Photosynthesis in IGCSE Science | IGCSE科学中的光合作用

📚 Photosynthesis in IGCSE Science | IGCSE科学中的光合作用

Photosynthesis is one of the most important biological processes in the IGCSE Science syllabus. It underpins life on Earth by converting light energy into chemical energy, producing both glucose and oxygen. In this revision article, we will explore its equation, mechanisms, limiting factors, and experiments in a clear, exam-focused way.

光合作用是IGCSE科学课程中最重要的生物过程之一。它通过将光能转化为化学能，同时产生葡萄糖和氧气，为地球上的生命提供了基础。在这篇复习文章中，我们将以清晰、紧扣考点的方式探讨其方程式、机制、限制因素和实验。

1. What is Photosynthesis? | 什么是光合作用？

Photosynthesis is the process by which green plants, algae and some bacteria use sunlight to synthesise carbohydrates from carbon dioxide and water. The oxygen produced is released into the atmosphere.

光合作用是绿色植物、藻类和某些细菌利用阳光，将二氧化碳和水合成碳水化合物的过程。产生的氧气被释放到大气中。

This process provides the primary source of organic matter for nearly all food chains. It also maintains atmospheric oxygen levels and removes carbon dioxide from the air, helping to regulate the climate.

这个过程为几乎所有食物链提供了主要的有机物质来源。它还维持大气中的氧气水平，并去除空气中的二氧化碳，有助于调节气候。

Photosynthesis takes place mainly in the leaves, where chlorophyll pigments capture light energy. The energy captured is stored in the chemical bonds of glucose, a simple sugar.

光合作用主要发生在叶片中，叶绿素色素捕获光能。捕获的能量储存在葡萄糖（一种单糖）的化学键中。

2. The Chemical Equation | 化学方程式

The word equation for photosynthesis is:

光合作用的文字方程式为：

carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)

二氧化碳 + 水 → 葡萄糖 + 氧气（在光和叶绿素的条件下）

The balanced chemical equation shows the exact numbers of atoms involved:

配平的化学方程式显示了所涉及原子的精确数量：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

In this equation, six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen. The reaction requires light energy absorbed by chlorophyll.

在这个方程式中，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。该反应需要叶绿素吸收的光能。

3. Leaf Structure and Photosynthesis | 叶片结构与光合作用

Leaves are the main photosynthetic organs of most plants. Their structure is highly adapted to maximise light capture and gas exchange.

叶片是大多数植物进行光合作用的主要器官。它们的结构高度适应于最大限度地捕获光和进行气体交换。

Tissue	Function
Epidermis	Thin protective layer; transparent to let light through.
Palisade mesophyll	Packed with chloroplasts; main site of photosynthesis.
Spongy mesophyll	Air spaces for gas diffusion; some photosynthesis.
Stomata	Pores controlling CO₂ intake and O₂ release.
Xylem	Transports water from roots to leaves.
Phloem	Transports glucose to other parts of the plant.

组织	功能
表皮	薄的保护层；透明，让光通过。
栅栏叶肉	富含叶绿体；光合作用的主要场所。
海绵叶肉	有气隙利于气体扩散；也有部分光合作用。
气孔	控制CO₂吸收和O₂释放的孔隙。
木质部	将水从根部输送到叶片。
韧皮部	将葡萄糖运输到植物其他部位。

The irregular shape of leaf cells and the presence of air spaces in the spongy mesophyll increase the surface area for carbon dioxide absorption. Stomata open and close to balance gas exchange with water loss.

叶细胞形状不规则，海绵叶肉中有气隙，增大了吸收二氧化碳的表面积。气孔的开闭在气体交换与失水之间取得平衡。

4. Chloroplasts and Pigments | 叶绿体与色素

Chloroplasts are the organelles where photosynthesis occurs. They contain chlorophyll, the green pigment that captures light energy.

叶绿体是发生光合作用的细胞器。它们含有叶绿素，这是一种捕获光能的绿色色素。

Chlorophyll absorbs light most strongly in the blue-violet and red regions of the spectrum. It reflects green light, which is why leaves appear green.

叶绿素在光谱的蓝紫光区域和红光区域吸收最强。它反射绿光，因此叶片呈绿色。

Inside a chloroplast, the thylakoid membranes contain chlorophyll and become the site of the light-dependent reactions. The surrounding fluid is called the stroma and is the site of the light-independent reactions.

在叶绿体内部，类囊体膜含有叶绿素，是光依赖反应的场所。周围的液体称为基质，是光不依赖反应的场所。

5. The Light-Dependent Stage | 光依赖阶段

The light-dependent stage occurs in the thylakoid membranes and requires light energy directly. Light energy is absorbed by chlorophyll and used to split water molecules into oxygen, protons and electrons.

光依赖阶段发生在类囊体膜上，需要直接的光能。叶绿素吸收光能，用来将水分子分解为氧气、质子和电子。

This splitting of water is called photolysis. The oxygen produced here comes from water, not from carbon dioxide.

水的这种分解称为光解。这里产生的氧气来自水，而不是二氧化碳。

The energy released from light is used to convert ADP and phosphate into ATP and to reduce the coenzyme NADP to reduced NADP. Both ATP and reduced NADP are then used in the next stage of photosynthesis.

释放的光能用于将ADP和磷酸基转化为ATP，并将辅酶NADP还原为还原型NADP。然后ATP和还原型NADP都用于光合作用的下一个阶段。

6. The Light-Independent Stage | 光不依赖阶段

The light-independent stage, also called the Calvin cycle, takes place in the stroma. It does not need light directly, but it depends on the ATP and reduced NADP produced in the light-dependent stage.

光不依赖阶段，也称为卡尔文循环，发生在基质中。它不需要光直接参与，但依赖于光依赖阶段产生的ATP和还原型NADP。

Carbon dioxide is captured by a five-carbon molecule called RuBP. This is then converted into two molecules of a three-carbon compound using the energy from ATP and the hydrogen from reduced NADP.

二氧化碳被一种称为RuBP的五碳分子捕获。然后利用ATP中的能量和还原型NADP中的氢，将其转化为两个三碳化合物分子。

Through a series of reactions, glucose and other carbohydrates are synthesised. The initial acceptor molecule, RuBP, is regenerated so the cycle can continue.

通过一系列反应，合成葡萄糖和其他碳水化合物。最初的受体分子RuBP被再生，从而使循环得以继续。

7. Limiting Factors of Photosynthesis | 光合作用的限制因素

A limiting factor is a condition that, when present at a low level, restricts the rate of photosynthesis. In IGCSE Science, the three main limiting factors are light intensity, carbon dioxide concentration and temperature.

限制因素是指当某种条件处于较低水平时，会限制光合作用速率的因素。在IGCSE科学中，三个主要限制因素是光强度、二氧化碳浓度和温度。

At low light intensity, increasing light intensity raises the rate of photosynthesis. However, once light is no longer limiting, further increases have little effect.

在低光强度下，增加光强度会提高光合作用速率。然而，一旦光不再成为限制因素，继续增加光强度影响很小。

Carbon dioxide is a raw material, so increasing its concentration generally speeds up photosynthesis until other factors become limiting. Temperature affects enzyme activity; photosynthesis works best at an optimum temperature, usually around 25-35°C for many plants, and stops if enzymes are denatured.

二氧化碳是原料，因此增加其浓度通常会加快光合作用，直到其他因素成为限制因素。温度影响酶活性；光合作用在适宜温度下效率最高，许多植物通常在25-35°C左右，如果酶变性则停止。

Examiners often ask you to interpret graphs of these limiting factors. Remember that the curve levels off when another factor becomes limiting.

考官经常要求你解读这些限制因素的曲线图。记住，当另一个因素成为限制因素时，曲线会趋于平缓。

8. Investigating Photosynthesis | 探究光合作用

A common experiment uses pondweed (Elodea) to measure the rate of photosynthesis by counting bubbles of oxygen released per minute.

一个常见实验使用池塘草（伊乐藻）通过计算每分钟释放的氧气气泡数来测量光合作用速率。

The variables that can be changed include light intensity (by moving a lamp closer or farther), carbon dioxide concentration (by adding sodium hydrogencarbonate), and temperature (by using a water bath).

可改变的变量包括光强度（通过移动灯靠近或远离）、二氧化碳浓度（通过添加碳酸氢钠）和温度（通过使用水浴）。

To make the experiment reliable, you must control all other factors. For example, keep the temperature constant and use the same piece of pondweed for each test.

为了使实验可靠，你必须控制所有其他因素。例如，保持温度恒定，并在每次测试中使用同一株伊乐藻。

Another method uses leaf discs: small discs are placed in a sodium hydrogencarbonate solution under vacuum so they sink. As photosynthesis produces oxygen, gas accumulates in the leaf discs and they float. The time taken for discs to rise is used to compare rates.

另一种方法使用叶片圆片：将小圆片放入碳酸氢钠溶液中并抽真空，使它们下沉。随着光合作用产生氧气，气体在叶片圆片中积累，圆片就会浮起。利用圆片浮起所需的时间来比较速率。

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

Photosynthesis and respiration are opposite processes in terms of overall energy change and gas exchange. Photosynthesis stores energy from sunlight in glucose; respiration releases energy from glucose.

光合作用和呼吸作用在总体能量变化和气体交换上是相反的过程。光合作用将来自太阳光的能量储存在葡萄糖中；呼吸作用则从葡萄糖中释放能量。

Photosynthesis	Respiration
Absorbs CO₂	Releases CO₂
Releases O₂	Absorbs O₂
Requires light	Occurs all the time
Stores energy	Releases energy

光合作用	呼吸作用
吸收CO₂	释放CO₂
释放O₂	吸收O₂
需要光	随时进行
储存能量	释放能量

Plants carry out both processes. During daytime, photosynthesis may exceed respiration, so the plant takes in CO₂ and releases O₂. At night, only respiration occurs, so the plant takes in O₂ and releases CO₂.

植物同时进行这两个过程。在白天，光合作用可能超过呼吸作用，因此植物吸入CO₂并释放O₂。在夜间，只进行呼吸作用，因此植物吸入O₂并释放CO₂。

10. The Fate of Glucose | 葡萄糖的去向

Glucose produced by photosynthesis is used in several ways. A major use is respiration to release energy for essential life processes.

光合作用产生的葡萄糖有多种用途。一个主要用途是呼吸作用，为生命活动释放能量。

Excess glucose can be converted into starch. Starch is insoluble and is stored in leaves, roots, stems and seeds. It can be hydrolysed back to glucose when needed.

多余的葡萄糖可以转化为淀粉。淀粉不溶于水，储存在叶、根、茎和种子中。需要时可以水解回葡萄糖。

Glucose is also used to make cellulose for cell walls and to synthesise proteins when combined with nitrate ions from the soil. Some glucose is converted into lipids for storage.

葡萄糖还用于制造细胞壁中的纤维素，并与从土壤中吸收的硝酸根离子结合合成蛋白质。部分葡萄糖转化为脂质储存。

11. Practical Exam Points | 实验考试要点

In the IGCSE practical paper, you may be tested on your understanding of photosynthesis experiments. Key terms include ‘rate of photosynthesis’, ‘independent variable’, ‘dependent variable’ and ‘control variable’.

在IGCSE实验考试中，你可能会被考察对光合作用实验的理解。关键术语包括’光合作用速率’、’自变量’、’因变量’和’控制变量’。

When measuring oxygen production, you should wait for a steady rate before counting bubbles. This ensures that the plant has adapted to the new conditions.

在测量氧气产生时，你应该等待速率稳定后再计数气泡。这确保植物已经适应了新的条件。

Remember to repeat your measurements and calculate a mean to reduce the effect of random errors. The lamp, if used, should be placed at the same height and distance in each trial unless it is the variable being tested.Published by TutorHao | IGCSE Science Revision Series | aleveler.com

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Photosynthesis in IGCSE Science | IGCSE科学中的光合作用