Photosynthesis: How Green Plants Make Food | 光合作用：绿色植物如何制造食物

📚 Photosynthesis: How Green Plants Make Food | 光合作用：绿色植物如何制造食物

Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy stored in glucose. It is one of the most important biological reactions on Earth because it provides food, oxygen, and raw materials for nearly all other organisms.

光合作用是绿色植物、藻类和某些细菌把光能转化为储存在葡萄糖中的化学能的过程。它是地球上最重要的生物反应之一，因为几乎所有其他生物都依赖它获得食物、氧气和基本原料。

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1. The Word and Symbol Equation | 文字方程式与符号方程式

The overall process of photosynthesis can be summarised using a word equation. Carbon dioxide and water react together in the presence of light and chlorophyll to produce glucose and oxygen.

光合作用的整体过程可以用文字方程式概括。二氧化碳和水在光照和叶绿素的存在下反应，生成葡萄糖和氧气。

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

The balanced chemical symbol equation shows the same reaction in terms of molecules:

配平的化学符号方程式从分子的角度表示了同一反应：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Six molecules of carbon dioxide react with six molecules of water to make one molecule of glucose and six molecules of oxygen. This equation shows that the mass of atoms is conserved during the reaction.

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。该方程式表明反应过程中原子的质量是守恒的。

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2. The Leaf as a Photosynthetic Organ | 叶片作为光合作用器官

The leaf is the main organ of photosynthesis in most green plants. Its structure is highly adapted to capture light, exchange gases and avoid water loss.

叶片是大多数绿色植物进行光合作用的主要器官。它的结构高度适应于捕获光、交换气体并减少水分流失。

• A large surface area helps absorb as much sunlight as possible.

  巨大的表面积有助于尽可能多地吸收阳光。

• A thin leaf shape means carbon dioxide does not have to travel far to reach photosynthesising cells.

  叶片薄，意味着二氧化碳不需要穿过很远距离就能到达进行光合作用的细胞。

• Transparent upper epidermal cells allow light to reach the palisade mesophyll layer below.

  透明的上表皮细胞让光能够到达下方的栅栏叶肉层。

• Palisade mesophyll cells contain many chloroplasts, which hold the green pigment chlorophyll.

  栅栏叶肉细胞含有大量叶绿体，叶绿体里有绿色色素叶绿素。

Stomata, or microscopic pores, are usually found on the lower leaf surface. They allow carbon dioxide to enter and oxygen to leave. Each stoma is surrounded by guard cells that control its opening and closing.

气孔是通常位于叶片下表面的微小孔隙，让二氧化碳进入，让氧气排出。每个气孔由一对保卫细胞包围，保卫细胞控制气孔的开放和关闭。

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3. Light and Chlorophyll | 光与叶绿素

Photosynthesis is an endothermic reaction because it takes in energy from the surroundings. The energy comes from light, usually sunlight.

光合作用是吸热反应，因为从周围环境吸收能量。这种能量来自光，通常是太阳光。

Chlorophyll is the pigment found inside chloroplasts. It absorbs light energy, especially blue and red light, and reflects green light. That is why leaves look green.

叶绿素是存在于叶绿体内的色素。它吸收光能，尤其是蓝光和红光，并反射绿光，因此叶片看起来是绿色的。

During the light-dependent stage, light energy is used to split water molecules into hydrogen and oxygen. The oxygen is released as a waste product, while the hydrogen is used to help build glucose in later stages.

在光依赖性阶段，光能用于将水分子分解为氢和氧。氧气作为废弃物释放，而氢则在后续阶段被用于构建葡萄糖。

Without light, photosynthesis cannot occur. If a leaf is kept in darkness overnight, it will use up much of its glucose through respiration and will no longer give a positive starch test.

没有光，光合作用便不能进行。如果叶片在黑暗中放置一夜，它会通过呼吸作用消耗掉大量葡萄糖，就不再呈阳性淀粉检测结果。

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4. Carbon Dioxide and Water | 二氧化碳和水

Carbon dioxide enters the leaf through the stomata by diffusion. It dissolves in the moist film on the surface of mesophyll cells before diffusing into the cell and then into the chloroplasts.

二氧化碳通过气孔扩散进入叶片。它溶解在叶肉细胞表面的湿润水膜中，然后扩散进入细胞，再进入叶绿体。

Inside a photosynthesising cell, the concentration of carbon dioxide is kept low because it is constantly used up. This maintains a steep diffusion gradient, encouraging more carbon dioxide to enter.

在进行光合作用的细胞内部，二氧化碳浓度很低，因为它不断被消耗。这保持了陡峭的扩散梯度，促进更多二氧化碳进入细胞。

Water is absorbed by root hair cells from the soil. It travels through the xylem vessels up the stem and into the leaves, where it is used in photosynthesis and also helps support the plant.

水通过根毛细胞从土壤中吸收。它沿着木质部导管向上输送，经过茎进入叶片，用于光合作用以及帮助支持植物。

If a plant does not receive enough water, the stomata close to reduce water loss. This also reduces the entry of carbon dioxide, so photosynthesis slows down.

如果植物水分不足，气孔会关闭以减少水分流失。这同时也减少了二氧化碳的进入，因此光合作用会减慢。

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5. Rate of Photosynthesis and Limiting Factors | 光合作用速率与限制因素

The rate of photosynthesis is affected by several environmental factors. The most important are light intensity, carbon dioxide concentration and temperature.

光合作用速率受多种环境因素影响，最重要的是光照强度、二氧化碳浓度和温度。

A limiting factor is the factor that is in shortest supply. If you increase that factor, the rate of photosynthesis increases until another factor becomes limiting.

限制因素是最短缺的因素。如果提高该因素，光合作用速率会增加，直到另一个因素变成限制因素。

• Light intensity: more light provides more energy, so photosynthesis increases. At very high light intensity, another factor such as carbon dioxide may become limiting.

  光照强度：光照越多，提供的能量越多，光合作用增强。在极高光照强度下，二氧化碳等因素可能成为限制因素。

• Carbon dioxide concentration: more carbon dioxide provides more raw material. However, once the plant has enough, increasing it higher has no effect.

  二氧化碳浓度：更多二氧化碳提供更多原料。但一旦达到足够浓度，继续增加便不再起作用。

• Temperature: as temperature rises, enzymes work faster and the rate increases. Above the optimum temperature, enzymes denature and the rate falls sharply.

  温度：温度升高时，酶活性增强，光合作用速率上升。超过最适温度后，酶会变性，速率急剧下降。

Farmers and gardeners often control these factors in greenhouses. For example, paraffin heaters provide heat and produce carbon dioxide as a waste product, which increases plant growth.

农民和园丁常在温室中控制这些因素。例如，石蜡加热器提供热量，同时产生二氧化碳副产品，从而促进植物生长。

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6. Investigating Photosynthesis | 探究光合作用

Several simple experiments can show that light, carbon dioxide and chlorophyll are needed for photosynthesis.

一些简单的实验可以证明光、二氧化碳和叶绿素是光合作用所必需的。

One classic method uses pondweed, such as Elodea. The plant is placed in water with dissolved sodium hydrogencarbonate to provide carbon dioxide. As photosynthesis happens, bubbles of oxygen are released from the cut stem.

一种经典方法使用水蕴草等水生植物。将植物放在含有溶解的碳酸氢钠的水中，以提供二氧化碳。光合作用进行时，氧气气泡从切开的茎端释放出来。

The number of bubbles per minute gives an estimate of the rate of photosynthesis. A more accurate method collects the gas in a graduated tube and measures the volume of oxygen produced in a fixed time.

每分钟的气泡数可以估算光合作用速率。更精确的方法是用刻度管收集气体，并测量固定时间内产生的氧气体积。

To test for starch, a leaf is boiled in water to kill it, then boiled in ethanol to remove the chlorophyll, then washed and tested with iodine solution. If starch is present, the iodine turns blue-black.

若要检测淀粉，先将叶片在沸水中煮以杀死细胞，再用乙醇煮沸以去除叶绿素，然后清洗并用碘液检测。如果存在淀粉，碘液会变成蓝黑色。

A leaf that has been partly covered with opaque paper and exposed to light will only show starch in the parts that received light. This proves that light is necessary for photosynthesis.

将叶片部分用不透光纸遮盖并给予光照，只有被照的部分会显示淀粉。这证明光是光合作用所必需的。

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7. Uses of Glucose in Plants | 葡萄糖在植物中的用途

Glucose produced during photosynthesis is an immediate soluble carbohydrate. Plants use it in many ways.

光合作用产生的葡萄糖是即时的可溶性碳水化合物。植物以多种方式利用它。

• Respiration: glucose is broken down in cells to release energy for metabolic reactions.

  呼吸作用：葡萄糖在细胞中被分解以释放能量，供代谢反应使用。

• Starch: excess glucose can be converted into starch and stored in roots, stems and leaves. Starch is insoluble, so it does not affect water balance in cells.

  淀粉：多余的葡萄糖可以转化为淀粉，储存在根、茎和叶片中。淀粉不溶于水，因此不会影响细胞的水分平衡。

• Sucrose: glucose can be converted into sucrose, which is transported in the phloem to other parts of the plant.

  蔗糖：葡萄糖可以转化为蔗糖，通过韧皮部运输到植物其他部位。

• Cellulose: glucose molecules are joined together to form cellulose, a strong structural material in plant cell walls.

  纤维素：葡萄糖分子连接形成纤维素，这是植物细胞壁中的坚固结构材料。

• Amino acids: glucose provides the carbon skeleton, and with nitrate ions absorbed from the soil, plants make amino acids for protein synthesis.

  氨基酸：葡萄糖提供碳骨架，与从土壤中吸收的硝酸根离子结合，植物可以制造氨基酸用于蛋白质合成。

• Fats and oils: glucose can be converted into lipids for storage, especially in seeds.

  脂肪和油：葡萄糖可以转化为脂质储存，尤其是储存在种子中。

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8. Mineral Nutrients Needed for Healthy Growth | 健康生长所需的无机矿物质

Plants need more than glucose to grow healthily. They also take up mineral ions through their roots. Two important examples are nitrate ions and magnesium ions.

植物健康生长不仅需要葡萄糖，还需要通过根系吸收矿物质离子。两个重要例子是硝酸根离子和镁离子。

• Nitrate ions provide nitrogen, which is needed to make amino acids, proteins and DNA. A plant that lacks nitrate ions has stunted growth and yellow older leaves.

  硝酸根离子提供氮，氮用于制造氨基酸、蛋白质和DNA。缺乏硝酸根离子的植物生长矮小，老叶发黄。

• Magnesium ions are a central component of the chlorophyll molecule. Without magnesium, leaves turn yellow in a condition called chlorosis and photosynthesis is reduced.

  镁离子是叶绿素分子的中心成分。缺少镁时，叶片变黄，称为失绿症，光合作用减弱。

These minerals are dissolved in soil water and absorbed by root hair cells. Farmers may add fertilisers to the soil to replace lost minerals and increase crop yield.

这些矿物质溶解在土壤水中，并通过根毛细胞吸收。农民可以向土壤施用肥料以补充流失的矿物质，提高作物产量。

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9. Photosynthesis and the Atmosphere | 光合作用与大气

Photosynthesis has a huge effect on the global carbon and oxygen cycles. It removes carbon dioxide from the atmosphere and releases oxygen.

光合作用对全球碳循环和氧循环有巨大影响。它清除大气中的二氧化碳并释放氧气。

Almost all living organisms depend, directly or indirectly, on photosynthesis. Plants are producers in food chains. Herbivores eat plants, and carnivores eat herbivores, so the glucose made by plants becomes the energy source for entire ecosystems.

几乎所有生物都直接或间接依赖光合作用。植物是食物链中的生产者。草食动物吃植物，肉食动物吃草食动物，因此植物制造的葡萄糖成为整个生态系统的能量来源。

Photosynthesis also helps reduce the greenhouse effect by absorbing carbon dioxide. However, deforestation and the burning of fossil fuels have reduced the balance of this cycle.

光合作用还通过吸收二氧化碳帮助减缓温室效应。然而，森林砍伐和化石燃料燃烧已经破坏了这一循环的平衡。

When plants and animals respire, they release carbon dioxide back into the atmosphere. This photosynthesising-respiring balance maintains a roughly steady level of atmospheric oxygen over long periods.

当植物和动物呼吸时，它们会释放二氧化碳回到大气中。这种光合作用与呼吸作用的平衡在长期内维持了大气中氧气水平的大致稳定。

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10. Summary of Key Points | 要点总结

• Photosynthesis converts light energy into chemical energy stored in glucose.

  光合作用将光能转化为储存在葡萄糖中的化学能。

• The word equation is: carbon dioxide + water → glucose + oxygen.

  文字方程式为：二氧化碳 + 水 → 葡萄糖 + 氧气。

• The balanced symbol equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

  配平符号方程式为：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。

• Leaves are adapted for photosynthesis with a broad, thin shape, stomata and palisade cells containing chloroplasts.

  叶片通过宽而薄的形状、气孔以及含有叶绿体的栅栏细胞，适应光合作用。

• The main limiting factors are light intensity, carbon dioxide concentration and temperature.

  主要限制因素是光照强度、二氧化碳浓度和温度。

• Glucose is used for respiration, starch storage, cellulose, sucrose, amino acids and lipids.

  葡萄糖用于呼吸作用、储存淀粉、形成纤维素、蔗糖、氨基酸和脂质。

Understanding photosynthesis is essential for explaining plant growth, crop production and the maintenance of atmospheric gases. It connects many topics in Edexcel IGCSE Science, including cell structure, enzymes, transport in plants and ecology.

理解光合作用对于解释植物生长、作物产量和大气气体维持至关重要。它将 Edexcel IGCSE 科学中的许多主题联系起来，包括细胞结构、酶、植物运输和生态学。

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