📚 Activity 1.3.2: Investigating the Effect of pH on Enzyme Activity | 探究 pH 对酶活性的影响
Enzymes are biological catalysts that speed up chemical reactions without being used up. In this OCR practical activity, you will investigate how changing pH affects the rate at which amylase breaks down starch. The iodine test is used as an indicator: when starch is present, iodine turns blue-black; when starch has been digested, the mixture stays orange-brown. This practical helps you link the active site model to experimental evidence.
酶是生物催化剂，能在不被消耗的情况下加速化学反应。在这项 OCR 实验活动中，你将探究改变 pH 如何影响淀粉酶分解淀粉的速率。实验使用碘液作为指示剂：当淀粉存在时，碘液变蓝黑色；当淀粉被完全消化后，混合物保持橙棕色。本实验帮助你将活性位点模型与实验证据联系起来。
1. Why pH Matters for Enzymes | 为什么 pH 对酶很重要
Enzymes have an active site with a specific three-dimensional shape. The shape is maintained by hydrogen bonds and ionic interactions between amino acids in the protein chain. If the pH is too high or too low, these bonds are disrupted. The active site changes shape, so the substrate can no longer bind. This permanent change is called denaturation.
酶有一个具有特定三维形状的活性位点。这个形状由蛋白质链中氨基酸之间的氢键和离子相互作用维持。如果 pH 过高或过低，这些键就会被破坏。活性位点形状改变，底物无法结合。这种永久性的变化称为变性。
Amylase is an enzyme found in saliva and the pancreas. It catalyses the breakdown of starch into maltose. In the mouth, amylase works best around pH 7. In the small intestine, it can still function in slightly alkaline conditions, but extreme pH values will stop its activity.
淀粉酶是一种存在于唾液和胰腺中的酶。它催化淀粉分解为麦芽糖。在口腔中，淀粉酶在 pH 7 左右活性最佳。在小肠中，它仍可在弱碱性条件下工作，但极端 pH 值会使其活性停止。
2. Core Practical Overview | 核心实验概述
In this activity, you mix amylase solution with starch solution at a chosen pH using a buffer solution. A buffer keeps the pH constant during the reaction. You then test small samples at regular time intervals with iodine solution. The end point is reached when iodine no longer turns blue-black, showing that all starch has been hydrolysed to maltose.
在本活动中，你使用缓冲液将淀粉酶溶液与淀粉溶液在选定 pH 下混合。缓冲液在反应过程中保持 pH 恒定。然后你每隔固定时间取小样，用碘液检测。当碘液不再变蓝黑色时即为终点，说明所有淀粉已水解为麦芽糖。
You repeat the procedure at different pH values, such as pH 4, 5, 6, 7, 8, 9 and 10, and record the time taken for the starch to disappear. This allows you to plot a graph of rate against pH and identify the optimum pH for amylase.
你在不同 pH 值（如 4、5、6、7、8、9、10）下重复实验，记录淀粉消失所需时间。这样你就可以绘制速率对 pH 的图形，并找出淀粉酶的最适 pH。
3. Variables and Control | 变量与控制
Independent variable: pH of the buffer solution. Dependent variable: time taken for starch to disappear, or the rate of reaction calculated as 1 ÷ time. Control variables: enzyme concentration, starch concentration, volume of solutions, temperature, and the number of iodine drops used in each test.
自变量：缓冲液的 pH。因变量：淀粉消失所需时间，或反应速率（用 1 ÷ 时间计算）。控制变量：酶浓度、淀粉浓度、溶液体积、温度，以及每次检测所用碘液滴数。
Temperature must be controlled carefully because enzyme activity is also affected by heat. In this practical, a water bath is usually set to 30 °C. If the temperature fluctuates, the rate of reaction will change and your pH results will no longer be valid. You must also use the same batch of enzyme and starch throughout to avoid variation in concentration.
温度必须小心控制，因为酶活性也受热的影响。在本实验中，水浴通常设置为 30 °C。如果温度波动，反应速率会改变，你的 pH 实验结果将不再有效。你还必须自始至终使用同一批酶和淀粉，以避免浓度变化。
4. Apparatus and Reagents | 仪器与试剂
- Amylase solution (1%) | 淀粉酶溶液（1%）
- Starch solution (1%) | 淀粉溶液（1%）
- Buffer solutions at pH 4, 5, 6, 7, 8, 9, 10 | pH 4、5、6、7、8、9、10 的缓冲液
- Iodine solution in a dropper bottle | 碘液（滴瓶中）
- Spotting tile or white tile | 点滴板或白瓷砖
- Test tubes and test tube rack | 试管和试管架
- Water bath or beaker of warm water | 水浴或装有温水的烧杯
- Thermometer, stopwatch, syringes or measuring cylinders | 温度计、秒表、注射器或量筒
- Glass rod or dropping pipettes | 玻璃棒或滴管
5. Step-by-Step Method | 分步实验方法
1. Set up a water bath at 30 °C. Label one test tube for each pH value you will test.
1. 将水浴设置为 30 °C。为你要测试的每个 pH 值标记一支试管。
2. Place 5 cm³ of starch solution, 5 cm³ of the appropriate buffer, and 5 cm³ of amylase solution into separate labelled test tubes. Allow all tubes to reach the temperature of the water bath.
2. 将 5 cm³ 淀粉溶液、5 cm³ 相应缓冲液和 5 cm³ 淀粉酶溶液分别放入标记好的试管中。让所有试管达到水浴温度。
3. Add 2–3 drops of iodine solution to each well of a spotting tile.
3. 在点滴板的每个孔中加入 2–3 滴碘液。
4. Mix the starch solution and buffer first, then add the amylase solution. Start the stopwatch immediately after adding amylase.
4. 先混合淀粉溶液和缓冲液，然后加入淀粉酶溶液。加入淀粉酶后立即启动秒表。
5. Every 30 seconds, use a clean dropper to transfer one drop of the reaction mixture to a fresh iodine drop. Observe the colour.
5. 每隔 30 秒，用干净的滴管将一滴反应混合物转移到新鲜的碘液滴上。观察颜色。
6. Continue until the iodine stays orange-brown and no blue-black colour appears. Record the time in seconds.
6. 继续操作，直到碘液保持橙棕色且不再出现蓝黑色。记录时间（秒）。
7. Repeat the entire procedure for each pH value. For reliability, repeat each pH at least three times and calculate a mean time.
7. 对每个 pH 值重复整个过程。为了提高可靠性，每个 pH 至少重复三次并计算平均时间。
8. Calculate the rate of reaction for each pH using: rate = 1 ÷ time.
8. 使用公式：速率 = 1 ÷ 时间，计算每个 pH 下的反应速率。
6. Recording Results | 记录结果
Record your results in a table with columns for pH, time for starch to disappear, and rate of reaction. The table below shows a typical set of results for amylase at 30 °C.
将结果记录在表格中，包含 pH、淀粉消失所需时间和反应速率等栏目。下表是淀粉酶在 30 °C 下的一组典型结果。
| pH | Time for starch to disappear / s | Rate = 1 ÷ time / s⁻¹ |
|---|---|---|
| 4 | >600 | <0.0017 |
| 5 | 180 | 0.0056 |
| 6 | 95 | 0.011 |
| 7 | 45 | 0.022 |
| 8 | 55 | 0.018 |
| 9 | 85 | 0.012 |
| 10 | 220 | 0.0045 |
7. Trend and Optimum pH | 趋势与最适 pH
The rate of reaction increases as pH approaches the optimum. After the optimum, the rate decreases sharply. For amylase, the fastest rate is usually around pH 7, although some bacterial amylases have optima near pH 6–8. At very low or very high pH, the enzyme is denatured and little or no reaction occurs.
随着 pH 接近最适值，反应速率上升。超过最适值后，速率迅速下降。淀粉酶的最快速率通常在 pH 7 左右，但某些细菌淀粉酶的最适 pH 在 6–8 之间。在极低或极高 pH 下，酶已变性，反应很少或完全不发生。
A graph of rate against pH is a bell-shaped curve, not a straight line. The peak of the curve shows the optimum pH. You should be able to read the optimum pH from the graph and explain the shape using the active site model.
速率对 pH 的图形呈钟形曲线，而不是直线。曲线最高点表示最适 pH。你应该能从图中读出最适 pH，并用活性位点模型解释曲线形状。
rate = 1 ÷ time (s⁻¹)
8. Errors and Improvements | 误差与改进
Random errors include judging the colour change by eye and small temperature fluctuations in the water bath. These errors can be reduced by taking repeat readings and calculating a mean. Larger random errors may occur if different people judge the end point differently.
随机误差包括用肉眼判断颜色变化和水浴中的微小温度波动。可以通过重复读数并计算平均值来减少这些误差。如果不同的人对终点的判断不同，就可能产生较大的随机误差。
Systematic errors include using a buffer solution with an inaccurate pH or using contaminated syringes that transfer small amounts of solution between tests. To improve accuracy, use a pH meter to check buffers, use a thermostatically controlled water bath, and use a colorimeter to measure the colour change objectively.
系统误差包括使用 pH 不准确的缓冲液，或使用被污染的注射器在测试之间转移少量溶液。为提高准确性，可使用 pH 计检查缓冲液，使用恒温控制水浴，并使用比色计客观地测量颜色变化。
9. Safety and Ethical Notes | 安全与伦理注意事项
Wear eye protection throughout the practical. Amylase powder may irritate the respiratory tract, so handle solutions carefully and avoid creating dust. Iodine solution is harmful if swallowed and can stain skin and clothing. If iodine spills on skin, wash immediately with plenty of water.
整个实验过程中佩戴护目镜。淀粉酶粉末可能刺激呼吸道，因此要小心操作溶液并避免产生粉尘。碘液吞食有害，且会染色皮肤和衣物。如果碘液溅到皮肤上，立即用大量清水冲洗。
This practical does not use live organisms, so there are no ethical issues. Dispose of all solutions according to your teacher’s instructions. Do not pour iodine solution down the sink unless told to do so.
本实验不使用活体生物，因此没有伦理问题。按照老师的指导处理所有废液。除非老师允许，否则不要将
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply