Using the right apparatus | 正确选用实验仪器

📚 Using the right apparatus | 正确选用实验仪器

In Cambridge A Level Biology, practical work is assessed not only by the biological conclusions you draw but also by your ability to select and justify the correct apparatus for each task. The right instrument reduces systematic and random error, improves resolution or precision, and ensures that the biological material is not damaged.

在剑桥 A Level 生物学中，实验操作不仅考查你得出的生物学结论，也考查你为每项任务选择并论证正确仪器的能力。合适的仪器可以减少系统误差和随机误差、提高分辨率或精密度，并确保生物材料不受损害。

1. Why apparatus choice matters | 为何仪器选择至关重要

Before planning any investigation, you must decide which apparatus gives the most valid and reliable data for the biological question. A Level examiners frequently award marks for justifying why one instrument is better than another, not just for naming it.

在规划任何探究之前，你必须确定哪种仪器能为该生物学问题提供最有效、最可靠的数据。A Level 考官常常会给“为什么一种仪器优于另一种”的论证打出分数，而不仅仅是写出仪器名称。

Accuracy describes how close a measurement is to the true value, while precision describes how repeatable repeated measurements are. An electronic balance may be both accurate and precise, whereas a measuring cylinder gives values that are precise enough only for approximate work.

准确度描述测量值与真实值的接近程度，而精密度描述重复测量的可重复性。电子天平可以兼具准确度和精密度，而量筒给出的数值只够用于粗略操作。

Choosing the wrong apparatus also increases percentage uncertainty. Percentage uncertainty is calculated as:

选择错误仪器还会增加百分比不确定度。百分比不确定度的计算公式为：

percentage uncertainty = (absolute uncertainty ÷ measured value) × 100%

A 100 cm³ measuring cylinder has an absolute uncertainty of about ±1 cm³, so measuring 5 cm³ gives a 20% uncertainty. A 10 cm³ measuring cylinder reduces this to 4%, and a 5 cm³ volumetric pipette reduces it much further.

一个 100 cm³ 量筒的绝对不确定度约为 ±1 cm³，因此量取 5 cm³ 会产生 20% 的不确定度。一个 10 cm³ 量筒可将不确定度降至 4%，而 5 cm³ 移液管则能将其大幅降低。

2. Measuring volumes: cylinders, pipettes and burettes | 测量体积：量筒、移液管与滴定管

Measuring cylinders are suitable when you need an approximate volume to make up a solution or add a reagent. They are not suitable for quantitative transfer because their wide bore and low resolution create large uncertainties.

量筒适用于需要大致体积来配制溶液或添加试剂的情况。它们不适合定量转移，因为其粗管径和低分辨率会造成较大不确定度。

A volumetric pipette delivers a fixed volume, such as 25.0 cm³, with high accuracy, typically ±0.06 cm³. Use a pipette filler or bulb, never your mouth, and allow the liquid to drain until the meniscus reaches the graduation mark.

移液管可准确量取固定体积，例如 25.0 cm³，其典型不确定度为 ±0.06 cm³。使用洗耳球或移液器吸球，切勿用嘴吸；让液体排出，直到弯月面到达刻度线。

A burette is the best choice when you need to deliver a variable volume accurately, for example during a titration or when adding small volumes of enzyme. Its scale has a resolution of 0.05 cm³, and the tip must be filled with no air bubbles before the initial reading is taken.

当需要准确量取可变体积时，滴定管是最佳选择，例如在滴定或加入少量酶时。其刻度分辨率为 0.05 cm³，在读取初始读数前必须确保尖端充满液体且无气泡。

For all transparent liquids in narrow apparatus, read the bottom of the meniscus at eye level to avoid parallax error. Rinse burettes and pipettes with the solution they will contain before use, but do not rinse volumetric flasks after making up to the mark.

对于窄口径仪器中的所有透明液体，应在视线水平处读取弯月面底部，以避免视差。使用前要先用待装溶液润洗滴定管和移液管，但容量瓶定容后不要再润洗。

3. Measuring mass and temperature | 测量质量与温度

Digital balances are preferred over mechanical balances because they usually give readings to 0.01 g or 0.001 g and have a tare function. Use a weighing boat or paper, and close the balance doors to avoid air currents.

数字天平优于机械天平，因为其读数通常可到 0.01 g 或 0.001 g，并具有去皮功能。使用称量舟或称量纸，并关闭天平防风门以避免气流影响。

For temperature, a liquid-in-glass thermometer is adequate when the value only needs to be recorded to the nearest 0.5 °C or 1 °C. A digital temperature probe or data logger is better when temperature must be measured continuously or when a fast response is needed.

对于温度，如果只需记录到 0.5 °C 或 1 °C 的精度，液体玻璃温度计就足够了。当需要连续测量温度或需要快速响应时，数字温度探头或数据记录仪则更合适。

In enzyme experiments, do not rely on the dial of a water bath. Place a thermometer or temperature probe in the reaction mixture itself, because the actual temperature may differ from the bath setting.

在酶实验中，不要只依赖水浴锅的表盘。应将温度计或温度探头插入反应混合物本身，因为实际温度可能与水浴设定值不同。

4. Making solutions: volumetric flasks and balances | 配制溶液：容量瓶与天平

A volumetric flask is used to make a solution of exactly known concentration, such as a 1.00 mol dm⁻³ sucrose stock. It has a single graduation mark and a narrow neck so the meniscus can be read with small uncertainty.

容量瓶用于配制浓度精确已知的溶液，例如 1.00 mol dm⁻³ 的蔗糖储备液。它只有一条刻度线，瓶颈较窄，因此读取弯月面的不确定度较小。

First dissolve the weighed solute in a beaker with distilled water, then transfer the solution into the volumetric flask. Rinse the beaker and funnel into the flask several times so all solute is transferred quantitatively.

先将称量好的溶质在烧杯中用蒸馏水溶解，然后将溶液转移至容量瓶。多次冲洗烧杯和漏斗并将洗液转入容量瓶，确保所有溶质被定量转移。

Add distilled water until the bottom of the meniscus just touches the graduation mark, then stopper the flask and invert it several times to mix thoroughly. Preparing a serial dilution is more accurate when fixed-volume pipettes and volumetric flasks are used instead of measuring cylinders.

加入蒸馏水直到弯月面底部刚好接触刻度线，然后塞紧瓶塞并倒转数次使溶液充分混合。进行系列稀释时，使用固定体积移液管和容量瓶比使用量筒更准确。

5. Microscopy and slide preparation | 显微镜使用与装片制作

A compound light microscope has two lenses: the eyepiece and the objective. Total magnification is the product of eyepiece magnification and objective magnification, for example 10 × 40 = ×400.

复式光学显微镜有两个透镜：目镜和物镜。总放大倍数是目镜放大倍数与物镜放大倍数的乘积，例如 10 × 40 = ×400。

Magnification enlarges the image, but resolution is what allows two close points to be seen as separate. The light microscope is limited by the wavelength of visible light, so an electron microscope is needed to see ribosomes, membranes at high resolution, or virus particles.

放大倍数能放大图像，但分辨率才决定相邻两点能否被分辨开。光学显微镜受可见光波长限制，因此要观察核糖体、高分辨率下的膜结构或病毒颗粒，需要使用电子显微镜。

To measure cell structures, calibrate the eyepiece graticule with a stage micrometer for each objective. This gives the true length of one graticule division, allowing you to convert eyepiece units into micrometres.

要测量细胞结构，需在每个物镜倍数下用镜台测微尺校准目镜测微尺。这样可得出目镜测微尺每分格的真实长度，从而将目镜单位换算成微米。

For temporary mounts, cut a very thin specimen, add a drop of water, and lower a coverslip at 45° with a mounted needle to reduce air bubbles. Stains such as iodine for starch, methylene blue for nuclei, or acetic orcein for chromosomes increase contrast.

制作临时装片时，应切取极薄的材料，加一滴水，然后用解剖针以 45° 角放下盖玻片以减少气泡。用碘液染淀粉、亚甲基蓝染细胞核、醋酸地衣红染染色体等染色剂可提高对比度。

6. Controlling the environment: water baths and incubators | 控制环境：水浴与培养箱

A thermostatically controlled water bath is essential for enzyme rate experiments because reaction rate is highly sensitive to temperature. The water circulates so the temperature remains uniform throughout the bath.

恒温水浴对酶速率实验至关重要，因为反应速率对温度高度敏感。水浴中的水循环流动，使整个水浴温度保持均匀。

Allow the water bath time to stabilise before beginning, and pre-incubate all reagents to the target temperature before mixing. This prevents the first few seconds of a reaction from occurring at the wrong temperature.

开始实验前要让水浴有足够时间稳定，并将所有试剂预热到目标温度后再混合。这样可以避免反应最初几秒在错误温度下进行。

An incubator is used for growing microorganisms, usually at 25 °C in schools because this reduces the chance of culturing human pathogens. In clinical or research contexts, 37 °C is used to mimic human body temperature.

培养箱用于培养微生物，学校中通常设定在 25 °C，因为这样可以降低培养人类病原体的风险。在临床或研究环境中，则使用 37 °C 来模拟人体体温。

7. Separation techniques: centrifuges and chromatography | 分离技术：离心机与色谱

A centrifuge separates organelles, cells, or precipitated DNA from a liquid by spinning at high speed. Heavier components form a pellet while the supernatant remains above, and the speed and time determine which components sediment.

离心机通过高速旋转将细胞器、细胞或沉淀的 DNA 从液体中分离。较重的组分形成沉淀，上清液留在上方；转速和时间决定哪些组分会沉降。

Always balance centrifuge tubes by mass and place them opposite one another. An unbalanced rotor causes vibration, poor separation, and damage to the centrifuge.

离心管必须按质量配平并相对放置。不平衡的转子会导致振动、分离效果差并损坏离心机。

Paper or thin-layer chromatography separates photosynthetic pigments such as chlorophyll a, chlorophyll b, carotenoids, and xanthophylls. The Rf value is calculated as:

纸色谱或薄层色谱可分离光合色素，如叶绿素 a、叶绿素 b、类胡萝卜素和叶黄素。Rf 值的计算公式为：

Rf = distance moved by pigment ÷ distance moved by solvent front

Draw the origin with pencil, not pen, because ink dyes dissolve in the solvent and contaminate the chromatogram. Run the chromatogram in a sealed container so the solvent vapour saturates the atmosphere and the solvent front rises evenly.

起点线应用铅笔绘制，不能用钢笔，因为墨水染料会溶于溶剂并污染色谱图。色谱应在密闭容器中进行，使溶剂蒸气饱和，溶剂前沿均匀上升。

8. Colorimetry and spectrophotometry | 比色法与分光光度法

A colorimeter measures the absorbance or transmittance of a coloured solution. Choose a filter with a complementary colour to the test solution, for example a red filter for a blue solution, because this gives maximum absorbance change.

比色计测量有色溶液的吸光度或透光率。应选择与待测溶液颜色互补的滤光片，例如蓝色溶液用红色滤光片，因为这样可获得最大的吸光度变化。

Before measuring, zero the colorimeter using a blank containing all reagents except the substance being tested. This removes absorption caused by the solvent and other reagent chemicals.

测量前，用除待测物质外含有所有试剂成分的空白液对比色计进行调零。这样可以扣除溶剂和其他试剂化学物质引起的吸收。

Handle cuvettes by the frosted or ridged sides, wipe the clear faces with lens tissue, and check that no air bubbles adhere to the inner wall. Use a standard curve of absorbance against known concentration to determine the concentration of an unknown sample.

拿取比色皿时要握住磨砂或有棱纹的两侧，用擦镜纸擦拭透明面，并检查内壁没有附着气泡。用吸光度对已知浓度作图的标准曲线来确定未知样品的浓度。

A spectrophotometer offers more versatility because the wavelength can be set to the exact absorption maximum of the substance. It is therefore preferable when high accuracy or specificity is required.

分光光度计更具灵活性，因为可将波长设定在物质吸收峰的精确位置。因此，当需要高准确度或特异性时，分光光度计更可取。

9. Aseptic technique and culturing apparatus | 无菌操作与培养器材

Aseptic technique prevents contamination of cultures by unwanted microorganisms and prevents contamination of the surroundings by the cultured organism. Autoclaves sterilise media and glassware at 121 °C under high pressure for about 15 minutes.

无菌操作可防止培养物被杂菌污染，同时防止培养微生物污染周围环境。高压灭菌器在 121 °C 高压下灭菌培养基和玻璃器皿约 15 分钟。

Work close to a Bunsen burner flame because the upward convection current carries airborne microbes away from the open culture. Flame the inoculating loop until it is red hot before and after transferring bacteria.

应在本生灯火焰附近操作，因为上升对流气流可将空气中的微生物带离敞开的培养物。接种环在转移细菌前后都要灼烧至红热。

Petri dishes containing sterile agar should be opened only slightly, and the lid should be taped but not completely sealed. This prevents aerobic pathogens from entering while still allowing air exchange, and dishes are incubated upside down to stop condensation dripping onto the culture.

含有无菌琼脂的培养皿只能稍微打开，盖子用胶带固定但不能完全密封。这样既能防止需氧病原体进入，又能保持空气交换；培养皿应倒置培养，以防止冷凝水滴落到培养物上。

10. Gas exchange, respirometers and data logging | 气体交换、呼吸计与数据记录

A respirometer measures the rate of oxygen uptake by a living organism. The organism is placed in a sealed chamber, and soda lime or potassium hydroxide absorbs the carbon dioxide produced, so any decrease in gas volume is due only to oxygen consumption.

呼吸计可测量活体生物的耗氧速率。将生物放入密封室中，钠石灰或氢氧化钾吸收产生的二氧化碳，因此气体体积的任何减少都只归因于氧气消耗。

The coloured liquid in the manometer moves toward the organism as oxygen is used. A control tube containing the same mass of glass beads or soda lime but no organism corrects for changes in atmospheric pressure and temperature.

随着氧气被消耗，压力计中的有色液体会向生物体方向移动。一个装有等质量玻璃珠或钠石灰但不含生物体的对照管，可校正大气压和温度的变化。

Use a syringe to reset the manometer fluid to its original position between readings, and place the whole respirometer in a water bath to keep temperature constant. Data loggers with oxygen, pH, or temperature sensors can record continuously, reducing human reading errors and allowing rate calculations from the slope of a graph.

每次读数之间用注射器将压力计液面复位到初始位置，并将整个呼吸计放入水浴中保持温度恒定。配备氧气、pH 或温度传感器的数据记录仪可连续记录，减少人工读数误差，并允许通过图形斜率计算速率。

11. Choosing apparatus for accuracy vs practicality | 在准确度与可行性间选择仪器

In a planning question, you should select the smallest apparatus that can hold the required volume, because this reduces percentage uncertainty. For example, use a 10 cm³ measuring cylinder rather than a 100 cm³ one to measure 8 cm³ of solution.

在实验设计题中，应选择能容纳所需体积的最小仪器，因为这样可以降低百分比不确定度

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Using the right apparatus | 正确选用实验仪器