📚 Enzymes: Biological Catalysts in IGCSE Science | 酶：IGCSE 科学中的生物催化剂
Enzymes are one of the most important topics in Edexcel IGCSE Science. They control nearly every chemical reaction in living organisms, from digestion to respiration, and they also have many industrial applications.
酶是 Edexcel IGCSE 科学中最重要的考点之一。生物体内几乎所有化学反应都由酶控制，从消化到呼吸，同时在工业上也有广泛应用。
1. What Are Enzymes? | 什么是酶？
Enzymes are protein molecules that act as biological catalysts. A catalyst speeds up the rate of a chemical reaction without being used up in the reaction. In living organisms, enzymes allow reactions to happen quickly at normal body temperature.
酶是充当生物催化剂的蛋白质分子。催化剂能加快化学反应速率，但自身并不在反应中被消耗。在生物体内，酶使反应能够在正常体温下快速进行。
Key points you must remember:
- Enzymes are biological catalysts.
- They are made of protein.
- They are not changed or destroyed by the reaction.
- 酶是生物催化剂。
- 它们由蛋白质组成。
- 它们在反应后不会被改变或破坏。
2. The Active Site and Enzyme-Substrate Complex | 活性位点与酶-底物复合物
Each enzyme has a special region called the active site. The reactant that joins to this region is called the substrate. When the substrate fits into the active site, an enzyme-substrate complex is formed. The reaction then occurs, and the products are released.
每种酶都有一个特殊区域，称为活性位点。与这个区域结合的反应物叫做底物。当底物与活性位点结合时，就形成了酶-底物复合物。随后反应发生，产物被释放出来。
Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product
酶 + 底物 → 酶-底物复合物 → 酶 + 产物
3. Specificity: One Enzyme, One Reaction | 专一性：一种酶，一种反应
Enzymes are specific. This means that an enzyme will usually only catalyse one reaction or one type of reaction. This is because the shape of the active site is complementary to only one substrate molecule. This is often described using the lock and key model: the enzyme is the lock, and the substrate is the key.
酶具有专一性。这意味着一种酶通常只能催化一个反应或一类反应。这是因为活性位点的形状只能与一种底物分子互补。这通常用“锁钥模型”来描述：酶是锁，底物是钥匙。
For Edexcel IGCSE, remember the term ‘complementary’. The substrate shape must fit the active site exactly, just like a key fitting a lock.
对于 Edexcel IGCSE，请记住“互补”这一术语。底物的形状必须与活性位点完全匹配，就像钥匙插入锁一样。
4. Temperature Effects | 温度的影响
Temperature has a major effect on enzyme activity. At low temperatures, particles move slowly, so fewer enzyme-substrate collisions happen. As temperature increases, particles gain more kinetic energy, collide more often, and the rate of reaction increases. This continues until the optimum temperature is reached.
温度对酶活性有很大影响。在低温下，分子运动缓慢，酶与底物碰撞次数少。随着温度升高，分子获得更多动能，碰撞更频繁，反应速率加快。这个趋势一直持续到最适温度。
For most human enzymes, the optimum temperature is about 37 °C. Beyond this point, heat breaks the weak bonds that hold the enzyme’s structure together. The active site changes shape, so the substrate cannot fit. The enzyme is said to be denatured.
大多数人体酶的最适温度约为 37 °C。超过该温度后，热量会破坏维持酶结构的弱键。活性位点发生改变，底物无法再结合，酶的活性丧失，这称为变性。
Rate increases up to optimum → sharp decrease after optimum → no activity when denatured
速率在最适温度前上升 → 超过最适温度后迅速下降 → 变性后活性消失
5. pH Effects | pH 的影响
Enzymes also have an optimum pH. Some work best in acidic conditions, others in neutral or alkaline conditions. A change in pH can alter the shape of the active site and cause the enzyme to denature.
酶也有最适 pH。有些酶在酸性条件下活性最高，有些则在中性或碱性条件下活性最高。pH 的改变会改变活性位点的形状，并可能导致酶变性。
| Enzyme / 酶 | Site / 作用部位 | Optimum pH / 最适 pH |
|---|---|---|
| Salivary amylase / 唾液淀粉酶 | Mouth / 口腔 | Around pH 7 / 约 pH 7 |
| Pepsin / 胃蛋白酶 | Stomach / 胃 | pH 2 |
| Trypsin / 胰蛋白酶 | Small intestine / 小肠 | Alkaline, about pH 8 / 碱性，约 pH 8 |
6. Denaturation | 变性
Denaturation is the permanent change in the structure of an enzyme, especially the active site, so that the enzyme no longer works. This can be caused by high temperatures or extremes of pH.
变性是指酶的结构，特别是活性位点发生永久性改变，使酶失去催化功能。高温或极端 pH 都可能引起变性。
Important exam point: denaturation is permanent, not reversible. When an enzyme is denatured, its active site is no longer complementary to the substrate.
考试要点：变性是不可逆的，而不是可逆的。当酶变性后，其活性位点不再与底物互补。
7. Calculating Rate of Reaction | 计算反应速率
In enzyme experiments, you may need to calculate the mean rate of reaction. You can measure the amount of product formed, or the amount of substrate used, over a period of time.
在酶促反应实验中，你可能需要计算平均反应速率。你可以测量一定时间内产物的生成量，或底物的消耗量。
Rate = Amount of product formed ÷ Time taken
反应速率 = 产物生成量 ÷ 反应时间
Units could be cm³ per second, g per minute or mol per second. You can also use rate = 1 ÷ time when measuring how long a reaction takes to finish.
单位可以是 cm³/s、g/min 或 mol/s。当测量反应完成所需时间时，也可以使用速率 = 1 ÷ 时间。
8. Investigating Enzymes: Core Practical | 酶的实验探究：核心实践
A common Edexcel IGCSE core practical involving enzymes is the breakdown of starch by amylase. Starch is a carbohydrate, and amylase breaks it down into maltose.
Edexcel IGCSE 常见的酶相关核心实验是淀粉酶分解淀粉。淀粉是一种碳水化合物，淀粉酶将其分解为麦芽糖。
Method outline:
- Add starch solution and amylase solution to a test tube.
- Keep the tube in a water bath at a specific temperature.
- Remove a drop every 30 seconds and test it with iodine solution.
- Iodine turns blue-black in the presence of starch.
- The reaction is complete when iodine stays orange-brown.
- 将淀粉溶液和淀粉酶溶液加入试管中。
- 将试管放入设定温度的水浴中。
- 每隔 30 秒取出一滴，用碘液测试。
- 碘液遇淀粉会变蓝黑色。
- 当碘液保持橙棕色时，说明反应已完成。
Control variables include the concentration of amylase, the volume of starch, and the temperature of each sample.
控制变量包括淀粉酶的浓度、淀粉的体积以及每个样品的温度。
9. Enzymes in Digestion | 消化中的酶
Digestion depends on enzymes that break large insoluble food molecules into small soluble molecules. These small molecules can then be absorbed into the bloodstream.
消化过程依赖酶将大分子不溶性食物分解为小分子可溶性物质。这些小分子随后可以被吸收进入血液。
| Enzyme / 酶 | Substrate / 底物 | Product / 产物 | Site / 部位 |
|---|---|---|---|
| Amylase / 淀粉酶 | Starch / 淀粉 | Maltose / 麦芽糖 | Mouth and small intestine / 口腔和小肠 |
| Protease / 蛋白酶 | Protein / 蛋白质 | Amino acids / 氨基酸 | Stomach, pancreas, small intestine / 胃、胰、小肠 |
| Lipase / 脂肪酶 | Fats / 脂肪 | Fatty acids and glycerol / 脂肪酸和甘油 | Pancreas and small intestine / 胰和小肠 |
10. Industrial Uses of Enzymes | 酶的工业应用
Enzymes are used in biological washing powders, food production and medicine. Proteases break down protein stains, and lipases break down fat and oil stains.
酶被用于生物洗衣粉、食品生产和医药等领域。蛋白酶分解蛋白质污渍，脂肪酶分解脂肪和油污渍。
Enzymes are useful in industry because they work at low temperatures, which saves energy, and they are specific, so fewer unwanted products are made. However, they are sensitive to temperature and pH, and can be expensive.
酶在工业中很有用，因为它们在较低温度下即可起作用，从而节省能源；同时酶具有专一性，副产物较少。然而，酶对温度和 pH 敏感，且可能较昂贵。
Example: lactase can be used to break down lactose in milk to glucose and galactose, making lactose-free milk for people who are lactose intolerant.
例如：乳糖酶可以将牛奶中的乳糖分解为葡萄糖和半乳糖，为乳糖不耐受人群生产无乳糖牛奶。
11. Common Exam Points and Pitfalls | 考试要点与常见错误
Students often lose marks because they use vague language. Instead of saying the enzyme is ‘killed’, say it is ‘denatured’. Instead of saying the active site is ‘destroyed’, say it ‘changes shape’ so the substrate can no longer bind.
学生常因表达模糊而失分。不要使用“酶被杀死了”，应说“酶变性”。不要说活性位点“被破坏”，而应说“形状改变”，导致底物无法结合。
You should also remember that rate of reaction increases with temperature only up to the optimum. After the optimum, the rate decreases because denaturation occurs.
还要记住，在最适温度之前，反应速率随温度升高而增加；超过最适温度后，速率下降，因为酶发生变性。
Finally, always use the word ‘specific’ to describe enzyme-substrate matching, and use the correct name for the model: the lock and key model.
最后，描述酶与底物匹配时要使用“专一性”，并正确写出模型名称：锁钥模型。
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