Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the process by which green plants, algae and some bacteria transform light energy into chemical energy. It is one of the most important biological reactions on Earth because it produces the oxygen we breathe and the organic molecules that feed almost all living organisms.

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，因为它产生了我们呼吸的氧气，也产生了供养几乎所有生物体的有机分子。

1. The Word Equation | 文字方程式

The overall process of photosynthesis can be summarised using a word equation. Carbon dioxide and water are the raw materials, and glucose and oxygen are the products. Light energy is required to drive the reaction, and chlorophyll acts as the catalyst-like pigment that absorbs light.

光合作用的总体过程可以用一个文字方程式来概括。二氧化碳和水是原料，葡萄糖和氧气是产物。反应需要光能驱动，而叶绿素作为吸收光的色素，起到类似催化剂的作用。

carbon dioxide + water → glucose + oxygen

In Edexcel IGCSE Science, this word equation is essential. You should be able to write it from memory and identify the reactants and products in exam questions.

在 Edexcel IGCSE 科学中，这个文字方程式是必需的。你应该能够凭记忆写出它，并在考试问题中识别反应物和产物。

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

The balanced symbol equation shows the same reaction using chemical formulae. Six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen.

平衡符号方程式用化学式表示同一个反应。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。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Notice that this equation is balanced: there are 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms on each side. The energy is not shown in the equation because light energy is not a substance, but it is an input.

注意这个方程式是平衡的：两边各有6个碳原子、12个氢原子和18个氧原子。能量没有显示在方程式中，因为光能不是一种物质，但它是一种输入。

3. The Requirements of Photosynthesis | 光合作用的条件

For photosynthesis to occur, four main requirements must be available. These are light, carbon dioxide, water and a suitable temperature. Without any of these, the rate of photosynthesis will slow down or stop.

要发生光合作用，必须具备四个主要条件：光、二氧化碳、水和适宜的温度。缺少其中任何一个，光合作用速率都会减慢或停止。

Light – provides the energy needed to split water molecules during the light-dependent stage.

光 – 提供光依赖阶段中分解水分子所需的能量。
Carbon dioxide – enters the leaf through stomata and is used to build glucose.

二氧化碳 – 通过气孔进入叶片，用于构建葡萄糖。
Water – absorbed by roots and transported to leaves via xylem vessels.

水 – 由根吸收，并通过木质部导管运输到叶片。
Suitable temperature – enzymes involved in photosynthesis work best at about 25–35 °C for most plants.

适宜温度 – 大多数植物中参与光合作用的酶在约25–35 °C时活性最高。

Chlorophyll is also required because it traps light energy. It is not consumed during the reaction, so it is not a reactant.

叶绿素也是必需的，因为它能捕获光能。它在反应中不被消耗，所以它不是反应物。

4. The Products and Their Uses | 产物及其用途

The two products of photosynthesis are glucose and oxygen. Glucose is a carbohydrate that provides chemical energy and is used to build other molecules. Oxygen is a waste product for the plant but is essential for aerobic respiration in most organisms.

光合作用的两种产物是葡萄糖和氧气。葡萄糖是一种碳水化合物，提供化学能并用于构建其他分子。氧气对植物来说是废物，但对大多数生物的有氧呼吸至关重要。

Product	In plants
Glucose	Used in respiration, converted to starch for storage, cellulose for cell walls, amino acids (with nitrates) and fats/lipids.
Oxygen	Released through stomata; used by the plant itself in respiration at night.

Starch is often stored in leaves as a way to keep glucose without affecting water balance. Testing a leaf for starch is a common practical in IGCSE Science.

淀粉通常储存在叶片中，作为在不影响水分平衡的情况下储存葡萄糖的方式。对叶片进行淀粉检测是 IGCSE 科学中的常见实验。

5. The Structure of the Leaf | 叶片的结构

The leaf is the main photosynthetic organ of a plant. Its structure is highly adapted to maximise light absorption and gas exchange.

叶片是植物主要的光合作用器官。它的结构高度适应以最大化光吸收和气体交换。

Layer	Function
Upper epidermis	Thin and transparent, allowing light to pass through; covered by a waxy cuticle to reduce water loss.
Palisade mesophyll	Elongated cells packed with chloroplasts, placed near the top to absorb maximum light.
Spongy mesophyll	Air spaces allow carbon dioxide to diffuse to palisade cells and oxygen to diffuse out.
Lower epidermis	Contains stomata and guard cells that control gas exchange and transpiration.

The internal leaf is like a solar panel: a large surface area, multiple chloroplast layers and a network of veins that supply water and remove products.

叶片内部就像一个太阳能电池板：巨大的表面积、多层的叶绿体以及输送水分和运走产物的维管系统。

6. The Role of Chlorophyll and Chloroplasts | 叶绿素和叶绿体的作用

Chloroplasts are organelles found mainly in palisade mesophyll cells. They contain chlorophyll, a green pigment that absorbs most wavelengths of visible light, especially red and blue, and reflects green light.

叶绿体是主要存在于栅栏组织细胞中的细胞器。它们含有叶绿素，这是一种绿色色素，能吸收大部分可见光波长，尤其是红光和蓝光，并反射绿光。

During photosynthesis, chlorophyll absorbs light energy and uses it to split water molecules into hydrogen and oxygen. This process is called photolysis. The hydrogen is then combined with carbon dioxide to form glucose.

在光合作用过程中，叶绿素吸收光能并利用它把水分子分解为氢和氧。这个过程称为光解。氢随后与二氧化碳结合形成葡萄糖。

Plants that lack chlorophyll cannot photosynthesise. For example, variegated plants have white areas without chlorophyll, and those areas do not produce starch in light.

缺乏叶绿素的植物不能进行光合作用。例如，斑叶植物有白色区域不含叶绿素，这些区域在光下不会产生淀粉。

7. Limiting Factors – An Overview | 限制因素概述

A limiting factor is a condition that is present in insufficient quantity to allow the reaction to proceed faster. In photosynthesis, the potential rate is controlled by the factor that is closest to its minimum.

限制因素是数量不足以致无法使反应更快进行的条件。在光合作用中，潜在速率受最接近其最小值的那种因素控制。

The three main limiting factors are light intensity, carbon dioxide concentration and temperature. In exams, you may be asked to read graphs and explain which factor is limiting at different points.

三个主要限制因素是光强度、二氧化碳浓度和温度。在考试中，你可能会被要求阅读曲线图并解释在不同点哪个因素是限制性的。

If light intensity is low, increasing temperature or CO₂ will have little effect. If light is not limiting, then increasing CO₂ or temperature can increase the rate until another factor becomes limiting.

如果光强度低，提高温度或 CO₂ 浓度几乎不会产生效果。如果光不是限制因素，那么增加 CO₂ 或温度可以提高速率，直到另一个因素成为限制因素。

8. Light Intensity and Rate | 光强度与速率

As light intensity increases, the rate of photosynthesis increases proportionally at first. This is because more light energy is available to excite chlorophyll and drive photolysis.

随着光强度增加，光合作用速率起初按比例增加。这是因为有更多的光能可用于激发叶绿素并驱动光解。

However, beyond a certain point, the rate stops increasing because light is no longer the limiting factor. At that point, temperature or CO₂ concentration usually limits the rate.

然而，超过某一点后，速率不再增加，因为光不再是限制因素。此时，通常由温度或 CO₂ 浓度限制速率。

rate of photosynthesis ∝ light intensity (at low light)

In practical experiments, the distance between a lamp and a plant can be changed. The inverse square law states that light intensity is inversely proportional to the square of the distance:

在实验实践中，可以改变灯与植物之间的距离。平方反比定律指出，光强度与距离的平方成反比：

light intensity = 1 ÷ (distance)²

So if the distance is doubled, the light intensity falls to one quarter of its original value.

所以，如果距离加倍，光强度下降到原来的四分之一。

9. Carbon Dioxide Concentration | 二氧化碳浓度

Carbon dioxide is a raw material for making glucose. In air, the CO₂ concentration is only about 0.04%, so it is often the limiting factor in bright, warm conditions.

二氧化碳是制造葡萄糖的原料。空气中 CO₂ 浓度仅为约0.04%，所以在光照充足、温度适宜的条件下，它常常是限制因素。

Increasing CO₂ concentration causes the rate of photosynthesis to rise steadily until a saturation point is reached. After that, further increases have no effect because another factor, such as light or temperature, becomes limiting.

增加 CO₂ 浓度会使光合作用速率稳步上升，直到达到饱和点。之后，再增加也没有效果，因为另一个因素，如光或温度，变成了限制因素。

In greenhouses, growers sometimes add extra CO₂ (e.g. from burning propane) to boost crop yields. This is only useful when light intensity and temperature are also high enough.

在温室中，种植者有时会额外增加 CO₂（例如通过燃烧丙烷）来提高作物产量。这只有在光强度和温度也足够高时才有用。

10. Temperature and Enzyme Activity | 温度与酶活性

Photosynthesis is controlled by enzymes, such as Rubisco, which are proteins that work best at an optimum temperature. For most temperate plants, the optimum is about 25–35 °C.

光合作用由酶控制，例如 Rubisco，这类蛋白质在最佳温度下活性最高。对大多数温带植物，最适温度约为25–35 °C。

At low temperatures, enzymes move slowly and molecular collisions are rare, so the rate is low. As temperature rises, molecules move faster and more enzyme-substrate complexes form, increasing the rate.

低温时，酶运动缓慢，分子碰撞少，所以速率低。随着温度升高，分子运动加快，形成更多酶-底物复合物，速率增加。

Above the optimum, the enzyme begins to denature. Its active site changes shape, and the enzyme can no longer bind to its substrate. Eventually, the rate falls to zero.

超过最适温度后，酶开始变性。活性位点形状改变，酶无法再与底物结合。最终速率降至零。

Too high temperature → enzymes denature → photosynthesis stops

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

Photosynthesis is the foundation of almost all food chains. Producers convert light energy into chemical energy stored in glucose, which is then passed on to consumers through feeding.

光合作用是几乎所有食物链的基础。生产者将光能转化为储存在葡萄糖中的化学能，然后通过摄食传递给消费者。

Photosynthesis also maintains atmospheric oxygen levels. The oxygen released by plants and algae is used by animals and plants in aerobic respiration. Without photosynthesis, aerobic life on Earth would not exist.

光合作用还维持大气中的氧气水平。植物和藻类释放的氧气被动物和植物用于有氧呼吸。没有光合作用，地球上的需氧生命将不存在。

In addition, photosynthesis removes carbon dioxide from the atmosphere, helping to regulate the global carbon cycle. Fossil fuels, formed from ancient photosynthesis, store energy that we use today.

此外，光合作用从大气中移除二氧化碳，有助于调节全球碳循环。由古代光合作用形成的化石燃料储存着我们今天使用的能量。

12. Investigating Photosynthesis | 探究光合作用

A common IGCSE practical is to investigate the effect of light intensity on the rate of photosynthesis using an aquatic plant such as Elodea (pondweed). The number of oxygen bubbles produced per minute is used as a measure of the rate.

一个常见的 IGCSE 实验是用伊乐藻等水生植物探究光强度对光合作用速率的影响，用每分钟产生的氧气气泡数作为速率的指标。

Place the pondweed in a beaker of water with sodium hydrogencarbonate (a source of CO₂).

将伊乐藻放入含有碳酸氢钠（CO₂ 来源）的水烧杯中。
Use a lamp at different distances to change light intensity, keeping temperature constant.

使用不同距离的灯来改变光强度，保持温度恒定。
Count the bubbles for a fixed time (e.g. 1 minute) at each distance.

在每个距离下固定时间（例如1分钟）计数气泡。
Repeat each measurement at least three times and calculate a mean to improve reliability.

每个测量至少重复三次并计算平均值，以提高可靠性。

To measure the volume of gas more accurately, you can collect the gas in a syringe or use a capillary tube to measure the length of a gas bubble. This gives a quantitative result.

为了更准确地测量气体体积，可以用注射器收集气体，或用毛细管测量气泡长度。这样可以获得定量结果。

When designing your own experiment, always identify the independent variable, dependent variable, and controlled variables such as temperature, CO₂ concentration and the size of the plant.

设计你自己的实验时，务必明确自变量、因变量和受控变量，例如温度、CO₂ 浓度和植物的大小。

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Photosynthesis | 光合作用