Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is one of the most essential biological processes on Earth. It is the process by which green plants, algae, and certain bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water.

光合作用是地球上最重要的生物过程之一。它是绿色植物、藻类和某些细菌将光能转化为化学能的过程，利用二氧化碳和水生成葡萄糖和氧气。

1. What is Photosynthesis? | 什么是光合作用？

Photosynthesis is an endothermic (energy-absorbing) reaction that takes place in the chloroplasts of plant cells, particularly in the palisade and spongy mesophyll layers of the leaf. The chloroplasts contain a green pigment called chlorophyll, which absorbs red and blue wavelengths of light and reflects green light, giving plants their characteristic colour.

光合作用是一种吸热反应，发生在植物细胞的叶绿体中，尤其是在叶片栅栏组织和海绵组织的叶肉层中。叶绿体含有一种称为叶绿素的绿色色素，它吸收红光和蓝光波长的光，反射绿光，使植物呈现特有的颜色。

Light energy is trapped by chlorophyll and used to split water molecules into hydrogen and oxygen through a process called photolysis. The hydrogen combines with carbon dioxide to form glucose, while oxygen is released as a by-product through the stomata.

叶绿素捕获光能，并通过一种称为光解的过程将水分子分解为氢和氧。氢与二氧化碳结合生成葡萄糖，而氧气则作为副产物通过气孔释放到大气中。

2. The Word Equation | 文字方程式

The word equation summarises the raw materials and products of photosynthesis:

光合作用的文字方程式概括了反应物和生成物：

Carbon dioxide + Water → Glucose + Oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

This process requires light energy, which is absorbed by chlorophyll, and it occurs most rapidly during daylight hours. In the Edexcel IGCSE specification, you must be able to write both the word equation and the balanced chemical equation from memory.

该过程需要光能，光能被叶绿素吸收，在白天光照充足时进行得最快。在 Edexcel IGCSE 考试大纲中，你必须能够凭记忆写出文字方程式和平衡化学方程式。

3. The Chemical Equation | 化学方程式

The balanced chemical equation for photosynthesis is:

光合作用的平衡化学方程式为：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

(in the presence of light energy and chlorophyll / 在光能和叶绿素的存在下)

This equation shows that six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen. The reaction is endothermic, absorbing light energy from the surroundings and storing it as chemical energy in glucose.

该方程式表明，六个二氧化碳分子与六个水分子反应，生成一个葡萄糖分子和六个氧分子。该反应是吸热反应，从周围环境中吸收光能，并将其以化学能的形式储存在葡萄糖中。

In examinations, take care to balance the equation correctly and use accurate chemical formulae. Carbon dioxide is CO₂, water is H₂O, glucose is C₆H₁₂O₆, and oxygen is O₂ — a frequent source of mark loss is writing O rather than O₂.

在考试中，请注意正确配平方程式并使用准确的化学式。二氧化碳是 CO₂，水是 H₂O，葡萄糖是 C₆H₁₂O₆，氧气是 O₂ —— 一个常见的失分点是写成 O 而不是 O₂。

4. The Role of Chlorophyll and Light | 叶绿素和光的作用

Chlorophyll is located in the thylakoid membranes of the chloroplasts. It acts as a pigment that absorbs light energy and transfers it to drive the light-dependent reactions of photosynthesis. Without chlorophyll, photosynthesis cannot occur, even in bright light.

叶绿素位于叶绿体的类囊体膜中。它作为一种色素吸收光能，并将光能传递以驱动光合作用的光依赖反应。没有叶绿素，即使在强光下光合作用也无法发生。

The absorption spectrum of chlorophyll shows that it absorbs light most strongly in the blue-violet (around 430–450 nm) and red (around 640–680 nm) regions of the electromagnetic spectrum. Green light (around 500–560 nm) is largely transmitted and reflected, which is why leaves appear green.

叶绿素的吸收光谱显示，它在电磁波谱的蓝紫光区域（约 430–450 nm）和红光区域（约 640–680 nm）吸收最强。绿光（约 500–560 nm）大部分被透射和反射，这就是叶子呈现绿色的原因。

If a plant is kept in darkness, photosynthesis stops because light energy is essential for the photolysis of water. However, the plant can continue to respire, using stored glucose as an energy source.

如果植物被放在黑暗环境中，光合作用会停止，因为光能对水的光解至关重要。然而，植物可以继续呼吸，利用储存的葡萄糖作为能量来源。

5. Factors Affecting the Rate of Photosynthesis | 影响光合作用速率的因素

The rate of photosynthesis is influenced by three key external factors: light intensity, carbon dioxide concentration, and temperature. Each of these factors can act as a limiting factor depending on the conditions.

光合作用速率受三个关键外部因素影响：光照强度、二氧化碳浓度和温度。根据条件不同，这些因素中的每一个都可能成为限制因子。

Factor	Effect on Rate
Light intensity	Rate increases as light intensity increases, until another factor becomes limiting.
CO₂ concentration	Rate increases with higher CO₂ levels, up to a saturation point.
Temperature	Rate increases with temperature up to the optimum (~25–35°C), then falls sharply as enzymes denature.

因素	对速率的影响
光照强度	光强度增加时速率增大，直到另一因素成为限制因子。
CO₂ 浓度	CO₂ 浓度升高时速率增大，直到达到饱和点。
温度	在达到最适温度（约 25–35°C）前，速率随温度升高而增大，之后因酶变性而急剧下降。

Beyond the optimum temperature, the rate declines sharply because enzymes such as Rubisco begin to denature. A common examination task is to draw and interpret graphs of rate against each factor, identifying the plateau regions.

超过最适温度后，由于 Rubisco 等酶开始变性，速率急剧下降。一个常见的考试任务是绘制并解读速率随各因素变化的曲线图，识别平台期。

6. Limiting Factors | 限制因子

A limiting factor is the factor that is in shortest supply and therefore restricts the rate of photosynthesis. According to Blackman’s Law of Limiting Factors, when several factors influence a process, the rate is limited by the factor that is most below its optimum level.

限制因子是指供应最不足、从而限制光合作用速率的因素。根据布莱克曼限制因子定律，当多个因素影响某一过程时，速率受最低于其最适水平的因素限制。

For example, if light intensity is low, increasing carbon dioxide concentration will not increase the rate of photosynthesis because light is the limiting factor. In exam questions, you may be asked to identify the limiting factor from a graph when the curve plateaus — the answer is the factor not being increased on the x-axis.

例如，如果光照强度较低，增加二氧化碳浓度不会提高光合作用速率，因为光是限制因子。在考试题目中，你可能会被要求在曲线趋于平缓时从图表中识别限制因子——答案就是 x 轴上没有被增加的那个因素。

This concept is crucial for agricultural applications: glasshouse growers commonly control light, CO₂, and temperature simultaneously to ensure no single factor slows crop growth. Adding CO₂ generators or using paraffin lamps in greenhouses raises both CO₂ concentration and temperature, boosting photosynthesis and yield.

这一概念对农业应用至关重要：温室种植者通常会同时控制光照、CO₂ 和温度，确保没有任何单一因素减缓作物生长。在温室中增加 CO₂ 发生器或使用石蜡灯，可同时提高 CO₂ 浓度和温度，从而促进光合作用并提高产量。

7. Investigating Photosynthesis | 光合作用实验探究

A common experiment to investigate photosynthesis uses an aquatic plant such as Elodea (pondweed). The number of oxygen bubbles released per minute is counted at different distances from a light source, allowing the rate of photosynthesis to be calculated.

一个常见的光合作用实验使用水生植物，如黑藻（伊乐藻）。在不同距离的光源下，统计每分钟释放的氧气气泡数量，从而计算光合作用速率。

The experimental setup involves placing the pondweed under a bright light, adding sodium hydrogen carbonate (bicarbonate) to ensure a constant and sufficient supply of CO₂, and maintaining a constant temperature using a water bath to avoid temperature as a variable.

实验装置包括将水草置于强光下，加入碳酸氢钠以确保 CO₂ 供应充足且恒定，并使用水浴维持恒温，以排除温度这一变量。

The distance between the lamp and the pondweed is varied. According to the inverse square law, light intensity is inversely proportional to the square of the distance, so intensity ∝ 1/d². Halving the distance quadruples the light intensity.

改变光源与水草之间的距离。根据平方反比定律，光强度与距离的平方成反比，即强度 ∝ 1/d²。距离减半会使光强度增加到原来的四倍。

More accurate results can be obtained by collecting the gas produced and testing it with a glowing splint — the splint relights, confirming the gas is oxygen. Alternatively, a data logger with an oxygen sensor can measure the oxygen concentration continuously over time.

更准确的结果可以通过收集产生的气体并用带火星的木条测试来获得——木条复燃，证明该气体是氧气。或者，使用带氧气传感器的数据记录器可以持续测量氧浓度随时间的变化。

To test whether light is required for photosynthesis, you can destarch a plant by keeping it in the dark for 24 hours, then cover part of a leaf with black paper, expose the plant to light, and test the leaf with iodine solution. The uncovered areas turn blue-black, confirming starch is produced only where light is available.

为测试光合作用是否需要光，可先将植物置于黑暗中 24 小时以消耗淀粉，然后用黑纸遮盖叶片的一部分，将植物置于光下，最后用碘液测试叶片。未遮盖的区域呈蓝黑色，证明只有光照到的区域才产生了淀粉。

8. Uses of Glucose | 葡萄糖的用途

Glucose produced during photosynthesis is used by plants in several important ways:

光合作用产生的葡萄糖在植物体内有多种重要用途：

Respiration: glucose is broken down in respiration to release energy for cellular activities such as active transport and protein synthesis.
Starch storage: glucose is converted to starch for storage in leaves, roots, stems, and seeds. Starch is insoluble, so it does not affect water potential.
Cellulose: glucose molecules are polymerised to form cellulose, which strengthens plant cell walls and provides structural support.
Amino acids and proteins: glucose combines with nitrate ions absorbed from the soil to form amino acids, which are then assembled into proteins.
Lipids: glucose is converted into lipids (oils and fats) for long-term energy storage, especially in seeds.

呼吸作用：葡萄糖在呼吸作用中分解，释放能量供细胞活动使用，如主动运输和蛋白质合成。
淀粉储存：葡萄糖被转化为淀粉储存在叶片、根、茎和种子中。淀粉不溶于水，因此不会影响水势。
纤维素：葡萄糖分子聚合形成纤维素，增强植物细胞壁并提供结构支持。
氨基酸和蛋白质：葡萄糖与从土壤中吸收的硝酸根离子结合形成氨基酸，再进一步组装成蛋白质。
脂质：葡萄糖被转化为脂质（油脂和脂肪），用于长期能量储存，尤其是在种子中。

Note: glucose is soluble and would draw water into cells by osmosis if left unmodified. Starch, by contrast, is osmotically inactive, making it an ideal storage molecule.

注意：葡萄糖是可溶的，如果不加以转化，会通过渗透作用将水分吸入细胞。相比之下，淀粉不具有渗透活性，因此是一种理想的储存分子。

9. Importance of Photosynthesis | 光合作用的重要性

Photosynthesis is fundamental to life on Earth for several reasons. First, it produces

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Photosynthesis | 光合作用