Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is one of the most important biological processes on Earth. Green plants, algae and some bacteria use this process to trap light energy and convert it into chemical energy stored in glucose. This process provides food for nearly all living organisms and produces the oxygen that most organisms need to survive. In Edexcel IGCSE Science, you are expected to recall the equations, explain leaf adaptations, interpret limiting-factor graphs and analyse practical experiments.

光合作用是地球上最重要的生命过程之一。绿色植物、藻类和某些细菌通过这一过程捕获光能，并将其转化为储存在葡萄糖中的化学能。光合作用为几乎所有生物提供食物，并产生大多数生物生存所需的氧气。在 Edexcel IGCSE 科学考试中，你需要熟记方程式、解释叶片适应性、解读限制因素曲线，并分析相关实验。

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1. What is Photosynthesis? | 什么是光合作用？

Photosynthesis is an endothermic reaction in which light energy is absorbed by chlorophyll and used to convert carbon dioxide and water into glucose and oxygen. The word “photosynthesis” comes from “photo” meaning light and “synthesis” meaning making something. The glucose produced is a sugar that acts as an energy source for the plant and for organisms that eat the plant.

光合作用是一种吸热反应：叶绿素吸收光能，并利用这些能量将二氧化碳和水转化为葡萄糖和氧气。光合作用一词由意为”光”的 photo 和意为”合成”的 synthesis 组成。产生的葡萄糖是一种糖类，为植物以及取食植物的生物提供能量来源。

Most photosynthesis occurs in the leaves, which are specialised for this process. The leaf is thin and flat, giving it a large surface area for absorbing light and exchanging gases. The glucose that is not used immediately for respiration is stored as starch or converted into other compounds such as cellulose and fats.

光合作用主要发生在叶片中，叶片是特化执行这一过程的结构。叶片薄而扁平，具有巨大的表面积，有利于吸收光线和交换气体。未被呼吸作用立即利用的葡萄糖会以淀粉形式储存，或转化为纤维素、脂肪等其他化合物。

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2. The Word Equation | 文字方程式

The word equation summarises the reactants and products of photosynthesis. You must be able to write it from memory and state that light energy absorbed by chlorophyll is the energy source for the reaction.

文字方程式概括了光合作用的反应物和生成物。你必须能够默写该方程式，并指出叶绿素吸收的光能是此反应的能量来源。

Carbon dioxide + Water → Glucose + Oxygen

(in the presence of light energy and chlorophyll)

Carbon dioxide is taken in through the stomata from the air, while water is absorbed by the roots and transported to the leaves through the xylem. The energy needed for this conversion comes only from light; without light, the reaction stops. Chlorophyll acts as the catalyst-like pigment that captures light energy, but it is not used up in the reaction.

二氧化碳通过气孔从空气中进入叶片，而水则由根系吸收，并经由木质部运输到叶片。这一转化所需的能量只来自光；没有光，反应即停止。叶绿素作为捕获光能的色素，起着类似催化剂的作用，但它在反应中不会被消耗。

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3. The Balanced Symbol Equation | 平衡符号方程式

The balanced symbol equation shows the relative number of atoms involved. You must be able to balance it and use state symbols where required.

平衡符号方程式展示了反应涉及的相对原子数目。你必须能够配平此方程式，并在需要时标注状态符号。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

On the left side there are 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms. On the right side, glucose contains 6 carbon, 12 hydrogen and 6 oxygen atoms, while the 6 oxygen molecules contain the remaining 12 oxygen atoms. Therefore both mass and atoms are conserved.

反应物一侧共有 6 个碳原子、12 个氢原子和 18 个氧原子。生成物一侧的葡萄糖含有 6 个碳、12 个氢和 6 个氧，而 6 个氧分子含有剩余的 12 个氧原子。因此，质量和原子数目均保持不变。

In the Edexcel course you may also be asked to write the equation with state symbols: 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g). Note that water is liquid, carbon dioxide is a gas, glucose is dissolved in the cell as an aqueous solution, and oxygen diffuses out as a gas.

在 Edexcel 课程中，你还可能被要求写出带状态符号的方程式：6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)。请注意：水为液态，二氧化碳为气态，葡萄糖以水溶液形式存在于细胞中，而氧气以气态扩散出去。

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4. Leaf Structure and Adaptations | 叶片结构与适应性

The leaf is the main photosynthetic organ. Its internal structure is highly adapted to absorb light, take in carbon dioxide and produce glucose efficiently.

叶片是主要的光合作用器官。其内部结构高度特化，能够高效地吸收光、摄取二氧化碳并制造葡萄糖。

Layer | 结构层	Adaptation | 适应性特征
Waxy cuticle | 蜡质角质层	Reduces water loss and acts as a barrier against infection. | 减少水分蒸发，并作为防感染的屏障。
Upper epidermis | 上表皮	Transparent so that light reaches the palisade layer below. | 透明，使光线能到达下方的栅栏层。
Palisade mesophyll | 栅栏组织	Long cells packed with many chloroplasts; located near the upper surface for maximum light capture. | 细胞细长且充满叶绿体，位于靠近上表面处以最大限度捕获光线。
Spongy mesophyll | 海绵组织	Air spaces allow carbon dioxide to diffuse rapidly between cells. | 细胞间有空隙，使二氧化碳能在细胞间快速扩散。
Stomata and guard cells | 气孔和保卫细胞	Pores on the lower surface allow CO₂ in and O₂ out; guard cells control their opening. | 下表面的小孔供 CO₂ 进入和 O₂ 排出；保卫细胞控制其开闭。

Table: Leaf structure and function | 表格：叶片结构与功能

The large surface area, thin shape and dense network of internal air spaces all speed up gas exchange. The position of stomata on the lower surface reduces water loss from direct sunlight while still allowing gas exchange.

巨大的表面积、薄而扁平的形状以及内部密集的气腔网络都能加速气体交换。气孔位于下表皮，可减少阳光直射造成的水分散失，同时仍能保证气体交换。

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5. Chloroplasts and Pigments | 叶绿体与色素

Chloroplasts are the organelles where photosynthesis takes place. Inside the chloroplast, the pigment chlorophyll is found in the thylakoid membranes. Chlorophyll absorbs light energy, mainly in the red and blue-violet regions of the spectrum, and reflects green light, which is why leaves appear green.

叶绿体是进行光合作用的细胞器。在叶绿体内部，叶绿素位于类囊体膜上。叶绿素主要吸收光谱中红光和蓝紫光区域的光能，并反射绿光，因此叶片呈现绿色。

Plants also contain accessory pigments such as carotenoids, which absorb light of different wavelengths and pass the energy to chlorophyll. Together these pigments allow plants to use a wider range of visible light and maximise energy capture in different light conditions.

植物还含有类胡萝卜素等辅助色素，这些色素吸收不同波长的光，并把能量传递给叶绿素。这些色素共同使植物能够利用更广泛的可见光谱，在不同光照条件下最大限度地捕获能量。

The number of chloroplasts in a plant cell can vary. Palisade cells contain far more chloroplasts than cells of the lower epidermis. When light intensity changes, the plant may also adjust chloroplast position within cells to receive more or less light.

植物细胞中叶绿体的数量可以不同。栅栏细胞所含叶绿体远多于下表皮细胞。当光强发生变化时，植物还可能调整叶绿体在细胞内的位置，以接收更多或更少的光。

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6. Limiting Factors | 限制因素

Any factor that slows down the rate of photosynthesis is called a limiting factor. The three main limiting factors are light intensity, carbon dioxide concentration and temperature. Water availability can also limit photosynthesis, but it is rarely tested as the main limiting factor in warm, well-watered conditions.

凡是会降低光合作用速率的因素都称为限制因素。三大主要限制因素是光强度、二氧化碳浓度和温度。水分供应也会限制光合作用，但在温暖湿润的条件下，它很少作为主要限制因素出现在题目中。

• Light intensity: Light provides the energy for photosynthesis. If light intensity is low, photosynthesis will be slow even if CO₂ and temperature are favourable.| 光强度：光为光合作用提供能量。若光强度较低，即使 CO₂ 和温度适宜，光合作用也会变慢。
• Carbon dioxide concentration: CO₂ is the raw material used to make glucose. At normal air levels (0.04%), CO₂ is often the limiting factor.| 二氧化碳浓度：CO₂ 是制造葡萄糖的原料。在正常空气水平（0.04%）下，CO₂ 往往是限制因素。
• Temperature: Photosynthesis is controlled by enzymes. As temperature rises towards the optimum (around 35°C), the rate increases; above 40-45°C enzymes denature and the rate falls sharply.| 温度：光合作用受酶控制。当温度升至最适温度（约 35°C）附近时，速率加快；超过 40-45°C 时酶会变性，速率急剧下降。

A limiting factor is only effective when it is below the level needed by the plant. At night, light is the limiting factor; on a bright but cold winter day, temperature may be the limiting factor; inside a greenhouse in summer, CO₂ concentration is often the limiting factor.

只有当某一因素低于植物所需水平时，它才会成为限制因素。在夜间，光是限制因素；在明亮但寒冷的冬日，温度可能是限制因素；在夏季温室中，CO₂ 浓度往往是限制因素。

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7. Interpreting Limiting-Factor Graphs | 解读限制因素曲线

In the Edexcel exam, you may be asked to interpret graphs showing the rate of photosynthesis against one variable. You must be able to describe the shape of the curve and explain why it plateaus.

在 Edexcel 考试中，你可能会被要求解读表示光合作用速率随某一变量变化的曲线图。你必须能够描述曲线形状，并解释速率为何趋于平稳。

Rate of photosynthesis → increases steeply, then plateaus as another factor becomes limiting

As light intensity increases from zero, the rate of photosynthesis rises steeply because more light energy is available. However, when the gradient becomes horizontal, light is no longer the limiting factor. At this point the rate is limited either by carbon dioxide concentration or by temperature.

随着光强度从零开始增加，光合作用速率迅速上升，因为可获得的光能增多。但是当曲线趋于水平时，光不再是限制因素。此时，限制速率的是二氧化碳浓度或温度。

• If the graph measures CO₂ concentration instead of light, the same plateau pattern appears when light or temperature becomes limiting.| 若曲线横坐标是 CO₂ 浓度而非光照，则当光照或温度成为限制因素时，会出现同样的平台期。
• If temperature is raised while all other factors stay constant, the plateau rises, then falls if the temperature becomes high enough to denature enzymes.| 若保持其他因素不变而升高温度，平台期会上升；当温度高到足以使酶变性时，曲线则会下降。
• A straight line through the origin means that the factor tested is the only limiting factor at that stage.| 曲线从原点作线性上升，说明在该阶段所测因素是唯一的限制因素。

Always quote the plateau region in your exam answer and name the new limiting factor clearly. Examiners award marks for identifying both the shape of the curve and the biological reason behind it.

在考试作答时务必描述平台期，并清楚指出新的限制因素。阅卷官会根据你是否识别出曲线形状及其背后的生物学原因来给分。

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8. Measuring the Rate of Photosynthesis | 测定光合作用速率

The classic Edexcel investigation uses pondweed (Elodea or Cabomba). The plant is placed in water containing dissolved sodium hydrogencarbonate, which supplies carbon dioxide. As photosynthesis occurs, oxygen bubbles are released from the cut stem.

Edexcel 经典实验使用伊乐藻（Elodea 或 Cabomba）作为实验材料。将植物放入含有溶解碳酸氢钠的水中，以提供二氧化碳。光合作用进行时，切开的茎端会释放出氧气气泡。

The rate can be measured by counting the number of bubbles released per minute or by collecting the gas in a measuring cylinder and recording the volume per unit time. To make the results more reliable, the light intensity is varied by changing the distance between the lamp and the pondweed.

可以通过每分钟释放的气泡数量来测定速率，也可用量筒收集气体并记录单位时间的体积。为了获得可靠结果，可以通过改变灯与伊乐藻之间的距离来改变光强度。

You must remember the controls: keep temperature constant with a water bath, keep the same pondweed, and use the same concentration of sodium hydrogencarbonate. The distance should be converted into light intensity using the inverse-square law.

你必须记住控制变量：用水浴保持温度恒定，使用相同的水生植物材料，并保持碳酸氢钠浓度一致。距离应依照平方反比定律换算为光强度。

Light intensity ∝ 1/d² (where d is the distance from the lamp)

Doubling the distance reduces light intensity to one-quarter, not one-half. This is a common exam point and many students lose marks by simply dividing by two instead of four.

距离加倍会使光强度降至原来的四分之一，而非二分之一。这是常见考点，许多学生因为只除以 2 而不是 4 而失分。

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9. Test for Starch | 淀粉检测实验

Glucose produced in photosynthesis is quickly converted to starch for storage. Therefore, the presence of starch in a leaf is evidence that photosynthesis has occurred. A leaf that has been kept in the dark for 24-48 hours is called a destarched leaf, because all its starch has been used up in respiration.

光合作用产生的葡萄糖会迅速转化为淀粉储存。因此，叶片中出现淀粉是曾经进行过光合作用的证据。在黑暗中放置 24-48 小时的叶片称为脱淀粉叶片，因为其淀粉已在呼吸作用中被消耗殆尽。

To test a leaf for starch, follow these steps.

检测叶片中是否有淀粉，需按以下步骤进行。

• Place the leaf in boiling water for 30 seconds to kill the cells and stop enzyme activity.| 将叶片放入沸水中 30 秒，杀死细胞并终止酶活性。
• Boil the leaf in ethanol for 1-2 minutes using a water bath (ethanol is flammable) to remove chlorophyll.| 用水浴加热酒精（酒精易燃）煮沸叶片 1-2 分钟，以去除叶绿素。
• Rinse the leaf with cold water to soften it and spread it on a white tile.| 用冷水冲洗叶片使其变软，并将其摊平在白瓷板上。
• Add a few drops of iodine solution. If starch is present, the iodine turns from brown to blue-black.| 滴加几滴碘液。若存在淀粉，碘液会从棕色变为蓝黑色。

A common experiment places a destarched leaf partly covered by black paper or a stencil. After a few hours of light exposure, only the exposed parts of the leaf turn blue-black, proving that light is required for photosynthesis.

常见实验是将脱淀粉叶片的一部分用黑纸或模板遮住。光照数小时后，只有暴露部分变为蓝黑色，从而证明光是光合作用的必要条件。

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10. Uses of Glucose in Plants | 葡萄糖在植物中的

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