Eukaryotes & Prokaryotes (AQA GCSE Biology): Revision Note
Exam code: 8461
Cells in biology
For a cell to be a cell, it has to have the following components:
Cell membrane
Holds the cell together, separating the inside of the cell from the environment outside, controlling what can and cannot enter or leave the cell
Cytoplasm
A jelly-like substance where many chemical reactions inside the cell happen
DNA
The genetic material that controls the activities of the cell
Ribosomes
The site of protein synthesis in the cell
There are two distinct types of cell:
Eukaryotic
Prokaryotic
Eukaryotic cells
Plant and animal cells are both eukaryotic cells
They have the components listed above (a cell membrane, cytoplasm and ribosomes), as well as others
A defining feature of eukaryotic cells is that their genetic material (DNA) is enclosed within a nucleus
Eukaryotic cells vary in size, usually between 10 and 100 µm
Prokaryotic cells
Bacterial cells are a type of prokaryotic cell
A defining feature of prokaryotic cells is that their genetic material is not enclosed within a nucleus
Instead, it is found as a single loop of DNA within the cytoplasm
Plasmids may also be present
Plasmids are smaller rings of DNA
All prokaryotic cells have a cytoplasm and a cell membrane, which is surrounded by a cell wall
Prokaryotic cells are much smaller in comparison to eukaryotic cells, with many measuring ~ 1 µm in size
Eukaryotic and prokaryotic cell comparison
Component | Eukaryotes | Prokaryotes |
|---|---|---|
Cell membrane | Y | Y |
Cytoplasm | Y | Y |
Genetic material | Y - in a nucleus | Y - in the cytoplasm |
Nucleus | Y | N |
Cell wall | Some types | Y |
Scale & the size of cells
Scientists measure the size of cells in micrometres (µm)
There are 1000 µm in 1 mm, so:
1 µm = 0.001 mm
1 µm = 1 x 10-3 mm
1 µm = 1 x 10-6 m
Exam papers require conversions between different units of measurement, particularly mm and µm
Exam papers also require an understanding of the size and scale of cells (and the sub-cellular structures within them)
Differences in size can sometimes be described using an order of magnitude. This is essentially the difference in size calculated by a factor of 10.
Worked Example
A bacterial cell is about 1 µm long. A typical animal cell is about 10 µm across.
What is the order of magnitude difference between the sizes of the bacterial and animal cell?
Answer
Step 1: Compare the size of the two numbers
Both measurements have the unit of µm
10 µm is 10 times larger than 1 µm
Therefore, the animal cell is 10 times larger than the bacterial cell
Step 2: Determine the order of magnitude of the size difference
This is a difference of 1 order of magnitude
Examiner Tips and Tricks
A common exam question is to ask you to calculate the size of sub-cellular structures and then to suggest why they may or may not be present in a certain type of cell. For example: Why do bacterial cells not contain mitochondria?
How to use standard form
When biologists talk about the size of cells and the microscopic structures within them, they are dealing with very small numbers
Very small (or very big) numbers are represented using standard form; this helps to avoid confusion
Powers of ten table
Number | Multiples of 10 | Power |
|---|---|---|
1000 | 10 × 10 × 10 | 103 |
100 | 10 × 10 | 102 |
10 | 10 | 101 |
1 | 1 | 100 |
0.1 | 1/10 | 10-1 |
0.01 | 1/100 | 10-2 |
0.001 | 1/1000 | 10-3 |
Examiner Tips and Tricks
Take care to look at the units in which measurements of cells and sub-cellular structures are given. Avoid common errors by:
checking whether you should be dividing or multiplying when converting between millimetres, micrometres and nanometres
ensuring you convert centimetres to millimetres first
leave rounding to the very final step
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