📚 Photosynthesis & Plant Nutrition | 光合作用与植物营养
Photosynthesis is the process by which green plants make their own food using light energy. This topic is essential for IGCSE Biology and covers both the reactions involved and the practical factors that affect plant growth.
光合作用是指绿色植物利用光能制造自身食物的过程。这个主题是 IGCSE 生物学的重点，既涉及参与的反应，也涉及影响植物生长的实际因素。
1. How Plants Get Nutrition | 植物如何获取营养
Unlike animals, plants do not need to consume other organisms. They are autotrophs: they produce glucose from simple inorganic molecules using energy from sunlight.
与动物不同，植物不需要摄食其他生物。它们是自养生物：利用阳光的能量，从简单的无机物分子合成葡萄糖。
Plants absorb water and minerals through their roots, and carbon dioxide from the air through leaves. Chlorophyll, the green pigment, captures light energy in the chloroplasts.
植物通过根部吸收水分和矿物质，通过叶片从空气中吸收二氧化碳。叶绿素是绿色色素，在叶绿体中捕获光能。
2. The Word Equation | 文字表达式
Photosynthesis can be written simply as a word equation. Six carbon dioxide molecules combine with six water molecules to produce one glucose molecule and six oxygen molecules.
光合作用可以用文字表达式简单表示。六个二氧化碳分子与六个水分子结合，生成一个葡萄糖分子和六个氧分子。
Carbon dioxide + Water → Glucose + Oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
3. The Chemical Equation | 化学方程式
The balanced chemical equation shows the exact number of atoms on each side. You should memorise this equation and be able to write it with correct state symbols.
配平的化学方程式显示了两边原子的精确数量。你应该记住这个方程式，并能写出正确的状态符号。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Light energy and chlorophyll are written above the arrow as reaction conditions, not as reactants. Energy from sunlight is absorbed by chlorophyll and converted into chemical potential energy stored in glucose.
光能和叶绿素写在箭头上方作为反应条件，而不是反应物。阳光能量被叶绿素吸收，转化为储存在葡萄糖中的化学势能。
4. Investigating Photosynthesis | 探究光合作用
A classic experiment uses an aquatic plant such as Elodea. As the plant photosynthesises, bubbles of oxygen are released; counting these bubbles or collecting the gas in a measuring tube gives an estimate of the rate.
经典实验使用诸如伊乐藻等水生植物。当植物进行光合作用时，会释放出氧气气泡；计算气泡数量或在量气管中收集气体，可估算光合作用速率。
Control variables: Keep temperature, light intensity (distance), and pH constant.
Scr控制变量：保持温度、光照强度（距离）和 pH 恒定。
Measurement: Count bubbles for one minute, or measure the volume of gas over a fixed time.
测量：在一分钟内计数气泡，或在固定时间内测量气体体积。
A positive test for oxygen: relight a glowing splint. This confirms the gas released is oxygen.
检验氧气的阳性实验：带火星的木条复燃。这证实释放的气体是氧气。
5. Limiting Factors | 限制因素
If a factor is in short supply, it limits the rate of photosynthesis. Three main factors affect the rate: light intensity, carbon dioxide concentration, and temperature.
如果某种因素供应不足，就会限制光合作用的速率。三个主要因素影响速率：光照强度、二氧化碳浓度和温度。
| Factor | 因素 | Effect | 影响 |
| Light intensity 光照强度 | As light increases, rate increases up to a plateau. |
| CO₂ concentration 二氧化碳浓度 | Higher CO₂ raises rate until another factor limits. |
| Temperature 温度 | Rate increases with temperature, but falls above the optimum as enzymes denature. |
Farmers use this knowledge by adding artificial lighting, CO₂ enrichment in greenhouses, and maintaining an optimum temperature to maximise crop yield.
农民利用这些知识，通过增加人工光照、在温室中增施 CO₂，并维持最适温度来最大化作物产量。
6. Uses of Glucose | 葡萄糖的用途
Glucose produced in photosynthesis is used in several ways. Some is used immediately in respiration to release energy; the rest may be converted for storage.
光合作用产生的葡萄糖有多种用途。部分立即用于呼吸作用释放能量；其余则可能转化储存。
Respiration: Glucose is broken down to release energy for cell activities.
呼吸作用：葡萄糖被分解以释放能量供细胞活动。
Starch: Stored in leaves, stems, roots, and seeds as a dense energy reserve.
淀粉：储存在叶、茎、根和种子中，作为致密的能量储备。
Sucrose: Transported in the phloem to other parts of the plant.
蔗糖：通过韧皮部运输到植物其他部位。
Cellulose: Used to build strong cell walls.
纤维素：用于构建坚固的细胞壁。
Fats/oils: Stored in seeds as a concentrated energy source.
脂肪/油：储存在种子中作为浓缩能源。
7. Mineral Requirements | 矿物质需求
Plants also need mineral ions absorbed from the soil. These are not used for energy, but are essential for building proteins and other molecules.
植物还需要从土壤中吸收矿物质离子。这些物质不用于提供能量，但对构建蛋白质和其他分子至关重要。
| Mineral 矿物质 | Importance 重要性 | Deficiency symptom 缺乏症状 |
| Nitrates (N) 硝酸盐 | For amino acids and proteins 用于氨基酸和蛋白质 | Stunted growth, yellowing leaves 生长受阻、叶片发黄 |
| Magnesium (Mg) 镁 | Central atom of chlorophyll 叶绿素中心原子 | Chlorosis (yellow leaves) 失绿（叶片变黄） |
Nitrate deficiency limits protein production, so plant growth is poor. Magnesium deficiency prevents chlorophyll synthesis, causing yellow leaves that cannot photosynthesise effectively.
缺乏硝酸盐会限制蛋白质合成，导致植物生长不良。缺乏镁会阻碍叶绿素合成，导致叶片变黄，无法有效进行光合作用。
8. Leaf Structure and Adaptation | 叶片结构与适应
The leaf is the main photosynthetic organ. Its structure is adapted to maximise light absorption, gas exchange, and water loss control.
叶片是主要的光合作用器官。其结构经过适应，能最大化光吸收、气体交换和水分流失控制。
Chlorophyll: Found in chloroplasts in palisade cells near the upper surface.
叶绿素：位于靠近上表皮的栅栏细胞的叶绿体中。
Thin shape: Short diffusion distance for gases.
扁平形状：气体扩散距离短。
Large surface area: More light capture and gas exchange.
大表面积：能捕获更多光并进行更多气体交换。
Stomata: Pores on the lower epidermis allowing CO₂ in and O₂ out.
气孔：下表皮上的孔隙，允许 CO₂ 进入并释放 O₂。
Spongy mesophyll: Loose cells with air spaces for gas circulation.
海绵组织：细胞排列疏松，有气室便于气体循环。
9. Photosynthesis and Respiration | 光合作用与呼吸作用
Photosynthesis and respiration are opposite processes chemically. Photosynthesis stores energy, while respiration releases energy. Photosynthesis uses CO₂ and water; respiration produces them.
光合作用和呼吸作用在化学上是相反过程。光合作用储存能量，呼吸作用释放能量。光合作用消耗 CO₂ 和水；呼吸作用产生它们。
At night, plants only respire, so oxygen is consumed and CO₂ is produced. In daylight, photosynthesis usually occurs faster than respiration, giving a net uptake of CO₂ and net release of O₂.
夜间，植物只进行呼吸作用，因此消耗氧气并产生 CO₂。在白天，光合作用通常快于呼吸作用，净效果是吸收 CO₂ 和释放 O₂。
10. Exam Tips | 考试技巧
When answering questions about photosynthesis, always use the correct equation and mention limiting factors clearly. For graph questions, describe the shape: rising at first, then plateau.
回答关于光合作用的问题时，务必使用正确的方程式并清晰说明限制因素。对于图表题，描述曲线形状：先上升，后趋平。
Common mistakes include forgetting that temperature is described as an enzyme-controlled factor, or confusing the word equation with the chemical equation. Practice writing both without looking.
常见错误包括忘记温度是酶控制的因素，或混淆文字表达式与化学方程式。练习时不看答案写出两者。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图