Photosynthesis and Plant Nutrition | 光合作用与植物营养

📚 Photosynthesis and Plant Nutrition | 光合作用与植物营养

Photosynthesis is the process by which green plants, algae and some bacteria use light energy to convert carbon dioxide and water into glucose and oxygen. This process stores energy in the chemical bonds of glucose, providing the energy source for nearly all living organisms on Earth.

光合作用是绿色植物、藻类和某些细菌利用光能，将二氧化碳和水转化为葡萄糖和氧气的过程。该过程将能量储存在葡萄糖的化学键中，为地球上几乎所有生物提供能量来源。

1. Word Equation for Photosynthesis | 光合作用的文字方程式

During photosynthesis, plants absorb carbon dioxide through the stomata and water through the roots. Light energy is absorbed by chlorophyll, the green pigment found in chloroplasts, and is used to combine these reactants into glucose. Oxygen is released as a waste product.

在光合作用过程中，植物通过气孔吸收二氧化碳，通过根系吸收水分。叶绿体中的绿色色素叶绿素吸收光能，并利用这些能量将反应物合成为葡萄糖，同时释放出氧气作为副产物。

Carbon dioxide + Water → Glucose + Oxygen

(light energy absorbed by chlorophyll)

In Edexcel IGCSE, you must be able to write this word equation from memory and identify the energy source and conditions above the arrow. The energy that drives the reaction comes from sunlight, not from the plant itself.

在 Edexcel IGCSE 考试中，你需要能够凭记忆写出这个文字方程式，并标出箭头上方的能量来源和反应条件。推动反应的能量来自阳光，而不是植物本身。

2. Balanced Symbol Equation | 平衡符号方程式

The balanced chemical equation shows the exact number of atoms involved. Six molecules of carbon dioxide react with six molecules of water to form one molecule of glucose and six molecules of oxygen.

平衡化学方程式显示了参与反应的原子的精确数目。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

Check the atom balance: 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms appear on both sides of the equation. This equation also demonstrates that photosynthesis is an endothermic reaction, because energy from light is converted into chemical energy stored in glucose.

检查原子守恒：方程式两侧各含有6个碳原子、12个氢原子和18个氧原子。该方程式还表明光合作用是一个吸热反应，因为光能被转化为储存在葡萄糖中的化学能。

3. Photosynthesis as an Endothermic Reaction | 光合作用——吸热反应

An endothermic reaction is one that takes in energy from its surroundings. During photosynthesis, light energy is absorbed and stored as chemical energy in glucose. This contrasts strongly with respiration, which is an exothermic reaction that releases energy from glucose.

吸热反应是从周围环境吸收能量的反应。在光合作用中，光能被吸收并以化学能的形式储存在葡萄糖中。这与呼吸作用形成鲜明对比——呼吸作用是从葡萄糖中释放能量的放热反应。

For this reason, photosynthesis can only occur in the presence of light and chlorophyll. Respiration, however, occurs continuously in all living cells, both during the day and at night. Many students lose marks by stating that plants photosynthesise at night; in the dark, plants only respire.

因此，光合作用只能在有光照和叶绿素的条件下发生。而呼吸作用在所有活细胞中持续进行，不分昼夜。许多学生因写到“植物在夜间进行光合作用”而丢分；在黑暗中，植物只进行呼吸作用。

4. Uses of Glucose | 葡萄糖的用途

Glucose produced during photosynthesis is used by the plant in several important ways. It may be converted into starch for storage, or into cellulose to build strong cell walls. Glucose can also be combined with nitrate ions to form amino acids, which are then assembled into proteins.

光合作用产生的葡萄糖在植物中有多种重要用途。它可以转化为淀粉用于储存，或转化为纤维素以构建坚硬的细胞壁。葡萄糖还可以与硝酸根离子结合形成氨基酸，进而组装成蛋白质。

• Starch: an insoluble storage molecule stored in roots, stems and seeds / 淀粉：不溶性储存分子，储存在根、茎和种子中。
• Cellulose: strengthens plant cell walls / 纤维素：增强植物细胞壁。
• Amino acids: made together with nitrate ions, then built into proteins / 氨基酸：与硝酸根离子一起合成，进一步构建蛋白质。
• Fats and oils: stored as energy reserves in seeds / 脂肪和油：作为能量储备储存在种子中。
• Sucrose: a soluble sugar used for transport in the phloem / 蔗糖：可溶性糖，用于在韧皮部中运输。

An exam question may ask why starch is used for storage rather than glucose. The answer is that starch is insoluble, so it does not affect the water potential of the cell contents and is easier to store without influencing osmosis.

考试题目可能会问：储存时为什么用淀粉而不用葡萄糖？答案是：淀粉不溶于水，因此不会影响细胞内容物的渗透势，储存起来不会干扰渗透作用。

5. Leaf Structure and Adaptations | 叶的结构与适应性

The leaf is the main photosynthetic organ of most plants. Its structure maximises light absorption, gas exchange and water supply. The following table summarises the key tissues found in a dicotyledonous leaf.

叶是大多数植物主要的光合作用器官。它的结构可以最大限度提高光能吸收、气体交换和水分供应的效率。下表总结了双子叶植物叶片中的主要组织。