📚 A-Level Biology Practicals: Common Experiments and Key Techniques | A-Level生物实验详解：常见实验与操作要点
Practical work is a core component of A-Level Biology. It tests your ability to handle apparatus, follow procedures, and interpret results accurately. This article outlines the most common experiments and the key operational points you must know for exams.
实验是A-Level生物学的核心部分。它考查你操作仪器、遵循步骤和准确解读结果的能力。本文总结了最常见的实验及考试中必须掌握的关键操作要点。
1. Microscope Use and Slide Preparation | 显微镜使用与制片
Always start with the lowest power objective lens. Focus using the coarse adjustment knob, then switch to a higher power and fine-focus. Never use the coarse knob on high power to avoid damaging the slide or lens.
务必从低倍物镜开始。使用粗准焦螺旋对焦，然后转到高倍镜并用细准焦螺旋微调。切勿在高倍镜下使用粗准焦螺旋，以免损坏玻片或镜头。
For a temporary mount, place a thin specimen on the slide, add a drop of water or stain, and lower a coverslip at an angle to prevent air bubbles. Excess stain can be removed with filter paper.
制作临时装片时，将薄样品放在载玻片上，滴一滴水或染液，然后倾斜盖玻片缓缓放下，避免产生气泡。多余染液可用滤纸吸去。
If measuring cells, calibrate the eyepiece graticule using a stage micrometer. Record the calibration factor in μm per division at each magnification.
若要测量细胞大小，需用载物台测微尺校准目镜测微尺。记录每个放大倍数下每格对应的微米数。
2. Enzyme Catalysis: Effects of Temperature and pH | 酶催化实验：温度与pH的影响
Use an enzyme such as catalase or amylase. For catalase, add hydrogen peroxide and measure the rate of oxygen production using a gas syringe or by counting bubbles. For amylase, test for starch with iodine at timed intervals.
使用过氧化氢酶或淀粉酶。对于过氧化氢酶，加入过氧化氢，用气体注射器测量氧气产生速率或数气泡；对于淀粉酶，每隔一定时间用碘液检测淀粉是否还存在。
Control temperature with a water bath. For pH, use buffer solutions. Keep enzyme and substrate concentrations constant. Always include a control without the enzyme to confirm that any reaction is due to the enzyme.
使用水浴控制温度。对于pH，使用缓冲液。保持酶和底物浓度恒定。始终设置一个不加酶的对照，以确认反应确实由酶引起。
Plot rate against temperature or pH. The graph should show an optimum, and for temperature a sharp decline after the optimum due to denaturation.
以反应速率对温度或pH作图。曲线应显示最适值，温度曲线在最适温度后因酶变性而急剧下降。
3. Osmosis and Cells | 渗透作用与细胞
Cut potato cylinders of equal size and mass. Place them in sucrose or sodium chloride solutions of different concentrations. After a set time, blot dry and reweigh. Calculate percentage change in mass.
切取等长等重的马铃薯条。将它们放入不同浓度的蔗糖或氯化钠溶液中。一段时间后，用吸水纸吸干表面水分并再次称重。计算质量变化百分比。
Use distilled water as a control. The percentage change in mass indicates the direction and magnitude of water movement. The concentration at which no change occurs is the tissue’s osmotic potential.
用蒸馏水作为对照。质量变化百分比反映了水分移动的方向和程度。质量不变时的外界浓度即为组织渗透势。
For animal cells, such as red blood cells, observe under a microscope. In hypotonic solutions they swell and burst (haemolysis); in hypertonic solutions they shrink (crenation).
对于动物细胞（如红细胞），可在显微镜下观察。在低渗溶液中，细胞膨胀并破裂（溶血）；在高渗溶液中，细胞皱缩。
4. Photosynthesis: Pigment Extraction and Separation | 光合作用：色素提取与分离
Grind fresh leaves with acetone or ethanol to extract pigments. Use a chromatography paper or thin-layer chromatography (TLC) plate. Draw a pencil baseline, apply concentrated pigment extract, and place the paper in a solvent such as petroleum ether and acetone.
用丙酮或乙醇研磨新鲜叶片以提取色素。使用层析纸或薄层层析板。用铅笔画基线，点上浓缩的色素提取液，然后将纸放入石油醚和丙酮等溶剂中。
Keep the solvent level below the baseline. The pigments separate by solubility and molecular size. Calculate Rf values: distance moved by pigment divided by distance moved by solvent front.
溶剂液面必须低于基线。色素按溶解度和分子大小分离。计算Rf值：色素移动距离除以溶剂前沿移动距离。
You should see carotenes (orange-yellow), xanthophylls (yellow), chlorophyll a (blue-green), and chlorophyll b (yellow-green).
通常可以看到胡萝卜素（橙黄色）、叶黄素（黄色）、叶绿素a（蓝绿色）和叶绿素b（黄绿色）。
5. Respiration: Measuring Gas Exchange | 呼吸作用：气体交换测定
Use a respirometer with germinating seeds or small invertebrates. The apparatus contains soda lime to absorb carbon dioxide, so any change in gas volume is due to oxygen uptake.
使用带有萌发种子或小型无脊椎动物的呼吸计。装置中含有钠石灰以吸收二氧化碳，因此气体体积的变化是由耗氧量引起的。
Set up a control with dead seeds or glass beads to correct for temperature and pressure changes. Equilibrium the apparatus before starting. Add a coloured fluid in a capillary tube and record its movement over time.
设置一个装有死亡种子或玻璃珠的对照，以校正温度和压力变化。开始前让装置平衡。在毛细管中加入有色液体，记录其随时间的移动。
Calculate oxygen uptake rate as volume per unit time. You can also measure carbon dioxide production by using a potash solution instead of soda lime and measuring the change.
计算耗氧速率为单位时间内的体积变化。也可用氢氧化钾溶液代替钠石灰，测量二氧化碳产生量。
6. DNA Extraction and Agarose Gel Electrophoresis | DNA提取与琼脂糖凝胶电泳
Crush a food sample (e.g., kiwi or strawberry) in a buffer containing detergent and salt. Detergent breaks down cell membranes, and salt helps DNA to precipitate. Filter the mixture, then add cold ethanol to precipitate the DNA.
将食物样品（如猕猴桃或草莓）在含有洗涤剂和盐的缓冲液中研磨。洗涤剂破坏细胞膜，盐有助于DNA沉淀。过滤混合物，然后加入冷乙醇使DNA沉淀。
For electrophoresis, load DNA samples into wells in an agarose gel immersed in a buffer. Apply an electric current. DNA is negatively charged and migrates toward the positive electrode.
进行电泳时，将DNA样品加入置于缓冲液中的琼脂糖凝胶加样孔内。通以电流。DNA带负电，向正极移动。
Use a DNA ladder (size marker) to estimate fragment sizes. Stain the gel with a fluorescent dye such as ethidium bromide or a safe alternative, and view under UV light.
使用DNA ladder（分子量标准）估算片段大小。用溴化乙锭等荧光染料或安全替代品对凝胶染色，并在紫外灯下观察。
7. Microbiology: Aseptic Technique and Counting | 微生物学：无菌操作与计数
Sterilise all equipment and media in an autoclave at 121 °C for 15 minutes. Work near a Bunsen burner to create an updraft that prevents contamination. Flame the inoculating loop before and after use.
所有器材和培养基应在121 °C高压蒸汽灭菌15分钟。在酒精灯旁操作以产生上升气流防止污染。接种环使用前后都要灼烧。
To count viable cells, use a dilution series and spread plates. Plate out samples onto agar, incubate at a suitable temperature for 24–48 hours, and count colonies. Each colony represents one cell or clump.
要计数活菌，需进行梯度稀释并涂布平板。将样品涂布在琼脂上，在适宜温度下培养24–48小时，统计菌落数。每个菌落代表一个细胞或细胞团。
Calculate the number of cells per cm³ using the dilution factor. Alternatively, use a haemocytometer for direct counting under a microscope, which counts both live and dead cells.
利用稀释因子计算每cm³中的细胞数。也可使用血球计数板在显微镜下直接计数，但该方法会同时计入死细胞和活细胞。
8. Biochemical Tests: Reducing Sugars, Proteins, and Starch | 生物化学测试：还原糖、蛋白质和淀粉
Benedict’s test: add Benedict’s reagent to a sample and heat in a water bath at 80–90 °C. A colour change from blue to green, yellow, orange, or brick-red indicates the presence of reducing sugars.
本尼迪特试验：向样品中加入本尼迪特试剂，并在80–90 °C水浴加热。颜色从蓝色变为绿色、黄色、橙色或砖红色，表明存在还原糖。
Biuret test: add equal volumes of sodium hydroxide and copper(II) sulfate to the sample. A purple/violet colour indicates the presence of proteins.
双缩脲试验：向样品中加入等体积的氢氧化钠和硫酸铜。出现紫色/紫罗兰色表明存在蛋白质。
Iodine test: add iodine–potassium iodide solution to the sample. A blue-black colour indicates starch.
碘液试验：向样品中加入碘-碘化钾溶液。出现蓝黑色表明存在淀粉。
9. Dissection: Heart or Kidney | 解剖实验：心脏或肾脏
Use a fresh sheep’s heart or kidney. Identify the external features first: for the heart, the atria, ventricles, coronary vessels, and the aorta, pulmonary artery, vena cava, and pulmonary vein. For the kidney, locate the ureter, renal artery, and renal vein.
使用新鲜羊心或羊肾。首先辨认外部结构：对于心脏，包括心房、心室、冠状动脉以及主动脉、肺动脉、腔静脉和肺静脉；对于肾脏，找到输尿管、肾动脉和肾静脉。
Cut along the coronal or sagittal plane to expose internal structures. In the heart, note the valves and the thickness of the ventricular walls. In the kidney, observe the cortex, medulla, renal pyramid, and renal pelvis.
沿冠状面或矢状面切开以暴露内部结构。在心脏中，注意瓣膜和心室壁的厚度；在肾脏中，观察皮质、髓质、肾锥体和肾盂。
Wear gloves and safety goggles. Dispose of biological material in the appropriate container. Draw and label your observations clearly.
佩戴手套和护目镜。将生物材料放入专用废弃容器中。清晰绘制并标注观察结果。
10. Data Recording and Error Analysis | 数据记录与误差分析
Record raw data in a table with units and uncertainties. Repeat measurements at least three times and calculate means. Identify anomalous results and investigate possible sources of error.
用表格记录原始数据，注明单位和不确定度。每个测量至少重复三次并计算平均值。识别异常结果，分析可能的误差来源。
Use appropriate graphs: line graphs for continuous variables, bar charts for discontinuous categories. Include error bars if you have calculated standard deviations.
选择合适的图形：连续变量用折线图，离散类别用条形图。若已计算标准差，可加上误差线。
In the conclusion, state whether your results support the hypothesis. Discuss limitations and suggest improvements such as using more precise instruments or controlling more variables.
在结论中说明结果是否支持假设。讨论局限性并提出改进建议，例如使用更精密的仪器或控制更多变量。
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