Photosynthesis and Plant Nutrition | 光合作用与植物营养

📚 Photosynthesis and Plant Nutrition | 光合作用与植物营养

Photosynthesis is one of the most vital chemical processes on Earth. It allows green plants, algae, and some bacteria to trap light energy and convert it into chemical energy stored in glucose. This glucose serves as food for the plant and forms the base of nearly all food chains. Understanding how plants make their own food is a key topic in the Cambridge Lower Secondary Science Stage 8 curriculum, and it explains why plants are called producers.

光合作用是地球上最重要的化学过程之一。它使绿色植物、藻类和某些细菌能够捕获光能，并将其转化为储存在葡萄糖中的化学能。这些葡萄糖既是植物自身的食物，也是几乎所有食物链的基础。理解植物如何制造食物是剑桥初中科学第8阶段课程的一个核心主题，它也解释了为什么植物被称为生产者。

1. What Is Photosynthesis? | 什么是光合作用？

Photosynthesis is the process by which green plants use light energy to combine carbon dioxide and water into glucose and oxygen. It takes place inside chloroplasts, which are tiny organelles found mainly in the cells of leaves. The word ‘photosynthesis’ comes from ‘photo’ meaning light and ‘synthesis’ meaning putting together.

光合作用是绿色植物利用光能把二氧化碳和水转变成葡萄糖和氧气的过程。这个过程发生在叶绿体内，叶绿体是主要存在于叶片细胞中的微小细胞器。“光合作用”这个词来源于“光”和“合成”，意思是利用光来合成物质。

Without photosynthesis, life as we know it would not exist. It produces the oxygen we breathe and removes carbon dioxide from the atmosphere. Virtually all energy entering ecosystems starts with the sun’s energy captured during photosynthesis.

没有光合作用，我们所知的生命将不复存在。它产生了我们呼吸所需的氧气，并从大气中移除了二氧化碳。进入生态系统的几乎所有能量都始于光合作用中捕获的太阳能。

2. The Photosynthesis Equation | 光合作用方程式

The overall chemical reaction of photosynthesis can be summarised using words or chemical symbols. In words, the process is:

光合作用的整体化学反应可以用文字或化学符号来概括。用文字表示为：

carbon dioxide + water → glucose + oxygen

When written with chemical formulae, the balanced equation is:

用化学式写出的配平方程式为：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

This reaction requires two essential conditions: the presence of chlorophyll (the green pigment in chloroplasts) and a supply of light energy. Without either, photosynthesis cannot proceed.

这个反应需要两个基本条件：叶绿素（叶绿体中的绿色色素）的存在和光能的供给。缺少任何一个，光合作用都无法进行。

Notice that the equation is balanced: there are six carbon atoms, twelve hydrogen atoms, and eighteen oxygen atoms on both sides of the arrow. This obeys the law of conservation of mass.

请注意方程式是配平的：箭头两侧都有六个碳原子、十二个氢原子和十八个氧原子。这遵循了质量守恒定律。

3. Where Photosynthesis Happens | 光合作用在哪里发生

Photosynthesis occurs mainly in the leaves of plants, although any green part of the plant can photosynthesise. Inside leaf cells are numerous chloroplasts, each containing the pigment chlorophyll. Chlorophyll absorbs light energy, predominantly from the blue and red parts of the spectrum, and gives leaves their green colour.

光合作用主要发生在植物的叶片中，不过植物的任何绿色部分都能进行光合作用。叶肉细胞内有许多叶绿体，每个叶绿体含有色素叶绿素。叶绿素吸收光能，主要是光谱中的蓝光和红光部分，并赋予叶片绿色。

Leaves are adapted to maximise photosynthesis. They are broad and flat to capture as much sunlight as possible, thin to allow gases to diffuse quickly, and packed with chloroplasts in the palisade layer near the upper surface.

叶片适应于最大限度地提高光合作用效率。它们宽阔且扁平，以尽可能多地捕获阳光；很薄，以便气体快速扩散；并在靠近上表皮的栅栏组织中密布叶绿体。

4. How Plants Obtain Raw Materials | 植物如何获取原料

For photosynthesis, plants need carbon dioxide and water. Carbon dioxide enters leaves through tiny pores called stomata, which are mainly found on the underside of leaves. Guard cells control the opening and closing of each stoma to regulate gas exchange and water loss.

植物进行光合作用需要二氧化碳和水。二氧化碳通过称为气孔的微小孔隙进入叶片，气孔主要分布在叶片的背面。保卫细胞控制每个气孔的开闭，以调节气体交换和水分流失。

Water is absorbed from the soil by root hair cells and travels up the stem through xylem vessels to reach the leaves. The xylem forms a continuous tube system that transports water and dissolved minerals from the roots to the rest of the plant.

水由根毛细胞从土壤中吸收，并通过木质部导管沿茎向上运输到达叶片。木质部形成了一个连续的管道系统，将水和溶解的矿物质从根部运输到植物体的其他部分。

5. Factors Affecting the Rate of Photosynthesis | 影响光合作用速率的因素

Three main factors limit the speed of photosynthesis: light intensity, carbon dioxide concentration, and temperature. At any given moment, the rate is limited by whichever factor is in shortest supply – this is called a limiting factor.

限制光合作用速度的主要因素有三个：光照强度、二氧化碳浓度和温度。在任一给定时刻，光合作用速率受限于供应量最不足的那个因素——这被称为限制因子。

As light intensity increases, the rate of photosynthesis increases, but only up to a certain point. Beyond that, further increases in light cause no extra rise in rate because another factor, such as CO₂ availability, becomes limiting.

随着光照强度增加，光合作用速率提高，但只在达到某一点之前如此。超过这一点，进一步增加光照不会使速率再升高，因为另一个因素（例如二氧化碳的可获得性）成了限制因子。

Similarly, raising the carbon dioxide level can boost photosynthesis until the plant reaches its maximum rate. Temperature affects the enzymes involved in photosynthesis; too low a temperature slows reactions, while too high a temperature denatures the enzymes and stops photosynthesis.

类似地，提高二氧化碳水平可以促进光合作用，直到植物达到其最大速率。温度影响参与光合作用的酶；温度太低会减慢反应，温度太高则会使酶变性并终止光合作用。

6. The Importance of Photosynthesis | 光合作用的重要性

Photosynthesis is fundamental to life on Earth. It provides the glucose that plants use for respiration to release energy, for growth, and for building other organic molecules such as cellulose, proteins, and lipids. It also produces oxygen, which is essential for respiration in most organisms.

光合作用是地球生命的基础。它提供了植物用于呼吸作用以释放能量、用于生长以及构建其他有机分子（如纤维素、蛋白质和脂质）的葡萄糖。它还产生氧气，这对大多数生物的呼吸作用至关重要。

Without photosynthesis, the atmosphere would contain almost no oxygen and carbon dioxide levels would rise enormously. This process also helps to keep the climate stable by removing the greenhouse gas CO₂ from the air and storing carbon in plant biomass.

如果没有光合作用，大气中几乎不会有氧气，二氧化碳浓度会大幅上升。这一过程还通过从空气中移除温室气体二氧化碳，并将碳储存在植物生物质中，帮助维持气候稳定。

7. Testing a Leaf for Starch | 检测叶片中的淀粉

Because glucose produced during photosynthesis is quickly converted into starch for storage, testing a leaf for starch is a reliable way to check whether photosynthesis has taken place. The standard experiment involves the following steps:

由于光合作用产生的葡萄糖会迅速转变成淀粉加以储存，因此检测叶片中的淀粉是验证光合作用是否发生的一个可靠方法。标准实验包括以下步骤：

Place a leaf in boiling water for about one minute to kill it and stop all chemical reactions.
将一片叶子放入沸水中约一分钟，以杀死叶片并停止所有化学反应。
Transfer the leaf into a test tube of ethanol and place the tube in a water bath of hot water. The ethanol boils and removes the green chlorophyll from the leaf.
将叶片转移到盛有乙醇的试管中，再将试管放入热水浴中。乙醇沸腾并溶去叶片的绿色叶绿素。
Rinse the decolourised leaf in cold water to soften it.
用冷水冲洗脱色后的叶片使其软化。
Spread the leaf on a white tile and cover it with iodine solution. A blue-black colour indicates the presence of starch.
将叶片平铺在白瓷砖上，滴加碘液。出现蓝黑色表示有淀粉存在。

This test is often used to show that light, chlorophyll, and carbon dioxide are all necessary for photosynthesis. Leaves that were covered or destarched give a negative result.

该测试常用来证明光、叶绿素和二氧化碳都是光合作用所必需的。被遮光或经脱淀粉处理的叶片会呈阴性结果。

8. Photosynthesis and Respiration | 光合作用与呼吸作用

Photosynthesis and respiration are complementary processes. Photosynthesis uses carbon dioxide and water to produce glucose and oxygen, storing energy. Respiration, in contrast, breaks down glucose using oxygen to release energy, producing carbon dioxide and water as waste products.

光合作用与呼吸作用是互补的过程。光合作用利用二氧化碳和水产生葡萄糖和氧气，并储存能量。相反，呼吸作用利用氧气分解葡萄糖来释放能量，产生二氧化碳和水作为废物。

Photosynthesis	Respiration
Uses carbon dioxide and water	Uses glucose and oxygen
Produces glucose and oxygen	Produces carbon dioxide and water
Occurs only in light	Occurs all the time
Stores energy	Releases energy

During daylight, plants photosynthesise faster than they respire, so they take in carbon dioxide and give out oxygen overall. At night, photosynthesis stops but respiration continues, so they take in oxygen and release carbon dioxide.

白天，植物的光合作用速率高于呼吸作用，因此总体上它们吸收二氧化碳并放出氧气。夜晚，光合作用停止，但呼吸作用仍在继续，因此植物吸收氧气并释放二氧化碳。

9. Adaptations of Leaves for Photosynthesis | 叶片对光合作用的适应

Leaves are highly adapted organs for efficient photosynthesis. Key structural features include:

叶片是高度适应高效光合作用的器官。关键结构特征包括：

A large surface area that captures maximum sunlight. A thin, flat shape that allows gases to diffuse quickly to the photosynthesising cells. Numerous stomata, usually on the lower epidermis, that allow carbon dioxide to enter and oxygen to exit, while guard cells control their opening to balance gas exchange with water loss.

具有较大的表面积，以捕获最多的阳光。薄而扁平的形状，使气体能快速扩散到进行光合作用的细胞。大量气孔（通常位于下表皮），使二氧化碳进入，氧气排出，同时保卫细胞控制其开闭，以平衡气体交换与水分流失。

Most chloroplasts are located in the palisade mesophyll cells just below the upper epidermis, where light intensity is highest. A network of xylem vessels delivers water to the leaf cells, and phloem tubes transport the sugar produced to other parts of the plant.

大部分叶绿体位于紧挨上表皮的栅栏叶肉细胞中，那里的光照强度最高。木质部导管网络将水输送到叶肉细胞，韧皮部则将产生的糖运输到植物其他部分。

10. Common Misconceptions about Photosynthesis | 关于光合作用的常见误区

Many students think that plants get their food from the soil. In fact, most of the plant’s dry mass comes from carbon dioxide absorbed from the air during photosynthesis, not from the soil. Minerals taken up by roots are essential for growth but do not make up the bulk of plant material.

许多学生认为植物从土壤中获取食物。事实上，植物的大部分干物质来自于光合作用中从空气中吸收的二氧化碳，而不是土壤。根部吸收的矿物质对生长至关重要，但并不构成植物材料的主体。

Another misconception is that plants only respire at night. Plants respire continuously, just like animals. During the day, photosynthesis usually produces more oxygen than respiration consumes, making it appear that they only release oxygen.

另一个误区是植物只在夜间进行呼吸作用。植物像动物一样，持续进行呼吸作用。白天，光合作用产生的氧气通常多于呼吸作用消耗的氧气，因此看起来植物似乎只放出氧气。

11. Investigating Photosynthesis with Pondweed | 用水草研究光合作用

A common practical investigation for Stage 8 uses an aquatic plant like Elodea (pondweed) to measure the rate of photosynthesis. The plant is placed in water with sodium hydrogen carbonate added to provide a plentiful supply of carbon dioxide, and a light source is shone on it from a close distance.

第8阶段的一个常见实验是利用水生植物如伊乐藻（水草）来测量光合作用速率。将植物置于水中，并加入碳酸氢钠以提供充足的二氧化碳，再用光源近距离照射。

As the plant photosynthesises, oxygen bubbles are released from the cut end of the stem. The number of bubbles produced per minute can be counted as a measure of the photosynthesis rate. By changing the distance of the lamp, the effect of light intensity can be investigated.

当植物进行光合作用时，氧气泡从茎的切口处释放。每分钟产生的气泡数量可作为光合作用速率的量度。通过改变灯的距离，就可以研究光照强度的影响。

This investigation can be adapted to explore the effect of colour of light or the concentration of carbon dioxide. It is important to control all other variables, such as temperature, and to allow the apparatus to equilibrate before taking measurements.

这个实验也可以调整来探究光色或二氧化碳浓度的影响。重要的是控制所有其他变量，如温度，并且让装置在测量前达到平衡状态。

12. Summary and Revision Checklist | 总结与复习清单

Make sure you can state the word and symbol equations for photosynthesis. Be able to describe how leaves are adapted for photosynthesis and explain the roles of stomata, xylem, and chloroplasts. Understand how light intensity, carbon dioxide, and temperature act as limiting factors, and know how to test a leaf for starch safely.

确保你能说出光合作用的文字方程式和化学方程式。能够描述叶片如何适应光合作用，并解释气孔、木质部和叶绿体的作用。理解光照强度、二氧化碳和温度如何成为限制因子，并知道如何安全地检测叶片中的淀粉。

Also be confident in comparing photosynthesis and respiration. Remember that plants respire all the time, while photosynthesis only occurs in the light. Reinforce your learning by drawing labelled diagrams of a leaf cross-section and by practising past paper questions on this topic.

还要能够自信地比较光合作用和呼吸作用。记住植物时时刻刻都在呼吸，而光合作用只在有光时进行。通过绘制叶片横截面标注图并练习此主题的历年试题来巩固你的学习。

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Photosynthesis and Plant Nutrition | 光合作用与植物营养