Eukaryotes & Prokaryotes (AQA GCSE Biology)

Cells in biology

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All cells have a number of features in common with each other

• For a cell to be a cell, it has to have the following components:

Cellular components & functions table

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• There are two distinct types of cell – eukaryotic and prokaryotic

Eukaryotic Cells

• Plant and animal cells are both eukaryotic cells

• They have the components listed in the table above (so 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

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Animal and plant cells are both eukaryotic cells as their genetic material is packaged in a nucleus

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, it is found as a single loop of DNA within the cytoplasm

• Additional smaller, circular pieces of DNA called plasmids may also be present

• The cell membranes of all prokaryotic cells are surrounded by a cell wall (usually made from a substance called peptidoglycan)

• Prokaryotic cells are much smaller in comparison to eukaryotic cells, with many measuring ~ 1 µm in size

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Prokaryotic cells do not have a nucleus, and are much smaller than eukaryotic cells

Prokaryotic cells table

Scale & the Size of Cells

• Cells are very small and require a microscope to be seen

• Scientists measure the size of cells in micrometers (µm)

• 1 µm is equivalent to 0.001 mm, or 1 x 10^-3 mm (or alternatively 1 millionth of a metre,  1 x 10^-6 mm)

• You need to be able to convert between different units of measurement, particularly mm and µm

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Make sure you are comfortable converting between different units

• You need to show an understanding of the size and scale of cells (and the subcellular structures within them)

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You need to be aware that many subcellular structures in eukaryotic cells are the same size as or bigger than prokaryotic cells!

• Differences in size can be described as differences in order of magnitude, essentially the difference in size calculated by a factor of 10

Size of cells table

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How to Use Standard Form

• When biologists talk about the size of cells and the 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

• Let’s say we want to represent the length of a Vibrio cholerae cell, which is 1.5µm, in mm

  • First, we need to convert the measurement in µm into mm (see image in Scale & the Size of Cells)

    • 1.5 µm ÷ 1000 = 0.0015 mm

  • Then we convert this into standard form

    • 0.0015 = 1.5 x 10^-3

• To convert numbers into standard form:

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Practise converting numbers into standard form – you may be asked to do this in the exam!