📚 Photosynthesis | 光合作用
Photosynthesis is the process by which green plants, algae and some bacteria use sunlight, carbon dioxide and water to produce glucose and oxygen. It is one of the most important biological reactions on Earth.
光合作用是绿色植物、藻类和某些细菌利用阳光、二氧化碳和水合成葡萄糖和氧气的过程。它是地球上最重要的生物反应之一。
1. What is Photosynthesis? | 什么是光合作用？
Photosynthesis is an endothermic reaction in which light energy is converted into chemical energy. The chemical energy is stored inside the bonds of glucose, a simple sugar.
光合作用是一个吸热反应，光能被转化为化学能。化学能储存在葡萄糖（一种单糖）的化学键中。
+ oxygen. However, the reaction actually happens in many small steps controlled by enzymes.
整个过程可概括为：二氧化碳 + 水 → 葡萄糖 + 氧气。但实际上该反应由酶控制，分许多小步骤进行。
2. Word and Symbol Equations | 文字和符号方程式
The word equation is:
文字方程式为：
carbon dioxide + water → glucose + oxygen
The chemical symbol equation is:
化学符号方程式为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
In the Edexcel IGCSE specification, you must be able to write both equations and identify the state symbols: (g) for gas and (l) for liquid.
在Edexcel IGCSE考纲中，你必须会写这两种方程式，并识别状态符号：(g)表示气体，(l)表示液体。
3. The Requirements of Photosynthesis | 光合作用的条件
Photosynthesis requires four essential factors:
光合作用需要四个基本因素：
Light energy – absorbed by chlorophyll in chloroplasts. 光能——被叶绿体中的叶绿素吸收。
Carbon dioxide – enters the leaf through stomata. 二氧化碳——通过气孔进入叶片。
Water – absorbed by roots and transported to leaves. 水——由根吸收并运输到叶片。
Chlorophyll – a green pigment that captures light energy. 叶绿素——一种捕获光能的绿色色素。
Without any one of these, the rate of photosynthesis is reduced or stopped completely.
缺少其中任何一个因素，光合作用的速率都会降低或完全停止。
4. The Products and Uses | 产物和用途
The immediate product of photosynthesis is glucose. Glucose can be used in several ways:
光合作用的直接产物是葡萄糖。葡萄糖有多种用途：
Used for respiration to release energy. 用于呼吸作用释放能量。
Converted into starch for storage. 转化为淀粉储存。
Converted into sucrose for transport around the plant. 转化为蔗糖在植物体内运输。
Converted into cellulose for cell walls. 转化为纤维素构成细胞壁。
Converted into amino acids when combined with nitrate ions. 与硝酸根离子结合转化为氨基酸。
Stored as oils or fats in seeds. 在种子中以油脂或脂肪形式储存。
Oxygen is a waste product and diffuses out of the leaf through stomata.
氧气是废物产物，通过气孔扩散出叶片。
5. The Rate of Photosynthesis | 光合作用速率
The rate of photosynthesis can be measured by the volume of oxygen produced per unit time, or by the rate of carbon dioxide uptake.
光合作用速率可以通过单位时间内产生的氧气体积或吸收二氧化碳的速率来测量。
In the lab, we often use pondweed and count the bubbles of oxygen produced. A more accurate method collects the gas in a capillary tube or syringe and measures its volume.
在实验室中，我们常用金鱼藻并数产生的氧气气泡。更准确的方法是用毛细管或注射器收集气体并测量其体积。
6. Limiting Factors | 限制因素
A limiting factor is a condition that, when in short supply, slows down the rate of a reaction. For photosynthesis, the main limiting factors are:
限制因素是当供应不足时会减慢反应速率的条件。对于光合作用，主要限制因素有：
Light intensity. 光照强度。
Carbon dioxide concentration. 二氧化碳浓度。
Temperature. 温度。
Light intensity is related to the distance from the light source by the inverse square law. If distance is doubled, light intensity falls to one quarter.
光照强度与光源距离的关系遵循平方反比定律。如果距离加倍，光照强度降为四分之一。
light intensity ∝ 1 / distance²
Temperature affects enzyme activity. At low temperatures, reactions are slow. As temperature rises, the rate increases up to an optimum (often around 30-35 °C). Above the optimum, enzymes denature and the rate falls sharply.
温度影响酶活性。低温时反应缓慢。随着温度升高，速率增加到最适温度（通常约30-35 °C）。超过最适温度后，酶变性，速率急剧下降。
7. Chlorophyll and Leaf Structure | 叶绿素与叶片结构
Chlorophyll is located in chloroplasts, which are most abundant in the palisade layer of the leaf. The leaf is adapted for photosynthesis in several ways:
叶绿素位于叶绿体中，叶绿体在叶片的栅栏层中最为丰富。叶片在多个方面适应光合作用：
Large surface area to absorb more light. 大表面积以吸收更多光。
Thin, so gases diffuse quickly. 薄，气体扩散快。
Transparent upper epidermis allows light to reach palisade cells. 透明的上表皮使光到达栅栏细胞。
Stomata allow gas exchange. 气孔允许气体交换。
Xylem and phloem transport water and glucose. 木质部和韧皮部运输水和葡萄糖。
You should be able to label a cross-section of a leaf and explain the function of each tissue.
你应该能够标注叶片横截面并解释每种组织的功能。
8. Experiments on Photosynthesis | 光合作用实验
Several classic experiments are tested in IGCSE:
IGCSE中会考查几个经典实验：
Testing a leaf for starch using iodine solution. 用碘液检测叶片中的淀粉。
Destarching a plant by keeping it in the dark for 24-48 hours. 将植物置于黑暗中24-48小时以消耗原有淀粉。
Investigating the effect of light by covering parts of a leaf with black paper. 用黑纸遮盖叶片一部分以研究光的影响。
Investigating the effect of carbon dioxide by using soda lime to absorb CO₂. 使用碱石灰吸收CO₂以研究二氧化碳的影响。
Investigating the effect of light intensity using pondweed and an LED lamp. 使用金鱼藻和LED灯研究光照强度的影响。
Remember to control variables such as temperature and time when comparing results.
在比较结果时，记得控制温度和时间等变量。
9. Industrial Applications | 工业应用（温室）
In greenhouses, farmers can increase the rate of photosynthesis to improve crop yield. Common methods include:
在温室中，农民可以增加光合作用速率以提高作物产量。常用方法包括：
Increasing carbon dioxide concentration using a burner or CO₂ generator. 使用燃烧器或CO₂发生器增加二氧化碳浓度。
Providing supplementary lighting, especially in winter. 提供补充光照，尤其是在冬季。
Maintaining an optimum temperature using heaters or ventilation. 使用加热器或通风保持最适温度。
However, increasing a factor beyond the limiting point gives no further benefit until another factor becomes limiting.
然而，在另一个因素成为限制因素之前，将某一因素增加到超过限制点并不会带来更多益处。
10. Exam Tips | 考试提示
In the exam, mark schemes often require specific keywords. Use these phrases to score full marks:
在考试中，评分标准通常要求特定关键词。使用这些短语以获得满分：
"Light energy is absorbed by chlorophyll." “光能被叶绿素吸收。”
"Carbon dioxide enters through stomata." “二氧化碳通过气孔进入。”
"Water is absorbed by root hairs." “水通过根毛吸收。”
"Glucose is stored as starch." “葡萄糖以淀粉形式储存。”
"Temperature affects enzymes; too high causes denaturation." “温度影响酶；过高会导致变性。”
Also be careful with equations: check that the equation is balanced and all subscripts are correct.
同时注意方程式：检查方程式是否配平，所有下标是否正确。
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