📚 Factors Necessary for Photosynthesis | 光合作用必需因素
Photosynthesis is the process by which photoautotrophs such as plants, algae and some bacteria convert light energy into chemical energy stored in glucose. In A-Level Cambridge Biology, you are expected to explain not only the light-dependent and light-independent stages but also the environmental and internal factors that are necessary for photosynthesis to proceed. These factors operate together, and any one of them can become limiting when it is in short supply.
光合作用是植物、藻类和某些细菌等光合自养生物将光能转化为储存在葡萄糖中的化学能的过程。在剑桥 A-Level 生物中，你不仅要解释光反应和暗反应阶段，还要说明光合作用进行所必需的环境因素和内部因素。这些因素共同作用，任何一个供应不足都可能成为限制因素。
1. Photosynthesis Overview | 光合作用概述
Photosynthesis occurs mainly in chloroplasts, where thylakoid membranes carry out light-dependent reactions and the stroma carries out the Calvin cycle. The process requires carbon dioxide, water, light energy and photosynthetic pigments. Products include glucose, oxygen and water.
光合作用主要发生在叶绿体中，类囊体膜上进行光反应，基质中进行卡尔文循环。该过程需要二氧化碳、水、光能和光合色素，产物包括葡萄糖、氧气和水。
ATP and reduced NADP produced in the light-dependent stage are essential for the Calvin cycle, linking energy capture to carbon fixation.
光反应阶段产生的 ATP 和还原型 NADP 是卡尔文循环所必需的，它们将能量捕获与碳固定联系起来。
2. Overall Chemical Equation | 光合作用总化学方程式
The overall word equation is carbon dioxide + water → glucose + oxygen in the presence of light and chlorophyll.
总文字方程式为二氧化碳 + 水 → 葡萄糖 + 氧气，需要光和叶绿素参与。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The equation shows that six molecules of carbon dioxide and six molecules of water produce one hexose sugar and six molecules of oxygen. However, the oxygen released comes from the splitting of water, not from carbon dioxide.
该方程表明六分子二氧化碳和六分子水生成一分子己糖和六分子氧。但释放的氧气来自水的裂解，而不是二氧化碳。
3. Light as an Energy Source | 光作为能量来源
Light provides the energy needed to excite electrons in chlorophyll and to split water during photolysis. Without sufficient light, ATP and reduced NADP cannot be generated, so the Calvin cycle slows or stops.
光提供激发叶绿素中电子和在光解过程中分解水所需的能量。如果光不足，就无法产生 ATP 和还原型 NADP，因此卡尔文循环会减慢或停止。
Light intensity affects the rate of photosynthesis only up to a certain point; beyond that another factor becomes limiting.
光强只在某一点之前影响光合作用速率；超过该点后另一因素变为限制因素。
4. Light Intensity and Limiting Factor | 光照强度与限制因素
At low light intensity, the rate of photosynthesis is directly proportional to light intensity because light is the limiting factor. At high light intensity, the rate plateaus as other factors such as CO₂ concentration or temperature limit the process.
在低光强下，光合速率与光强成正比，因为光是限制因素。在高光强下，速率趋于平稳，因为 CO₂ 浓度或温度等其他因素限制了过程。
This can be represented by a graph with a linear increase followed by a plateau.
这可以用一条先线性上升后趋于平稳的曲线来表示。
5. Light Wavelength and Photosynthetic Pigments | 光波长与光合色素
Chlorophyll a absorbs mainly red and blue-violet light, while accessory pigments such as chlorophyll b and carotenoids extend the range of wavelengths absorbed. Green light is reflected or transmitted, which is why leaves appear green.
叶绿素 a 主要吸收红光和蓝紫光，而叶绿素 b 和类胡萝卜素等辅助色素扩展了可吸收光的波长范围。绿光被反射或透射，因此叶片呈绿色。
An action spectrum shows the rate of photosynthesis at different wavelengths, and an absorption spectrum shows which wavelengths are absorbed by pigments. The two spectra correlate closely.
作用光谱显示不同波长下的光合速率，吸收光谱显示色素吸收哪些波长。这两种光谱密切相关。
| Pigment | Main absorption | Function |
|---|---|---|
| Chlorophyll a | Red and blue-violet light | Reaction centre pigment |
| Chlorophyll b | Blue and red-orange light | Accessory pigment |
| Carotenoids | Blue-violet light | Accessory and photoprotection |
The table summarises the main photosynthetic pigments and their roles in absorbing light and protecting the photosynthetic apparatus.
上表总结了主要光合色素及其在吸收光和保护光合结构方面的作用。
6. Carbon Dioxide Concentration | 二氧化碳浓度
Carbon dioxide is the carbon source for the Calvin cycle. RuBP carboxylase, also called Rubisco, fixes CO₂ onto RuBP to form two molecules of GP. At low CO₂ concentration, the rate of photosynthesis is limited because carbon fixation is slow.
二氧化碳是卡尔文循环的碳源。RuBP 羧化酶（Rubisco）将 CO₂ 固定到 RuBP 上，形成两分子 GP。在低 CO₂ 浓度下，由于碳固定缓慢，光合速率受到限制。
Above an optimum CO₂ concentration, the rate plateaus because RuBP regeneration or other enzyme steps become limiting.
超过最适 CO₂ 浓度后，光合速率趋于平稳，因为 RuBP 再生或其他酶促步骤成为限制因素。
7. Temperature and Enzyme Activity | 温度与酶活性
Photosynthesis involves enzymes such as Rubisco and ATP synthase. As temperature increases, enzyme activity and reaction rate rise until an optimum. Above the optimum, enzymes denature and the rate falls sharply.
光合作用涉及 Rubisco 和 ATP 合酶等酶。随着温度升高，酶活性和反应速率增加，直到达到最适温度。超过最适温度后，酶变性，速率急剧下降。
The light-independent reactions are more temperature-sensitive than the light-dependent reactions because they depend on enzymes.
暗反应比光反应对温度更敏感，因为它们依赖酶。
8. Water Supply and Stomatal Control | 水分供应与气孔控制
Water is a raw material for photolysis and supplies electrons, protons and oxygen. It also maintains turgor in guard cells; when water is scarce, stomata close to reduce transpiration, but this also reduces CO₂ uptake and slows photosynthesis.
水是光解的原料，提供电子、质子和氧气。水还维持保卫细胞的膨压；缺水时气孔关闭以减少蒸腾，但这也减少 CO₂ 吸收并降低光合速率。
2H₂O → 4H⁺ + 4e⁻ + O₂
Therefore water shortage affects photosynthesis mainly indirectly through stomatal closure, except when severe enough to inhibit photolysis.
因此缺水主要通过气孔关闭间接影响光合作用，除非严重到足以抑制光解。
9. Chlorophyll and Accessory Pigments | 叶绿素与辅助色素
Chlorophyll molecules contain a magnesium ion at the centre of a porphyrin ring. This structure allows them to absorb light energy and transfer excited electrons. Without chlorophyll, photosynthesis cannot capture light effectively.
叶绿素分子在卟啉环中心含有镁离子。这种结构使其能够吸收光能并传递激发电子。没有叶绿素，光合作用就无法有效捕获光。
Accessory pigments pass energy to the reaction centre in photosystems and also protect chlorophyll from photo-oxidation.
辅助色素将能量传递给光系统中的反应中心，并保护叶绿素免受光氧化。
10. Mineral Nutrients: Magnesium and Nitrogen | 矿质营养：镁和氮
Magnesium is required to synthesise chlorophyll. Nitrogen is needed to make proteins, including Rubisco, and nucleic acids. Deficiency of magnesium causes chlorosis, so light absorption decreases and photosynthesis slows.
镁是合成叶绿素所必需的。氮用于制造蛋白质（包括 Rubisco）和核酸。缺镁会引起萎黄病，因此光吸收减少，光合作用减慢。
Nitrogen deficiency reduces the quantity of photosynthetic enzymes and can cause stunted growth.
缺氮会减少光合酶的数量，并可能导致生长受阻。
11. The Law of Limiting Factors | 限制因素定律
Blackman’s law of limiting factors states that when a process is affected by more than one factor, its rate is limited by the factor closest to its minimum value. Increasing other factors will not increase the rate until that limiting factor is relieved.
布莱克曼限制因素定律指出，当一个过程受多个因素影响时，其速率受最接近最小值的因素限制。在解除该限制因素之前，增加其他因素不会提高速率。
For example, at low light intensity, raising CO₂ concentration has little effect because light is limiting.
例如，在低光强下，提高 CO₂ 浓度几乎没有效果，因为光是限制因素。
12. Investigating Factors Experimentally | 实验探究影响因素
A common practical uses Elodea or Cabomba in water, with sodium hydrogencarbonate as a CO₂ source. The rate of photosynthesis can be measured by counting oxygen bubbles produced per minute or by collecting gas in a capillary tube.
常用实验使用水中的伊乐藻或金鱼藻，并以碳酸氢钠作为 CO₂ 来源。光合速率可以通过每分钟产生的氧气泡数或使用毛细管收集气体来测量。
Changing light intensity by moving a lamp, or changing CO₂ concentration or temperature, allows a limiting-factor graph to be plotted. A control uses boiled water or no hydrogencarbonate.
通过移动灯改变光强，或改变 CO₂ 浓度或温度，可以绘制限制因素曲线。对照使用煮沸过的水或不加碳酸氢盐。
Care must be taken to keep other variables constant; if temperature changes, enzymes may be affected.
必须注意保持其他变量不变；如果温度变化，酶可能受到影响。
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