Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is one of the most important chemical processes on Earth. It allows green plants to trap light energy and convert it into chemical energy stored in glucose. This process is essential for plant growth and for providing oxygen to the atmosphere.

光合作用是地球上最重要的化学过程之一。绿色植物利用它捕获光能，并将其转化为储存在葡萄糖中的化学能。这一过程对植物生长以及向大气提供氧气至关重要。

1. Definition and Word Equation | 定义与文字方程式

Photosynthesis is the process by which green plants, algae and some bacteria use sunlight, carbon dioxide and water to produce glucose and oxygen. The glucose can be used for respiration, stored as starch, or converted into cellulose and other substances.

光合作用是绿色植物、藻类和某些细菌利用阳光、二氧化碳和水制造葡萄糖和氧气的过程。葡萄糖可用于呼吸作用、以淀粉形式储存，或转化为纤维素及其他物质。

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 and products in words. It is important to learn it exactly as written for the IGCSE examination.

该方程式用文字表示反应物和生成物。在IGCSE考试中，准确记住它的写法非常重要。

2. Chemical Equation | 化学方程式

The balanced chemical equation for photosynthesis shows the same relationship using symbols and numbers. Carbon dioxide and water are converted into glucose and oxygen in the presence of light and chlorophyll.

光合作用的平衡化学方程式使用符号和数字表示相同的关系。二氧化碳和水在光和叶绿素的存在下转化为葡萄糖和氧气。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Here, six molecules of carbon dioxide react with six molecules of water to form one molecule of glucose and six molecules of oxygen. The energy from sunlight drives this endothermic reaction.

这里，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气分子。来自阳光的能量驱动这一吸热反应。

Note the use of subscripts in the formula C₆H₁₂O₆. You must write them correctly to show the number of atoms in each molecule.

注意C₆H₁₂O₆中的下标。必须正确书写，以显示每个分子中的原子数目。

3. Requirements: Light, Chlorophyll and Raw Materials | 必要条件：光、叶绿素和原料

For photosynthesis to occur, four main conditions must be met. These are light energy, chlorophyll, carbon dioxide and water. Without any of these, the process slows down or stops.

光合作用发生需要满足四个主要条件：光能、叶绿素、二氧化碳和水。缺少其中任何一个，该过程都会减慢或停止。

Light – provides the energy needed to split water molecules and drive the reaction.

光——提供分解水分子并驱动反应所需的能量。
Chlorophyll – green pigment found in chloroplasts that absorbs light energy.

叶绿素——存在于叶绿体中的绿色色素，能吸收光能。
Carbon dioxide – enters the leaf through stomata and diffuses into the mesophyll cells.

二氧化碳——通过气孔进入叶片，并扩散到叶肉细胞中。
Water – absorbed by roots from the soil and transported to leaves via xylem vessels.

水——由根部从土壤中吸收，并通过木质部导管输送到叶片。

Chlorophyll is not used up in the reaction. It acts as a catalyst that absorbs light and transfers its energy to the reactants.

叶绿素在反应中不会被消耗。它作为催化剂，吸收光并将其能量传递给反应物。

4. The Site: Chloroplasts in Leaves | 反应场所：叶片中的叶绿体

Photosynthesis takes place mainly in the palisade mesophyll layer of a leaf. These cells are packed with chloroplasts, which contain chlorophyll. The upper epidermis is transparent, allowing light to reach the palisade cells.

光合作用主要发生在叶片的栅栏叶肉层中。这些细胞充满了含有叶绿素的叶绿体。上表皮透明，能使光线到达栅栏细胞。

The internal structure of a leaf is adapted for efficient photosynthesis. Large surface area, thin structure, stomata, and air spaces all help maximise gas exchange.

叶片的内部结构是为高效光合作用而适应形成的。大表面积、薄的结构、气孔和细胞间隙都有助于最大化气体交换。

Stomata allow carbon dioxide in and oxygen out.

气孔允许二氧化碳进入，氧气排出。
Air spaces inside the leaf provide a large surface for gas diffusion.

叶片内部的细胞间隙为气体扩散提供了大面积表面。
Xylem and phloem transport water and sugars to and from the leaf.

木质部和韧皮部在叶片与植物其他部分之间运输水分和糖分。

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

Light intensity is a major limiting factor in photosynthesis. When light intensity increases, the rate of photosynthesis increases, provided other factors are sufficient. However, at high light intensities, the rate plateaus because another factor becomes limiting.

光强度是光合作用的主要限制因素。当光强度增加时，光合作用速率增加，前提是其他因素充足。然而，在高光强度下，速率达到平台，因为另一个因素成为限制因素。

On a graph of rate versus light intensity, the curve rises steeply at first and then flattens. This flattening indicates that light is no longer the limiting factor.

在光合速率对光强度的关系图中，曲线先急剧上升，然后趋于平缓。这种平缓表明光不再是限制因素。

Rate of photosynthesis ∝ light intensity (at low intensities)

光合速率 ∝ 光强度（在低光强度下）

In experiments, a lamp is placed at different distances from a plant. The inverse square law states that light intensity is proportional to 1/distance². Therefore, doubling the distance reduces light intensity to one quarter.

在实验中，将灯放在离植物不同距离处。平方反比定律指出，光强度与1/距离²成正比。因此，距离加倍会使光强度降为原来的四分之一。

6. Carbon Dioxide Concentration | 二氧化碳浓度

Carbon dioxide is a raw material for photosynthesis. Increasing its concentration will increase the rate of photosynthesis until another factor becomes limiting. Normally, atmospheric CO₂ is only about 0.04%, so it often limits photosynthesis in air.

二氧化碳是光合作用的原料。增加其浓度会提高光合作用速率，直到另一个因素成为限制因素。通常，大气中的CO₂仅为约0.04%，因此空气环境常常限制光合作用。

Farmers and gardeners sometimes increase CO₂ concentration in greenhouses to boost crop growth. This is an example of manipulating limiting factors in controlled environments.

农民和园丁有时会在温室中增加CO₂浓度以提高作物生长。这是人为调节限制因素的一个例子。

Increasing CO₂ concentration generally produces a rising curve, but the effect diminishes at higher concentrations. There is a saturation point beyond which further increases have no effect.

增加CO₂浓度通常会产生上升曲线，但在较高浓度时效应减小。存在一个饱和点，超过该点后进一步增加没有效果。

7. Temperature and the Role of Enzymes | 温度与酶的作用

Photosynthesis is an enzyme-controlled process. The enzymes involved work best at an optimal temperature, usually around 25–35°C for most plants. Below this range, the rate decreases because enzymes have less kinetic energy.

光合作用是酶控制的过程。参与其中的酶在最佳温度下工作最好，通常对大多数植物而言约为25–35°C。低于此范围，速率降低，因为酶具有较少的动能。

Above the optimal temperature, the rate rapidly decreases because enzymes begin to denature. The shape of the active site changes irreversibly, so the reaction slows dramatically.

高于最佳温度时，速率迅速下降，因为酶开始变性。活性位点的形状发生不可逆变化，因此反应大大减慢。

Optimum temperature → maximum rate; too hot → enzymes denature

最佳温度 → 最大速率；过热 → 酶变性

In greenhouses, heating can improve yields in winter, but excessive heat must be avoided to prevent enzyme damage.

在温室中，冬季加热可以提高产量，但必须避免过热以防止酶损伤。

8. Chlorophyll and Other Factors | 叶绿素及其他因素

Chlorophyll concentration affects photosynthesis. Plants that have less chlorophyll, such as variegated leaves or plants suffering from mineral deficiency, show reduced glucose production. Magnesium is needed to make chlorophyll, so a lack of magnesium causes chlorosis.

叶绿素浓度影响光合作用。叶绿素较少的植物，如斑叶植物或缺乏矿物质的植物，葡萄糖产量降低。镁是合成叶绿素所必需的，因此缺镁会导致失绿症。

Other factors include water availability. If water is scarce, stomata close to reduce water loss, which also limits CO₂ entry. This indirectly reduces photosynthesis.

其他因素包括水分供应。如果水分不足，气孔关闭以减少水分流失，这也限制了CO₂的进入。这间接降低了光合作用。

Oxygen accumulation may also slow photosynthesis in sealed environments, although it is rarely a limiting factor in nature.

在封闭环境中，氧气积累也可能减缓光合作用，尽管在自然界它很少成为限制因素。

9. Investigating Photosynthesis: Rate Measurement | 探究光合作用：速率测定

Several methods can be used to measure the rate of photosynthesis. The most common IGCSE experiment is counting oxygen bubbles released by an aquatic plant such as Elodea.

有几种方法可用于测定光合作用速率。IGCSE中最常见的实验是计算水生植物（如伊乐藻）释放的氧气气泡数。

Alternatively, the volume of oxygen collected in a gas syringe can be measured over time. The slope of the volume–time graph gives the rate.

或者，可以测量在注射器中收集的氧气体积随时间的变化。体积–时间图的斜率即为速率。

Keep the temperature constant to ensure a fair test.

保持温度恒定以确保公平测试。
Use a lamp at a fixed distance to control light intensity.

在固定距离处使用灯光以控制光强度。
Repeat readings and calculate the mean for reliability.

重复读数并计算平均值，以确保可靠性。

Another method uses leaf disks. Disks float when photosynthesis produces oxygen, and the time taken for half the disks to float can indicate the rate.

另一种方法使用叶圆片。当光合作用产生氧气时，叶片浮起，半数叶片浮起所需时间可指示速率。

10. Uses of Glucose from Photosynthesis | 光合作用产物的用途

The glucose produced in photosynthesis is used by the plant in several ways. It can be used immediately for respiration to release energy. It can also be converted into insoluble starch for storage in leaves, roots or seeds.

光合作用产生的葡萄糖在植物中有多种用途。它可立即用于呼吸作用以释放能量。也可转化为不溶性淀粉，储存在叶片、根或种子中。

Glucose is used to make cellulose for cell walls, amino acids (when combined with nitrate ions) for proteins, and lipids for cell membranes. These biosynthetic pathways are essential for growth and development.

葡萄糖用于制造细胞壁的纤维素、氨基酸（与硝酸根离子结合时）以合成蛋白质，以及细胞膜的脂质。这些合成代谢途径对生长发育至关重要。

Glucose → starch, cellulose, amino acids, lipids

葡萄糖 → 淀粉、纤维素、氨基酸、脂质

When animals eat plants, they obtain glucose indirectly. Thus, photosynthesis is the ultimate source of food for almost all life on Earth.

当动物食用植物时，它们间接获得葡萄糖。因此，光合作用是地球上几乎所有生命最终的食物来源。

11. Importance of Photosynthesis to the Ecosystem | 光合作用对生态系统的重要性

Photosynthesis produces oxygen, which most living organisms need for aerobic respiration. It also removes carbon dioxide from the atmosphere, helping to regulate the carbon cycle.

光合作用产生氧气，这是大多数生物进行有氧呼吸所需的。它还将二氧化碳从大气中移除，有助于调节碳循环。

The chemical energy stored in glucose is passed along food chains when organisms are consumed. This energy flow sustains all heterotrophic organisms, including humans.

当生物被食用时，储存在葡萄糖中的化学能沿食物链传递。这种能量流动维持着所有异养生物，包括人类。

Fossil fuels are formed from the remains of plants and algae that lived millions of years ago. These fuels contain energy originally captured by photosynthesis, making this process a cornerstone of energy resources.

化石燃料由数百万年前生活的植物和藻类残骸形成。这些燃料含有最初通过光合作用捕获的能量，使这一过程成为能源资源的基石。

12. Common Exam Tips | 考试常见提示

In the IGCSE Science examination, students are often asked to identify limiting factors from graphs, write the equations, and describe experiments. You should be able to explain why the rate plateaus at high light intensity.

在IGCSE科学考试中，学生经常被要求从图中识别限制因素、书写方程式以及描述实验。你应该能够解释为什么速率在高光强度下达到平台。

Always mention both reactant and product when defining photosynthesis.

定义光合作用时，务必同时提及反应物和生成物。
Use the correct arrow in equations: → (not =).

在方程式中使用正确的箭头：→（而不是=）。
Remember the chlorophyll and light are conditions, not reactants in the chemical equation.

记住叶绿素和光是条件，而不是化学方程式中的反应物。
When describing experiments, mention control variables and repeats.

描述实验时，提及控制变量和重复测量。

Practising past-paper questions on photosynthesis will help you apply this knowledge quickly and accurately in the exam.

练习光合作用的真题可以帮助你在考试中快速准确运用这些知识。

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Photosynthesis | 光合作用