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First exams 2025

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Using Units, Symbols & Numerical Values in Biology (SL IB Biology)

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Marlene

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Marlene

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Using Appropriate Units

  • The International System of Units (SI) is also called the metric system
    • This is the international standard for measurement
  • There are several SI base units that are used in science

SI Base Units Table

Quantity SI base unit Symbol
length metre m
mass kilogram kg
time second s
temperature Kelvin K
amount of substance mole mol
current  Ampere A
luminous intensity candela cd

  • Measurements of physical quantities can require very large and very small values, for example:
    • The diameter of an atom is about 10–10 m or 0.0000000001 m
    • One mole of a substance contains 6.02 × 1023  or 602 000 000 000 000 000 000 000 particles
  • Powers of ten are numbers that can be achieved by multiplying 10 times itself
  • These come under two categories of units:
    • Multiples e.g. 102, 103
    • Sub-multiples e.g. 10-1, 10-2
  • Each power of ten is defined by a prefix, the most common ones used in biology are listed in the table below 

Table of common prefixes in biology

Prefix Abbreviation Power of ten
kilo- k 103 
centi- c 10–2 
milli- m 10–3 
micro- μ 10–6 
nano- n 10–9 
  • It essential that the correct scientific measurements are used when discussing biological experiments
  • Ensure that the correct symbols are used in conjunction with the unit of measurement
    • E.g. m3 for cubic metres

Units of Measurement Table

Measurement Base unit Symbol Units used
Length Metre m 1000 m = 1 km
0.01 m = 1 cm
0.001 m = 1 mm
0.000001 m = 1 µm
Volume Cubic metre m3 109 m3 = 1 km3
0.000001 m3 = 1 cm3
10-9 m3 = 1 mm3
10-18 m3 = 1 µm3
Volume Cubic decimetre dm3 0.001 dm3 = 1 cm3
Area Square metre m2 10 000 m2 = 1 ha
0.0001 m2 = 1 cm2
Mass Kilogram kg 1000 kg = 1 tonne
0.001 kg = 1 g
0.000001 kg = 1 mg
10-9 kg = 1 µg
Time Second s 60 s = 1 min
60 min = 1 hour
Pressure pascal Pa 1000 Pa = 1 kPa
Energy joule J 1000 J = 1 kJ
Temperature degree Celcius °C  
Amount of substance mole mol 0.001 mol = 1 millimole
  • cm3 is the same as millilitre (ml)
  • dm3 is the same as litre (l)

Examiner Tip

Be careful when using the word "amount" in your answers. "Amount" has a very specific meaning in science - "mole". Instead refer to the mass, volume or concentration of a substance!

Significant figures

  • Significant figures must be used when dealing with quantitative data
  • Significant figures are the digits in a number that are reliable and absolutely necessary to indicate the quantity of that number
  • There are some important rules to remember for significant figures
    • All non-zero digits are significant
    • Zeros between non-zero digits are significant
      • 4107 (4.s.f.)
      • 29.009 (5.s.f)
    • Zeros that come before all non-zero digits are not significant
      • 0.00079 (2.s.f.)
      • 0.48 (2.s.f.)
    • Zeros after non-zero digits within a number without decimals are not significant
      • 57,000 (2.s.f)
      • 640 (2.s.f)
    • Zeros after non-zero digits within a number with decimals are significant
      • 689.0023 (7.s.f)
  • When rounding to a certain number of significant figures:
    • Identify the significant figures within the number using the rules above
    • Count from the first significant figure to the specified number
    • Use the next number as the ‘rounder decider’
    • If the decider is 5 or greater, increase the previous value by 1

Worked example

Write 1.0478 to 3 significant figures.

Answer:

Step 1: Identify the significant figures

They are all significant figures

Step 2: Count to the specified number (3rd s.f.)

1.0478

Step 3: Round up or down

1.05

Examiner Tip

An exam question may sometimes specify how many significant figures the answer should be, make sure you keep an eye out for this!

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Marlene

Author: Marlene

Expertise: Biology

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.