Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the process by which green plants and certain bacteria use light energy, carbon dioxide and water to produce glucose and oxygen. It is the foundation of almost all food chains on Earth and a core topic in the Edexcel IGCSE Science specification.

光合作用是绿色植物和某些细菌利用光能、二氧化碳和水制造葡萄糖与氧气的过程。它是地球上几乎所有食物链的基础，也是 Edexcel IGCSE 科学考纲的核心内容。

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

Photosynthesis is an endothermic reaction in which light energy is absorbed by chlorophyll and converted into chemical energy stored in glucose. The process can be summarised by a word equation that you must be able to write from memory.

光合作用是一个吸热反应：叶绿素吸收光能，并将其转化为储存在葡萄糖中的化学能。这个过程可以用文字方程式概括，你必须能够凭记忆写出该方程式。

Carbon dioxide + Water → Glucose + Oxygen

This equation shows that the raw materials are carbon dioxide and water, while the products are glucose and oxygen. Light energy and chlorophyll act as conditions, not reactants.

该方程式表明原料是二氧化碳和水，产物是葡萄糖和氧气。光能和叶绿素是反应条件，而不是反应物。

You should also know where each raw material comes from: carbon dioxide enters the leaf from the air through stomata, and water is absorbed by root hairs from the soil and transported to the leaves in xylem vessels.

你还应知道每种原料的来源：二氧化碳通过气孔从空气进入叶片；水分由根毛从土壤中吸收，并通过木质部导管运输到叶片。

2. Chemical Equation and Energy | 化学方程式与能量

The balanced chemical equation for photosynthesis shows the exact number of molecules involved and is frequently examined in IGCSE questions.

光合作用的配平化学方程式表明了参与反应的分子的精确数量，在 IGCSE 考试中经常考查。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

In this equation, six molecules of carbon dioxide react with six molecules of water to form one molecule of glucose and six molecules of oxygen. The equation is balanced because the number of atoms of each element is the same on both sides.

在该方程式中，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。方程式两边各元素的原子数量相同，因此是配平的。

Energy is transferred from the environment to the glucose molecules. This stored chemical energy can later be released by respiration when the plant or an animal that eats the plant needs it.

能量从环境转移到葡萄糖分子中。这些储存的化学能以后可以在植物或取食植物的动物需要时，通过呼吸作用释放出来。

3. Structure of Leaves | 叶片结构

The leaf is a specialised organ adapted to maximise the rate of photosynthesis. Each tissue has a clear function that examiners expect you to link directly to the process.

叶片是特化的器官，其结构适应于最大化光合作用速率。每种组织都有明确的功能，考官期望你将它们与光合作用直接联系起来。

Upper epidermis: a transparent protective layer that allows light to pass through.
Palisade mesophyll: contains many chloroplasts and is located near the upper surface to absorb maximum light.
Spongy mesophyll: has large air spaces that allow carbon dioxide to diffuse quickly to photosynthesising cells.
Stomata and guard cells: control gas exchange and water loss through transpiration.
Xylem and phloem in veins: transport water to the leaf and glucose away from it.

上表皮：透明的保护层，允许光线穿透。

栅栏组织：含有大量叶绿体，位于靠近上表面的位置，以吸收最多的光。

海绵组织：具有大的气隙，使二氧化碳能够快速扩散到进行光合作用的细胞。

气孔和保卫细胞：控制气体交换以及蒸腾作用造成的水分散失。

叶脉中的木质部和韧皮部：将水分运入叶片，并将葡萄糖运走。

4. Chloroplasts and Pigments | 叶绿体与色素

Photosynthesis takes place inside chloroplasts, which contain the green pigment chlorophyll. Chlorophyll is essential because it absorbs light energy and uses it to drive the reaction.

光合作用发生在叶绿体内部，叶绿体含有绿色色素——叶绿素。叶绿素至关重要，因为它吸收光能并利用光能驱动反应。

Chlorophyll absorbs red and blue-violet wavelengths most effectively and reflects green light, which is why leaves appear green. This is a common multiple-choice question in Edexcel past papers.

叶绿素最有效地吸收红光和蓝紫光波长的光，并反射绿光，这就是叶片呈绿色的原因。这是 Edexcel 历年真题中常见的单选题。

During autumn, chlorophyll breaks down and other pigments such as carotenoids become visible, giving leaves yellow-orange colours. You do not need to memorise pigment names, but understanding absorption is useful.

在秋季，叶绿素分解，类胡萝卜素等其他色素显现出来，使叶片呈现黄橙色。你无需记住色素名称，但理解吸收特性是有用的。

5. Rate of Photosynthesis | 光合作用速率

The rate of photosynthesis can be measured in several ways. The most common laboratory method uses an aquatic plant such as Cabomba or Elodea.

光合作用速率可以通过多种方法测量。最常用的实验室方法使用水生植物，例如金鱼藻或伊乐藻。

Count the number of bubbles of oxygen produced per minute.
Collect the oxygen gas in a measuring cylinder and record its volume per unit time.
Use a data logger or probe to measure oxygen concentration or pH changes in the water.

计算每分钟产生的氧气气泡数量。

将氧气收集在量筒中，记录单位时间内的体积。

使用数据记录器或传感器测量水中氧气浓度或 pH 值的变化。

When calculating rate, you should rearrange the formula rate = quantity ÷ time. Temperature must be kept constant in these experiments because it affects enzyme activity and bubble size.

计算速率时，应使用公式：速率 = 数量 ÷ 时间。实验中必须保持温度恒定，因为温度影响酶活性和气泡大小。

6. Limiting Factors | 限制因素

A limiting factor is a condition that, when present at too low a level, restricts the rate of photosynthesis. Increasing the limiting factor will speed up the reaction until another factor becomes limiting.

限制因素是指当水平过低时限制光合作用速率的条件。提高限制因素可以在另一个因素成为限制因素之前加快反应速率。

Factor	Effect
Light intensity	As light intensity increases, rate increases until another factor becomes limiting.
Carbon dioxide concentration	A higher CO₂ concentration speeds up photosynthesis until the graph plateaus.
Temperature	Rate rises as enzymes become more active, then falls sharply when enzymes denature.

因素：光强度、二氧化碳浓度、温度；对应影响：光强度增加速率增加直到另一因素成为限制因素；二氧化碳浓度升高加速光合作用直至曲线趋于平缓；温度升高酶活性增强速率上升，酶变性后速率急剧下降。

7. Light Intensity Experiment | 光强度实验

In the pondweed experiment, a lamp is placed at different distances from the plant and the bubbles produced per minute are counted. The closer the lamp, the higher the light intensity and the faster the rate.

在伊乐藻实验中，将灯放置在与植物不同的距离处，并计算每分钟产生的气泡数。灯越近，光强度越高，速率越快。

The relationship between distance and light intensity follows the inverse square law. If you double the distance, the light intensity becomes one quarter of its original value.

距离与光强度之间的关系遵循平方反比定律。如果距离加倍，光强度将变为原来的四分之一。

Light intensity ∝ 1 ÷ d²

To make the experiment fair, you must control the temperature, the concentration of carbon dioxide in the water, and use the same piece of pondweed. Sodium hydrogencarbonate is often added to the water to provide a constant supply of carbon dioxide.

为确保实验公平，必须控制温度、水中二氧化碳浓度，并使用同一株伊乐藻。通常向水中添加碳酸氢钠，以提供恒定的二氧化碳供应。

8. Uses of Glucose | 葡萄糖的用途

Glucose produced in photosynthesis has several uses in the plant. You should be able to list at least four of them and explain each briefly in a two-mark or four-mark question.

光合作用产生的葡萄糖在植物体内有多种用途。你应该能够列出至少四种，并在两分或四分的题目中简要解释每一种。

Used in respiration to release energy for growth and reproduction.
Converted to starch and stored in roots, stems and seeds.
Converted to cellulose for strengthening cell walls.
Converted to amino acids combined with nitrate ions for making proteins.
Converted to fats and oils for storage in seeds.

用于呼吸作用，释放能量以支持生长和繁殖。

转化为淀粉，储存在根、茎和种子中。

转化为纤维素，用于加固细胞壁。

与硝酸根离子结合转化为氨基酸，用于制造蛋白质。

转化为脂肪和油脂，储存在种子中。

9. Mineral Requirements | 矿物质需求

Plants cannot make all molecules from glucose alone. They require mineral ions absorbed from the soil to synthesise essential biological molecules.

植物不可能仅靠葡萄糖制造所有分子。它们需要从土壤中吸收矿物质离子来合成必需的生物分子。

Nitrate ions (NO₃⁻) + glucose → amino acids → proteins

Nitrate deficiency causes stunted growth and yellowing of older leaves because proteins, DNA and enzymes cannot be made. Magnesium ions are needed to make chlorophyll, so magnesium deficiency causes chlorosis, where leaves turn yellow.

缺氮会导致生长矮小和老叶发黄，因为无法制造蛋白质、DNA 和酶。镁离子用于制造叶绿素，因此缺镁会引起失绿症，叶片变黄。

Phosphate ions are also required for the production of DNA, ATP and cell membranes. A shortage of phosphate results in poor root growth and dark or purple tinted leaves.

磷酸根离子也是合成 DNA、ATP 和细胞膜所必需的。缺磷会导致根系生长不良和叶片呈暗色或紫色。

10. Photosynthesis versus Respiration | 光合作用与呼吸作用对比

Photosynthesis and respiration are complementary processes. Photosynthesis stores energy, while respiration releases energy. Examiners often ask you to compare them in a table.

光合作用和呼吸作用是互补的过程。光合作用储存能量，而呼吸作用释放能量。考官经常要求你以表格形式比较二者。

Feature	Photosynthesis	Respiration
Energy	Stores energy (endothermic)	Releases energy (exothermic)
Equation	CO₂ + H₂O → C₆H₁₂O₆ + O₂	C₆H₁₂O₆ + O₂ → CO₂ + H₂O
When it occurs	Only in daylight	All the time
Where it occurs	Chloroplasts	Cytoplasm and mitochondria

能量方面光合作用储存能量（吸热），呼吸作用释放能量（放热）。方程式互为逆过程。光合作用只在白天有光时发生，呼吸作用随时进行。光合作用发生在叶绿体中，呼吸作用发生在细胞质和线粒体中。

11. Exam Tips | 考试要点

The following points are frequently tested in Edexcel IGCSE Science papers. Memorising them will help you secure full marks on photosynthesis questions.

以下几点在 Edexcel IGCSE 科学试卷中经常考查。牢记它们将帮助你在光合作用题目中获得满分。

Always write glucose as C₆H₁₂O₆ and oxygen as O₂ when balancing equations.
Remember that light intensity decreases by the square of the distance.
On a limiting factor graph, the horizontal section means another factor is now limiting.
Use the phrase “enzymes denature” only above optimum temperature, not for low temperature.
State that photosynthesis is an endothermic reaction because it absorbs light energy.

配平方程式时，始终将葡萄糖写作 C₆H₁₂O₆，氧气写作 O₂。

牢记光强度随距离的平方而减小。

在限制因素曲线图中，水平段意味着另一个因素现在起限制作用。

只有温度超过最适温度时才使用”酶变性”这一表述，低温时不能说酶变性。

说明光合作用是吸热反应，因为它吸收光能。

In any six-mark question, structure your answer: state the dependent variable, name at least two control variables, describe how to change the independent variable, and explain how you would improve reliability by repeating and calculating the mean.

在六分题中，请组织好答案结构：说明因变量，至少写出两个控制变量，描述如何改变自变量，并解释如何通过重复实验和计算平均值来提高可靠性。

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Photosynthesis | 光合作用