📚 Units of measurement | 测量单位
Accurate measurement is the backbone of experimental biology. In A‑Level Cambridge Biology, you are expected to record, convert, and interpret data using standard SI units and their derived forms. From the nanometre scale of a ribosome to the metre scale of an ecosystem transect, every measurement must be expressed with the correct unit and an appropriate number of significant figures. Mastering these units not only eliminates careless errors but also deepens your understanding of biological scales, concentrations, rates, and microscopy calculations.
精确测量是实验生物学的支柱。在A‑Level剑桥生物学中，你需要使用标准国际单位制（SI）及其导出单位来记录、转换和解读数据。从核糖体的纳米尺度到生态系统样线的米尺度，每一个测量值都必须以正确的单位和恰当的有效数字表示。掌握这些单位不仅能避免粗心错误，还能加深你对生物学尺度、浓度、速率和显微镜计算的理解。
1. SI units and their importance | 国际单位制及其重要性
The International System of Units (SI) provides a universal language for scientific measurement. The base units most relevant to biology are the metre (m) for length, kilogram (kg) for mass, second (s) for time, and mole (mol) for the amount of substance.
国际单位制（SI）为科学测量提供了一种通用语言。与生物学最相关的基本单位包括长度的米（m）、质量的千克（kg）、时间的秒（s）和物质的量的摩尔（mol）。
Using SI units guarantees that experiments conducted in different laboratories can be compared directly. Prefixes such as milli- (10⁻³), micro- (10⁻⁶), and nano- (10⁻⁹) are added to base units to handle the enormous range of sizes encountered in living systems.
使用SI单位可确保不同实验室开展的实验能够直接比较。通过添加毫（10⁻³）、微（10⁻⁶）和纳（10⁻⁹）等前缀，可以处理生命系统中极其宽广的尺寸范围。
In written answers and practical write‑ups, always include units unless a quantity is truly dimensionless (e.g. magnification). Leaving out a unit or using the wrong prefix is a frequent source of lost marks.
在书面答案和实验报告中，除非某一量确实没有量纲（如放大倍数），否则一定要写明单位。遗漏单位或使用错误前缀是失分的常见原因。
2. Length units: from metres to nanometres | 长度单位：从米到纳米
Length in biology spans at least nine orders of magnitude. The metre (m) is the SI base unit, but most cellular and subcellular structures require smaller divisions.
生物学中的长度跨越至少九个数量级。米（m）是SI基本单位，但大多数细胞和亚细胞结构需要更小的刻度。
Tissue dimensions and field‑study measurements are usually given in millimetres (mm, 10⁻³ m) or centimetres (cm, 10⁻² m). Individual cells are measured in micrometres (µm, 10⁻⁶ m), while organelles and macromolecules are expressed in nanometres (nm, 10⁻⁹ m).
组织尺寸和野外研究测量通常以毫米（mm，10⁻³ m）或厘米（cm，10⁻² m）给出。单个细胞以微米（µm，10⁻⁶ m）测量，而细胞器和大分子则以纳米（nm，10⁻⁹ m）表示。
| Unit (单位) | Symbol (符号) | Value in metres (米值) | Biological example (生物学实例) |
|---|---|---|---|
| centimetre 厘米 | cm | 10⁻² m | leaf length 叶片长度 |
| millimetre 毫米 | mm | 10⁻³ m | stomatal pore width 气孔宽度 |
| micrometre 微米 | µm | 10⁻⁶ m | red blood cell diameter 红细胞直径 |
| nanometre 纳米 | nm | 10⁻⁹ m | ribosome 核糖体 |
When moving between units, multiply by 1000 for each step: 1 mm = 1000 µm, 1 µm = 1000 nm. It is helpful to recall that a typical bacterial cell is about 1–2 µm long, while a mitochondrion is roughly 0.5–1 µm in diameter.
在单位之间转换时，每跨一步就乘以1000：1 mm = 1000 µm，1 µm = 1000 nm。记住典型细菌细胞长约1–2 µm，而线粒体的直径约为0.5–1 µm，这些常见数值有助于快速判断。
3. Volume and mass units | 体积和质量单位
Volume measurements in biology often use the litre (L) and its sub‑multiples. 1 L = 1 dm³ = 1000 mL = 1000 cm³. In molecular biology and microbiology, microlitres (µL, 10⁻⁶ L) are routine.
生物学中的体积测量常使用升（L）及其分倍数。1 L = 1 dm³ = 1000 mL = 1000 cm³。在分子生物学和微生物学中，微升（µL，10⁻⁶ L）是常规单位。
Mass is recorded in kilograms (kg), grams (g), milligrams (mg) or micrograms (µg). A laboratory balance usually reads to 0.01 g or 0.001 g; microgram quantities require an analytical balance.
质量以千克（kg）、克（g）、毫克（mg）或微克（µg）记录。实验室天平通常可读至0.01 g或0.001 g；微克级数量则需要分析天平。
When preparing a solution, the mass of solute (in g) and the final volume (in dm³ or cm³) determine the concentration in g dm⁻³. Never confuse mL with L when entering values into a formula.
配制溶液时，溶质的质量（以g计）和最终体积（以dm³或cm³计）决定其g dm⁻³浓度。在将数值代入公式时，永远不要混淆mL和L。
4. Concentration units | 浓度单位
Concentration expresses the quantity of solute dissolved in a given volume of solvent. Two common forms are mass concentration (g dm⁻³) and molar concentration (mol dm⁻³).
浓度表示溶解在给定体积溶剂中的溶质量。两种常见形式是质量浓度（g dm⁻³）和摩尔浓度（mol dm⁻³）。
A 1 mol dm⁻³ solution contains one mole of solute per cubic decimetre of solution. Since the mole is the amount of substance containing 6.02 × 10²³ particles, molarity allows chemists and biologists to work with equal numbers of reacting entities.
1 mol dm⁻³溶液每立方分米溶液中含有1摩尔溶质。因为摩尔是约含6.02 × 10²³个微粒的物质的量，所以使用摩尔浓度能让化学家和生物学家处理相同数量的反应实体。
In dilution calculations, use C₁V₁ = C₂V₂, where C is concentration and V is volume. Both sides must be in the same units; for example, if C₁ is in mol dm⁻³ and V₁ in cm³, convert V₁ to dm³ by dividing by 1000, or express both volumes in cm³ and keep C in mol cm⁻³, which is less common. Exam advice: always convert volumes to dm³ when dealing with molarity.
在稀释计算中，使用C₁V₁ = C₂V₂，其中C为浓度，V为体积。两边必须使用相同单位；例如，若C₁为mol dm⁻³而V₁为cm³，则应将V₁除以1000转化为dm³，或者将两个体积都保留为cm³并将浓度表示为mol cm⁻³，但后者不常用。应试建议：涉及摩尔浓度时，务必将体积转换为dm³。
5. Temperature, time and derived units | 温度、时间及导出单位
Temperature in biology is recorded in degrees Celsius (°C), but thermodynamic equations require the Kelvin scale (K). The conversion is K = °C + 273.15. Enzyme‑controlled reactions are extremely temperature‑sensitive; the Q₁₀ coefficient describes the rate increase over a 10 °C rise.
生物学中的温度以摄氏度（°C）记录，但热力学方程需要使用开尔文温标（K）。换算关系为 K = °C + 273.15。酶控反应对温度极其敏感；Q₁₀系数描述了温度每升高10 °C反应速率的增加倍数。
Time is measured in seconds (s), minutes (min), or hours (h). For rate calculations, such as initial reaction rate or transpiration rate, seconds are preferred because they are the SI base unit.
时间以秒（s）、分钟（min）或小时（h）测量。在计算速率时，例如初始反应速率或蒸腾速率，优先使用秒，因为秒是SI基本单位。
Derived units frequently appear: rate of oxygen production in cm³ s⁻¹ or cm³ min⁻¹, magnification as a pure number, density in g cm⁻³, and pressure in kPa (e.g. water potential components). Using derived units correctly demonstrates a clear understanding of the relationship between quantities.
导出单位经常出现：氧气产生速率可用cm³ s⁻¹或cm³ min⁻¹，放大倍数为纯数，密度用g cm⁻³，压力用kPa（如水势组分）。正确使用导出单位能体现你对各量之间关系的清晰理解。
6. Units in microscopy: eyepiece graticule and stage micrometer | 显微镜单位：目镜测微尺与镜台测微尺
An eyepiece graticule is a tiny ruler fitted inside the eyepiece, but its divisions are arbitrary and must be calibrated for each objective lens using a stage micrometer.
目镜测微尺是安装在目镜内部的微小标尺，但其刻度是任意的，必须使用镜台测微尺针对每个物镜进行校准。
The stage micrometer has a precise scale, typically 1 mm long and subdivided into 100 divisions, so each division = 10 µm. By superimposing the two scales, you calculate the value of one eyepiece unit (epu) in micrometres.
镜台测微尺具有精确刻度，通常长度1 mm并细分为100格，因此每格 = 10 µm。通过叠加两个标尺，即可计算出一个目镜单位（epu）对应的微米值。
For example, if 40 graticule divisions align with 16 stage‑micrometer divisions, the length in µm for the graticule interval is (16 × 10 µm) ÷ 40 = 4 µm per division. Always recalibrate after changing magnification.
例如，若40个目镜刻度与16个镜台测微尺刻度重合，则每个目镜刻度代表 (16 × 10 µm) ÷ 40 = 4 µm。每次改变放大倍数后都必须重新校准。
7. Calculating magnification and actual size | 计算放大倍数与实际大小
The fundamental relationship is expressed as:
Magnification = Image size ÷ Actual size
基本关系可表示为：放大倍数 = 图像大小 ÷ 实际大小。
This formula works only when image size and actual size are in the same units. If a photomicrograph shows a mitochondrion measuring 30 mm and its actual length is 3 µm, first convert 30 mm to 30,000 µm, then magnification = 30,000 ÷ 3 = ×10,000.
该公式仅在图像大小和实际大小使用相同单位时才有效。若一张显微照片中的线粒体长30 mm，而实际长度为3 µm，首先将30 mm转换为30,000 µm，然后放大倍数 = 30,000 ÷ 3 = ×10,000。
To find actual size, rearrange the formula:
Actual size = Image size ÷ Magnification
计算实际大小时，可变换公式：实际大小 = 图像大小 ÷ 放大倍数。
Always express your answer in the most appropriate unit, usually µm for cells and nm for organelles. Prefix the magnification value with ‘×’, e.g. ×400, ×12,000.
始终用最合适的单位表达答案，细胞通常用µm，细胞器用nm。放大倍数前加“×”，如×400、×12,000。
8. Standard form and significant figures | 科学记数法与有效数字
Standard form (scientific notation) writes a number as A × 10ⁿ, where 1 ≤ A < 10. For example, 0.000045 m becomes 4.5 × 10⁻⁵ m. This avoids counting long strings of zeros.
科学记数法（标准形式）将数字写成A × 10ⁿ，其中1 ≤ A < 10。例如，0.000045 m写作4.5 × 10⁻⁵ m，从而避免数长串的零。
Significant figures indicate the precision of a measurement. If a balance reads to 0.01 g, a mass of 3.50 g has three significant figures. When multiplying or dividing, the final answer should contain the same number of significant figures as the least precise measurement used.
有效数字指示测量的精度。若天平可读至0.01 g，则质量3.50 g具有三位有效数字。在乘除运算中，最终答案的有效数字位数应与所使用的最不精确测量值的位数一致。
In exam questions, leaving an answer as ‘0.00001 g’ is less acceptable than ‘1.0 × 10⁻⁵ g’. Using standard form and appropriate significant figures demonstrates quantitative confidence.
在考试中，将答案写为“1.0 × 10⁻⁵ g”远比“0.00001 g”更容易被接受。运用科学记数法和恰当的有效数字能展示你的定量表达能力。
9. Practical use of units in biological experiments | 生物实验中单位的实际应用
Serial dilutions are a common technique for obtaining a range of concentrations. If you need 0.1 mol dm⁻³, 0.01 mol dm⁻³, and 0.001 mol dm⁻³ sucrose solutions, you must calculate volumes accurately and label each tube with the correct unit.
连续稀释是获取一系列浓度的常用技术。若你需要0.1 mol dm⁻³、0.01 mol dm⁻³和0.001 mol dm⁻³的蔗糖溶液
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