Topic 613: Photosynthesis | 课题613：光合作用

📚 Topic 613: Photosynthesis | 课题613：光合作用

Photosynthesis is the process by which green plants and some other organisms use light energy to convert carbon dioxide and water into glucose and oxygen. It is the ultimate source of food and energy for nearly all life on Earth.

光合作用是绿色植物和一些其他生物利用光能将二氧化碳和水转化为葡萄糖和氧气的过程。它是地球上几乎所有生命食物和能量的最终来源。

1. Definition and Importance | 定义与重要性

Photosynthesis takes place in the chloroplasts of plant cells, which contain the green pigment chlorophyll. Chlorophyll absorbs light energy, usually from the Sun, and uses it to drive a series of chemical reactions.

光合作用发生在植物细胞的叶绿体中，叶绿体含有绿色色素叶绿素。叶绿素吸收光能（通常来自太阳），并利用光能驱动一系列化学反应。

The glucose produced by photosynthesis is used by plants in several ways: it can be converted into starch for storage, used in respiration to release energy, or built into cellulose for cell walls. Oxygen is released as a waste product through the stomata.

光合作用产生的葡萄糖被植物以多种方式利用：可以转化为淀粉储存，用于呼吸作用释放能量，或构建为细胞壁中的纤维素。氧气作为副产物通过气孔释放。

It provides the starting material for all food chains and food webs.

它为所有食物链和食物网提供了起点物质。
It maintains atmospheric oxygen levels that higher organisms need to respire.

它维持了高等生物呼吸所需的大气氧气水平。
It removes carbon dioxide from the atmosphere, helping to regulate the greenhouse effect.

它从大气中移除二氧化碳，有助于调节温室效应。

2. The Overall Equation | 总反应方程式

The balanced chemical equation for photosynthesis is straightforward, but you must know both the word equation and the symbol equation.

光合作用的平衡化学方程式很直接，但你必须同时掌握文字方程式和符号方程式。

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Note that the reaction requires light energy and chlorophyll. Neither the energy nor the chlorophyll appears in the chemical equation because they are conditions or catalysts, not reactants.

注意该反应需要光能和叶绿素。光能和叶绿素都不出现在化学方程式中，因为它们只是反应条件或催化剂，不是反应物。

3. Light-Dependent and Light-Independent Stages | 光依赖与光不依赖阶段

In higher biology courses you will study photosynthesis as two main stages, but at IGCSE you need to understand the basic roles of light and enzymes.

在更高阶的生物课程中，光合作用分为两个主要阶段；但在IGCSE中，你只需要理解光和酶的基本作用。

The light-dependent stage requires light energy directly. It occurs in the thylakoid membranes of the chloroplast, where light splits water into oxygen, hydrogen ions and electrons.

光依赖阶段直接需要光能。它发生在叶绿体的类囊体膜上，在那里光将水分解为氧气、氢离子和电子。

The light-independent stage does not need light directly. It occurs in the stroma of the chloroplast, where carbon dioxide is combined with hydrogen ions to make glucose, using enzymes.

光不依赖阶段不直接需要光。它发生在叶绿体的基质中，在那里二氧化碳与氢离子结合生成葡萄糖，这个过程需要酶。

light-dependent stage: H₂O → ½O₂ + 2H⁺ + 2e⁻

光依赖阶段：H₂O → ½O₂ + 2H⁺ + 2e⁻

light-independent stage: CO₂ + 4H⁺ + 4e⁻ → (CH₂O) + H₂O

光不依赖阶段：CO₂ + 4H⁺ + 4e⁻ → (CH₂O) + H₂O

For IGCSE, it is enough to remember that light is needed to split water and produce oxygen, while carbon dioxide is fixed into carbohydrate using enzyme-controlled reactions.

就IGCSE而言，记住光是分解水并产生氧气所必需的，而二氧化碳则通过酶控制的反应固定为碳水化合物，这就足够了。

4. Limiting Factors | 限制因素

Photosynthesis is affected by three main limiting factors: light intensity, carbon dioxide concentration and temperature. A limiting factor is the one that is in shortest supply and therefore controls the rate of the process.

光合作用受到三个主要限制因素的影响：光照强度、二氧化碳浓度和温度。限制因素是最缺乏的那个，因此控制着反应速率。

When a factor is not limiting, increasing it will not increase the rate. If you increase all other factors, the limiting factor will eventually become the one that is still in short supply.

当某个因素不是限制因素时，增加它不会提高速率。如果你增加其他所有因素，最终限制因素将是仍然缺乏的那个因素。

Factor	Effect
Light intensity 光照强度	As light intensity increases, rate rises until another factor limits it. 随着光照强度增加，速率上升直到另一个因素限制它。
CO₂ concentration 二氧化碳浓度	Higher CO₂ levels increase rate, but only up to a saturation point. 较高CO₂水平提高速率，但只到饱和点为止。
Temperature 温度	Rate increases with temperature until enzymes denature above optimum (usually ~35-40°C). 速率随温度升高而增加，直到酶在超过最适温度（通常约35-40°C）时变性。

Rate of photosynthesis = f(light, CO₂, temperature)

光合作用速率 = f(光照、CO₂、温度)

5. Leaf Structure and Photosynthesis | 叶片结构与光合作用

The leaf is a specialised organ for photosynthesis. Its structure maximises light capture, gas exchange and water supply.

叶片是进行光合作用的特化器官。其结构最大化光捕获、气体交换和水分供应。

Epidermis is transparent, allowing light to pass through to the mesophyll.

表皮是透明的，允许光线穿透到达叶肉。
Palisade mesophyll cells are packed with chloroplasts, especially near the upper surface.

栅栏叶肉细胞充满叶绿体，特别是靠近上表面。
Spongy mesophyll has large air spaces for gas diffusion.

海绵叶肉具有大的气隙，用于气体扩散。
Stomata allow CO₂ in and O₂ out, regulated by guard cells.

气孔允许CO₂进入和O₂排出，由保卫细胞调节。
Xylem transports water; phloem transports sugars away.

木质部运输水分；韧皮部把糖运输出去。

These adaptations are frequently examined in multiple-choice and short-answer questions.

这些适应特征常出现在选择题和简答题中。

6. Testing for Starch in Leaves | 叶片中淀粉的检测

Glucose produced by photosynthesis is often stored as starch. To test whether a leaf has been photosynthesising, you can test for starch using iodine solution.

光合作用产生的葡萄糖通常以淀粉形式储存。要检测叶片是否进行了光合作用，可以用碘液测试淀粉。

The procedure is:

步骤如下：

Boil the leaf in water to kill it and break down cell membranes.

将叶片在沸水中煮沸以杀死细胞并破坏细胞膜。
Boil the leaf in ethanol to remove chlorophyll (this makes the leaf white).

将叶片在乙醇中煮沸以去除叶绿素（使叶片变白）。
Rinse with water to soften the leaf.

用水冲洗使叶片变软。
Add iodine solution. Blue-black colour indicates starch.

滴加碘液。蓝黑色表明有淀粉。

To show that light is needed, use a leaf that was partly covered with black paper. Only the exposed parts turn blue-black.

为了证明需要光，可以使用部分被黑纸遮盖的叶片。只有暴露部分变蓝黑色。

7. The Relationship Between Photosynthesis and Respiration | 光合作用与呼吸作用的关系

Photosynthesis and respiration are complementary processes. Photosynthesis stores energy by building glucose, while respiration releases energy by breaking glucose down.

光合作用和呼吸作用是互补的过程。光合作用通过构建葡萄糖储存能量，而呼吸作用通过分解葡萄糖释放能量。

Photosynthesis produces oxygen and glucose; respiration consumes them. Photosynthesis consumes CO₂ and water; respiration produces them.

光合作用产生氧气和葡萄糖；呼吸作用消耗它们。光合作用消耗CO₂和水；呼吸作用产生它们。

At night, plants carry out respiration only, so they take in oxygen and release CO₂. During the day, photosynthesis usually dominates, so the net gas exchange reverses.

在夜间，植物只进行呼吸作用，所以它们吸入氧气并释放CO₂。白天，光合作用通常占主导，因此净气体交换相反。

Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

光合作用：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

呼吸作用：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

8. Agricultural Applications | 农业应用

Farmers and horticulturists can increase photosynthesis rates to improve crop yields by controlling the limiting factors.

农民和园艺家可以通过控制限制因素来提高光合作用速率，从而增加作物产量。

Greenhouses allow sunlight in while maintaining a higher temperature than outside.

温室让阳光进入，同时保持比外部更高的温度。
Burning paraffin in a greenhouse raises CO₂ concentration and adds warmth.

在温室中燃烧石蜡可提高CO₂浓度并增加热量。
Artificial lighting can extend the hours of photosynthesis in winter.

人工照明可以延长冬季光合作用的时间。
Proper spacing and pruning ensure leaves do not shade one another.

合适的间距和修剪确保叶片不会互相遮阴。

These measures are cost/benefit decisions; farmers must weigh extra costs against the additional yield gained.

这些措施是成本/效益决策；农民必须权衡额外成本与增产收益。

9. Measuring the Rate of Photosynthesis | 测量光合作用速率

The rate of photosynthesis can be measured by following the uptake of CO₂, the release of O₂, or the production of biomass.

光合作用速率可以通过跟踪CO₂吸收、O₂释放或生物量产生来测量。

A common practical is counting oxygen bubbles from an aquatic plant like Elodea. The number of bubbles per minute gives a relative rate.

一个常见的实验是计数水生植物（如伊乐藻）释放的氧气气泡。每分钟的气泡数给出相对速率。

You can change light intensity by moving a lamp closer or further, or changing the lamp wattage. Remember that light intensity follows the inverse square law:

可以通过移动台灯靠近或远离，或改变台灯功率来改变光照强度。记住光照强度遵循平方反比定律：

Light intensity ∝ 1 / distance²

光照强度 ∝ 1 / 距离²

When using the bubble method, keep temperature constant to avoid it becoming a second uncontrolled variable.

使用气泡法时，保持温度恒定，避免温度成为第二个不受控制的变量。

10. Common Exam Mistakes and Tips | 常见考试错误与提示

Students often lose marks in photosynthesis questions due to incorrect terminology or missing conditions. Here are critical points.

学生在光合作用题中常因术语错误或漏写条件而失分。以下是关键点。

Write the full word equation, not just “plants make glucose”.

写出完整的文字方程式，而不仅仅是”植物制造葡萄糖”。
State that chlorophyll is essential, but is not used up.

说明叶绿素是必需的，但不会被消耗。
Use ‘limiting factor’ correctly: the factor that is at the lowest level relative to need.

正确使用”限制因素”：相对于需求处于最低水平的因素。
When describing experiments, mention control variables such as temperature, pH and type of plant.

描述实验时，提及控制变量如温度、pH和植物类型。
Do not confuse the direction of gas exchange in light vs. dark conditions.

不要混淆光照与黑暗条件下气体交换的方向。

Practise drawing graphs of rate against each factor, and label the plateau where another factor becomes limiting.

练习绘制速率随各因素变化的曲线图，并标注出另一因素变为限制因素时的平台期。

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Topic 613: Photosynthesis | 课题613：光合作用