Eukaryotes & Prokaryotes

Cells

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

• 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

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

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

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)

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

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)
• 5 µm = 0.0015 mm
• To write this in standard form: