Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the process by which green plants, algae and some bacteria use sunlight to convert carbon dioxide and water into glucose and oxygen. It is the fundamental biological reaction that supplies energy to almost all life on Earth.

光合作用是绿色植物、藻类和某些细菌利用阳光，将二氧化碳和水转化为葡萄糖和氧气的过程。这一基本的生物化学反应为地球上几乎所有生命提供了能量来源。

1. What Is Photosynthesis? | 什么是光合作用？

Photosynthesis is an endothermic chemical reaction in which light energy is absorbed by chlorophyll and transformed into chemical energy stored in glucose. The word ‘photosynthesis’ comes from ‘photo’ meaning light and ‘synthesis’ meaning making.

光合作用是一个吸热的化学反应，叶绿素吸收光能并将其转化为储存在葡萄糖中的化学能。’photosynthesis’一词源自希腊语，’photo’意为光，’synthesis’意为制造。

The overall process can be summarised as the conversion of inorganic raw materials (carbon dioxide and water) into an organic product (glucose), with oxygen released as a waste product.

此过程总体上可将无机原料（二氧化碳和水）转化为有机产物（葡萄糖），并释放出副产物氧气。

2. The Word Equation | 文字方程式

The word equation for photosynthesis shows the reactants on the left and the products on the right:

光合作用的文字方程式左侧为反应物，右侧为产物：

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

This equation is essential in IGCSE exams. Make sure you can write it exactly as above, including the arrow pointing to the right.

这个方程式在IGCSE考试中非常重要，请务必准确写出，包括指向右侧的箭头。

3. The Balanced Chemical Equation | 化学方程式

The balanced symbol equation provides the same information in a quantitative form. For photosynthesis in a typical plant:

配平的化学方程式以定量形式表达相同的信息。对于典型植物中的光合作用：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Here, carbon dioxide (CO₂) and water (H₂O) in the presence of light and chlorophyll produce glucose (C₆H₁₂O₆) and oxygen (O₂). The coefficients must be balanced: six molecules of CO₂ and six molecules of H₂O are needed to make one molecule of glucose and six molecules of oxygen.

在光照和叶绿素的存在下，二氧化碳（CO₂）和水（H₂O）生成葡萄糖（C₆H₁₂O₆）和氧气（O₂）。化学计量数必须配平：需要6个二氧化碳分子和6个水分子，才能生成1个葡萄糖分子和6个氧气分子。

4. The Role of Chlorophyll | 叶绿素的作用

Chlorophyll is a green pigment found in chloroplasts, mainly in the cells of plant leaves. Its role is to absorb light energy, usually most strongly in the blue and red regions of the spectrum.

叶绿素是一种绿色色素，存在于叶绿体中，主要分布在植物叶片的细胞中。它的作用是吸收光能，通常在光谱的蓝光和红光区域吸收最强。

Without chlorophyll, photosynthesis cannot begin. Chlorophyll acts as an ‘energy transformer’: it captures light energy and transfers it into chemical energy which is then used to combine carbon dioxide and water.

没有叶绿素，光合作用便无法启动。叶绿素充当’能量转换器’：它捕获光能并将其转化，用于将二氧化碳和水合成有机物。

In an IGCSE context, you must know that chlorophyll is not used up in the reaction; it is a catalyst-like pigment that enables the reaction to happen.

在IGCSE考试中，你需要知道叶绿素在反应中不会消耗，它像催化剂一样促使反应发生。

5. Required Conditions and Raw Materials | 所需条件与原料

For photosynthesis to take place, three external conditions must be met:

光合作用的发生需要满足三个外部条件：

Light of sufficient intensity and appropriate wavelengths | 足够强度和适当波长的光
Carbon dioxide in the atmosphere or dissolved in water | 大气中或水中的二氧化碳
Water available to the plant | 植物可获得的水

The plant must also have chlorophyll in its cells and a suitable temperature range (usually 5–40 °C for most crop plants).

植物细胞中还必须含有叶绿素，并处于适宜的温度范围内（大多数农作物通常在5–40 °C之间）。

6. The Leaf as an Organ of Photosynthesis | 叶片——光合作用的器官

The leaf is specially adapted for efficient photosynthesis. Its broad, flat shape gives a large surface area for absorbing sunlight and for gas exchange.

叶片为高效光合作用发生了特化。其宽大扁平的形状提供了很大的表面积，有利于吸收阳光和进行气体交换。

Key adaptations of the leaf include:

叶片的主要适应性结构包括：

Cuticle – transparent layer preventing water loss | 角质层——透明层，防止水分蒸发
Upper epidermis – transparent cells that let light pass through | 上表皮——透明细胞，使光线透入
Palisade mesophyll – tightly packed cells rich in chloroplasts | 栅栏组织——紧密排列、富含叶绿体的细胞
Spongy mesophyll – air spaces for gas diffusion | 海绵组织——提供气体扩散的气隙
Stomata – microscopic pores allowing CO₂ in and O₂ out | 气孔——使二氧化碳进入、氧气排出的微小孔隙
Veins – transport water and minerals in, glucose out | 叶脉——运输水和矿物质入内，葡萄糖外运

7. Investigating Photosynthesis | 探究光合作用

One common IGCSE experiment tests for starch in a leaf that has been exposed to light. Starch is stored glucose, so its presence proves that photosynthesis occurred.

一个常见的IGCSE实验是用碘液检验曝光叶片的淀粉。淀粉是葡萄糖的储存形式，因此它的存在证明光合作用已经发生。

The procedure includes destarching a plant by keeping it in darkness for 24 hours, then covering part of a leaf with black paper. After exposure to light, the leaf is boiled, softened and tested in ethanol to remove chlorophyll. Iodine solution turns blue-black if starch is present.

实验步骤包括：将植物在黑暗中放置24小时以消耗淀粉，然后用黑纸遮盖部分叶片。在光照后，将叶片煮沸、软化，并用乙醇脱去叶绿素。若存在淀粉，碘液会变蓝黑色。

Another simple experiment measures the volume of oxygen produced by pondweed under different light intensities. A lamp placed at different distances provides a measurable independent variable.

另一个简单实验通过测量池塘藻类在不同光照强度下产生氧气的体积。通过将灯放在不同距离，可以控制并测量自变量。

8. Rate of Photosynthesis | 光合作用的速率

The rate of photosynthesis can be measured by the volume of oxygen produced, the rate of CO₂ uptake, or the increase in biomass over time. In experiments, the most convenient method is counting oxygen bubbles released by an aquatic plant.

光合作用的速率可通过氧气产量、二氧化碳吸收速率或生物量随时间增加来衡量。在实验中，最方便的方法是计算水生植物释放的气泡数量。

When drawing a graph of rate against light intensity, the curve is a straight line through the origin up to a point where it levels off. This plateau indicates that another factor has become limiting.

绘制速率与光强关系图时，曲线呈现起源于零点的直线，到达某一点后趋于平缓。这一平台表明另一个因素变为限制因素。

9. Limiting Factors | 限制因素

A limiting factor is something that is in short supply and therefore causes the rate of reaction to slow down. In photosynthesis, the three main limiting factors are light intensity, carbon dioxide concentration and temperature.

限制因素是指供应不足、从而减慢反应速率的因素。光合作用的三大限制因素是光照强度、二氧化碳浓度和温度。

At night, light is the limiting factor. In bright light, a low CO₂ concentration often limits the rate. At very high temperatures, enzymes become denatured and the rate drops sharply.

在夜间，光是限制因素。在强光下，低CO₂浓度常常限制速率。在高温下，酶变性，速率急剧下降。

Understanding limiting factors is important for improving crop yields in greenhouses. Farmers may add artificial light, enrich the air with CO₂ or use heaters to increase the rate of photosynthesis.

理解限制因素对于提高温室作物产量很重要。农民可通过增加人工光照、补充CO₂或使用加热器来提高光合作用速率。

10. Uses of Glucose in Plants | 葡萄糖在植物中的用途

Glucose produced during photosynthesis is used by the plant in several ways:

植物利用光合作用产生的葡萄糖有多种途径：

Respiration – glucose is broken down to release energy | 呼吸作用——分解葡萄糖释放能量
Starch storage – insoluble starch stored in roots and leaves for later use | 淀粉储存——以不溶性储存在根和叶中备用
Cellulose – used to build strong cell walls | 纤维素——用于构建坚固的细胞壁
Sucrose – transported in phloem to other parts of the plant | 蔗糖——在韧皮部中运输到植物其他部位
Amino acids – combined with nitrate ions to form proteins | 氨基酸——与硝酸根离子结合形成蛋白质
Lipids and oils – stored in seeds and fruits as energy reserves | 脂质和油——储存在种子和果实中作为能量储备

11. Mineral Requirements for Healthy Plants | 植物健康所需的矿质元素

Plants need a few more nutrients from the soil to make useful substances from glucose. Nitrate ions are needed to make amino acids and proteins; a deficiency causes stunted growth and yellow leaves.

植物还需要从土壤中获取少量营养元素，以利用葡萄糖合成有用物质。硝酸根离子用于制造氨基酸和蛋白质；缺乏时会导致植株矮小和叶片发黄。

Magnesium is an element in chlorophyll; without it the leaves become pale yellow (chlorosis). Magnesium deficiency is a common IGCSE exam question.

镁是叶绿素中的一种元素；缺镁时叶片会变淡黄色（失绿）。镁缺乏是IGCSE常见的考试题目。

Potassium and phosphate are also required for healthy growth, affecting enzyme activity and energy transfer. These mineral deficiencies show distinct symptoms and are often tested using plants grown in sand culture.

钾和磷也是健康生长所必需的，影响酶活性和能量转化。这些矿质元素的缺乏表现出特定症状，通常可用砂培试验进行检测。

12. Summary | 总结

Photosynthesis is a key topic in Edexcel IGCSE Science. You should be able to write the word and symbol equations, describe the leaf adaptations, explain the effects of light, CO₂ and temperature on the rate, and use the concept of limiting factors in practical contexts.

光合作用是Edexcel IGCSE科学中的关键主题。你应当能够写出文字方程式和化学方程式，描述叶片的结构适应，解释光、CO₂和温度对速率的影响，并在实际情境中应用限制因素的概念。

Practising experimental design and graph interpretation will help you score full marks on extended response questions. Remember that chlorophyll, light, carbon dioxide and water must all be present, and that the products are glucose and oxygen.

练习实验设计和图表解读将帮助你在扩展答题中获得满分。请记住，叶绿素、光、二氧化碳和水都必须存在，产物是葡萄糖和氧气。

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Photosynthesis | 光合作用