📚 706: Photosynthesis | 706：光合作用
Photosynthesis is the fundamental process by which green plants, algae and some bacteria convert light energy into chemical energy stored in glucose. In IGCSE Edexcel Science, you must understand the word and balanced chemical equation, the leaf structures involved, and the factors that limit the rate of this essential reaction. Mastery of experimental design — especially using aquatic plants to measure oxygen production — is crucial for exam success.
光合作用是绿色植物、藻类及某些细菌将光能转化为储存在葡萄糖中的化学能的基本过程。在 IGCSE Edexcel 科学课程中，你需要掌握文字方程式、配平的化学方程式，了解参与反应的叶片结构，以及限制这一关键反应速率的因素。熟练掌握实验设计——尤其是利用水生植物测量氧气释放量——对考试成功至关重要。
1. What is Photosynthesis? | 什么是光合作用？
Photosynthesis is an endothermic reaction in which carbon dioxide and water are combined in the presence of light energy and chlorophyll to produce glucose and oxygen. The energy is absorbed from sunlight, making this process the direct or indirect energy source for almost all life on Earth.
光合作用是一种吸热反应，在光能和叶绿素的参与下，二氧化碳和水结合生成葡萄糖和氧气。能量从阳光中吸收，使这一过程成为地球上几乎所有生命的直接或间接能量来源。
It is an anabolic pathway, meaning it builds larger molecules from smaller ones. The glucose produced can be used immediately for respiration, stored as starch, or converted into cellulose, proteins and lipids.
这是一个合成代谢途径，即从小分子构建大分子。生成的葡萄糖可立即用于呼吸作用，以淀粉形式储存，或转化为纤维素、蛋白质和脂质。
The ability to photosynthesise is what makes plants autotrophic — they manufacture their own food using simple inorganic substances.
光合作用的能力使植物成为自养生物——它们利用简单的无机物制造自身所需的食物。
2. The Word and Chemical Equations | 文字方程式与化学方程式
For IGCSE Edexcel, you must be able to state both forms. The word equation is:
在 IGCSE Edexcel 考试中，你必须能够写出这两种形式。文字方程式为：
carbon dioxide + water → glucose + oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
The balanced chemical equation requires correct state symbols and conditions:
配平的化学方程式需要正确的状态符号和反应条件：
6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)
Light energy and chlorophyll are written above the arrow, as they are necessary conditions, not reactants. The arrow (→) indicates the direction of the reaction, and you may also see ‘light’ and ‘chlorophyll’ noted above it.
光能和叶绿素写在箭头上方，因为它们是必要条件而非反应物。箭头（→）表示反应方向，你也会看到“光”和“叶绿素”标注在箭头上方。
Remember that glucose dissolves in the cell cytoplasm, hence (aq); oxygen is released as a gas, and carbon dioxide is taken in as a gas.
记住葡萄糖溶解在细胞质中，因此标为 (aq)；氧气以气体形式释放，二氧化碳以气体形式摄入。
3. Where Does Photosynthesis Occur? | 光合作用发生的场所
Photosynthesis takes place inside chloroplasts, organelles that are most abundant in the palisade mesophyll cells of leaves. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy, primarily in the red and blue regions of the spectrum, and reflects green light — hence leaves appear green.
光合作用在叶绿体内进行，叶绿体是叶片栅栏组织细胞中含量最丰富的细胞器。叶绿体含有绿色色素叶绿素，它能吸收红光和蓝光区域的光能，并反射绿光——因此叶片呈现绿色。
The internal membranes of chloroplasts, called thylakoid membranes, provide a large surface area for the light-dependent reactions. The stroma, a fluid-filled matrix, contains enzymes needed for the light-independent reactions (the Calvin cycle).
叶绿体的内膜称为类囊体膜，为光反应提供了巨大的表面积。基质是一种充满液体的基质，含有暗反应（卡尔文循环）所需的酶。
In a typical IGCSE question, you may be asked to label a leaf cross-section and identify the main photosynthetic tissue. Always recognise that palisade cells are tightly packed near the upper epidermis, maximising light capture.
在典型的 IGCSE 题目中，可能会要求你标注叶片横切面并识别主要光合组织。要始终记住栅栏细胞紧密排列在上表皮附近，以最大限度地捕获光线。
4. Structure of a Leaf | 叶片的结构
Leaves are adapted for efficient photosynthesis. Key adaptations include a large surface area, thinness for short diffusion distances, and specialised layers.
叶片具有适应高效光合作用的结构。关键适应特征包括较大的表面积、薄以缩短扩散距离，以及特化的层次结构。
Upper epidermis: transparent to allow light to penetrate. Waxy cuticle reduces water loss without blocking light.
上表皮：透明以允许光线穿透。蜡质角质层可减少水分流失而不阻挡光线。
Palisade mesophyll: columnar cells densely packed with chloroplasts, located just beneath the upper epidermis for maximum light absorption.
栅栏组织：柱状细胞，富含叶绿体，紧靠上表皮下方，以最大限度地吸收光能。
Spongy mesophyll: loosely arranged cells with air spaces, allowing gas exchange — carbon dioxide diffuses in, oxygen diffuses out.
海绵组织：细胞排列松散，有气隙，便于气体交换——二氧化碳扩散进入，氧气扩散排出。
Veins (vascular bundles): contain xylem for water and mineral transport, and phloem for removing glucose as sucrose.
叶脉（维管束）：包含木质部用于运输水和矿物质，以及韧皮部用于将葡萄糖以蔗糖形式运走。
Guard cells and stomata (mainly on lower epidermis): regulate gas exchange and minimise water loss. Stomata open during the day for carbon dioxide entry and close at night.
保卫细胞和气孔（主要在下表皮）：调节气体交换并减少水分流失。气孔白天开启以允许二氧化碳进入，夜间关闭。
5. Factors Affecting the Rate of Photosynthesis | 影响光合作用速率的因素
The rate of photosynthesis is influenced by several environmental conditions. At IGCSE, you need to interpret graphs showing how light intensity, carbon dioxide concentration, and temperature affect the rate.
光合作用的速率受多种环境条件影响。在 IGCSE 中，你需要解读显示光强度、二氧化碳浓度和温度如何影响速率的图表。
Light intensity: as light intensity increases, the rate initially rises linearly because more energy is available for the light-dependent reactions. Eventually, another factor becomes limiting.
光强度：随着光强度增加，速率起初线性上升，因为光反应可利用更多能量。最终，另一个因素会成为限制因子。
Carbon dioxide concentration: CO₂ is a substrate. Raising its concentration speeds up the Calvin cycle up to a plateau.
二氧化碳浓度：CO₂ 是反应物。提高其浓度可加快卡尔文循环，直至达到一个平稳期。
Temperature: enzymes control the dark reactions. The rate increases with temperature up to an optimum (around 25°C for many plants). Above this, enzymes denature, the rate drops sharply.
温度：酶控制暗反应。速率随温度升高而增加，直至最适温度（许多植物约为25°C）。高于此温度，酶变性，速率急剧下降。
Chlorophyll concentration: insufficient chlorophyll due to magnesium deficiency or disease will reduce the rate even if other factors are favourable.
叶绿素浓度：因缺镁或病害导致叶绿素不足，即使其他条件适宜也会降低速率。
6. Limiting Factors | 限制因子
A limiting factor is the variable that is in shortest supply and prevents the rate of photosynthesis from increasing, even if other factors are abundant. Identifying limiting factors from data or graphs is a core skill.
限制因子是指供应量最少，即使其他因素充足也会阻止光合作用速率增加的变量。从数据或图表中识别限制因子是一项核心技能。
If light intensity is increased and the rate rises, light was limiting. If adding CO₂ causes an increase, then CO₂ was limiting. At very low temperatures, temperature becomes the limiting factor because enzymes work too slowly.
如果增加光强度后速率上升，说明光强度是限制因子。如果增加 CO₂ 导致速率上升，则 CO₂ 是限制因子。在极低温度下，温度成为限制因子，因为酶的工作速度太慢。
An important principle: only one factor limits the rate at any one time. Raising a non‑limiting factor will not increase the rate.
一个重要原则是：在任何时刻只有一个因素限制速率。提高非限制因子不会增加速率。
You might be given a graph with two curves (low and high CO₂) at different light intensities. If, at high CO₂, the curve still stops rising, then light intensity or temperature is now limiting. Interpret this with care.
你可能会看到一张图，在低 CO₂ 和高 CO₂ 条件下，不同光强度有两条曲线。如果在高 CO₂ 时曲线仍停止上升，那么现在光强度或温度成为限制因子。请小心解读。
7. Investigating Photosynthesis Using Pondweed | 使用水草探究光合作用
One of the most common practicals in IGCSE Edexcel is measuring the effect of light intensity on the rate of photosynthesis using pondweed (usually Elodea or Cabomba). The rate is determined by counting oxygen bubbles released per minute.
IGCSE Edexcel 最常见的实验之一是利用水草（通常为伊乐藻或金鱼藻）测量光强度对光合作用速率的影响。速率通过计数每分钟释放的氧气气泡数来确定。
Procedure: place a fresh piece of pondweed in a boiling tube filled with sodium hydrogencarbonate solution (to supply CO₂). Place a light source at a measured distance. After an acclimation period, count bubbles for a fixed time, e.g., one minute. Vary the distance and record.
步骤：将一段新鲜水草放入装有碳酸氢钠溶液（提供 CO₂）的沸腾管中。在设定的距离处放置一个光源。适应一段时间后，在固定的时间内（例如一分钟）计数气泡个数。改变距离并记录。
As distance increases, light intensity decreases — not linearly, but according to the inverse square law (light intensity ∝ 1/d²). The number of bubbles per minute initially decreases with distance.
随着距离增加，光强度降低——并非线性，而是遵循平方反比定律（光强度 ∝ 1/d²）。每分钟气泡数起初随距离增加而减少。
Control variables: temperature (use a water bath or heat shield), wavelength of light, CO₂ concentration (use same solution), and the size/health of the pondweed.
控制变量：温度（使用水浴或隔热板）、光的波长、CO₂ 浓度（使用同一溶液）以及水草的规格/健康状况。
An alternative method: measure the volume of oxygen produced using a gas syringe, which is more accurate for quantitative analysis.
替代方法：使用气体注射器测量氧气产生体积，这在定量分析中更精确。
8. The Importance of Photosynthesis | 光合作用的重要性
Photosynthesis is the ultimate source of all biological energy. It converts inorganic carbon (CO₂) into organic compounds, forming the basis of food chains. The oxygen released maintains the atmospheric oxygen level essential for aerobic respiration.
光合作用是一切生物能量的最终来源。它将无机碳（CO₂）转变为有机化合物，构成食物链的基础。释放的氧气维持了大气中的氧气水平，这对需氧呼吸至关重要。
In the carbon cycle, photosynthesis removes CO₂ from the atmosphere and locks it into biomass. This balance is critical in regulating Earth’s climate.
在碳循环中，光合作用从大气中吸收 CO₂ 并将其封存在生物量中。这种平衡对调节地球气候至关重要。
Understanding photosynthesis also underpins agriculture and horticulture: optimising light, CO₂, and temperature in greenhouses can maximise crop yields.
理解光合作用也是农业和园艺的基础：在温室中优化光照、CO₂ 和温度可最大限度地提高作物产量。
Exam questions often ask for an explanation of why photosynthesis is described as the most important chemical process on Earth. Be prepared to link it to energy flow, oxygen production and food supply.
考试题目常要求解释为什么光合作用被称为地球上最重要的化学过程。要准备好将其与能量流动、氧气产生和食物供应联系起来。
9. Products of Photosynthesis & Their Uses | 光合作用产物及其利用
The primary product is glucose, which can be used in multiple ways: 1) immediately in respiration to release energy; 2) converted to starch for storage (insoluble, so does not affect osmotic balance); 3) converted to cellulose for cell wall formation; 4) combined with nitrogen and other elements to form amino acids and proteins; 5) converted to lipids for storage in seeds.
初级产物是葡萄糖，它可以通过多种方式利用：1）立即用于呼吸作用释放能量；2）转化为淀粉储存（不溶，因此不影响渗透平衡）；3）转化为纤维素用于构建细胞壁；4）与氮及其他元素结合形成氨基酸和蛋白质；5）转化为脂质储存在种子中。
Oxygen is a by-product released into the atmosphere or used internally for respiration. During the day, the rate of photosynthesis often exceeds respiration, so there is a net output of oxygen.
氧气是释放到大气中或用于内部呼吸的副产物。白天，光合作用速率通常超过呼吸速率，因此有净氧气输出。
The starch test on a variegated leaf is a standard IGCSE practical: after boiling in ethanol, rinsing and adding iodine solution, only the green parts (chlorophyll‑containing) turn blue‑black, proving that chlorophyll is essential for starch formation.
对花叶进行淀粉检验是标准 IGCSE 实验：在乙醇中煮沸、漂洗并加入碘液后，只有绿色部分（含叶绿素）变成蓝黑色，证明叶绿素是形成淀粉所必需的。
10. Key Experiments and Tests | 关键实验与检验
Destarching a plant: place it in a dark cupboard for 24–48 hours to ensure any existing starch in leaves is translocated and used up. This is the starting point for many photosynthesis investigations.
对植物进行暗处理：将其放在黑暗的柜子里 24–48 小时，确保叶片中现有的淀粉被运走并耗尽。这是许多光合作用实验的起点。
Testing a leaf for starch: boil the leaf in water to soften it, then boil in ethanol using a water bath (ethanol is flammable) to remove chlorophyll. Rinse in hot water to soften and place on a tile. Add iodine solution; a blue‑black colour indicates starch, confirming photosynthesis occurred in exposed areas.
检验叶片淀粉：将叶片在水中煮沸软化，然后在水浴中使用乙醇（易燃）煮沸以去除叶绿素。在热水中漂洗软化后放在白瓷板上。滴加碘液；蓝黑色表明存在淀粉，证实暴露区域发生了光合作用。
Investigating the need for light: cover part of a leaf with aluminium foil or black paper before placing the plant in bright light. After a few hours, test that leaf for starch. Only the uncovered areas turn blue‑black.
探究光的需求：将叶片的一部分用铝箔或黑纸遮住，然后把植物放在强光下。几小时后，对该叶片进行淀粉检验。只有未遮光区域变蓝黑色。
Investigating the need for carbon dioxide: place a leaf inside a flask containing potassium hydroxide (absorbs CO₂), while a control leaf is exposed to normal air. After time, the CO₂‑deprived leaf shows no starch formation.
探究二氧化碳的需求：将一片叶置于装有氢氧化钾（吸收 CO₂）的烧瓶内，对照叶暴露在正常空气中。一段时间后，缺乏 CO₂ 的叶片不产生淀粉。
Using hydrogencarbonate indicator: this pH indicator changes colour with CO₂ concentration. In photosynthesis experiments, it shifts from red to purple as CO₂ is consumed, indicating photosynthetic activity.
使用碳酸氢盐指示剂：这种 pH 指示剂会随 CO₂ 浓度改变颜色。在光合作用实验中，随着 CO₂ 被消耗，它会由红变紫，表明光合作用活动。
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