Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy. It is one of the most important biological reactions on Earth because it produces oxygen and forms the base of nearly every food chain.

光合作用是绿色植物、藻类和一些细菌将光能转化为化学能的过程。它是地球上最重要的生物反应之一，因为它产生氧气，并构成了几乎所有食物链的基础。

1. What Is Photosynthesis? | 什么是光合作用？

Photosynthesis takes place mainly in the leaves of green plants. The leaf contains specialised cells called palisade mesophyll cells, which are packed with chloroplasts. Inside the chloroplasts is a green pigment called chlorophyll.

光合作用主要发生在绿色植物的叶片中。叶片含有称为栅栏组织细胞的特殊细胞，这些细胞中充满叶绿体。叶绿体内有一种名为叶绿素的绿色色素。

During photosynthesis, carbon dioxide and water are converted into glucose and oxygen using light energy. The energy from sunlight is trapped by chlorophyll and stored in the chemical bonds of glucose.

在光合作用过程中，二氧化碳和水在光能的作用下转化为葡萄糖和氧气。阳光中的能量被叶绿素捕获，并储存在葡萄糖的化学键中。

The overall process can be summarised by two equations: a word equation and a balanced symbol equation. You must be able to write and interpret both for the Edexcel IGCSE Science examination.

整个过程可以用两个方程式概括：文字方程式和平衡符号方程式。在 Edexcel IGCSE 科学考试中，你必须能够写出并解释这两个方程式。

2. The Word Equation | 文字方程式

The word equation for photosynthesis is:

光合作用的文字方程式为：

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

This equation shows the reactants (carbon dioxide and water) and the products (glucose and oxygen). In the IGCSE course, you should remember that light energy and chlorophyll are also required, but they are not written as reactants or products.

该方程式显示了反应物（二氧化碳和水）和产物（葡萄糖和氧气）。在 IGCSE 课程中，你应记住光和叶绿素也是必需的，但它们不写作反应物或产物。

Carbon dioxide enters the leaf through small pores called stomata. Water is absorbed by the roots and transported to the leaves through the xylem vessels.

二氧化碳通过称为气孔的小孔进入叶片。水分由根部吸收，并通过木质部导管输送到叶片。

3. The Balanced Chemical Equation | 平衡化学方程式

The balanced symbol equation for photosynthesis is:

光合作用的平衡符号方程式为：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

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 numbers of atoms on both sides must be equal, obeying the law of conservation of mass.

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。方程式两边的原子数量必须相等，遵循质量守恒定律。

You should also be able to state that the energy required for this reaction comes from sunlight, and that chlorophyll acts as a catalyst-like substance that captures the light energy. Strictly speaking, chlorophyll is a pigment, not a catalyst, but it is essential for light absorption.

你还应能够指出，该反应所需的能量来自阳光，叶绿素作为捕获光能的物质。严格来说，叶绿素是一种色素，而不是催化剂，但它对光吸收至关重要。

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

Chlorophyll is a green pigment found in chloroplasts. It absorbs light most strongly in the red and blue-violet regions of the spectrum, while reflecting green light, which is why leaves appear green.

叶绿素是叶绿体中的绿色色素。它最强烈地吸收光谱中红色和蓝紫色区域的光，同时反射绿光，这就是叶子呈绿色的原因。

Light energy is used to split water molecules (photolysis) during the light-dependent stage of photosynthesis. This releases oxygen as a waste product and provides hydrogen ions and electrons to form glucose.

在光合作用的光依赖阶段，光能用于分解水分子（光解）。这一过程释放出氧气作为废物，并提供氢离子和电子来合成葡萄糖。

Without light, photosynthesis cannot take place. In dim light, the rate is slow; as light intensity increases, the rate generally increases until another factor becomes limiting.

没有光，光合作用就无法进行。在弱光下，速率缓慢；随着光照强度增加，速率通常会提高，直到另一个因素成为限制因素。

5. Limiting Factors | 限制因素

A limiting factor is a condition that, when in short supply, slows down the rate of photosynthesis. The three main limiting factors are light intensity, carbon dioxide concentration and temperature.

限制因素是指供应不足时会降低光合作用速率的条件。三个主要限制因素为光照强度、二氧化碳浓度和温度。

The relationship can be illustrated using graphs. At low light intensity, increasing the light intensity increases the rate. At higher light intensities, the rate plateaus because either CO₂ concentration or temperature is now the limiting factor.

这种关系可以用图表来说明。在低光照强度下，增加光照强度会提高反应速率。在较高光照强度下，速率趋于平稳，因为此时二氧化碳浓度或温度成为限制因素。

For the Edexcel exam, you need to describe and explain how each factor affects the rate. You also need to interpret graphs showing these effects.

对于 Edexcel 考试，你需要描述并解释每个因素如何影响速率，还需要解读显示这些影响的图表。

6. The Rate of Photosynthesis | 光合作用速率

The rate of photosynthesis can be measured by the volume of oxygen produced, the rate of carbon dioxide uptake, or the increase in biomass. A simple experiment uses an aquatic plant such as Elodea and counts the bubbles released per minute.

光合作用速率可以通过氧气的产生量、二氧化碳的吸收速率或生物量的增加来测量。一个简单的实验使用池塘藻类（如黑藻），并计算每分钟释放的气泡数。

When light intensity increases, the number of bubbles per minute increases until the rate becomes constant. The same principle applies when increasing carbon dioxide concentration at constant light and temperature.

当光照强度增加时，每分钟气泡数增加，直到速率变为恒定。在光和温度恒定时增加二氧化碳浓度，也遵循同样的原理。

You may be asked to calculate the rate from a set of data using the formula:

你可能会被要求使用以下公式从一组数据中计算速率：

Rate = quantity of product ÷ time

速率 = 产物量 ÷ 时间

7. How Plants Use Glucose | 植物如何利用葡萄糖

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

光合作用产生的葡萄糖在植物体内有几种重要用途：

Respiration – Glucose is broken down in respiration to release energy for life processes.

呼吸作用 – 葡萄糖在呼吸中被分解，释放能量以维持生命活动。
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。
Starch – Glucose is stored as starch in roots, stems and leaves as an insoluble energy reserve.

淀粉 – 葡萄糖以不溶性淀粉形式储存在根、茎和叶中，作为能量储备。
Sucrose – Glucose is transported as sucrose in the phloem to other parts of the plant.

蔗糖 – 葡萄糖在韧皮部中以蔗糖形式运输到植物其他部分。
Proteins and fats – Glucose combines with nitrate ions to form amino acids, which build proteins; excess glucose may be converted into fats or oils.

蛋白质和脂肪 – 葡萄糖与硝酸根离子结合形成氨基酸，进而构建蛋白质；多余的葡萄糖可转化为脂肪或油。

8. Mineral Requirements | 矿物质需求

Plants need more than just carbon dioxide and water for healthy growth. They also absorb mineral ions from the soil through their roots.

植物健康生长不仅需要二氧化碳和水，还需要通过根部从土壤中吸收矿物质离子。

Nitrate ions (NO₃⁻) are essential for making amino acids and therefore proteins. A lack of nitrates causes stunted growth and yellowing of older leaves.

硝酸根离子（NO₃⁻）对制造氨基酸和蛋白质至关重要。缺乏硝酸盐会导致生长缓慢和老叶变黄。

Magnesium ions (Mg²⁺) are a central component of chlorophyll. Without magnesium, leaves become pale or yellow and photosynthesis cannot occur efficiently.

镁离子（Mg²⁺）是叶绿素的核心成分。缺乏镁，叶片会变白或变黄，光合作用无法有效进行。

In the exam, you should be able to state the deficiency symptoms for nitrate and magnesium ions, and explain why these ions are needed.

在考试中，你应该能够陈述硝酸根离子和镁离子的缺乏症状，并解释为什么需要这些离子。

9. Investigating Gas Exchange | 气体交换的探究

Hydrogencarbonate indicator (bicarbonate indicator) can be used to investigate gas exchange in aquatic plants. The colour changes indicate whether photosynthesis or respiration is dominant.

碳酸氢盐指示剂可用于研究水生植物的气体交换。颜色变化表明光合作用或呼吸作用哪个占主导。

In bright light, a plant photosynthesising rapidly will take up CO₂ faster than it produces it in respiration. The indicator becomes red/purple, indicating that CO₂ concentration is lower than in air.

在强光下，植物光合作用消耗 CO₂ 的速度快于呼吸作用产生它的速度。指示剂变为红色/紫色，表明 CO₂ 浓度低于空气中的浓度。

In darkness, respiration is the only gas exchange process, and CO₂ accumulates. The indicator turns yellow, showing a high CO₂ concentration.

在黑暗中，呼吸作用是唯一的气体交换过程，CO₂ 积累，指示剂变黄，显示 CO₂ 浓度高。

This experiment is common in IGCSE practical exams. You may be asked to describe how to set it up or to explain the colour changes.

该实验在 IGCSE 实践考试中常见。你可能会被要求描述如何设置实验或解释颜色变化。

10. Environmental and Agricultural Importance | 环境与农业重要性

Photosynthesis is responsible for removing carbon dioxide from the atmosphere and releasing oxygen, helping to maintain atmospheric balance. It is also the source of all food energy for animals and humans.

光合作用负责从大气中移除二氧化碳并释放氧气，有助于维持大气平衡。它也是动物和人类所有食物能量的来源。

Farmers use their understanding of limiting factors to improve crop yields. Greenhouses often supply extra carbon dioxide and use heating and artificial lighting to maintain optimal conditions.

农民利用对限制因素的理解来提高作物产量。温室通常补充额外的二氧化碳，并使用加热和人工照明来维持最佳条件。

In a greenhouse, increasing CO₂ concentration from 0.03% to 0.1% can significantly increase the rate of photosynthesis and thus crop growth, as long as light and temperature are not limiting.

在温室中，只要光和温度不受限制，将 CO₂ 浓度从 0.03% 增加到 0.1% 可以显著提高光合作用速率，从而促进作物生长。

11. Common Exam Misconceptions | 常见考试误区

Many students confuse respiration with photosynthesis. Remember that photosynthesis uses carbon dioxide and water, and produces glucose and oxygen. Respiration is the opposite overall process, using glucose and oxygen to produce carbon dioxide and water and releasing energy.

许多学生将呼吸作用与光合作用混淆。记住，光合作用消耗二氧化碳和水，产生葡萄糖和氧气。呼吸作用总体上是相反的过程，消耗葡萄糖和氧气，产生二氧化碳和水，并释放能量。

Another common mistake is to say that photosynthesis occurs at night. It does not, because light is required. Plants may still respire at night, but they cannot photosynthesise in darkness.

另一个常见错误是说光合作用在夜间发生。事实并非如此，因为需要光。植物在夜间可能仍在呼吸，但在黑暗中不能进行光合作用。

Also, be careful with the term ‘limiting factor’. A factor is only limiting when it is below the optimum level. For example, at night light is the limiting factor, not CO₂ concentration.

同时，注意“限制因素”这个术语。只有当某个因素低于最适水平时，它才是限制因素。例如，在夜间，光是限制因素，而不是 CO₂ 浓度。

12. Quick Revision Summary | 快速复习总结

Use this summary to check your knowledge before the exam:

在考试前使用此总结检查你的知识：

Topic	Key point
Word equation	carbon dioxide + water → glucose + oxygen
Symbol equation	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Requirements	light, chlorophyll, CO₂, water
Limiting factors	light intensity, CO₂ concentration, temperature
Glucose uses	respiration, starch, cellulose, sucrose, proteins, fats
Ions needed	nitrate (for proteins), magnesium (for chlorophyll)

Make sure you can draw and interpret graphs of rate against light intensity, CO₂ concentration and temperature. These are frequently tested in multiple-choice and long-answer questions.

确保你能够绘制并解读速率与光照强度、二氧化碳浓度和温度的关系图。这些在选择题和长答题中经常出现。

Published by TutorHao | IGCSE Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Photosynthesis | 光合作用