Eukaryotes & Prokaryotes (AQA GCSE Biology): Revision Note

Cells in biology

• Cells are the basic building blocks of all living organisms

• Different cell types have different features, but most cells contain:

  • a cell membrane

  • cytoplasm

  • genetic material

  • ribosomes

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• There are two distinct types of cell:

  • eukaryotic

  • prokaryotic

Eukaryotic cells

• Plant and animal cells are eukaryotic cells

• They have the following components:

  • cell membrane

  • cytoplasm

  • genetic material (DNA) enclosed within a nucleus

• Eukaryotic cells vary in size, usually between 10 and 100 µm

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Prokaryotic cells

• Bacterial cells are prokaryotic cells

• 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 than eukaryotic cells, with many measuring ~ 1 µm in size

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Eukaryotic and prokaryotic cell comparison

Cytoplasm, cell membrane and plain "ribosomes" are found in both cell types, so they are similarities, not differences.

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

  • 1 µm = 1000 nm, so 1 nm = 1 x 10^-9 m

  • Sub-cellular structures and viruses are often measured in nanometres (nm)

  • 1 cm = 10 mm

• Exam papers require conversions between different units of measurement, particularly mm and µm

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• Exam papers also require an understanding of the size and scale of cells (and the sub-cellular structures within them)

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• 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.

• Size differences can also be given as a ratio. First put both sizes in the same unit, e.g. bacterial cell 1 µm and mesophyll cell 0.05 mm = 50 µm, so the ratio is 1 : 50. A ratio has no unit, so do not write 50 µm

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

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