Photosynthesis and Limiting Factors | 光合作用与限制因素

📚 Photosynthesis and Limiting Factors | 光合作用与限制因素

Photosynthesis is one of the most important biological processes on Earth. It is the way in which green plants, algae and some bacteria convert light energy into chemical energy, producing glucose and oxygen. In Edexcel IGCSE Science, you need to know the equation, the limiting factors and how to investigate the rate of photosynthesis.

光合作用是地球上最重要的生物过程之一。绿色植物、藻类和某些细菌通过光合作用将光能转化为化学能，生成葡萄糖和氧气。在爱德思 IGCSE 科学考试中，你需要掌握光合作用的方程式、限制因素以及如何探究光合作用的速率。

1. What Is Photosynthesis? | 什么是光合作用？

Photosynthesis is an endothermic reaction because it takes in energy from the surroundings in the form of light. It happens in the chloroplasts of plant cells, which contain the green pigment chlorophyll. Chlorophyll absorbs light energy, mainly red and blue light, and uses it to combine carbon dioxide and water.

光合作用是一种吸热反应，因为它以光的形式从周围环境吸收能量。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。叶绿素主要吸收红光和蓝光，并利用这些光能把二氧化碳和水合成有机物。

The word equation and the chemical equation are:

光合作用的文字方程式和化学方程式如下：

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Glucose is a simple sugar that plants can use immediately for respiration or store as starch. Oxygen is released as a waste product through the stomata.

葡萄糖是一种单糖，植物可以立即用于呼吸作用，也可以储存为淀粉。氧气作为副产物通过气孔释放到空气中。

2. Leaf Structure and Adaptations | 叶片结构与适应性

Leaves are the main photosynthetic organs of a plant. Their structure is highly adapted to capture light and exchange gases efficiently.

叶片是植物进行光合作用的主要器官。其结构高度适应于高效捕获光线和交换气体。

Part / 结构	Adaptation / 适应性
Waxy cuticle 角质层	Reduces water loss 减少水分蒸发
Upper epidermis 上表皮	Transparent so light passes through 透明，利于光线透过
Palisade mesophyll 栅栏组织	Tightly packed cells with many chloroplasts 细胞排列紧密，叶绿体多
Spongy mesophyll 海绵组织	Large air spaces for gas diffusion 气室大，利于气体扩散
Stomata 气孔	Allow CO₂ in and O₂ out 允许 CO₂ 进入、O₂ 排出

The large surface area of a leaf and the thin shape mean that carbon dioxide can diffuse quickly to the photosynthesising cells.

叶片表面积大、形状薄，这意味着二氧化碳能够快速扩散到进行光合作用的细胞中。

3. Light Intensity as a Limiting Factor | 光照强度作为限制因素

In Edexcel IGCSE, a limiting factor is a condition that is in short supply and therefore prevents the rate of a reaction from increasing further. When light intensity is low, increasing it will increase the rate of photosynthesis because more light energy is available for chlorophyll to absorb.

在爱德思 IGCSE 中，限制因素是指供应不足、从而阻止反应速率进一步提高的条件。当光照强度较低时，提高光照强度会加快光合作用速率，因为叶绿素能够吸收更多的光能。

However, beyond a certain point, light intensity is no longer the limiting factor. The graph reaches a plateau: adding more light does not increase the rate because another factor, such as carbon dioxide concentration or temperature, has become limiting.

然而，超过某一数值后，光照强度不再是限制因素。曲线趋于平缓：继续增加光照并不会提高速率，因为另一个因素（如二氧化碳浓度或温度）已成为新的限制因素。

Rate of photosynthesis ∝ light intensity (at low intensity)

光合作用速率 ∝ 光照强度（在低光照下）

4. Carbon Dioxide Concentration | 二氧化碳浓度

Carbon dioxide is one of the reactants of photosynthesis. In normal air, the CO₂ concentration is only about 0.04 %, so it is often the limiting factor in nature.

二氧化碳是光合作用的反应物之一。在正常空气中，CO₂ 浓度仅为 0.04% 左右，因此在自然界中它常常是限制因素。

If CO₂ concentration is increased while light and temperature stay the same, the rate of photosynthesis rises until a new plateau is reached. At that plateau, light or temperature has become the limiting factor. Farmers often use this idea in glasshouses by burning paraffin or releasing bottled CO₂.

如果光照和温度不变而提高 CO₂ 浓度，光合作用速率会上升，直到达到新的平台。在平台上，光照或温度成为限制因素。农民在温室中常利用这一原理，通过燃烧石蜡或释放瓶装 CO₂ 来提高产量。

5. Temperature | 温度

Photosynthesis is controlled by enzymes, so temperature has a strong effect on its rate. As temperature rises from cold conditions, the rate increases because particles have more kinetic energy and enzyme-substrate collisions happen more often.

光合作用由酶控制，因此温度对其速率影响很大。从低温开始升温时，速率加快，因为粒子动能增大，酶与底物的碰撞更加频繁。

At very high temperatures, the enzymes that control photosynthesis become denatured. The shape of the active site changes permanently, the rate drops sharply, and the plant may be damaged. For most plants, the optimum temperature is around 25–35 °C.

在很高温度下，控制光合作用的酶会变性。活性位点的形状发生不可逆改变，速率急剧下降，植物可能受到损伤。对大多数植物而言，最适温度约为 25–35 °C。

6. How Limiting Factors Interact | 限制因素如何相互作用

Limiting factors never work alone. If you increase light intensity, the rate may increase until CO₂ or temperature becomes the limiting factor. Then you must increase that factor too, or the rate will not rise further.

限制因素从不是单独起作用的。如果你提高光照强度，速率可能会上升，直到 CO₂ 或温度成为限制因素。这时必须同时提高那个因素，否则速率不会再上升。

The table below shows a typical exam scenario with pondweed:

下表展示了一个典型的考试场景（使用金鱼藻）：

Conditions / 条件	Result / 结果
Low light, low CO₂ / 弱光、低 CO₂	Limiting factor: light / 限制因素：光
High light, low CO₂ / 强光、低 CO₂	Limiting factor: CO₂ / 限制因素：CO₂
High light, high CO₂, low temperature / 强光、高 CO₂、低温	Limiting factor: temperature / 限制因素：温度
High light, high CO₂, optimum temperature / 强光、高 CO₂、最适温度	Rate is maximum / 速率最大

7. What Happens to the Glucose? | 葡萄糖的用途

The glucose produced by photosynthesis is used in several ways. Understanding these uses is a common multiple-choice and short-answer question.

光合作用产生的葡萄糖有多种用途。理解这些用途是选择题和简答题中的常见考点。

Respiration: glucose is broken down to release energy for cell activities.

呼吸作用：葡萄糖被分解，为细胞活动释放能量。
Starch: insoluble starch is stored in roots, stems and leaves.

淀粉：不溶性的淀粉储存在根、茎和叶中。
Cellulose: glucose is converted into cellulose for cell walls.

纤维素：葡萄糖转化为纤维素，用于构建细胞壁。
Amino acids and proteins: glucose combines with mineral ions, especially nitrate ions absorbed from the soil, to make amino acids and proteins.

氨基酸和蛋白质：葡萄糖与从土壤中吸收的矿质离子（尤其是硝酸根离子）结合，生成氨基酸和蛋白质。
Lipids: fats and oils are stored in seeds and fruits as a compact energy store.

脂质：脂肪和油以紧凑的能量形式储存在种子和果实中。

8. Investigating the Rate of Photosynthesis | 探究光合作用速率

The classic Edexcel IGCSE practical uses pondweed, such as Elodea or Cabomba. The pondweed is placed in water with dissolved sodium hydrogencarbonate to supply CO₂, and a lamp

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Photosynthesis and Limiting Factors | 光合作用与限制因素