Eukaryotes & Prokaryotes (AQA GCSE Biology)

Revision Note

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Biology Lead

Cells

general-cell-features-igcse-and-gcse-biology-revision-notes

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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

cellular-components-and-functions-table-igcse-and-gcse-biology-revision-notes

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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

 

typical-animal-and-plant-cells-igcse-and-gcse-biology-revision-notes

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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

a-typical-prokaryotic-cell-igcse-and-gcse-biology-revision-notes

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

 

Prokaryotic cells tableprokaryotic-cells-table-igcse-and-gcse-biology-revision-notes

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

converting-units-aqa-igcse-and-gcse-biology-revision-notesconverting-units-aqa-igcse-and-gcse-biology-revision-notes

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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)

comparison-of-size-aqa-igcse-and-gcse-biology-revision-notes

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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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Exam Tip

A common exam question is to ask you to calculate the size of subcellular 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 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:

standard-form-correct

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

Exam Tip

Take care to look at the units in which measurements of cells and subcellular structures are given.

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Lára

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.