Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is one of the most important biological processes on Earth. In the Edexcel IGCSE Science specification, you need to understand its word and symbol equations, the role of chlorophyll, 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。在 Edexcel IGCSE 科学考纲中，你需要掌握其文字方程式和符号方程式、叶绿素的作用、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 the process by which green plants and some other organisms use light energy to convert carbon dioxide and water into glucose and oxygen. The chemical energy stored in glucose is then available for plant respiration or for other organisms that eat the plant.

光合作用是绿色植物和某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。葡萄糖中储存的化学能随后可供植物呼吸作用使用，或被取食植物的其他生物利用。

This process takes place mainly in the leaves of green plants, where the mesophyll cells contain many chloroplasts. The overall reaction is endothermic, meaning it absorbs light energy from the environment.

该过程主要发生在绿色植物的叶片中，其中叶肉细胞含有许多叶绿体。总反应是吸热的，意味着它从环境中吸收光能。

2. The Equations for Photosynthesis | 光合作用的方程式

The word equation summarises the reactants and products:

文字方程式概括了反应物和产物：

Carbon dioxide + Water → Glucose + Oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

The balanced symbol equation is:

平衡的符号方程式是：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

You should be able to write both equations from memory. In the symbol equation, glucose is C₆H₁₂O₆ and oxygen is produced as O₂. The equation is balanced because there are 18 oxygen atoms on each side, 6 carbon atoms, and 12 hydrogen atoms.

你应该能够凭记忆写出这两个方程式。在符号方程式中，葡萄糖是 C₆H₁₂O₆，生成的氧气是 O₂。方程式是平衡的，因为两边各有 18 个氧原子、6 个碳原子和 12 个氢原子。

3. Chloroplasts and Chlorophyll | 叶绿体与叶绿素

Chloroplasts are organelles found in plant cells, especially in the palisade mesophyll of leaves. Each chloroplast contains green pigments called chlorophyll, which are essential for capturing light energy.

叶绿体是植物细胞中的细胞器，尤其存在于叶片栅栏组织中。每个叶绿体都含有称为叶绿素的绿色色素，它对于捕获光能至关重要。

Chlorophyll absorbs light mainly in the blue and red parts of the visible spectrum, while reflecting green light, which is why leaves appear green. The light energy absorbed is used to split water molecules (photolysis) and to generate ATP and reduced NADP for the Calvin cycle.

叶绿素主要吸收可见光谱中蓝光和红光的能量，同时反射绿光，因此叶子呈现绿色。吸收的光能用于分解水分子（光解），并产生 ATP 和还原性 NADP 供卡尔文循环使用。

In IGCSE-level work, you do not need to know the detailed light-dependent and light-independent stages, but you should know that chlorophyll is the site of light absorption and is located in the chloroplast membranes (thylakoids).

在 IGCSE 阶段，你不需要了解光反应和暗反应的详细步骤，但你应该知道叶绿素是吸收光能的部位，位于叶绿体膜（类囊体）上。

4. Raw Materials Required | 光合作用所需的原料

Photosynthesis requires three essential inputs: carbon dioxide, water, and light energy. Carbon dioxide enters the leaf through small pores called stomata, while water is absorbed by root hairs and transported to the leaves via the xylem.

光合作用需要三种基本输入：二氧化碳、水和光能。二氧化碳通过叶片上称为气孔的小孔进入，而水由根毛吸收，并通过木质部运输到叶片。

Raw material	Source	Role
Carbon dioxide	Air through stomata	Carbon source for glucose
Water	Soil via root hairs	Provides hydrogen and releases oxygen
Light energy	Sun / artificial lamps	Drives the reaction

Chlorophyll is also a raw material in the sense that it is necessary as a catalyst, but it is not consumed in the reaction. Minerals such as magnesium are needed to make chlorophyll, so magnesium deficiency leads to yellow leaves and reduced photosynthesis.

从某种意义上说，叶绿素也是原料，因为它作为催化剂是必需的，但在反应中不被消耗。镁等矿物质是制造叶绿素所必需的，因此缺镁会导致叶子发黄并降低光合作用。

5. Products of Photosynthesis | 光合作用的产物

The main product is glucose, a simple sugar that can be used immediately in respiration or converted into starch for storage. The other product is oxygen, which is released as a waste product through the stomata.

主要产物是葡萄糖，一种单糖，可以立即用于呼吸作用，或转化为淀粉储存。另一种产物是氧气，作为一种废物通过气孔释放。

Glucose can also be used to make sucrose for transport, cellulose for cell walls, amino acids when combined with nitrate ions, and lipids for cell membranes. These conversions are important in understanding how plants are able to grow and develop.

葡萄糖还可以用于制造运输用的蔗糖、细胞壁用的纤维素、与硝酸根离子结合时生成氨基酸，以及细胞膜用的脂质。这些转化对于理解植物如何生长和发育很重要。

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

The rate of photosynthesis is affected by several environmental factors. The three main ones are light intensity, carbon dioxide concentration, and temperature. If any of these is in short supply, it can limit the rate, even if the other conditions are ideal.

光合作用的速率受多种环境因素影响。最主要的三种是光强度、二氧化碳浓度和温度。如果其中任何一个供应不足，即使其他条件理想，也会限制速率。

This concept is known as the law of limiting factors. For example, on a cloudy day, light intensity is the limiting factor; at night, photosynthesis stops almost completely. If light is plentiful but carbon dioxide is low, then CO₂ concentration becomes the limiting factor.

这个概念称为“限制因素定律”。例如，在多云天气，光强度是限制因素；在夜间，光合作用几乎完全停止。如果光充足但二氧化碳低，则二氧化碳浓度成为限制因素。

7. Light Intensity as a Limiting Factor | 光强度作为限制因素

As light intensity increases, the rate of photosynthesis increases proportionally at first. This is because more light provides more energy for the light-dependent reactions, producing more ATP and reduced NADP.

随着光强度增加，光合作用速率起初成正比增加。这是因为更多的光为光反应提供了更多能量，产生更多的 ATP 和还原性 NADP。

However, beyond a certain point, the rate plateaus because another factor has become limiting, such as temperature or carbon dioxide concentration. A typical graph shows a steep rise and then a horizontal line.

然而，超过某一点后，速率趋于平稳，因为另一个因素（如温度或二氧化碳浓度）成为限制因素。典型图形显示先急剧上升，然后水平线。

Rate ∝ Light intensity (until saturation)

速率 ∝ 光强度（直到饱和）

In experiments, changing the distance between a lamp and a plant changes light intensity according to the inverse square law: intensity ∝ 1/d². Doubling the distance makes the intensity four times smaller.

在实验中，改变灯与植物之间的距离会按照平方反比定律改变光强度：强度 ∝ 1/d²。距离加倍会使强度减小为原来的四分之一。

8. Carbon Dioxide Concentration and Temperature | 二氧化碳浓度与温度

Carbon dioxide is one of the raw materials, so increasing its concentration will increase the rate of photosynthesis, but only up to a saturation point. In nature, atmospheric CO₂ is about 0.03–0.04%, so it is often a limiting factor.

二氧化碳是原料之一，因此增加其浓度会提高光合作用速率，但只能达到饱和点。自然大气中的 CO₂ 约为 0.03%–0.04%，因此它常常是限制因素。

Temperature affects the enzymes involved in photosynthesis. As temperature rises from low values, the rate increases because the molecules move faster and more collisions occur. The optimum temperature is usually around 25–35°C for many plants.

温度影响光合作用所涉及的酶。当温度从较低值升高时，速率增加，因为分子运动加快，碰撞次数增多。许多植物的最适温度通常在 25–35°C 左右。

Above the optimum, the rate rapidly declines because enzymes begin to denature. Denaturation changes the shape of the active site and permanently stops the enzyme from working. This is why photosynthesis stops in very hot conditions.

超过最适温度后，速率迅速下降，因为酶开始变性。变性改变活性位点的形状，并永久性地使酶停止工作。这就是为什么在非常炎热的条件下光合作用会停止的原因。

9. Investigating Photosynthesis: The Bubbling Pondweed Experiment | 探究光合作用：金鱼藻冒泡实验

A common IGCSE practical uses the water plant Elodea (Canadian pondweed). As it photosynthesises, it produces oxygen bubbles, which can be counted to estimate the rate of photosynthesis.

一个常见的 IGCSE 实验使用水生植物加拿大伊乐藻（金鱼藻）。它在光合作用时产生氧气气泡，可通过计数气泡来估算光合作用速率。

Place a piece of pondweed in a beaker of water with a thermometer.
中文：将一段金鱼藻放入装有水和温度计的烧杯中。
Use a lamp at a fixed distance and count bubbles per minute.
中文：使用距离固定的灯，计数每分钟产生的气泡数。
Change the distance of the lamp and repeat the count.
中文：改变灯的距离并重复计数。
Repeat under different light intensities or CO₂ concentrations (adding sodium hydrogencarbonate).
中文：在不同光强度或二氧化碳浓度（加入碳酸氢钠）下重复。

Bubble counting is a simple method, but it can be inaccurate because bubbles vary in size. A more accurate method is to collect the gas in a capillary tube and measure the volume over a fixed time, or use a gas syringe.

气泡计数是简单的方法，但可能不准确，因为气泡大小不一。更准确的方法是收集气体到毛细管中并在固定时间内测量体积，或使用气体注射器。

10. Photosynthesis and Respiration | 光合作用与呼吸作用

Photosynthesis and aerobic respiration are complementary processes. Photosynthesis stores energy by building glucose, while respiration releases energy by breaking glucose down. The products of one are the reactants of the other.

光合作用和有氧呼吸是互补的过程。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。一者的产物是另一者的反应物。

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

光合作用：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

有氧呼吸：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

During the day, plants photosynthesise faster than they respire, so they take in carbon dioxide and release oxygen. At night, photosynthesis stops, and only respiration occurs, so plants take in oxygen and release carbon dioxide.

白天，植物光合作用快于呼吸作用，因此吸收二氧化碳并释放氧气。夜间，光合作用停止，只有呼吸作用发生，因此植物吸收氧气并释放二氧化碳。

11. Applications in Agriculture | 光合作用在农业中的应用

Farmers and greenhouse growers maximise photosynthesis to increase crop yield. They can control several limiting factors in a greenhouse environment.

农民和温室种植者通过最大限度地促进光合作用来提高作物产量。他们可以在温室环境中控制多个限制因素。

Use artificial lighting (often LED) to extend daylight hours and increase light intensity.
中文：使用人工光照（通常为 LED）延长日照时间并增加光强度。
Add carbon dioxide generators (e.g., paraffin burners) to raise CO₂ concentration.
中文：添加二氧化碳发生器（例如石蜡燃烧器）以提高二氧化碳浓度。
Use heaters to keep the greenhouse at an optimal temperature, without overheating.
中文：使用加热器维持温室处于最适温度，同时避免过热。
These measures are cost-effective because the increase in yield outweighs the energy costs.
中文：这些措施具有成本效益，因为产量增加超过了能源成本。

In addition, ensuring that plants receive enough mineral ions, especially magnesium and nitrate, helps maintain healthy chlorophyll and protein synthesis, further supporting photosynthesis.

此外，确保植物获得足够的矿质离子，特别是镁和硝酸盐，有助于维持健康的叶绿素和蛋白质合成，进一步支持光合作用。

12. Key Points for Revision | 复习要点总结

For the Edexcel IGCSE exam, remember the following:

针对 Edexcel IGCSE 考试，请记住以下要点：

Photosynthesis converts light energy to chemical energy in glucose.
中文：光合作用将光能转化为葡萄糖中的化学能。
The word and symbol equations must be memorised: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
中文：必须记住文字方程式和符号方程式：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。
Chlorophyll in chloroplasts absorbs light, mainly red and blue light.
中文：叶绿体中的叶绿素吸收光，主要是红光和蓝光。
Light intensity, CO₂ concentration and temperature are the main limiting factors.
中文：光强度、二氧化碳浓度和温度是主要限制因素。
Graphs show a plateau when a factor is no longer limiting.
中文：当一个因素不再成为限制因素时，图形呈现平台。
Use the inverse square law when relating distance to light intensity.
中文：将距离与光强度关联时，使用平方反比定律。
Pondweed experiments are used to measure oxygen production and estimate rates.
中文：金鱼藻实验用于测量氧气产生量并估算速率。

Make sure you can interpret graphs and experimental data, and that you can explain how greenhouses control limiting factors. Practice writing the equations and predicting what will happen to the rate when conditions change.

确保你能解读图表和实验数据，并能解释温室如何控制限制因素。练习书写方程式，并预测条件改变时速率会如何变化。

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Photosynthesis | 光合作用