Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the process by which green plants, algae and some bacteria use light energy to convert carbon dioxide and water into glucose and oxygen. It is one of the most important chemical reactions on Earth because it supplies the organic compounds and oxygen that most living organisms depend on.

光合作用是绿色植物、藻类和某些细菌利用光能，将二氧化碳和水转化为葡萄糖和氧气的过程。这是地球上最重要的化学反应之一，因为它为大多数生物提供了所需的有机物和氧气。

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1. Word Equation and Chemical Equation | 文字方程式与化学方程式

In IGCSE Edexcel Science, you must be able to recall both the word equation and the balanced chemical equation for photosynthesis.

在IGCSE Edexcel 科学中，你必须能够熟练写出光合作用的文字方程式和配平的化学方程式。

The word equation is:

Carbon dioxide + Water → Glucose + Oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

The balanced chemical equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Notice that the arrow points from reactants to products. Light energy and chlorophyll are not written as part of the equation, but they are essential conditions: light provides the energy, and chlorophyll absorbs it.

注意箭头从反应物指向生成物。光能和叶绿素不写在化学方程式中，但它们是必需条件：光提供能量，叶绿素吸收光能。

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2. Chlorophyll and Photosynthesis Pigments | 叶绿素与光合色素

Chlorophyll is a green pigment found in chloroplasts, especially in the mesophyll cells of leaves. It absorbs light most strongly in the red and blue-violet regions of the spectrum, and reflects green light, which is why leaves appear green.

叶绿素是一种存在于叶绿体中的绿色色素，尤其在叶片的叶肉细胞中。它主要吸收光谱中的红光和蓝紫光，反射绿光，因此叶片看起来是绿色的。

• Chlorophyll transfers light energy into chemical energy during the light-dependent stage.

  叶绿素在光依赖阶段将光能转化为化学能。

• Other pigments such as carotenoids also absorb light and pass energy to chlorophyll.

  类胡萝卜素等其他色素也能吸收光能，并将其传递到叶绿素。

• A plant that lacks chlorophyll cannot photosynthesise; variegated leaves have white areas where no photosynthesis occurs.

  缺少叶绿素的植物无法进行光合作用；斑叶植物的白色区域无法进行光合作用。

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3. Leaf Structure Adaptations | 叶片结构的适应性

Leaves are well adapted for efficient photosynthesis. Their flat shape provides a large surface area for absorbing light and for gas exchange through stomata.

叶片的结构非常适合高效进行光合作用。它们的扁平形状提供了巨大的表面积，有利于吸收光能以及通过气孔进行气体交换。

Structure | 结构	Adaptation | 适应性
Cuticle | 角质层	Transparent and waterproof, reduces water loss while letting light pass | 透明且防水，在透光的同时减少水分流失
Upper epidermis | 上表皮	Thin and transparent for light penetration | 薄而透明，利于光线穿透
Palisade mesophyll | 栅栏叶肉	Many chloroplasts near the top surface to absorb maximum light | 靠近上表面含大量叶绿体，吸收最多光线
Spongy mesophyll | 海绵叶肉	Air spaces allow CO₂ to diffuse to palisade cells | 气隙使二氧化碳扩散到栅栏细胞
Stomata | 气孔	Openings for CO₂ in and O₂ out, controlled by guard cells | 供二氧化碳进入和氧气排出的开口，由保卫细胞控制

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4. Limiting Factors | 限制因素

A limiting factor is a condition that, when present in short supply, slows down or stops the rate of photosynthesis. The three main limiting factors are light intensity, carbon dioxide concentration and temperature.

限制因素是指当某条件供应不足时，会减慢或停止光合作用速率的因素。三大主要限制因素是光照强度、二氧化碳浓度和温度。

• Light intensity: as light intensity increases, the rate of photosynthesis increases until another factor becomes limiting.

  光照强度：随着光照强度增加，光合作用速率随之增加，直到另一个因素成为限制因素。

• Carbon dioxide concentration: increasing CO₂ often boosts photosynthesis, especially in bright light and warm conditions.

  二氧化碳浓度：提高CO₂浓度通常会促进光合作用，尤其在光照充足且温暖的条件下。

• Temperature: photosynthesis is controlled by enzymes; too low a temperature slows enzyme activity, while too high a temperature denatures enzymes.

  温度：光合作用由酶控制；温度过低会降低酶活性，温度过高会使酶变性。

Graphs of photosynthesis rate against light intensity show a plateau after the compensation point, indicating that another factor is limiting. This is a common examination question.

光照强度与光合作用速率的关系图中，在达到平台期后速率不再上升，说明另一因素成为限制因素。这是常见的考题。

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5. Rate of Photosynthesis: Practical Skills | 光合作用速率：实验技能

You may be asked to describe a simple experiment to measure the rate of photosynthesis. The easiest method uses an aquatic plant such as Elodea or Cabomba and counts the bubbles of oxygen produced per minute.

你可能会被要求描述测量光合作用速率的简单实验。最简便的方法是利用水生植物如伊乐藻或黑藻，计算每分钟产生的氧气气泡数。

Rate = volume of oxygen produced ÷ time

速率 = 产生氧气的体积 ÷ 时间

To investigate a limiting factor, change one variable at a time:

要研究某个限制因素，应一次只改变一个变量：

• Vary the distance of a lamp from the plant to change light intensity (remember light intensity follows the inverse square law).

  通过改变光源与植物的距离来改变光照强度（记住光照强度遵循平方反比定律）。

• Vary the concentration of sodium hydrogencarbonate solution to change CO₂ availability.

  改变碳酸氢钠溶液浓度来改变CO₂供应量。

• Vary the water bath temperature while controlling light and CO₂.

  在控制光照和CO₂的条件下，改变水浴温度。

Always keep controls: use the same plant size, same lamp, same water volume, and allow the plant to acclimatise before timing.

务必设置对照：使用相同大小的植物、相同的光源、相同体积的水，并在计时前让植物适应环境。

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6. Testing for Starch | 检测淀粉

Starch is often stored in leaves as an insoluble product of glucose. To prove that photosynthesis has occurred, we can test a leaf for starch using iodine solution.

叶片常常以不溶性的淀粉形式储存葡萄糖产物。为了证明光合作用已经发生，我们可以用碘液检测叶片中的淀粉。

The method involves these steps:

该方法的步骤如下：

1. Boil the leaf in water to kill the cells and break down cell membranes.

   将叶片在沸水中煮沸，杀死细胞并破坏细胞膜。

2. Boil the leaf in ethanol to remove chlorophyll (do this in a water bath because ethanol is flammable).

   在乙醇中煮沸叶片以去除叶绿素（由于乙醇易燃，必须在水浴中加热）。

3. Rinse the leaf in warm water to soften it.

   用温水冲洗叶片使其变软。

4. Spread the leaf on a white tile and add a few drops of iodine solution.

   将叶片平铺在白瓷板上，滴加几滴碘液。

If starch is present, the leaf turns blue-black; if not, it stays orange-brown. For a controlled experiment, use a destarched plant by keeping it in the dark for 24–48 hours before the test.

如果存在淀粉，叶片会变成蓝黑色；如果不存在，则保持棕黄色。为了进行对照实验，应先将植物置于黑暗中24–48小时以消耗原有淀粉。

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7. Uses of Glucose in Plants | 植物中葡萄糖的用途

Glucose produced during photosynthesis is used in multiple ways. This is a key area where examiners ask students to link biology to plant growth and metabolism.

光合作用产生的葡萄糖有多种用途。这是考官常考查学生将生物学与植物生长和代谢联系起来的关键领域。

• Respiration: glucose is broken down in respiration to release energy for cell activities.

  呼吸作用：葡萄糖在呼吸作用中被分解，释放能量供细胞活动使用。

• Starch: stored as insoluble starch in roots, stems and seeds as a long-term energy store.

  淀粉：以不溶性淀粉形式储存在根、茎和种子中，作为长期能量储备。

• Sucrose: transported in the phloem to other parts of the plant.

  蔗糖：通过韧皮部运输到植物其他部位。

• Cellulose: used to build plant cell walls for structural support.

  纤维素：用于构建植物细胞壁，提供结构支持。

• Proteins: combined with nitrate ions absorbed from the soil to make amino acids and proteins.

  蛋白质：与从土壤中吸收的硝酸根离子结合，生成氨基酸和蛋白质。

• Fats and oils: stored in seeds as energy reserves for germination.

  脂肪和油：储存在种子中，作为萌发时的能量储备。

This explains why plants need nitrate ions in addition to the products of photosynthesis – a common link to mineral nutrition.

这解释了为什么植物除了光合作用产物之外还需要硝酸根离子——这是与矿物质营养相关的常见联系。

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8. Mineral Nutrition and Chlorosis | 矿物质营养与缺绿症

Photosynthesis depends on healthy leaves. Mineral ions are absorbed from the soil through root hairs, and a lack of certain minerals can reduce photosynthesis.

光合作用依赖健康的叶片。矿物质离子通过根毛从土壤吸收，缺乏某些矿物质会降低光合作用能力。

Mineral | 矿质元素	Deficiency symptom | 缺乏症状
Nitrate (NO₃⁻) | 硝酸根	Stunted growth, yellowing of older leaves due to lack of protein | 生长受阻，老叶发黄（因蛋白质缺乏）
Magnesium (Mg²⁺) | 镁离子	Chlorosis – yellow leaves because magnesium is needed to make chlorophyll | 缺绿症——叶片发黄，因为镁是合成叶绿素的原料
Phosphate (PO₄³⁻) | 磷酸根	Poor root growth and dark green leaves | 根系生长不良，叶片呈暗绿色
Potassium (K⁺) | 钾离子	Yellow leaf edges and weak stems | 叶缘发黄，茎秆软弱

When answering questions, mention how each mineral is used in the plant, not just the symptom.

答题时要说明每种矿质元素在植物中的用途，而不仅仅是列出症状。

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9. Photosynthesis and Respiration Compared | 光合作用与呼吸作用比较

Photosynthesis and respiration are often confused. Remember that photosynthesis is anabolic and stores energy, while respiration is catabolic and releases energy.

光合作用和呼吸作用经常被混淆。记住：光合作用是合成代谢，储存能量；呼吸作用是分解代谢，释放能量。

• Photosynthesis: uses CO₂ and water to produce glucose and oxygen, requires light, occurs only in cells with chloroplasts.

  光合作用：利用CO₂和水产生葡萄糖和氧气，需要光，只发生在含叶绿体的细胞中。

• Respiration: uses glucose and oxygen to produce CO₂, water and energy, occurs in all living cells all the time.

  呼吸作用：利用葡萄糖和氧气产生CO₂、水和能量，发生在一切活细胞中，持续进行。

A useful way to compare is to write them side by side:

一个有用的比较方式是把二者并排写出：

Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

Note that respiration is essentially the reverse of photosynthesis in terms of raw materials and products.

注意，从反应物和生成物来看，呼吸作用基本上是光合作用的逆过程。

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10. Global Importance and Conservation | 全球重要性及生态保护

Photosynthesis is the basis of almost all food chains. It removes carbon dioxide from the atmosphere and releases oxygen, helping to maintain the balance of gases in the air.

光合作用几乎是所有食物链的基础。它从大气中吸收二氧化碳并释放氧气，有助于维持空气中气体的平衡。

Deforestation and burning of fossil fuels increase CO₂ levels and reduce photosynthetic capacity. Protecting forests and planting more trees enhances photosynthesis and helps mitigate climate change.

森林砍伐和化石燃料燃烧会增加CO₂水平并降低光合作用能力。保护森林和植树造林可以增强光合作用，有助于减缓气候变化。

In IGCSE questions, you may be asked to evaluate measures such as rewilding or sustainable agriculture in the context of the carbon cycle. Always connect the idea back to the rate of photosynthesis.

在IGCSE考题中，可能会要求你从碳循环的角度评价再野化或可持续农业等措施。一定要将观点与光合作用速率联系起来。

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