Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It is the foundation of almost all food chains on Earth.

光合作用是绿色植物、藻类以及某些细菌将光能转化为储存在葡萄糖中的化学能的过程。它是地球上几乎所有食物链的基础。

1. The Word Equation | 文字方程式

Carbon dioxide and water react, in the presence of light and chlorophyll, to produce glucose and oxygen. The oxygen released comes from the splitting of water, not from carbon dioxide.

二氧化碳和水在光照和叶绿素的存在下反应，生成葡萄糖和氧气。释放的氧气来自水的分解，而不是来自二氧化碳。

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

In symbols: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

用符号表示：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

2. The Chemical Equation | 化学方程式

Six molecules of carbon dioxide combine with six molecules of water using light energy absorbed by chlorophyll. This yields one molecule of glucose and six molecules of oxygen.

六个二氧化碳分子与六个水分子在叶绿素吸收的光能作用下结合，生成一个葡萄糖分子和六个氧分子。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

This equation is balanced: 6 carbon atoms, 12 hydrogen atoms, and 18 oxygen atoms appear on both sides.

该方程式是平衡的：两侧各有6个碳原子、12个氢原子和18个氧原子。

3. Required Conditions | 必要条件

Photosynthesis cannot occur without light, chlorophyll, water, and carbon dioxide. Temperature also affects the rate because enzymes control the reactions.

没有光、叶绿素、水和二氧化碳，光合作用无法进行。温度也会影响速率，因为酶控制着这些反应。

Light energy (photons) drives the reaction.

光能（光子）驱动反应。
Chlorophyll absorbs light, mainly red and blue wavelengths.

叶绿素吸收光线，主要是红光和蓝光波段。
Water is absorbed by roots and transported to leaves.

水由根部吸收并运输到叶片。
Carbon dioxide enters leaves through stomata.

二氧化碳通过气孔进入叶片。

4. Light-Dependent Stage | 光依赖阶段

This stage occurs in the thylakoid membranes of chloroplasts. Light energy splits water molecules into oxygen, protons, and electrons. ATP and reduced NADP are produced.

该阶段发生在叶绿体的类囊体膜上。光能分解水分子，产生氧气、质子和电子。同时生成ATP和还原型NADP。

H₂O → ½O₂ + 2H⁺ + 2e⁻

Oxygen is released as a by-product. ATP and reduced NADP are used in the next stage.

氧气作为副产物释放。ATP和还原型NADP用于下一阶段。

5. Light-Independent Stage (Calvin Cycle) | 光不依赖阶段（卡尔文循环）

This stage takes place in the stroma of the chloroplast. Carbon dioxide is fixed by ribulose bisphosphate (RuBP) to form two molecules of glycerate-3-phosphate (GP). ATP and reduced NADP provide energy to reduce GP to triose phosphate (TP).

该阶段发生在叶绿体基质中。二氧化碳被核酮糖二磷酸（RuBP）固定，形成两个甘油酸-3-磷酸（GP）分子。ATP和还原型NADP提供能量，将GP还原为三碳糖磷酸（TP）。

CO₂ + RuBP → 2 × GP

CO₂ + RuBP → 2 × GP
GP + ATP + reduced NADP → TP

GP + ATP + 还原型NADP → TP
Most TP is recycled to RuBP; some is converted to glucose.

大多数TP被再生成RuBP；部分转化为葡萄糖。

6. Factors Affecting the Rate | 影响速率的因素

Light intensity, carbon dioxide concentration, and temperature each limit the rate of photosynthesis. The law of limiting factors states that at any moment, the rate is limited by the factor closest to its minimum value.

光照强度、二氧化碳浓度和温度各自限制光合作用速率。限制因子定律指出，在任何时刻，速率受最接近其最小值的因子所限制。

Factor	Effect
Light intensity	More light → more photons → faster rate until saturation
CO₂ concentration	Higher CO₂ → more substrate for carbon fixation
Temperature	Optimum around 25–35°C; above 40°C enzymes denature
Chlorophyll availability	Lack of magnesium reduces chlorophyll production

光照强度

光照越多 → 光子越多 → 速率越快，直至饱和

二氧化碳浓度

CO₂越高 → 碳固定底物越多

温度

最适约25–35°C；超过40°C酶失活

叶绿素可用性

缺镁会减少叶绿素合成

7. Limiting Factor Graphs | 限制因子曲线

When light intensity is increased, the rate rises steeply, then plateaus. At the plateau, CO₂ or temperature becomes the limiting factor.

当光照强度增加时，速率先急剧上升，然后趋于平缓。在平缓阶段，CO₂或温度成为限制因子。

If CO₂ concentration is increased at the plateau, the rate rises again until another factor limits it. This confirms the principle of limiting factors.

如果在平台期增加CO₂浓度，速率会再次上升，直到另一个因子限制它。这验证了限制因子的原理。

8. Practical: Investigating Photosynthesis | 实验：探究光合作用

The number of oxygen bubbles produced per minute by an aquatic plant (e.g., Elodea) can be used to estimate the rate of photosynthesis. Change the distance of a lamp to vary light intensity, or add sodium hydrogen carbonate solution to vary CO₂ concentration.

每分钟由水生植物（如伊乐藻）产生的氧气气泡数可用于估算光合作用速率。改变灯的距离可改变光照强度，或添加碳酸氢钠溶液以改变CO₂浓度。

Count bubbles for a fixed time, e.g., 1 minute.

在固定时间内数气泡，例如1分钟。
Repeat at different distances or CO₂ concentrations.

在不同距离或CO₂浓度下重复。
Control temperature by keeping the water bath at a constant value.

通过保持水浴温度恒定来控制温度。

A more accurate method collects the oxygen in a graduated test tube and measures its volume.

更精确的方法是将氧气收集在刻度试管中并测量其体积。

9. Uses of Glucose in Plants | 葡萄糖在植物中的用途

Glucose produced during photosynthesis is used for respiration, converted into starch for storage, cellulose for cell walls, sucrose for transport, and lipids for membranes. It also provides carbon skeletons for amino acids and proteins when combined with nitrate ions.

光合作用产生的葡萄糖用于呼吸作用、转变成淀粉储存、纤维素构成细胞壁、蔗糖运输以及脂质构成膜。与硝酸根离子结合时，还提供氨基酸和蛋白质的碳骨架。

Respiration releases energy from glucose.

呼吸作用从葡萄糖中释放能量。
Starch is insoluble and stored in chloroplasts.

淀粉不溶于水，储存在叶绿体中。
Cellulose strengthens plant cell walls.

纤维素增强植物细胞壁。
Sucrose travels in the phloem.

蔗糖在韧皮部运输。

10. Adapted Leaf Structure | 叶片结构的适应

Leaves are broad and thin to maximise light absorption and reduce diffusion distance. The upper epidermis is transparent, palisade mesophyll cells are packed with chloroplasts, and spongy mesophyll has air spaces for gas exchange.

叶片宽而薄，以最大化吸收光照并减少扩散距离。上表皮透明，栅栏组织细胞富含叶绿体，海绵组织具有气室以利于气体交换。

Structure	Adaptation
Waxy cuticle	Prevents water loss
Palisade layer	Many chloroplasts near the upper surface
Stomata	Allow CO₂ in and O₂ out
Xylem/Phloem	Transport water and products

蜡质角质层

防止水分蒸发

栅栏层

近上表面有许多叶绿体

气孔

允许CO₂进入和O₂释放

木质部/韧皮部

运输水和产物

11. Mineral Requirements for Photosynthesis | 光合作用的矿物质需求

Magnesium is essential for chlorophyll synthesis, and nitrogen is required for amino acids and enzymes. Without these minerals, plants show chlorosis and reduced photosynthetic capacity.

镁是叶绿素合成的必需元素，氮是氨基酸和酶的必需元素。缺乏这些矿物质，植物会出现失绿症，光合能力下降。

Nitrate ions (NO₃⁻) are used to build proteins, which are needed for the enzymes of photosynthesis. Phosphate ions are part of ATP and the phospholipid bilayer of chloroplast membranes.

硝酸根离子（NO₃⁻）用于构建蛋白质，而蛋白质是光合作用酶所必需的。磷酸根离子是ATP和叶绿体膜磷脂双分子层的一部分。

12. Summary | 总结

Photosynthesis converts light energy into chemical energy. Its rate is controlled by light intensity, CO₂ concentration, temperature, and chlorophyll availability. Understanding these factors helps farmers improve crop yields.

光合作用将光能转化为化学能。其速率受光照强度、CO₂浓度、温度和叶绿素可用性控制。理解这些因素有助于农民提高作物产量。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Remember the key equation and the limiting factors — they are essential for both written and practical exam questions.

记住关键方程式和限制因子——它们是笔试和实验题的核心。

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Photosynthesis | 光合作用