📚 Common Exam Points in Biochemistry Experiments | 生物化学实验常见考点梳理
Biochemistry experiments are a vital part of the A-Level chemistry curriculum, testing students' ability to handle biological molecules with precision while applying fundamental chemical principles. This article summarises the most frequently examined techniques and concepts, ensuring you are fully prepared for both practical assessments and written exam questions.
生物化学实验是 A-Level 化学课程中的重要组成部分，考查学生在处理生物分子时的操作精确性以及对基础化学原理的应用能力。本文系统梳理了最常考的实验技巧与核心概念，帮助你从容应对实验操作考核与笔试题目。
1. Qualitative Test for Reducing Sugars | 还原糖的定性检测
Benedict's reagent is used to detect reducing sugars such as glucose, fructose and maltose. When heated with a reducing sugar, the blue Cu²⁺ ions are reduced to a brick-red Cu₂O precipitate.
本尼迪特试剂常用于检测葡萄糖、果糖和麦芽糖等还原糖。与还原糖共热时，蓝色的 Cu²⁺ 离子被还原为砖红色的 Cu₂O 沉淀。
R-CHO + 2Cu²⁺ + 2H₂O → R-COOH + Cu₂O ↓ + 4H⁺
- Procedure: Mix equal volumes of the sample and Benedict's reagent in a test tube, then heat in a water bath at 80–100°C for 2–5 minutes.
- 操作步骤：在试管中加入等体积的样品与本尼迪特试剂，置于 80–100°C 水浴中加热 2–5 分钟。
- Colour change: Blue → Green → Yellow → Orange → Brick-red, with the intensity proportional to sugar concentration.
- 颜色变化：蓝色 → 绿色 → 黄色 → 橙色 → 砖红色，颜色深浅与糖浓度成正比。
- A negative control using distilled water must remain blue; a warm water bath, not direct flame, is essential to avoid decomposing the reagent.
- 使用蒸馏水的阴性对照应保持蓝色；必须用水浴加热而非直接火焰，以免试剂分解。
2. Iodine Test for Starch | 淀粉的碘液检测
Iodine dissolved in potassium iodide solution (I₂/KI) produces a blue-black colour when starch is present. This arises from the formation of a complex between iodine molecules and the helical amylose structure of starch.
碘-碘化钾溶液（I₂/KI）遇到淀粉时会呈现蓝黑色，这是由于碘分子与直链淀粉的螺旋结构形成了包合物。
- Positive result: blue-black colouration; no heating is required.
- 阳性结果：呈蓝黑色，无需加热。
- Amylopectin gives a weaker red-purple colour because it has fewer helical regions.
- 支链淀粉因螺旋区较少，呈较弱的红紫色。
- Cool hot starch solutions before testing, because the helical structure is destabilised at high temperature.
- 测试前应将热淀粉溶液冷却，因为高温下螺旋结构不稳定。
3. Biuret Test for Proteins | 蛋白质的双缩脲检测
The Biuret test detects peptide bonds. Under alkaline conditions, copper(II) sulfate reacts with peptide bonds to form a violet-purple complex. The reaction is specific to molecules containing two or more peptide linkages.
双缩脲测试用于检测肽键。在碱性条件下，硫酸铜与肽键反应生成紫红色络合物。该反应对含有两个或以上肽键的分子具有特异性。
- Procedure: Add dilute NaOH (or KOH) to the sample first, then add a few drops of copper(II) sulfate solution dropwise with gentle shaking.
- 操作步骤：先向样品中加入稀 NaOH（或 KOH），再逐滴加入少量硫酸铜溶液并轻轻摇匀。
- Positive result: violet/purple colouration appears at room temperature; do not overheat.
- 阳性结果：室温下出现紫色；切勿加热。
- Excess CuSO₄ produces a blue colour that masks the positive result; the NaOH must be in excess to maintain alkaline conditions.
- 过量的 CuSO₄ 会产生蓝色并掩盖阳性结果；NaOH 需过量以维持碱性环境。
4. Emulsion Test for Lipids | 脂质的乳化检测
The emulsion test (ethanol solubility test) is used to identify lipids. The sample is mixed with ethanol to dissolve the lipid, then poured into cold water; a white milky emulsion confirms the presence of lipids.
乳化测试（乙醇溶解测试）用于鉴别脂质。先用乙醇溶解样品中的脂质，再将其倒入冷水中，出现白色乳浊液即证明脂质存在。
- Always shake the mixture thoroughly before adding water to ensure the lipid is fully dissolved in ethanol.
- 加水前务必充分振荡混合物，确保脂质完全溶解于乙醇中。
- A cloudy white emulsion forms because the ethanol-water mixture has reduced solubility, causing tiny lipid droplets to disperse.
- 白色乳浊液的形成是因为乙醇-水混合液的溶解能力下降，使微小脂滴分散悬浮。
- Unsaturated lipids give a more translucent emulsion than saturated ones due to their different physical states.
- 不饱和脂质因常温下多为液态，形成的乳浊液比饱和脂质更为半透明。
5. Investigating Enzyme Activity: Temperature | 探究酶活性与温度的关系
Enzymes are biological catalysts, usually proteins. Temperature affects their activity significantly: at low temperatures molecular motion decreases, while at high temperatures the tertiary structure unfolds irreversibly. The optimum temperature for most human enzymes is around 37°C.
酶是生物催化剂，通常为蛋白质。温度对酶活性影响显著：低温下分子运动减慢，高温下蛋白质三级结构不可逆地展开。人体内大多数酶的最适温度约为 37°C。
Enzyme activity ∝ 1 / t (t = time for a fixed amount of product to appear)
酶活性 ∝ 1 / t （t 为生成一定量产物所需的时间）
- Use catalase with hydrogen peroxide: H₂O₂ → H₂O + O₂. Measure the rate of O₂ production using a gas syringe or by collecting gas over water.
- 用过氧化氢酶分解过氧化氢：H₂O₂ → H₂O + O₂。使用气体注射器或排水集气法测量 O₂ 的生成速率。
- Control variables: pH, substrate concentration, enzyme volume and the temperature of the water bath.
- 控制变量：pH、底物浓度、酶用量及水浴温度。
- Below the optimum, activity roughly doubles for every 10°C rise (Q₁₀ effect); above the optimum, activity falls sharply due to denaturation.
- 低于最适温度时，温度每升高 10°C 活性约增加一倍（Q₁₀ 效应）；高于最适温度时，活性因变性而急剧下降。
6. Effect of pH on Enzyme Activity | pH 对酶活性的影响
The optimum pH varies for different enzymes: pepsin works best at pH 2, catalase at pH 7, while trypsin is most active at pH 8–9. Deviation from the optimum pH alters ionic interactions and hydrogen bonds that maintain the enzyme's three-dimensional structure.
不同酶的最适 pH 各异：胃蛋白酶最适 pH 为 2，过氧化氢酶为 7，胰蛋白酶在 pH 8–9 时活性最高。偏离最适 pH 会改变维持酶三维结构的离子键和氢键。
- Buffer solutions must be used to maintain a constant pH throughout each trial.
- 每次实验必须使用缓冲溶液以维持恒定的 pH。
- Measure the time taken for a fixed amount of reaction to occur at each pH value, then convert to rate (1/time).
- 记录每个 pH 值下完成一定量反应所需的时间，再换算为速率（1/时间）。
- Buffers also provide constant ionic strength, preventing non-specific ionic effects on the enzyme.
- 缓冲溶液还可维持恒定的离子强度，避免离子对酶产生非特异性影响。
7. Colorimetry: Quantitative Analysis | 比色法：定量分析
Colorimetry measures the absorbance or transmission of light by a coloured solution using a colorimeter or spectrophotometer. It is commonly used to quantify reducing sugars or protein concentrations by reference to a calibration curve.
比色法利用比色计或分光光度计测定有色溶液对特定波长光的吸光度或透光率，常用于对照标准曲线定量测定还原糖或蛋白质浓度。
A = ε c l
- A is the absorbance; ε is the molar absorptivity; c is the concentration; l is the path length of the cuvette.
- A 为吸光度；ε 为摩尔吸光系数；c 为浓度；l 为比色皿的光程长度。
- Construct a calibration curve by measuring absorbance of known standard concentrations, then interpolate the unknown sample from the linear region.
- 先测定各已知标准溶液的吸光度绘制标准曲线，再在直线区域内插值求取未知样品浓度。
- Use a filter or wavelength corresponding to the complementary colour of the test solution for maximum sensitivity.
- 选择与待测液颜色互补的滤光片或波长，以获得最大灵敏度。
8. Paper Chromatography of Amino Acids | 氨基酸的纸色谱法
Amino acids can be separated on chromatography paper using a suitable solvent system such as butan-1-ol, ethanoic acid and water. After chromatographic separation, ninhydrin spray is applied and heated to visualise the separated amino acids as purple spots.
氨基酸可在层析纸上利用适当的溶剂系统（如正丁醇-乙酸-水）进行分离。展开后喷洒茚三酮显色剂并加热，分离的氨基酸呈紫色斑点。
- Rf = distance moved by the solute ÷ distance moved by the solvent front.
- Rf = 溶质迁移距离 ÷ 溶剂前沿迁移距离。
- Identify unknown amino acids by comparing Rf values with those of standard amino acids under identical conditions.
- 在完全相同的条件下，将未知样品的 Rf 值与标准氨基酸对照，即可鉴定氨基酸种类。
- Mark the solvent front immediately after the paper is removed, before it dries.
- 层析结束后应立即标记溶剂前沿，避免其挥发后无法判断。
- Keep the chromatographic tank closed so that the atmosphere becomes saturated with solvent vapour, preventing edge effects and uneven solvent flow.
- 展开过程中保持层析缸密闭，使缸内气氛被溶剂蒸气饱和，防止边缘效应和不均匀迁移。
9. DNA Extraction and Purification | DNA 的提取与纯化
A classic practical involves extracting DNA from plant tissue such as onion or kiwi fruit. The procedure blends cells, disrupts the cell membrane with detergent, digests proteins with enzymes or salt, and precipitates DNA with cold ethanol.
经典实验是从洋葱或猕猴桃等植物组织中提取 DNA。操作流程包括破碎细胞、用去污剂破坏细胞膜、用酶或盐消化蛋白质，最后用冷乙醇沉淀 DNA。
- Detergent (e.g., SDS) disrupts the phospholipid bilayer of the cell and nuclear membranes by hydrophobic interaction.
- 去污剂（如 SDS）通过疏水作用破坏细胞膜和核膜的磷脂双分子层。
- Sodium chloride helps DNA to separate from histone proteins by providing positive Na⁺ ions that shield the negatively charged phosphate groups.
- 氯化钠提供的 Na⁺ 可中和 DNA 磷酸基团的负电荷，帮助 DNA 与组蛋白分离。
- Cold ethanol causes DNA to precipitate at the interface between the aqueous layer and the ethanol layer; DNA appears as a white, cotton-like fibrous mass.
- 冷乙醇使 DNA 在水层与乙醇层的界面处析出，形成白色棉絮状纤维团块。
- The precipitated DNA can be spooled onto a glass rod and later dissolved in buffer for further analysis.
- 析出的 DNA 可用玻璃棒缠绕挑出，随后溶于缓冲液供进一步分析。
10. Investigating Respiration Rate | 探究呼吸速率
Respiration rate can be measured by oxygen uptake using a respirometer. The apparatus consists of a sealed chamber containing organisms, with soda
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