📚 IGCSE Biology Practical Exam Points and Key Techniques | IGCSE生物实验考点及操作要点总结
Practical work is a compulsory part of the IGCSE Biology syllabus, and experimental questions appear in both Paper 3 (Practical) and Paper 6 (Alternative to Practical). Students are expected to know not only the procedures, but also the underlying scientific reasoning: why a step is done, what control is needed, and how to judge the reliability of results. This guide summarises the core experiments, common pitfalls, and the precise vocabulary examiners look for in markschemes.
实验操作是IGCSE生物考纲的必考内容，实验题同时出现在Paper 3（实验实操）与Paper 6（实验笔试替代）中。学生不仅需要记住操作步骤，更要理解背后的科学原理：为什么要做这一步、需要什么对照、以及如何判断结果的可靠性。本文总结了核心实验、常见错误以及阅卷评分标准所要求的关键表述，帮助你在考试中稳定拿分。
1. Safe Use of Laboratory Equipment | 实验室设备安全使用
Before carrying out any practical, examiners expect you to describe the correct handling of basic equipment. A Bunsen burner should be lit with the air hole partially closed, then opened to give a roaring blue flame for heating. When boiling liquids that may spit, such as ethanol in photosynthesis tests, always use a water bath rather than direct heat. Measuring cylinders must be read at eye level at the bottom of the meniscus; do not estimate more precisely than the scale allows.
做任何实验前，阅卷者都期望你掌握基础设备的正确使用方法。点燃本生灯时先关闭空气孔，点燃后再打开空气孔以获得用于加热的蓝色火焰。在加热可能飞溅的液体（如光合作用实验中的乙醇）时，务必使用水浴而非直接加热。量筒读数时要平视凹液面的最低处，且不能超出刻度尺允许的精度进行估算。
Use heat-proof gloves when handling hot glassware; never point a boiling tube towards yourself or others.
拿取加热后的玻璃器皿要戴隔热手套；加热试管口切勿朝向自己或他人。
The blue flame is hotter and non-luminous; the yellow flame is cooler and produces soot.
蓝色火焰温度更高且不发亮光；黄色火焰温度较低并产生烟灰。
Wash any chemical spills off the skin immediately and report to the teacher; wear goggles when using acids, alkalis or hot liquids.
皮肤沾到化学药品应立即冲洗并告知老师；使用酸、碱或热液体时要戴护目镜。
2. Food Tests | 食物成分检测
Food tests are among the most frequently examined practicals. For reducing sugars, add Benedict's reagent to the sample and heat in a water bath at about 80 °C; a positive result is a colour change from blue to green, then yellow, then brick-red. For starch, add iodine solution; a blue-black colour indicates starch. For protein, add Biuret reagent (sodium hydroxide followed by copper sulfate); a purple colour is positive. For lipids, shake the sample with ethanol, then pour into water; a milky white emulsion confirms the presence of lipid.
食物成分检测是最高频的实验考点之一。还原糖检测：向样品中加入本尼迪克特试剂，在80°C左右水浴加热，阳性结果颜色由蓝色变为绿色、黄色，最终变为砖红色。淀粉检测：滴加碘液，出现蓝黑色即为阳性。蛋白质检测：加入双缩脲试剂（先加氢氧化钠，再加硫酸铜），出现紫色即为阳性。脂肪检测：用乙醇振荡样品后倒入水中，出现乳白色乳浊液即证明含有脂肪。
| Food component | Test/reagent | Positive result |
|---|---|---|
| Reducing sugar | Benedict's reagent + heat | Blue → brick-red |
| Starch | Iodine solution | Blue-black |
| Protein | Biuret reagent | Purple/violet |
| Lipid | Ethanol + water | Milky white emulsion |
3. Enzyme Experiments | 酶促反应实验
The classic enzyme experiment uses amylase and starch at different temperatures. Prepare five water baths at 10 °C, 20 °C, 30 °C, 40 °C and 50 °C. Place a test tube with 2 cm³ of amylase solution and another with 2 cm³ of starch solution in each bath for two minutes to allow the contents to reach the same temperature. Then mix them, and every ten seconds remove one drop of the mixture and test it with iodine on a white tile. Record the time taken for the blue-black colour to disappear.
经典酶实验是在不同温度下用淀粉酶分解淀粉。准备五个水浴锅，温度分别为10°C、20°C、30°C、40°C和50°C。在每个水浴中各放一支含2 cm³淀粉酶溶液的试管和一支含2 cm³淀粉溶液的试管，静置两分钟使内容物达到相同温度。随后混合两种液体，每隔十秒取一滴混合液放在白瓷板上滴碘液检测，记录蓝色完全消失所需的时间。
A key variable to control is the concentration and volume of enzyme and substrate; only temperature should change. The pH is kept constant using a buffer solution at pH 7. For a pH experiment, use a series of buffer solutions from pH 2 to pH 10 with catalase and hydrogen peroxide, measuring the height of the oxygen bubble column or the rate of bubble production as the dependent variable.
需要控制的关键变量是酶与底物的浓度和体积，只改变温度。用pH 7的缓冲液保持酸碱度恒定。若做pH实验，则用pH 2至pH 10的一系列缓冲液配合过氧化氢酶与过氧化氢，以氧气气泡柱高度或每分钟气泡产生速率为因变量。
Reaction rate = 1 ÷ time taken (s⁻¹)
4. Photosynthesis Experiments | 光合作用实验
To test a leaf for starch, first kill the leaf by placing it in boiling water for 30 seconds, then boil it in ethanol in a water bath to remove chlorophyll. The leaf becomes white and brittle. Rinse it with hot water to soften it, spread it on a white tile, and add iodine solution. Blue-black areas contain starch; areas that remain brown show no starch production. You must explain that ethanol is flammable and therefore heated in a water bath, never directly over a flame.
检测叶片中的淀粉时，先将叶片放入沸水中煮30秒杀死细胞，再在水浴中用乙醇煮沸脱去叶绿素。叶片变为白色且脆硬。用热水冲洗使叶片软化，平铺在白瓷板上滴加碘液。变蓝黑色的区域含有淀粉，保持褐色的区域则没有产生淀粉。必须说明乙醇易燃，因此要用水浴加热而非直接在火焰上加热。
For the need for light: cover part of a leaf with black paper, destarch the plant for 48 hours first, then test after exposing to light.
验证光是否为必要条件：先用黑纸遮住叶片的一部分，将植物暗处理48小时以消耗原有淀粉，见光后再检验。
For the need for CO₂: use soda lime to absorb carbon dioxide in one sealed bag and keep another bag as a control with normal air.
验证CO₂是否为必要条件：在一个密封袋内放置碱石灰以吸收二氧化碳，另一袋保持正常空气作为对照。
For the need for chlorophyll: use a variegated leaf such as a variegated geranium; only the green parts give a positive starch test.
验证叶绿素是否必需：使用斑叶植物（如花叶天竺葵），只有绿色部分能测得淀粉阳性。
5. Osmosis and Diffusion | 渗透与扩散实验
In the potato osmometer experiment, use a cork borer to cut uniform cylinders of potato, blot them dry, and record the initial mass. Place each cylinder in a different sucrose solution, for example 0.0, 0.2, 0.4, 0.6 and 0.8 mol dm⁻³, and leave them for 30 minutes. After blotting dry again, record the final mass and calculate the percentage change in mass.
在马铃薯渗透实验中，用软木打孔器切取粗细均匀的马铃薯条，用吸水纸吸干表面水分并记录初始质量。将每条马铃薯分别放入不同浓度的蔗糖溶液中，例如0.0、0.2、0.4、0.6与0.8 mol dm⁻³，浸泡30分钟。再次吸干表面水分并记录终质量，计算质量变化百分率。
Percentage change in mass = (final mass − initial mass) ÷ initial mass × 100%
Plot percentage change against sucrose concentration. The point where the line crosses the x-axis gives the concentration at which there is no net movement of water; this is approximately equal to the solute concentration of the potato cell contents. Using percentage change rather than absolute change allows fair comparison because the initial masses are not identical.
以蔗糖浓度为横坐标、质量变化百分率为纵坐标作图。曲线与x轴的交点表示水分净移动为零的浓度，这一浓度约等于马铃薯细胞内容物的溶质浓度。使用百分率而非绝对质量变化是为了公平比较，因为各马铃薯条的初始质量并不完全相同。
6. Respiration Experiments | 呼吸作用实验
To demonstrate heat production by respiration, soak pea seeds overnight and place them in a vacuum flask. Insert a thermometer through a cotton wool plug. In a control flask use dead (boiled) seeds. Record the temperature in both flasks over several days; the living seeds show a rise in temperature due to heat released from aerobic respiration. The cotton wool allows gas exchange but reduces heat loss, and the flask is lagged to prevent heat escaping.
为证明呼吸作用产热，将豌豆浸泡过夜后放入保温瓶，通过棉花塞插入温度计。对照瓶中使用已煮沸杀死的种子。连续几天记录两瓶温度，活种子因有氧呼吸释放热量而温度上升。棉花塞允许气体交换但减少热量散失，保温瓶外层包裹也用于防止热量流失。
To detect carbon dioxide, bubble the gas produced by germinating seeds through limewater; it turns milky. Alternatively, use hydrogencarbonate indicator, which is orange in air, turns yellow with a high concentration of CO₂, and turns red if CO₂ is removed. For anaerobic respiration, place yeast in glucose solution and cover with liquid paraffin; the production of ethanol and carbon dioxide can be shown by the smell of ethanol and the balloon inflating over the tube.
为检测二氧化碳，可将萌发种子产生的气体通入石灰水，石灰水变浑浊。另一种方法是使用碳酸氢盐指示剂：在空气中呈橙色，CO₂浓度高时变黄色，CO₂被去除时变红色。验证无氧呼吸时，将酵母放入葡萄糖溶液中并用液体石蜡密封隔绝空气，气球鼓起及酒精气味可以证明产生了CO₂和乙醇。
7. Transpiration and Plant Transport | 蒸腾作用与植物运输
A potometer is used to measure the rate of water uptake by a leafy shoot. Set up the apparatus underwater to exclude air bubbles, cut the shoot stem at an angle to prevent air locks, and record the distance moved by the air meniscus in the capillary tube per unit time. Change one environmental factor at a time: increasing light intensity, increasing temperature, increasing air movement (wind) or decreasing humidity all increase the transpiration rate.
蒸腾计用于测量带叶枝条的吸水速率。在水下组装装置以排尽气泡，并将枝条茎部斜切以防气栓堵塞。记录毛细管中气栓界面单位时间移动的距离。每次只改变一个环境因素：增强光照、升高温度、加大风力或降低湿度均会加快蒸腾速率。
For observing transport in xylem, place a stalk of celery or a white carnation in water containing red dye (eosin) for a few hours. Cut thin sections of the stem and observe under a microscope: only the xylem vessels are stained red, which provides evidence that xylem is responsible for transporting water upwards. The phloem remains unstained because it transports sucrose, not water.
为观察木质部运输，将芹菜茎或白色康乃馨放入含红色染料（曙红）的水中数小时。用刀切取茎的薄切片置于显微镜下观察：只有木质部导管被染成红色，这证明木质部负责向上运输水分。韧皮部不被染色，因为它运输的是蔗糖而非水分。
8. Microscope Techniques | 显微镜操作技术
To prepare a temporary mount of onion epidermal cells, peel a thin layer from the inner surface of an onion scale leaf, place it flat on a microscope slide, add a drop of iodine solution, and lower a coverslip slowly using a mounted needle to avoid trapping air bubbles. Iodine stains the nuclei yellow-brown and makes the cell wall more visible.
制作洋葱表皮临时装片时，从洋葱鳞片叶内表面撕取一层薄膜，平铺于载玻片上，滴一滴碘液，用解剖针缓慢放下盖玻片以避免产生气泡。碘液能将细胞核染成黄褐色，并让细胞壁更加清晰可见。
Always use the lowest power objective first to locate the specimen, then increase magnification.
先用最低倍物镜找到标本，再逐步增大放大倍数。
Draw with a sharp pencil, use clear unbroken lines, and avoid shading; label the cell wall, cell membrane, cytoplasm and nucleus.
绘图用削尖的铅笔，线条清晰连续，不要涂阴影；标注细胞壁、细胞膜、细胞质与细胞核。
Magnification is calculated as image size divided by actual size; always show your working and include units.
放大倍数等于像的大小除以实际大小；必须写出计算过程并带单位。
Magnification = image size ÷ actual size
mm is 50 μm in diameter. If drawn 10 mm wide, the magnification is 10 mm ÷ 0.05 mm = 200×.
毫米换算为微米时乘以1000，例如细胞直径0.05 mm即50 μm。若图中画为10 mm宽，则放大倍数 = 10 mm ÷ 0.05 mm = 200×。
9. Ecology Sampling Methods | 生态学样方调查方法
t randomly. Random sampling is achieved by generating random coordinates or throwing the quadrat without bias. Record the frequency, percentage cover or the number of individuals of each species. Repeat the sampling many times; the larger the sample size, the more reliable the estimate.
估算某种植物的分布时，使用50 cm × 50 cm的样方。随机取样可以通过随机坐标或任意抛掷样方实现，做到无偏见选择。记录每个物种的出现频率、盖度或个体数。重复多次取样，样本量越大，估算结果越可靠。
For measuring the change in species distribution along a habitat, for example from a shoreline inland, use a belt transect. Place a tape measure along the ground and record the species touching the tape at regular intervals. This method clearly shows the gradient of environmental conditions and the corresponding change in species distribution.
若要测量物种分布沿生境的变化，例如从岸边向内陆，可使用样带法。沿地面拉一卷尺，每隔固定间隔记录接触卷尺的物种。该方法能清楚显示环境条件的梯度变化以及物种随之发生的改变。
10. Common Errors and Exam Tips | 常见错误与考试技巧
Candidates frequently lose marks by failing to repeat readings, not identifying the independent, dependent and control variables in their answers, or confusing the words 'describe' and 'explain'. When a question says 'describe', state what is seen in the results; when it says 'explain', use biological knowledge to give a reason. For example, describing the graph shows the rate increases then plateaus, while explaining reasons involves enzyme denaturation or substrate exhaustion.
考生最常见的失分原因是：没有重复实验读数、不能正确识别自变量、因变量与控制变量，以及混淆"描述"与"解释"。题目要求"描述"时，只需说明结果中的现象；要求"解释"时，则需用生物学知识给出原因。例如描述曲线时说明速率先上升后趋于平稳，而解释原因时才能提及酶变性或底物耗尽。
Repeat each reading at least three times and calculate a mean to improve reliability; state this in your answer.
每组读数至少重复三次并计算平均值以提高结果可靠性；在答案中写明这一点。
Use a control to improve validity; for example, boiled enzyme solution in an enzyme experiment shows that the observed effect is due to the enzyme itself.
使用对照组提高有效性；例如在酶实验中使用煮沸的酶溶液，证明观察到的效果确实由酶本身引起。
Record all results in a table with units in the column headings and correct significant figures; plot a graph with labelled axes and a suitable scale.
将所有结果记录在表格中，表头标明单位并注意有效数字；绘制图表时标注坐标轴并选择合适的刻度。
If an anomalous result appears, do not ignore it; state that it is anomalous and suggest a possible reason such as human timing error or a temperature fluctuation.
出现异常数据时不要回避，应指出其为异常值并给出可能的解释，如人为计时误差或温度波动。
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