——分离光合色素并计算它们的Rf值。公式为：

Rf = distance moved by pigment spot / distance moved by solvent front

Light-Dependent Reactions | 光反应

The light-dependent reactions convert light energy into chemical energy in the form of ATP and reduced NADP (NADPH). These reactions occur in the thylakoid membranes and involve two photosystems working in series.

光反应将光能转化为ATP和还原型NADP（NADPH）形式的化学能。这些反应发生在类囊体膜上，涉及两个串联工作的光系统。

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

This is the main pathway and produces ATP, NADPH, and oxygen. The process occurs in four key stages:

这是主要途径，产生ATP、NADPH和氧气。该过程分为四个关键阶段：

Stage 1 — Photoionisation of chlorophyll (叶绿素的光电离): Light energy is absorbed by PSII, exciting electrons in chlorophyll a to a higher energy level. These high-energy electrons are released from the chlorophyll molecule and captured by an electron acceptor. The chlorophyll is now oxidised (it has lost electrons) and positively charged.

Stage 2 — Photolysis of water (水的光解): To replace the electrons lost from PSII, water molecules are split in a process catalysed by the oxygen-evolving complex. The equation is:

2H₂O → 4H⁺ + 4e⁻ + O₂

This produces: (1) electrons that replace those lost from chlorophyll; (2) protons (H⁺) that contribute to the proton gradient across the thylakoid membrane; and (3) oxygen gas, which is released as a by-product.

Stage 3 — Electron Transport Chain (电子传递链): The excited electrons pass through a series of electron carriers (including plastoquinone, cytochrome b6f complex, and plastocyanin) embedded in the thylakoid membrane. As electrons move down the chain, the energy released is used to actively pump H⁺ ions from the stroma into the thylakoid lumen, creating a proton gradient (higher H⁺ concentration inside the thylakoid). This is chemiosmosis.

Stage 4 — ATP Synthesis and NADP Reduction: The electrons reach PSI, where they are re-excited by light energy. These re-excited electrons are passed to the enzyme NADP reductase, which catalyses the reduction of NADP to NADPH:

NADP⁺ + 2e⁻ + H⁺ → NADPH

Meanwhile, protons flow back into the stroma through the enzyme ATP synthase (a process called chemiosmosis). This flow of protons drives the synthesis of ATP from ADP and inorganic phosphate (Pi):

ADP + Pi → ATP

The overall equation for non-cyclic photophosphorylation is:

2H₂O + 2NADP⁺ + 3ADP + 3Pi → 2NADPH + 2H⁺ + 3ATP + O₂

Cyclic Photophosphorylation | 循环光合磷酸化

In cyclic photophosphorylation, only PSI is involved. The excited electrons from PSI are not passed to NADP but instead return to the electron transport chain and back to PSI. This process produces ATP only (no NADPH, no oxygen). It occurs when the plant needs more ATP than NADPH for the Calvin cycle, as the Calvin cycle uses more ATP per NADPH than is produced in non-cyclic phosphorylation.

在循环光合磷酸化中，只有PSI参与。PSI的激发电子不传递给NADP，而是返回电子传递链并回到PSI。该过程仅产生ATP（不产生NADPH，不产生氧气）。当植物需要比非循环磷酸化产生更多的ATP用于卡尔文循环时，就会发生这种情况，因为卡尔文循环每个NADPH消耗的ATP比非循环磷酸化产生的更多。

Light-Independent Reactions (Calvin Cycle) | 暗反应（卡尔文循环）

The Calvin cycle uses the ATP and NADPH produced in the light-dependent reactions to fix carbon dioxide into organic molecules. It occurs in the stroma of the chloroplast and does not require light directly — although it typically runs during the day when ATP and NADPH are available.

卡尔文循环利用光反应中产生的ATP和NADPH将二氧化碳固定为有机分子。它发生在叶绿体基质中，不直接需要光——尽管它通常在白天当ATP和NADPH可用时运行。

The Three Stages of the Calvin Cycle | 卡尔文循环的三个阶段

1. Carbon Fixation (碳固定): CO₂ combines with a 5-carbon sugar called ribulose bisphosphate (RuBP). This reaction is catalysed by the enzyme RuBisCO (ribulose bisphosphate carboxylase/oxygenase) — probably the most abundant enzyme on Earth! The product is an unstable 6-carbon intermediate that immediately splits into two molecules of glycerate-3-phosphate (GP), a 3-carbon compound.

RuBP (5C) + CO₂ → 2 × GP (3C)

2. Reduction (还原): Each GP molecule is reduced to glyceraldehyde-3-phosphate (GALP), also known as triose phosphate (TP). This requires ATP (for phosphorylation) and NADPH (for reduction):

GP → GALP (using ATP + NADPH)

This is the point where the products of the light-dependent reactions (ATP and NADPH) are used. NADPH provides the reducing power, and ATP provides the energy.

3. Regeneration of RuBP (RuBP的再生): Out of every six GALP molecules produced, five are used to regenerate RuBP (using ATP), and one is available for the synthesis of organic molecules — primarily glucose, but also amino acids, lipids, and nucleic acids. The regeneration of RuBP ensures the cycle can continue.

5 × GALP (3C) → 3 × RuBP (5C) [using ATP]

The remaining one GALP molecule (out of every six) is the net gain, used to produce hexose sugars like glucose. Two GALP molecules are needed to synthesise one glucose molecule, so the Calvin cycle must turn six times to produce one glucose molecule.

每六个GALP分子中，五个用于再生RuBP（使用ATP），一个可用于合成有机分子——主要是葡萄糖，但也包括氨基酸、脂质和核酸。RuBP的再生确保循环可以继续。剩余的一个GALP分子（每六个中的）是净收益，用于产生己糖如葡萄糖。需要两个GALP分子来合成一个葡萄糖分子，因此卡尔文循环必须循环六次才能产生一个葡萄糖分子。

Factors Affecting Photosynthesis | 影响光合作用的因素

Several environmental factors limit the rate of photosynthesis. Understanding limiting factors is a key concept for A-Level exams:

几个环境因素限制光合作用速率。理解限制因素是A-Level考试的关键概念：

• Light intensity (光照强度): As light intensity increases, the rate of photosynthesis increases until another factor becomes limiting. At very high light intensities, the rate plateaus because all available chlorophyll molecules are saturated. The light compensation point is the light intensity at which photosynthesis equals respiration (net CO₂ exchange = 0).
• Carbon dioxide concentration (二氧化碳浓度): CO₂ is the substrate for carbon fixation. At low CO₂ concentrations, the rate is limited by RuBisCO activity. At around 0.1% CO₂, the rate typically reaches its maximum. Atmospheric CO₂ is approximately 0.04%, so CO₂ is often the limiting factor in natural conditions.
• Temperature (温度): Photosynthesis is enzyme-controlled (RuBisCO and others). The rate increases with temperature up to an optimum (typically 25-30°C for C3 plants). Above the optimum, enzymes denature and the rate falls sharply. At low temperatures, kinetic energy is low, and enzyme-substrate collisions are less frequent.

Key Exam Tips | 关键考试技巧

1. Use precise terminology: Exam markers look for specific terms. Say “photoionisation” not “electrons become excited”, say “photolysis” not “water splitting”, say “chemiosmosis” not “protons move across the membrane”.

2. Link structure to function: Always connect the chloroplast structure to its function. The thylakoid membranes provide a large surface area for photosystems and electron carriers; the stroma contains all Calvin cycle enzymes; the grana maximise light capture.

3. Know the key products: Light-dependent reactions produce ATP, NADPH, and O₂. The Calvin cycle produces GALP/TP, which can be converted to glucose, starch, cellulose, amino acids, and lipids. ADP and NADP are recycled back to the light-dependent reactions.

4. Understand limiting factors graphs: Be able to interpret and draw graphs showing the effect of light intensity, CO₂ concentration, and temperature on the rate of photosynthesis. Know that the rate is limited by the factor in shortest supply — this is the Law of Limiting Factors.

5. Chromatography practical: Be prepared to describe the method for separating photosynthetic pigments using paper or thin-layer chromatography, calculating Rf values, and explaining why different pigments separate (different solubilities in the solvent).

1. 使用精确术语：阅卷官寻找特定术语。说”photoionisation”而不是”electrons become excited”，说”photolysis”而不是”water splitting”，说”chemiosmosis”而不是”protons move across the membrane”。

2. 连接结构与功能：始终将叶绿体结构与其功能联系起来。类囊体膜为光系统和电子载体提供大表面积；基质包含所有卡尔文循环酶；基粒最大化光捕获。

3. 了解关键产物：光反应产生ATP、NADPH和O₂。卡尔文循环产生GALP/TP，可转化为葡萄糖、淀粉、纤维素、氨基酸和脂质。ADP和NADP被回收到光反应中。

4. 理解限制因素图表：能够解释和绘制显示光照强度、CO₂浓度和温度对光合作用速率影响的图表。知道速率受供应最短的因素限制——这是限制因素定律。

5. 色谱实验：准备好描述使用纸色谱或薄层色谱分离光合色素的方法，计算Rf值，并解释不同色素分离的原因（在溶剂中的溶解度不同）。

Practice Questions | 练习题

Q1: Describe the role of water in the light-dependent reactions of photosynthesis. (3 marks)

Q2: Explain how the products of the light-dependent reactions are used in the Calvin cycle. (4 marks)

Q3: A student investigates the effect of light intensity on the rate of photosynthesis using pondweed. Suggest why the rate of photosynthesis does not continue to increase beyond a certain light intensity. (2 marks)

Q4: Compare and contrast cyclic and non-cyclic photophosphorylation. (5 marks)

Q5: Explain the importance of the enzyme RuBisCO in the Calvin cycle. (3 marks)

问题1：描述水在光合作用光反应中的作用。（3分）

问题2：解释光反应的产物如何在卡尔文循环中使用。（4分）

问题3：学生使用水草研究光照强度对光合作用速率的影响。请说明为什么超过一定光照强度后，光合作用速率不再继续增加。（2分）

问题4：比较和对比循环和非循环光合磷酸化。（5分）

问题5：解释RuBisCO酶在卡尔文循环中的重要性。（3分）

Glossary of Key Terms | 关键术语词汇表

English	中文	Definition
Photoionisation	光电离	The process by which light energy causes electrons to be emitted from chlorophyll
Photolysis	光解作用	The splitting of water molecules using light energy
Chemiosmosis	化学渗透	The movement of protons across a membrane through ATP synthase, driving ATP synthesis
RuBisCO	核酮糖-1,5-二磷酸羧化酶/加氧酶	The enzyme that catalyses carbon fixation in the Calvin cycle
RuBP	核酮糖-1,5-二磷酸	Ribulose bisphosphate — the 5-carbon CO₂ acceptor in the Calvin cycle
GP	甘油酸-3-磷酸	Glycerate-3-phosphate — the 3-carbon product of carbon fixation
GALP / TP	甘油醛-3-磷酸 / 磷酸丙糖	Glyceraldehyde-3-phosphate / triose phosphate — the reduced 3-carbon product of the Calvin cycle
NADP / NADPH	NADP / NADPH	Nicotinamide adenine dinucleotide phosphate — the electron carrier in photosynthesis
Photosystem	光系统	A protein complex containing photosynthetic pigments that absorbs light energy

Summary | 总结

Photosynthesis is a beautifully orchestrated two-stage process. The light-dependent reactions capture solar energy and convert it into chemical energy (ATP and NADPH), while the Calvin cycle uses that energy to fix CO₂ into organic carbon. Understanding how these stages are linked — and how environmental factors regulate the overall rate — is fundamental to A-Level Biology. Master the terminology, practise drawing the Z-scheme and the Calvin cycle, and be ready to interpret data from experiments investigating limiting factors. Good luck!

光合作用是一个精心编排的两阶段过程。光反应捕获太阳能并将其转化为化学能（ATP和NADPH），而卡尔文循环利用这些能量将CO₂固定为有机碳。理解这些阶段如何连接——以及环境因素如何调节整体速率——是A-Level生物学的基础。掌握术语，练习绘制Z方案和卡尔文循环，并准备好解释研究限制因素的实验数据。祝你好运！