📚 Photosynthesis: The Engine of Life | 光合作用：生命的引擎
Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into stored chemical energy. This process is endothermic, meaning it takes in energy from the surroundings. The light energy absorbed by chlorophyll is used to build glucose from carbon dioxide and water.
光合作用是绿色植物、藻类和某些细菌将光能转化为储存的化学能的过程。这是一个吸热反应，意味着它从环境中吸收能量。叶绿素吸收的光能被用来将二氧化碳和水合成葡萄糖。
1. What Is Photosynthesis? | 什么是光合作用？
Photosynthesis takes place mainly in the leaves of plants. Carbon dioxide enters the leaf through tiny pores called stomata, while water is absorbed by the roots and transported to the leaves through the xylem. The glucose produced can be used immediately as an energy source, stored as starch, or converted into other substances such as cellulose and oils.
光合作用主要发生在植物的叶片中。二氧化碳通过叶片上叫做气孔的小孔进入，水分则被根部吸收并通过木质部输送到叶片。生成的葡萄糖可以被立即用作能量来源，以淀粉形式储存，或转化为纤维素、油脂等其他物质。
For plants, photosynthesis is not just about food. The oxygen released during the process is essential for most living organisms on Earth to respire aerobically.
对于植物来说，光合作用不仅关乎食物。过程中释放的氧气对地球上大多数生物进行有氧呼吸至关重要。
2. The Equations Behind Photosynthesis | 光合作用的化学方程式
You need to remember both the word equation and the balanced symbol equation for photosynthesis. The word equation can be written as:
你需要熟记光合作用的文字方程式和配平的符号方程式。文字方程式可以写成：
carbon dioxide + water → glucose + oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
The balanced chemical equation below shows the same reaction using symbols. This reaction requires light energy and chlorophyll to take place.
下面的配平化学方程式用符号表示了同一反应。这个反应需要光能和叶绿素才能进行。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Notice that there are six atoms of carbon, twelve atoms of hydrogen and eighteen atoms of oxygen on each side of the arrow. The equation is balanced because the number of each type of atom is the same in the reactants and the products.
注意，箭头两边各有6个碳原子、12个氢原子和18个氧原子。因为反应物和产物中每种原子的数量相同，所以这个方程式是配平的。
3. Chlorophyll: The Essential Pigment | 叶绿素：关键色素
Chlorophyll is the green pigment found inside chloroplasts. It is essential because it absorbs the light energy needed to drive photosynthesis. Chlorophyll absorbs red and blue light most strongly and reflects green light, which is why leaves appear green to our eyes.
叶绿素是存在于叶绿体中的绿色色素。它至关重要，因为它吸收了驱动光合作用所需的光能。叶绿素对红光和蓝光的吸收最强，而反射绿光，所以我们的眼睛看到的叶片是绿色的。
Chloroplasts are usually most concentrated in the palisade mesophyll cells near the upper surface of a leaf. This position lets them receive maximum sunlight. Without chlorophyll, the light energy cannot be captured and the reaction will not happen.
叶绿体通常最集中分布在靠近叶片上表面的栅栏组织细胞中。这种位置使它们能够获得最多的阳光。没有叶绿素，光能就无法被捕获，反应也就不会发生。
4. Light as a Driving Force | 光作为驱动力
Light provides the energy that powers photosynthesis. When light particles, called photons, hit chlorophyll, electrons in the pigment become excited. This energy is then used to split water molecules in a process known as photolysis, producing oxygen, protons and electrons.
光提供了推动光合作用的能量。当被称为光子的光粒子撞击叶绿素时，色素中的电子被激发。然后这部分能量被用来分解水分子，这个过程称为光解，产生氧气、质子和电子。
In very low light intensity, photosynthesis is limited by light energy. In high light intensity, light is no longer the factor that limits the rate. This idea is the basis of limiting factor graphs, which you will often see in IGCSE Science exams.
在非常低的光照强度下，光合作用受到光能的限制。在高光照强度下，光不再成为限制速率的因素。这一概念是限制因子图的基础，你在 IGCSE 科学考试中会经常看到。
5. The Two Stages of Photosynthesis | 光合作用的两个阶段
Photosynthesis can be divided into two main stages. In the light-dependent stage, light energy is absorbed by chlorophyll and is used to split water, releasing oxygen. This stage also produces energy-carrying molecules such as ATP.
光合作用可以分为两个主要阶段。在光依赖阶段，光能被叶绿素吸收，并用于分解水，释放氧气。这个阶段还会产生 ATP 等携带能量的分子。
In the light-independent stage, carbon dioxide is added to a five-carbon compound in the stroma of the chloroplast. Using the energy stored in ATP, this carbon dioxide is converted into glucose. This stage does not directly need light, but it depends on the products of the light-dependent stage.
在光不依赖阶段，二氧化碳在叶绿体基质中与一种五碳化合物结合。利用储存在 ATP 中的能量，二氧化碳被转化为葡萄糖。这个阶段并不直接需要光，但它依赖光依赖阶段的产物。
6. Rate of Photosynthesis: Measuring It | 光合作用速率的测量
There are several practical ways to measure the rate of photosynthesis. One common method is to measure the volume of oxygen produced in a given time. Another is to measure the number of oxygen bubbles released from a water plant.
测量光合作用速率有多种实验方法。一种常见方法是测量在给定时间内产生的氧气体积。另一种方法是测量水生植物释放的氧气气泡数量。
You can also measure the rate by observing how quickly plant tissues gain biomass, or by measuring the uptake of carbon dioxide using a carbon dioxide sensor. Each method has advantages, but the pondweed experiment is the simplest one for school laboratories.
你也可以通过观察植物组织增加生物量的快慢，或使用二氧化碳传感器测量二氧化碳的吸收速率。每种方法都有优点，但金鱼藻实验是学校实验室中最简单的方法。
7. The Pondweed Experiment | 金鱼藻实验
In the pondweed experiment, a piece of Elodea or Cabomba is placed in a beaker of water with a small amount of sodium hydrogencarbonate. This provides a steady supply of carbon dioxide. A lamp is placed at a known distance from the plant, and the number of bubbles produced per minute is counted.
在金鱼藻实验中，将一段伊乐藻或茨藻放入含有少量碳酸氢钠的水中。碳酸氢钠可以提供稳定的二氧化碳来源。将台灯放在离植物已知距离的位置，记录每分钟产生的气泡数量。
As the lamp is moved closer to the plant, the light intensity increases and the bubble rate usually increases. To make the experiment fair, you should keep the temperature and the concentration of sodium hydrogencarbonate constant. Repeating the count and calculating a mean will improve reliability.
当台灯移近植物时，光照强度增加，气泡速率通常也会增加。为保证公平，应保持温度和碳酸氢钠浓度不变。重复计数并计算平均值可以提高结果的可靠性。
8. Limiting Factors: Light Intensity | 限制因子：光照强度
Light intensity is one of the main limiting factors for photosynthesis. At low light intensity, the rate of photosynthesis is slow because there is not enough energy to drive the reaction. As light intensity increases, the rate increases steadily at first.
光照强度是光合作用的主要限制因子之一。在低光照强度下，光合作用速率很慢，因为没有足够的能量驱动反应。随着光照强度增加，速率起初会稳定上升。
However, once a certain light intensity is reached, the rate stops increasing because another factor, such as carbon dioxide concentration or temperature, is now limiting. You should remember the inverse square law relating to light intensity and distance:
然而，当光照强度达到一定值后，速率不再增加，因为另一个因素，如二氧化碳浓度或温度，这时候成为限制因子。你应该记住光照强度与距离之间的平方反比定律：
light intensity ∝ 1 / d²
光照强度 ∝ 1 / d²
This means that if the distance from the lamp to the plant is halved, the light intensity becomes four times greater.
这意味着，如果灯与植物之间的距离减半，光照强度将变为原来的四倍。
9. Limiting Factors: Carbon Dioxide and Temperature | 限制因子：二氧化碳与温度
Carbon dioxide is needed as a reactant in the light-independent stage. Increasing the carbon dioxide concentration will increase the rate of photosynthesis until a plateau is reached. At that point, light intensity or temperature is limiting the reaction.
二氧化碳是光不依赖阶段的反应物。增加二氧化碳浓度会使光合作用速率增大，直到达到平台期。此时，光照强度或温度成为限制反应的因素。
Temperature affects photosynthesis mainly through enzymes. In the range of about 5 °C to 35 °C, increasing temperature speeds up the rate of photosynthesis because enzymes work faster. Above the optimum temperature, enzymes begin to denature, so the rate falls quickly.
温度主要通过酶来影响光合作用。在大约5°C到35°C范围内，升高温度会使光合作用速率加快，因为酶的活性增强。超过最适温度后，酶开始变性，速率会迅速下降。
In the greenhouse, growers control these limiting factors to increase crop yields. They may add artificial lighting, supply extra carbon dioxide, or use heaters to keep the temperature at the optimum level.
在温室中，种植者通过控制这些限制因子来提高作物产量。他们可能会补充人工光照、增加二氧化碳浓度，或使用加热器使温度保持在最适水平。
10. Leaf Structure and Adaptations | 叶片结构与适应性
A leaf is well adapted to carry out photosynthesis efficiently. The table below summarises the key structures and their functions.
叶片具有很好的结构适应性，可以高效地进行光合作用。下表总结了关键结构及其功能。
| Structure | 结构 | Function | 功能 |
| Broad, flat blade | Provides a large surface area to absorb light. |
| Broad, flat blade | 宽大扁平的叶片 |
| Large surface area | 提供大面积吸收光。 |
| Thin leaf | Short diffusion distance for gases such as CO₂ and O₂. |
| Thin leaf | 叶片较薄 |
| Short diffusion distance | 缩短 CO₂ 和 O₂
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