Using Units, Symbols & Numerical Values in Biology (DP IB Biology)
Revision Note
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 |
Volume | Cubic metre | m3 | 109 m3 = 1 km3 |
Volume | Cubic decimetre | dm3 | 0.001 dm3 = 1 cm3 |
Area | Square metre | m2 | 10 000 m2 = 1 ha |
Mass | Kilogram | kg | 1000 kg = 1 tonne |
Time | Second | s | 60 s = 1 min |
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 Tips and Tricks
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 Tips and Tricks
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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