📚 Photosynthesis | 光合作用
Photosynthesis is the process by which green plants and some algae use light energy to convert carbon dioxide and water into glucose and oxygen. It is an endothermic reaction because energy is taken in from the surroundings.
光合作用是绿色植物和部分藻类利用光能，将二氧化碳和水转化为葡萄糖和氧气的过程。由于反应从周围环境吸收能量，因此这是一个吸热反应。
1. The Importance of Photosynthesis | 光合作用的重要性
Photosynthesis is the foundation of almost all food chains on Earth. Plants act as producers, forming glucose from simple inorganic substances.
光合作用是地球上几乎所有食物链的基础。植物作为生产者，利用简单的无机物合成葡萄糖。
Photosynthesis also releases oxygen, which most living organisms need for aerobic respiration.
光合作用还释放氧气，而绝大多数生物进行有氧呼吸都需要氧气。
In this way, photosynthesis sustains both the energy supply and the oxygen supply for life on Earth.
因此，光合作用为地球上生命提供了能量来源和氧气来源。
2. The Word and Symbol Equation | 光合作用的化学方程式
The word equation for photosynthesis can be written as:
光合作用可以用下面的文字方程式表示：
Carbon dioxide + Water → Glucose + Oxygen
The balanced symbol equation is:
配平的符号方程式为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Light energy and chlorophyll are not written as reactants because they are not used up. However, photosynthesis can only take place when both are present.
光能和叶绿素不应写在反应物一侧，因为它们不会被消耗。但是，只有在两者都存在时，光合作用才能进行。
3. Conditions Needed for Photosynthesis | 光合作用所需条件
Four main conditions are needed for photosynthesis to occur:
光合作用的发生需要以下四个主要条件：
Light energy is captured by chlorophyll and used to power the reaction.
光能被叶绿素捕获，并为反应提供能量。
Chlorophyll is the green pigment found in chloroplasts that absorbs light.
叶绿素是叶绿体中的绿色色素，能够吸收光能。
Carbon dioxide enters the leaf through stomata and diffuses into the photosynthetic cells.
二氧化碳通过气孔进入叶片，然后扩散到进行光合作用的细胞中。
Water is absorbed by roots and carried to the leaves in the xylem.
水分由根吸收，通过木质部运输到叶片。
If any one of these conditions is missing, photosynthesis will slow down or stop.
如果缺少其中任何一个条件，光合作用就会减慢或停止。
4. Where Photosynthesis Occurs | 光合作用发生的场所
Photosynthesis occurs inside chloroplasts, which are organelles found mainly in the cells of the leaf mesophyll.
光合作用主要发生在叶绿体中。叶绿体是细胞器，主要存在于叶片叶肉细胞中。
Chloroplasts contain chlorophyll, which absorbs light energy. Chlorophyll absorbs red and blue light most strongly and reflects green light, which is why leaves appear green.
叶绿体含有叶绿素，可以吸收光能。叶绿素对红光和蓝光的吸收最强，而反射绿光，因此叶片看起来是绿色的。
The reactions of photosynthesis take place both in the thylakoid membranes and in the stroma of the chloroplast, but a detailed study of these stages is beyond the IGCSE core requirement.
光合作用的相关反应既发生在叶绿体的类囊体膜上，也发生在叶绿体的基质中，但更细致的阶段划分超出了 IGCSE 核心考纲的要求。
5. Leaf Adaptations for Photosynthesis | 叶片对光合作用的适应
Leaves are highly adapted to carry out photosynthesis efficiently.
叶片高度适应高效进行光合作用。
A large surface area absorbs as much light as possible.
巨大的表面积可以尽可能多地吸收光能。
A thin shape reduces the diffusion distance for carbon dioxide and oxygen.
叶片很薄，可以缩短二氧化碳和氧气的扩散距离。
An upper layer of palisade cells contains many chloroplasts, packed close to the upper surface.
上方的栅栏层细胞含有大量叶绿体，紧密分布在叶片上表皮附近。
Stomata allow carbon dioxide to enter and oxygen to leave.
气孔允许二氧化碳进入，氧气离开。
Air spaces inside the spongy mesophyll allow gases to diffuse quickly around the leaf.
海绵组织中的细胞间隙可以让气体在叶片内部快速扩散。
The xylem transports water to the leaf, while the phloem carries sucrose and amino acids away.
木质部将水运输到叶片，韧皮部则将蔗糖和氨基酸运输出去。
6. The Effect of Light Intensity | 光照强度的影响
Light intensity is a major limiting factor. As light intensity increases, the rate of photosynthesis increases, provided other conditions remain constant.
光照强度是一个重要的限制因素。在其他条件保持不变时，随着光照强度增大，光合作用速率也随之增大。
At low light intensity, light is the limiting factor. Adding more light increases the rate directly.
在弱光下，光就是限制因素。增加光照可以直接提高光合作用速率。
At high light intensity, a further increase may not raise the rate because another factor, such as carbon dioxide concentration or temperature, is now limiting.
在强光下，继续增强光照可能不会再提高速率，因为此时二氧化碳浓度或温度等因素成为了限制因素。
For a point source of light, the light intensity follows an inverse square law with distance:
对于点光源，光照强度与距离的平方成反比：
light intensity ∝ 1/d²
This means that if the distance from the light source is doubled, the light intensity falls to one quarter of its original value.
这意味着，如果光源距离加倍，光照强度会下降到原来的四分之一。
7. The Effect of Carbon Dioxide Concentration | 二氧化碳浓度的影响
Carbon dioxide is one of the raw materials for photosynthesis. An increase in CO₂ concentration usually increases the rate of photosynthesis until another factor becomes limiting.
二氧化碳是光合作用的原料之一。提高二氧化碳浓度通常会加快光合作用，直到另一个因素成为限制因素。
If all other conditions are kept constant, the rate of photosynthesis rises steeply at first and then gradually levels off.
如果保持其他条件不变，光合作用速率最初会急剧上升，然后逐渐趋于平稳。
The graph eventually plateaus because at high CO₂ concentration, the enzyme-controlled reactions inside the chloroplast become saturated.
曲线最终趋于平缓，是因为在较高二氧化碳浓度下，叶绿体内由酶控制的反应已经达到饱和。
In glasshouses, farmers sometimes release extra carbon dioxide to increase crop yields.
在温室中，农民有时会释放额外的二氧化碳以提高作物产量。
8. The Effect of Temperature | 温度的影响
Photosynthesis is controlled by enzymes, so temperature affects the rate of photosynthesis in a typical enzyme-dependent way.
光合作用受酶控制，因此温度对光合作用速率的影响符合典型的酶反应规律。
As temperature increases, particles gain kinetic energy and enzyme-substrate complexes form more often.
随着温度升高，分子获得更多动能，酶与底物的结合机会增多。
The rate increases up to an optimum temperature, which is usually around 25–35 °C for many plants.
在达到最适温度之前，速率持续增加。许多植物的最适温度约为 25–35 °C。
Above the optimum temperature, enzymes begin to denature. The active site changes shape and the enzyme can no longer catalyse the reaction.
超过最适温度后，酶开始变性。活性位点的形状发生改变，酶无法再催化反应。
The rate of photosynthesis therefore falls steeply at high temperatures.
因此在高温下，光合作用速率会急剧下降。
9. Limiting Factors in Action | 限制因素的实际应用
In a greenhouse, a farmer can control all three main factors to maximise photosynthesis.
在温室中，种植者可以控制三个主要因素，从而最大化光合作用速率。
Factor 因素 |
How a farmer can increase it 种植者如何提高 |
Why it helps 为什么有效 |
Light intensity 光照强度 |
Use artificial lamps, especially in winter. 使用人工光源，尤其是在冬季。 |
More light energy reaches the chlorophyll. 更多光能到达叶绿素。 |
CO₂ concentration 二氧化碳浓度 |
Burn a clean fuel or use a CO₂ generator. 燃烧清洁燃料或使用二氧化碳发生器。 |
More raw material is available for glucose synthesis. 有更多原料合成葡萄糖。 |
Temperature 温度 |
Use greenhouse heaters or ventilation on hot days. 使用温室加热器，炎热天气时通风。 |
Keeps temperature near the optimum for enzymes. 使温度接近酶的最适温度。 |
10. Investigating Photosynthesis with Pondweed | 用金鱼藻探究光合作用
Canadian pondweed is often used to measure the rate of photosynthesis because it releases oxygen as bubbles.
加拿大金鱼藻常用于测定光合作用速率，因为它会释放出氧气气泡。
One simple method is to count bubbles of oxygen produced per minute, or to collect the gas in a small measuring tube and record its volume per minute.
一种简单的方法是计算每分钟产生氧气气泡的数量，或者用小型收集管收集气体，记录每分钟产气的体积。
The rate can be found by dividing the volume of oxygen produced by the time taken.
速率可以通过“产生的氧气体积 ÷ 所用时间”来计算。
rate = volume of O₂ ÷ time
To make the investigation fair, keep the temperature constant, use the same plant, and keep the concentration of sodium hydrogencarbonate solution the same.
为了确保实验公平，应保持温度不变，使用同一株植物，并保持碳酸氢钠溶液的浓度相同。
Changing the distance between the lamp and the pondweed changes the light intensity. Move the lamp closer to increase the rate of bubble production; move it farther away to decrease the rate.
改变灯泡与金鱼藻之间的距离可以改变光照强度。将灯泡移近会加快气泡产生速率；移远则会降低速率。
11. Testing a Leaf for Starch | 检测叶片中的淀粉
Starch is a safe storage form of glucose, so a leaf that has been photosynthesising will contain starch.
淀粉是葡萄糖的安全储存形式，因此进行过光合作用的叶片中会含有淀粉。
The iodine test can be used to show whether starch is present.
可以用碘液检测叶片中是否含有淀粉。
The leaf is boiled in water to stop chemical reactions and break down cell membranes.
先将叶片在沸水中煮，以停止化学反应并破坏细胞膜。
The leaf is then boiled in ethanol to remove chlorophyll. This takes out the green colour so any colour change in the iodine test can be seen clearly.
然后将叶片放在乙醇中煮沸，以去除叶绿素。这样叶片会失去绿色，便于观察碘液的颜色变化。
Finally, the leaf is rinsed in water and iodine solution is added.
最后将叶片用水冲洗，并滴加碘液。
If starch is present, the leaf turns blue-black. If starch is absent, it stays orange-brown.
如果含有淀粉，叶片会变成蓝黑色；如果不含淀粉，则仍为橙棕色。
This test can be used to show that light, carbon dioxide and chlorophyll are needed for photosynthesis by comparing destarched leaves under different conditions.
该实验可以通过比较不同条件下经过“饥饿处理”的叶片，来证明光、二氧化碳和叶绿素是光合作用所必需的。
12. The Uses of the Products | 产物的用途
Glucose produced by photosynthesis is used in many ways inside the plant.
光合作用产生的葡萄糖在植物体内有多种用途。
Glucose can be used in respiration to release energy for cell activities.
葡萄糖可用于呼吸作用，释放能量供细胞活动使用。
It may be converted into starch for storage in roots, stems and seeds.
葡萄糖可转化为淀粉，储存在根、茎和种子中。
It is converted into sucrose for transport around the plant.
葡萄糖会转化为蔗糖，以便在植物体内运输。
It is used to make cellulose for strengthening plant cell walls.
葡萄糖被用于制造纤维素，以增强植物细胞壁。
Combined with nitrate ions, glucose is converted into amino acids, which are then used to build proteins.
葡萄糖与硝酸根离子结合，可以转化为氨基酸，进而用于合成蛋白质。
Some glucose is converted into fats and oils for energy storage.
部分葡萄糖会被转化为脂肪和油脂，用于储存能量。
As a by-product, oxygen is released from the leaf through stomata into the atmosphere, and it is also used by plant cells during respiration.
光合作用的副产物氧气通过气孔释放到大气中，同时也会被植物细胞呼吸作用所利用。
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