📚 Photosynthesis: Making Food from Sunlight | 光合作用：用阳光制造食物
Photosynthesis is the process by which green plants, algae and some bacteria use light energy to make glucose from carbon dioxide and water.
光合作用是绿色植物、藻类和某些细菌利用光能，将二氧化碳和水转化为葡萄糖的过程。
This process is essential to almost all life on Earth, because it produces the organic molecules used as food and releases oxygen into the atmosphere.
这一过程对地球上几乎所有生命都至关重要，因为它制造了作为食物的有机分子，并向大气中释放氧气。
1. What Is Photosynthesis? | 什么是光合作用？
Photosynthesis takes place in the chloroplasts of plant cells, especially in the palisade mesophyll cells of leaves. Chloroplasts contain chlorophyll, a green pigment that absorbs light energy.
光合作用发生在植物细胞的叶绿体里，尤其是叶片栅栏组织细胞中。叶绿体含有叶绿素，这是一种能吸收光能的绿色色素。
Photosynthesis is an endothermic reaction, meaning it takes in energy. Light energy is converted into chemical energy stored in glucose molecules.
光合作用是吸热反应，意味着它需要吸收能量。光能被转化为化学能，储存在葡萄糖分子中。
Without photosynthesis, green plants would not be able to grow, and animals would have no food or oxygen.
没有光合作用，绿色植物就无法生长，动物也就没有食物和氧气。
2. The Balanced Equation | 配平的化学方程式
The word equation for photosynthesis is:
光合作用的文字表达式为：
Carbon dioxide + Water → Glucose + Oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
The balanced symbol equation is:
配平的符号方程式为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Light energy and chlorophyll are required for this reaction, even though they do not appear as chemical reactants or products.
该反应需要光能和叶绿素，尽管它们不写进化学反应的物质项中。
3. Light Energy and Chlorophyll | 光能与叶绿素
Chlorophyll absorbs light mainly in the red and blue parts of the spectrum. Green light is reflected, which is why most leaves appear green.
叶绿素主要吸收光谱中的红光和蓝光。绿光被反射，所以大多数叶子看起来是绿色的。
The absorbed light energy provides the energy needed to split water and to join carbon dioxide molecules into glucose.
吸收的光能为分解水分子以及将二氧化碳分子合成为葡萄糖提供能量。
Plants kept in darkness cannot photosynthesise because chlorophyll cannot absorb light energy. If a plant is left in the dark for a long time, it loses its green colour.
在黑暗中生长的植物无法进行光合作用，因为叶绿素无法吸收光能。如果植物长期置于黑暗中，它会失去绿色。
4. Leaf Structure and Photosynthesis | 叶片结构与光合作用
Leaves are specialised organs for photosynthesis. Their broad, flat shape gives a large surface area for absorbing light and for gas exchange.
叶片是专门进行光合作用的器官。它宽大扁平的形状提供了很大的表面积，有利于吸收光和进行气体交换。
Key leaf structures include:
叶片的关键结构包括：
- Epidermis: a transparent protective layer that lets light pass through.
- 角质层/表皮：一层透明的保护层，允许光线透过。
- Palisade mesophyll: closely packed cells with many chloroplasts; most photosynthesis happens here.
- 栅栏组织：排列紧密的细胞，含大量叶绿体；光合作用主要在这里进行。
- Spongy mesophyll: loosely packed cells with air spaces for carbon dioxide diffusion.
- 海绵组织：排列疏松的细胞，有细胞间隙，便于二氧化碳扩散。
- Xylem and phloem: xylem carries water; phloem carries sugars away.
- 木质部与韧皮部：木质部运输水分；韧皮部运输糖类。
Water is needed for photosynthesis. It enters the plant through root hairs by osmosis and is carried to the leaves in the xylem.
光合作用需要水。水通过根毛以渗透作用进入植物体，再由木质部运输到叶片。
5. Gas Exchange and Stomata | 气体交换与气孔
Photosynthesis consumes carbon dioxide and produces oxygen. Gas exchange mainly occurs through stomata, which are tiny pores found mostly on the lower surface of leaves.
光合作用消耗二氧化碳并产生氧气。气体交换主要通过气孔进行，气孔是主要分布在叶片下表面的微小孔隙。
Each stoma is surrounded by two guard cells. When guard cells become turgid, the stoma opens; when they lose water, the stoma closes.
每个气孔由两个保卫细胞包围。当保卫细胞吸水膨胀时，气孔打开；当它们失水时，气孔关闭。
During the day, stomata usually open so that carbon dioxide can enter for photosynthesis and oxygen can leave. Water vapour also escapes through stomata during transpiration.
白天气孔通常打开，以便二氧化碳进入用于光合作用，同时让氧气排出。水蒸气也会通过气孔在蒸腾作用中散失。
6. Limiting Factors | 限制因素
A limiting factor is a factor that slows down the rate of a reaction when it is in short supply. If more of that factor is supplied, the rate of photosynthesis increases until another factor becomes limiting.
限制因素是指当它供应不足时会减慢反应速率的因素。如果增加该因素的供应量，光合作用速率会提高，直到另一个因素成为新的限制因素。
The three main limiting factors of photosynthesis are:
光合作用的三个主要限制因素是：
- Light intensity: provides the energy for the reaction.
- 光强度：为反应提供能量。
- Carbon dioxide concentration: supplies carbon for glucose.
- 二氧化碳浓度：为葡萄糖提供碳元素。
- Temperature: affects the enzymes that control photosynthesis.
- 温度：影响控制光合作用的酶。
At night, light is usually the limiting factor. In very bright greenhouses, carbon dioxide or temperature may become the limiting factor.
在夜间，光通常是限制因素。在光线很充足的温室中，二氧化碳或温度可能成为限制因素。
7. Effect of Light Intensity | 光强度的影响
As light intensity increases, the rate of photosynthesis also increases, provided light is the limiting factor. However, eventually the rate plateaus because light is no longer the limiting factor.
当光作为限制因素时，随着光强度增加，光合作用速率也会增加。但最终速率会趋于稳定，因为光不再是限制因素。
Light intensity obeys the inverse square law when a lamp is used as the light source:
当使用灯泡作为光源时，光强度遵循平方反比定律：
Light intensity ∝ 1 / d²
光强度 ∝ 1 / 距离²
If the distance between the lamp and the plant is doubled, the light intensity falls to one quarter of its original value. This can be investigated using pondweed.
如果灯与植物之间的距离加倍，光强度会下降到原来的四分之一。这可以用金鱼藻等水生植物来探究。
8. Effect of Carbon Dioxide and Temperature | 二氧化碳浓度与温度的影响
Carbon dioxide is a raw material for photosynthesis. Increasing its concentration raises the rate of photosynthesis up to a saturation point, after which other factors become limiting.
二氧化碳是光合作用的原料。提高其浓度会使光合作用速率上升，直到达到饱和点，之后其他因素会成为限制因素。
Temperature affects the enzymes involved in photosynthesis. As temperature rises, the rate increases because the enzymes work faster. If the temperature becomes too high, enzymes denature and the rate falls sharply.
温度影响参与光合作用的酶。温度升高时，酶活性增强，速率加快。如果温度过高，酶会变性，速率会急剧下降。
For most plants, the optimum temperature for photosynthesis is around 25–35°C. Above about 40°C, photosynthesis slows down and may stop.
对大多数植物来说，光合作用的最适温度约为25–35°C。超过约40°C时，光合作用会减慢甚至停止。
9. Investigating Photosynthesis | 探究光合作用
A common practical uses pondweed, such as Elodea, to measure the rate of photosynthesis. As the plant photosynthesises, bubbles of oxygen are released from the cut stem.
一个常见的实验是使用金鱼藻类如 Elodea 来测量光合作用速率。植物进行光合作用时，从切开的茎部释放出氧气气泡。
Method summary:
实验方法概要：
- Place pondweed in a beaker of water with some sodium bicarbonate, which provides carbon dioxide.
- 把金鱼藻放入含有少量碳酸氢钠的水中，碳酸氢钠可提供二氧化碳。
- Use a lamp at a fixed distance and count the number of bubbles produced in one minute.
- 将灯泡固定在某一距离，记录一分钟内产生的气泡数量。
- Repeat at different distances and calculate the mean number of bubbles per minute.
- 在不同距离下重复实验，并计算每分钟气泡数量的平均值。
- Keep temperature constant using a water bath or by controlling the lamp heating.
- 用水浴恒温或控制灯泡发热来保持温度恒定。
The gas collected can be tested with a glowing splint. If the splint relights, the gas is oxygen.
收集到的气体可以用带火星的木条检验。如果木条重新燃烧起来，说明气体是氧气。
10. Testing a Leaf for Starch | 检测叶片中的淀粉
Starch is a good way to test whether photosynthesis has happened because glucose is quickly changed into starch in the leaf.
淀粉是检验光合作用是否发生的良好指标，因为葡萄糖在叶片中会很快转化为淀粉。
Before this experiment, a plant should be destarched by keeping it in darkness for 24–48 hours. This makes sure that no starch is already present.
实验前应先将植物置于黑暗中24–48小时进行“饥饿处理”，以确保叶中原本没有淀粉。
Steps for testing a leaf:
检测叶片的步骤：
- Boil the leaf in water to kill the cells and break down cell membranes.
- 将叶片放入沸水中煮沸，以杀死细胞并破坏细胞膜。
- Boil the leaf in ethanol using a water bath to remove the chlorophyll. Ethanol is flammable, so use a water bath, not direct heat.
- 用水浴加热乙醇以去除叶绿素。乙醇易燃，
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