A-Level Biology: Photosynthesis — Light-Dependent & Light-Independent Reactions | 光合作用：光反应与暗反应详解

Introduction to Photosynthesis | 光合作用简介

Photosynthesis is arguably the most important biochemical process on Earth. It is the mechanism by which plants, algae, and some bacteria convert light energy from the Sun into chemical energy stored in glucose. This process not only sustains the organisms that perform it but also produces the oxygen that aerobic organisms — including humans — depend on for respiration. For A-Level Biology students, understanding photosynthesis in detail is essential, as it features prominently in examinations across all major exam boards, including AQA, Edexcel, CIE, and OCR.

光合作用可以说是地球上最重要的生物化学过程。它是植物、藻类和一些细菌将太阳光能转化为储存在葡萄糖中的化学能的机制。这个过程不仅维持了进行光合作用的生物体，而且还产生了包括人类在内的需氧生物进行呼吸所依赖的氧气。对于A-Level生物学学生来说，详细理解光合作用至关重要，因为它是所有主要考试局（包括AQA、Edexcel、CIE和OCR）考试中的重要内容。

The overall balanced equation for photosynthesis is deceptively simple:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

However, this single equation masks a complex series of reactions divided into two main stages: the light-dependent reactions and the light-independent reactions (Calvin cycle). In this article, we will explore both stages in depth, examining the structures involved, the key molecules and enzymes, and the exam techniques required to achieve top marks.

然而，这个单一的方程式掩盖了一系列复杂的反应，分为两个主要阶段：光反应（光依赖反应）和暗反应（光独立反应，即卡尔文循环）。在本文中，我们将深入探讨这两个阶段，研究相关的结构、关键分子和酶，以及获得高分所需的考试技巧。

The Chloroplast: The Site of Photosynthesis | 叶绿体：光合作用的场所

Before we can understand the reactions of photosynthesis, we must first familiarise ourselves with the organelle where it all takes place: the chloroplast. Chloroplasts are found in the mesophyll cells of plant leaves, particularly in the palisade mesophyll layer where light intensity is highest.

在我们理解光合作用的反应之前，我们必须先熟悉这一切发生的地方：叶绿体。叶绿体存在于植物叶片的叶肉细胞中，特别是在光照强度最高的栅栏组织层中。

A typical chloroplast is surrounded by a double membrane — an outer membrane and an inner membrane — called the chloroplast envelope. Inside, the chloroplast contains two distinct regions critical to photosynthesis:

1. The Grana (singular: Granum): Stacks of flattened membrane sacs called thylakoids. The thylakoid membranes contain photosynthetic pigments (chlorophyll a, chlorophyll b, and carotenoids) arranged in photosystems, as well as electron carrier molecules and ATP synthase enzymes. The light-dependent reactions occur here.
2. The Stroma: The fluid-filled matrix surrounding the grana. It contains enzymes for the Calvin cycle, starch grains, lipid droplets, and the chloroplast’s own DNA and ribosomes. The light-independent reactions take place here.

1. 基粒：扁平的膜囊（称为类囊体）堆叠而成。类囊体膜含有排列在光系统中的光合色素（叶绿素a、叶绿素b和类胡萝卜素），以及电子载体分子和ATP合酶。光反应在此发生。
2. 基质：围绕基粒的充满液体的基质。它含有卡尔文循环的酶、淀粉粒、脂滴以及叶绿体自身的DNA和核糖体。暗反应在此发生。

The Light-Dependent Reactions | 光反应

The light-dependent reactions take place on the thylakoid membranes of the chloroplast. Their primary purpose is to capture light energy and convert it into chemical energy in the form of ATP (adenosine triphosphate) and reduced NADP (NADPH). These two products are then used to drive the Calvin cycle. The light-dependent reactions also produce oxygen as a by-product through the photolysis of water.

光反应发生在叶绿体的类囊体膜上。其主要目的是捕获光能并将其转化为ATP（三磷酸腺苷）和还原型NADP（NADPH）形式的化学能。这两种产物然后被用来驱动卡尔文循环。光反应还通过水的光解产生氧气作为副产物。

Photosystems and Light Absorption | 光系统与光吸收

Embedded in the thylakoid membrane are two types of photosystems — protein complexes that contain photosynthetic pigments. These are:

• Photosystem II (PSII): Contains chlorophyll a with an absorption peak at 680 nm (P680). It is located mainly in the appressed regions of granal thylakoids.
• Photosystem I (PSI): Contains chlorophyll a with an absorption peak at 700 nm (P700). It is found mainly in the stromal thylakoids and the non-appressed regions of granal thylakoids.

Despite the numbering, PSII functions first in the light-dependent reactions — the numbering reflects the order in which they were discovered, not the order of operation.

尽管编号如此，PSII在光反应中最先起作用——这个编号反映了它们被发现的顺序，而非作用顺序。

Each photosystem consists of an antenna complex — a network of hundreds of pigment molecules (chlorophyll a, chlorophyll b, and carotenoids) that act as light-harvesting antennae. When a photon of light strikes one of these pigment molecules, the energy is passed from pigment to pigment by resonance energy transfer until it reaches the reaction centre, where a specialised pair of chlorophyll a molecules are located.

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

This is the primary pathway of the light-dependent reactions that produces ATP, reduced NADP, and oxygen. It involves both photosystems working in series:

Step 1: Photoionisation in PSII
When light energy reaches the reaction centre of PSII (P680), it excites electrons in the chlorophyll a molecules. These electrons become so energetic that they leave the chlorophyll molecule entirely — a process called photoionisation. The chlorophyll a becomes oxidised (positively charged) and is now a strong oxidising agent.

Step 2: Photolysis of Water
The oxidised P680 must regain electrons to continue functioning. It does so by splitting water molecules in a process called photolysis:

2H₂O → 4H⁺ + 4e⁻ + O₂

This reaction is catalysed by the oxygen-evolving complex (OEC), which contains manganese ions. The electrons replace those lost by PSII, the protons are released into the thylakoid lumen (contributing to the proton gradient), and the oxygen is released as a waste product or used in respiration.

Step 3: Electron Transport Chain
The excited electrons from PSII are passed along a series of electron carriers embedded in the thylakoid membrane, including plastoquinone (PQ), the cytochrome b6f complex, and plastocyanin (PC). As electrons move through this chain, their energy is used to pump protons (H⁺) from the stroma into the thylakoid lumen, creating a proton gradient.

Step 4: Photoionisation in PSI
Light energy also excites electrons in PSI (P700). These electrons are passed to a primary electron acceptor and then to the enzyme NADP reductase, which uses the electrons and protons from the stroma to reduce NADP to NADPH:

NADP + 2H⁺ + 2e⁻ → NADPH + H⁺

The electrons lost by PSI are replaced by the electrons that have travelled through the electron transport chain from PSII.

Step 5: Chemiosmosis and ATP Synthesis
The proton gradient established across the thylakoid membrane creates a proton motive force. Protons cannot diffuse back through the membrane because it is impermeable to them. Instead, they flow back into the stroma through ATP synthase — a transmembrane enzyme complex. This flow of protons (chemiosmosis) drives the phosphorylation of ADP to ATP:

ADP + Pi → ATP

Together, the products of non-cyclic photophosphorylation are: ATP, reduced NADP, and O₂.

Cyclic Photophosphorylation | 循环光合磷酸化

In some circumstances, only PSI is involved in a process called cyclic photophosphorylation. Here, the excited electrons from PSI are passed back to the electron transport chain (via ferredoxin and the cytochrome b6f complex) rather than being used to reduce NADP. This process produces ATP but not reduced NADP or oxygen.

在某些情况下，只有PSI参与循环光合磷酸化过程。在这里，来自PSI的激发电子被传回电子传递链（通过铁氧还蛋白和细胞色素b6f复合体），而不是用于还原NADP。这个过程产生ATP，但不产生还原型NADP或氧气。

Cyclic photophosphorylation occurs when the concentration of NADP in the stroma is low (i.e., most NADP has already been reduced). It allows the plant to continue producing ATP for the Calvin cycle even when NADPH is abundant.

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

The light-independent reactions — collectively known as the Calvin cycle — take place in the stroma of the chloroplast. Despite being called “light-independent,” these reactions depend on the products of the light-dependent reactions: ATP and reduced NADP. The Calvin cycle uses these energy-carrying molecules to fix carbon dioxide and synthesise glucose.

暗反应——统称为卡尔文循环——发生在叶绿体的基质中。尽管被称为”光独立反应”，这些反应依赖于光反应的产物：ATP和还原型NADP。卡尔文循环利用这些能量载体分子来固定二氧化碳并合成葡萄糖。

The Calvin cycle can be divided into three main phases:

1. Carbon Fixation | 碳固定

Carbon dioxide from the atmosphere diffuses into the stroma of the chloroplast. Here, it combines with a 5-carbon sugar called ribulose bisphosphate (RuBP) — a reaction catalysed by the enzyme rubisco (ribulose bisphosphate carboxylase/oxygenase). This produces 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)

Rubisco is often cited as the most abundant enzyme on Earth, reflecting the sheer scale of carbon fixation that occurs globally. However, it is also a relatively slow enzyme with a low affinity for CO₂, which explains why plants need so much of it.

2. Reduction | 还原

In this phase, each molecule of GP is reduced and phosphorylated to form glyceraldehyde 3-phosphate (GALP), also known as triose phosphate (TP). This process requires energy from ATP and reducing power from reduced NADP — both products of the light-dependent reactions:

GP + ATP + NADPH → GALP (TP) + ADP + Pi + NADP

For every 6 molecules of GALP produced, 5 are used to regenerate RuBP, and 1 is available for the synthesis of useful organic molecules such as glucose, amino acids, and lipids.

3. Regeneration of RuBP | RuBP的再生

The 5 molecules of GALP (3C each = 15 carbons in total) are rearranged through a series of reactions to regenerate 3 molecules of RuBP (5C each = 15 carbons). This regeneration requires ATP from the light-dependent reactions. Once RuBP is regenerated, the cycle can continue.

5 × GALP (3C) + 3 ATP → 3 × RuBP (5C) + 3 ADP

Summary of the Calvin Cycle Inputs and Outputs | 卡尔文循环输入输出总结

For the net synthesis of one molecule of glucose (C₆H₁₂O₆), the Calvin cycle must turn six times (since each turn fixes one CO₂ molecule and produces two GALP, with one-sixth of GALP being used for glucose synthesis):

Photosynthetic Pigments and Absorption Spectra | 光合色素与吸收光谱

Understanding the pigments involved in photosynthesis is essential for A-Level Biology. There are three main classes of photosynthetic pigments found in the thylakoid membranes:

• Chlorophyll a: The primary pigment, present in all photosynthetic organisms. It absorbs light most strongly in the blue-violet (430 nm) and red (662 nm) regions of the spectrum. There are two forms: P680 (in PSII) and P700 (in PSI).
• Chlorophyll b: An accessory pigment that absorbs light in the blue (453 nm) and orange-red (642 nm) regions. It transfers the absorbed energy to chlorophyll a and broadens the range of wavelengths that can be used for photosynthesis.
• Carotenoids: Accessory pigments (including beta-carotene and xanthophylls) that absorb light primarily in the blue-violet region (400–500 nm). They also have a protective role — they dissipate excess light energy as heat to prevent damage to chlorophyll from photo-oxidation.

The absorption spectrum shows the wavelengths of light absorbed by each pigment, while the action spectrum shows the rate of photosynthesis at different wavelengths. The two spectra closely overlap, confirming that the pigments identified are indeed responsible for photosynthesis. The fact that chlorophyll reflects green light (around 500–550 nm) explains why plants appear green and why photosynthesis rates are lowest in the green region of the spectrum.

Comparative Photosynthesis: C3, C4, and CAM Plants | 光合作用比较：C3、C4与CAM植物

While the Calvin cycle is the universal mechanism for carbon fixation, plants have evolved different strategies to cope with environmental challenges, particularly high temperatures and water limitation. A-Level syllabi, especially CIE and OCR, often ask students to compare these pathways.

C3 Plants | C3植物

Most plants (including rice, wheat, and soybeans) are C3 plants. The first stable product of carbon fixation is the 3-carbon compound GP. C3 plants perform the entire Calvin cycle in mesophyll cells. However, they are susceptible to photorespiration — when stomata close in hot, dry conditions, CO₂ levels inside the leaf drop while O₂ levels rise (due to ongoing light reactions). Under these conditions, rubisco fixes O₂ instead of CO₂, leading to wasteful photorespiration that can reduce photosynthetic efficiency by up to 25%.

C4 Plants | C4植物

C4 plants (such as maize, sugarcane, and sorghum) have evolved a spatial separation of carbon fixation. They possess a specialised leaf anatomy called Kranz anatomy, where bundle sheath cells are surrounded by mesophyll cells. The initial carbon fixation occurs in mesophyll cells, where CO₂ combines with phosphoenolpyruvate (PEP) — catalysed by the enzyme PEP carboxylase — to form the 4-carbon compound oxaloacetate. This is then converted to malate and transported to the bundle sheath cells, where it is decarboxylated to release CO₂. The released CO₂ enters the Calvin cycle as normal.

The key advantage is that PEP carboxylase has a much higher affinity for CO₂ than rubisco and does not bind O₂ at all. This means C4 plants can maintain high rates of photosynthesis even when stomata are partially closed, making them well-adapted to hot, dry environments. However, the C4 pathway requires additional ATP (5 ATP per CO₂ fixed vs 3 ATP in C3), making it less energy-efficient in cool, moist conditions.

CAM Plants | CAM植物

Crassulacean Acid Metabolism (CAM) plants (such as cacti, pineapples, and succulents) separate carbon fixation temporally rather than spatially. At night, when temperatures are lower and humidity is higher, stomata open and CO₂ is fixed into malate using PEP carboxylase. The malate is stored in vacuoles. During the day, stomata close to conserve water, and the stored malate is decarboxylated to release CO₂ for the Calvin cycle, which runs using ATP and NADPH from the light reactions.

This temporal separation allows CAM plants to survive in extremely arid environments where water conservation is paramount. The trade-off is slower growth rates due to the limited capacity for malate storage.

Practical Investigations | 实验探究

A-Level Biology courses place significant emphasis on practical skills, and photosynthesis is a rich topic for investigation. Here are two classic experiments you should be familiar with:

Investigating the Effect of Light Intensity on Photosynthesis | 探究光照强度对光合作用的影响

Using an aquatic plant such as Elodea (pondweed), the rate of photosynthesis can be measured by counting the number of oxygen bubbles produced per minute. The plant is placed in a beaker of water with sodium hydrogen carbonate (to provide CO₂), and a light source is placed at varying distances. As the distance decreases (light intensity increases), the rate of bubble production increases — up to the point where another factor (such as CO₂ concentration or temperature) becomes limiting.

Key practical considerations: Use a water bath as a heat filter between the lamp and the plant to prevent temperature from becoming a confounding variable. Cut the stem of the Elodea at an angle to ensure bubbles can escape freely. Measure the volume of gas produced using a gas syringe or an inverted measuring cylinder for more accurate results than simply counting bubbles.

Chromatography of Photosynthetic Pigments | 光合色素的色谱分析

Paper chromatography or thin-layer chromatography (TLC) can be used to separate the photosynthetic pigments from a leaf extract. The leaf is ground with propanone (acetone) using a pestle and mortar, and the extract is spotted onto chromatography paper. The paper is then placed in a solvent (such as petroleum ether and propanone mixture). As the solvent rises, it carries the pigments with it — each pigment travels at a different rate, producing distinct bands.

The Rf value (retention factor) for each pigment is calculated as:

Rf = distance travelled by pigment spot ÷ distance travelled by solvent front

Typically, from the origin upwards, the bands appear as: chlorophyll b (yellow-green, lowest Rf), chlorophyll a (blue-green), xanthophyll (yellow), and beta-carotene (orange, highest Rf).

Limiting Factors of Photosynthesis | 光合作用的限制因素

Understanding the factors that limit the rate of photosynthesis is a key A-Level concept and a frequent exam topic. The three primary limiting factors are:

• Light Intensity: As light intensity increases, the rate of photosynthesis increases proportionally — until another factor becomes limiting. At the light compensation point, the rate of photosynthesis equals the rate of respiration. Beyond the light saturation point, further increases in light intensity have no effect.
• Carbon Dioxide Concentration: CO₂ is the substrate for carbon fixation. At low CO₂ concentrations, rubisco cannot work at its maximum rate. In commercial greenhouses, CO₂ levels are often artificially elevated to boost crop yields.
• Temperature: Photosynthesis involves enzyme-catalysed reactions, including the action of rubisco. As temperature rises, the rate initially increases due to increased kinetic energy and enzyme activity. However, above a certain temperature (typically around 25–30°C for C3 plants), enzymes begin to denature, and the rate drops sharply. High temperatures also increase photorespiration (see below).

Photorespiration | 光呼吸

A level-appropriate detail: rubisco can also bind to oxygen instead of carbon dioxide in a process called photorespiration. When rubisco fixes O₂ instead of CO₂, RuBP is oxidised rather than carboxylated, producing one molecule of GP and one molecule of phosphoglycolate. The phosphoglycolate must be recycled through a wasteful process that consumes ATP and releases previously fixed CO₂. Photorespiration effectively reduces the efficiency of photosynthesis and is more likely to occur when temperatures are high (because the stomata close to conserve water, reducing CO₂ uptake) and when CO₂ concentrations are low relative to O₂.

Exam-Style Questions and Model Answers | 考试题型与标准答案

Question 1 (AQA-style): Describe the role of the thylakoid membrane in the light-dependent reactions of photosynthesis. (4 marks)

Model Answer: The thylakoid membrane provides a large surface area for the attachment of photosynthetic pigments, electron carriers, and ATP synthase enzymes (1). It contains photosystems I and II where light energy is absorbed by chlorophyll and used to excite electrons in photoionisation (1). The membrane holds electron carriers that form an electron transport chain, allowing energy from electrons to be used to pump protons from the stroma into the thylakoid space, creating a proton gradient (1). This proton gradient drives the synthesis of ATP via chemiosmosis through ATP synthase, also embedded in the thylakoid membrane (1).

Question 2 (Edexcel-style): Explain why the light-independent reactions of photosynthesis cannot continue indefinitely in the dark. (3 marks)

Model Answer: The light-independent reactions require ATP and reduced NADP, which are only produced during the light-dependent reactions (1). In the dark, the light-dependent reactions cannot take place, so the supply of ATP and reduced NADP would quickly be exhausted (1). Without these products, the Calvin cycle cannot reduce GP to GALP or regenerate RuBP, so the cycle grinds to a halt (1).

Question 3 (OCR-style): Explain the importance of the stroma in the light-independent reactions. (3 marks)

Model Answer: The stroma contains the enzyme rubisco, which catalyses the fixation of carbon dioxide by combining it with RuBP to form GP (1). It also contains all the other enzymes required for the Calvin cycle, including those needed to reduce GP to GALP and to regenerate RuBP (1). The stroma fluid surrounds the grana, placing it in close proximity to the thylakoid membranes where ATP and reduced NADP are produced, allowing these products to diffuse rapidly into the stroma (1).

Common Misconceptions and Pitfalls | 常见误区与陷阱

• “The light-independent reactions only occur at night.” This is false. They occur during the day as well — they are called “light-independent” because they do not directly require light, not because light inhibits them. In fact, they occur most actively during daylight when ATP and NADPH supplies are high.
• “Oxygen is produced in the Calvin cycle.” This is incorrect. Oxygen is produced only during the light-dependent reactions, specifically during the photolysis of water at PSII. The Calvin cycle produces no gaseous products.
• “Chlorophyll absorbs green light.” Actually, chlorophyll reflects green light (which is why plants appear green) and absorbs primarily red and blue-violet light. This is a common trap in multiple-choice questions.
• Confusing GP and GALP: GP (glycerate 3-phosphate) is the 3-carbon acid produced when CO₂ is fixed to RuBP. GALP (glyceraldehyde 3-phosphate) or TP (triose phosphate) is the 3-carbon sugar produced when GP is reduced. Knowing the difference is essential for accurate biochemical descriptions.
• “ATP is produced in the Calvin cycle.” No — ATP is consumed in the Calvin cycle. It is produced only in the light-dependent reactions.

Final Summary | 最后总结

Photosynthesis is a beautifully orchestrated two-stage process that converts light energy into chemical energy. The light-dependent reactions occur on the thylakoid membranes, using light energy to split water, generate ATP via chemiosmosis, and reduce NADP to NADPH. The light-independent reactions (Calvin cycle) occur in the stroma, using ATP and NADPH to fix CO₂ into glucose through the action of the enzyme rubisco.

光合作用是一个精心编排的两阶段过程，将光能转化为化学能。光反应发生在类囊体膜上，利用光能分解水，通过化学渗透产生ATP，并将NADP还原为NADPH。暗反应（卡尔文循环）发生在基质中，利用ATP和NADPH，通过rubisco酶的作用将CO₂固定为葡萄糖。

For A-Level success, focus on: (1) the precise locations of each process; (2) the role of membrane structure in creating proton gradients; (3) the specific names of key molecules and enzymes; (4) linking structure to function at every opportunity; and (5) practising the application of knowledge to unfamiliar scenarios — a skill that distinguishes A* candidates from the rest.

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