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 every food chain on Earth.

光合作用是绿色植物、藻类和一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 Definition and Importance | 定义与重要性

Photosynthesis is an endothermic reaction in which carbon dioxide and water are combined, using energy from sunlight trapped by chlorophyll, to produce glucose and oxygen. The glucose can then be used for respiration or converted into starch, cellulose and other substances.

光合作用是吸热反应，二氧化碳和水在叶绿素捕获的太阳光能作用下合成葡萄糖和氧气。生成的葡萄糖随后可用于呼吸作用，或转化为淀粉、纤维素等物质。

This process is essential for life because it produces oxygen for animals to breathe and creates biomass that supports food webs. Without photosynthesis, most living organisms, including humans, would not exist.

这一过程对生命至关重要，因为它为动物呼吸提供氧气，同时产生支持食物网的生物量。没有光合作用，包括人类在内的大多数生物将不复存在。

2. The Word and Symbol Equations | 文字式和符号方程式

The balanced chemical equation for photosynthesis is often written in two forms.

光合作用的化学方程式通常有两种写法。

Word equation:

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

Symbol equation:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The equation shows that six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen. The reaction takes place in the chloroplasts of plant cells.

该方程式表明，六个二氧化碳分子与六个水分子反应生成一个葡萄糖分子和六个氧分子。反应发生在植物细胞的叶绿体中。

3. The Structure of the Leaf | 叶片的结构

Leaves are specially adapted organs that maximise the rate of photosynthesis. Their flat shape provides a large surface area for absorbing light, and their thinness allows carbon dioxide to diffuse rapidly to all cells.

叶片是专为最大化光合作用速率而特化的器官。其扁平形状提供了吸收光线的巨大表面积，而薄薄的叶片使二氧化碳能迅速扩散到所有细胞。

The upper epidermis is transparent and covered with a waxy cuticle that reduces water loss. The palisade mesophyll layer, located just below, contains most of the chloroplasts and is the main site of photosynthesis.

上表皮透明，并覆有减少水分蒸发的蜡质角质层。位于其下方的栅栏组织含有大部分叶绿体，是光合作用的主要场所。

Spongy mesophyll cells are more loosely packed, creating large air spaces that allow gases to circulate. Stomata (pores) on the lower surface open and close to control gas exchange and water loss.

海绵组织细胞排列疏松，形成大量气室，便于气体流通。下表面的气孔通过开闭调节气体交换和水分蒸发。

4. Root Hair Cells and Transport of Water | 根毛细胞与水分运输

Water enters the plant through root hair cells, which are elongated projections on the surface of roots. They have a large surface area and thin cell walls, allowing water to be absorbed efficiently by osmosis.

水分通过根毛细胞进入植物体。根毛细胞是根表面伸长的突起，具有巨大的表面积和薄的细胞壁，可通过渗透作用高效吸水。

From the root hair cells, water moves through the root cortex and into the xylem vessels. The xylem transports water and dissolved minerals upwards to the leaves, driven by transpiration pull and root pressure.

水从根毛细胞穿过皮层进入木质部导管。木质部靠蒸腾拉力与根压将水和溶解的矿物质向上输送到叶片。

In the leaf, water is needed as a reactant in photosynthesis, and it also keeps cells turgid so that the leaf stays firm and spread out to capture light. Deficiency of water causes stomata to close, reducing carbon dioxide uptake and slowing photosynthesis.

在叶片中，水既是光合作用的反应物，又可使细胞保持膨压，使叶片挺立展开以捕捉光。缺水时气孔关闭，二氧化碳吸收减少，光合作用减慢。

5. Light Absorption and Chlorophyll | 光吸收与叶绿素

Chlorophyll is a green pigment found in chloroplasts. It absorbs red and blue light most efficiently and reflects green light, which is why leaves appear green. Light energy is transferred into chemical energy during the light-dependent reactions.

叶绿素是存在于叶绿体中的绿色色素。它最有效地吸收红光和蓝光，而反射绿光，因此叶片呈现绿色。在光反应阶段，光能被转化为化学能。

There are several accessory pigments, such as carotenoids, that broaden the range of wavelengths absorbed. In autumn, chlorophyll breaks down and the carotenoids become visible, causing leaves to turn yellow or orange.

叶黄素等辅助色素拓宽了吸收的波长范围。秋天叶绿素分解，类胡萝卜素显现，叶片因而变黄或变橙。

For accurate experiments on photosynthesis, the wavelength of light is an important variable. Red and blue light generally produce the highest photosynthetic rates, while green light produces a much lower rate.

在准确的光合作用实验中，光波长是关键变量。红光和蓝光通常产生最高的光合速率，而绿光的效率低得多。

6. The Rate of Photosynthesis | 光合作用速率

The rate of photosynthesis can be measured by the volume of oxygen produced, the rate of carbon dioxide uptake, or the mass increase of the plant. In aquatic plants such as Elodea, counting bubbles per minute gives an approximate rate, but measuring the gas volume in a syringe is more accurate.

光合速率可通过产氧量、二氧化碳吸收速率或植物干重增加来测量。对金鱼藻等水生植物，每分钟气泡数可粗略估算，但用注射器收集气体体积更精确。

Several factors affect the rate, including light intensity, carbon dioxide concentration, temperature, chlorophyll amount and water availability. The interaction of these factors is described by the concept of limiting factors.

影响光合速率的因素包括光照强度、二氧化碳浓度、温度、叶绿素含量和可用水分。这些因素的交互作用可用限制因素的概念来描述。

A simple experiment involves placing a lamp at different distances from a pondweed. Since light intensity follows the inverse square law, the rate is proportional to 1/distance².

一个简单实验是将灯放在离水生植物不同距离处。由于光强度服从平方反比定律，光合速率与 1/距离² 成正比。

7. Limiting Factors: Light, CO₂ and Temperature | 限制因素：光、二氧化碳与温度

When a factor is present in such low amounts that it prevents the rate from increasing, it is called a limiting factor. Increasing a non-limiting factor has no effect until the limiting factor is raised.

当某种因素含量过低、阻碍光合速率继续升高时，它便成为限制因素。增加非限制因素不会改变速率，直到限制因素被提高。

Light intensity is often the most common limiting factor in nature. As light intensity increases, the rate of photosynthesis rises until a plateau is reached, where another factor becomes limiting.

光强在自然界中常是最常见的限制因素。随着光强增大，光合速率上升直至达到平台期，此时另一因素变为限制因素。

Carbon dioxide concentration also limits photosynthesis in still air or closed greenhouses. Farmers can boost yields by adding CO₂ generators. Temperature affects enzyme-controlled reactions; for every 10 °C rise up to optimum, the rate roughly doubles, but very high temperatures denature enzymes.

二氧化碳浓度在密闭温室或静止空气中也会限制光合作用。农民通过增加二氧化碳发生器提高产量。温度影响酶促反应；在到达最适温度前，每升高 10 °C，速率约翻倍，但过高会令酶变性。

8. Investigating Photosynthesis | 探究光合作用的实验

The classic validation experiment uses a plant such as a potted geranium. After destarching the plant in darkness for 48 hours, a stencil is placed on a leaf and the leaf is exposed to light. The leaf is then boiled in ethanol to remove chlorophyll, washed, and tested with iodine solution.

经典验证实验以天竺葵为材料。将植株在黑暗中饥饿处理 48 小时去除淀粉，然后用模板遮住叶片的一部分并照光。之后用乙醇煮沸脱色，冲洗后滴加碘液。

The blue-black colour appears only in the areas that received light, proving that light is necessary for photosynthesis and that starch is a product of photosynthesis. Other experiments can test for oxygen production and carbon dioxide uptake.

碘液变蓝紫色只出现在被光照到的部位，证明光是光合作用所必需的，且淀粉是产物之一。其他实验可检测氧气生成和二氧化碳消耗。

It is important to control variables such as temperature, light intensity and carbon dioxide concentration. A water bath with a transparent container can keep temperature constant while a coloured filter can change the wavelength of light.

控制变温、光强和二氧化碳浓度至关重要。使用透明水浴可保持温度恒定，而滤光片可改变光照波长。

9. Uses of Glucose in the Plant | 植物体内葡萄糖的用途

Glucose produced in photosynthesis is used in several ways. Some is used immediately for respiration to release energy; the rest is converted into insoluble storage products that can be used later.

光合作用产生的葡萄糖有多种用途。一部分立即用于呼吸作用释放能量；其余部分转化为不溶性储存物质，供后续使用。

Starch is stored in roots, stems and leaves, because it is insoluble and does not affect cell water potential.
纤维素（cellulose）成为植物细胞壁的主要成分，提供结构支持。
Sucrose is transported in the phloem to growing regions and storage organs.
Other carbon compounds, such as amino acids, are formed by combining glucose with nitrate ions.

淀粉储存于根、茎和叶中，因其不溶于水且不影响细胞水势。纤维素构成细胞壁，提供结构支撑。蔗糖可在韧皮部中运输至生长区或储存器官。氨基酸由葡萄糖和硝酸根离子结合形成。

Fats and oils are often stored in seeds as concentrated energy reserves, and these are valuable to humans as food products.

脂肪和油脂常储存于种子中作为浓缩的能量储备，也是人类重要的食物来源。

10. Mineral Requirements for Healthy Growth | 健康生长所需的矿物质

Plants also need small quantities of mineral ions absorbed from soil. Two major ions are nitrate and magnesium.

植物也需要从土壤中吸收少量矿质离子。两种主要离子是硝酸根和镁离子。

Nitrate ions are needed to make amino acids, which are then assembled into proteins for enzymes and cell structure. A shortage of nitrate causes stunted growth and yellowing of older leaves.

硝酸根离子用于合成氨基酸，进而组装成蛋白质，供酶和细胞结构使用。缺氮会导致矮小生长和老叶黄化。

Magnesium ions are an essential component of chlorophyll molecules. Without magnesium, chlorophyll cannot be produced, so leaves become yellow (chlorosis) and photosynthesis slows drastically.

镁离子是叶绿素分子的必需成分。没有镁，叶绿素无法合成，叶片会变黄（缺绿症），光合作用急剧减慢。

Other essential minerals include phosphorus for DNA and membranes, and potassium for enzyme activity and stomatal opening.

其他必需矿物质还包括磷，用于DNA和膜；钾，用于酶活性和气孔开闭。

11. Photosynthesis and the Environment | 光合作用与环境

Photosynthesis is central to the global carbon cycle. It removes carbon dioxide from the atmosphere and returns oxygen, thereby helping to regulate the Earth’s climate and atmospheric composition.

光合作用是全球碳循环的核心。它从大气中去除二氧化碳并释放氧气，从而帮助调节地球气候和大气组成。

Deforestation reduces the overall rate of photosynthesis, leading to higher atmospheric CO₂ levels and contributing to global warming. Reforestation and sustainable management of green spaces are therefore important climate mitigation strategies.

森林砍伐降低了总光合速率，导致大气CO₂水平升高，加剧全球变暖。因此，造林和绿色空间可持续管理是重要的气候减缓策略。

In agriculture, understanding the limiting factors of photosynthesis allows farmers to create optimum conditions in greenhouses, including controlled lighting, CO₂ enrichment and heating, to increase crop yields.

在农业中，理解光合作用的限制因素使农民能够在温室中创造最佳条件，如控制光照、增施CO₂和加热，从而提高作物产量。

12. Common Mistakes in Exams | 考试常见错误

Students often make similar mistakes when answering photosynthesis questions. Identifying them can help gain marks.

学生在回答光合作用问题时常犯一些类似错误。识别它们有助于得分。

Writing the symbol equation incorrectly: forgetting the 6 in front of O₂ or CO₂, or using the wrong formula for glucose (must be C₆H₁₂O₆, not C₆H₁₂O₃).
Confusing the gas produced: plants produce oxygen during photosynthesis in light, but they also take in oxygen during respiration at all times.
Stating ‘temperature increases the rate’ without mentioning the optimum and denaturation above it.
Ignoring the word ‘limiting factor’ when a plateau is shown on a graph.
Mixing up the roles of xylem and phloem: xylem carries water, phloem carries glucose.

写错符号方程式：忘记O₂或CO₂前的系数6，或葡萄糖化学式写错（必须是C₆H₁₂O₆，而不是C₆H₁₂O₃）；混淆气体：植物在光照下光合作用产生氧气，但任何时候都在呼吸作用吸收氧气；忽略温度最适值和高温变性；看到曲线平台期时想不到“限制因素”；混淆木质部与韧皮部的功能：木质部运输水分，韧皮部运输葡萄糖。

Remember that photosynthesis is a multiple-step process, so any reference to a single ‘chlorophyll reaction’ is not accurate. Always use the correct terminology: light-dependent stage and light-independent stage (Calvin cycle).

记住光合作用是多步骤过程，因此只提一个“叶绿素反应”是不准确的。务必使用正确术语：光反应阶段和暗反应阶段（卡尔文循环）。

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Photosynthesis | 光合作用