Eukaryotic and prokaryotic cells

​​In a nutshell

Cells are the basic structural unit of all organisms. There are different types of cells, which can be generally be classified as eukaryotic or prokaryotic. Animal cells and plant cells are eukaryotic, whereas prokaryotic cells may include bacteria. 

Cells

​​Definition

Cells are the smallest unit in living organisms. They can make up unicellular organisms such as bacteria, or multicellular organisms like plants and animals.

​​​​Eukaryotic cells

Eukaryotic cells are those which contain genetic material within a nucleus. They are mainly found in animal and plant cells. 

Animal cells

All animal cells have the following features.

Plant cells

All plant cells have all the structures in animal cells as mentioned (nucleus, cell membrane, cytoplasm, mitochondria and ribosomes) as well as three additional ones.

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Prokaryotic cells

Prokaryotic cells are those that do not have a nucleus; instead, the genetic information is floating freely in the cytoplasm. 

Example

A bacterium is an example of a prokaryotic cell. Bacteria also have a cell wall. Sometimes they will have a tail called a flagellum which helps them swim or hook onto things in watery environments, and sometimes they'll have a mucus coat around it to stop itself from dehydrating. Bacteria often also have small loops of DNA called plasmids. 

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Estimating the size of cells

A microscope has to be used to view cells. However, it is very unlikely that you will view a singular cell; rather you will know the size of the slide you have prepared, and you can count the number of cells within this area. Counting the number of cells is often made easier by counting the number of nuclei for eukaryotic cells, or number of cell membranes for prokaryotic cells. You can then apply the following formula: 

​$$cell\ size = \frac{size\ of\ measured\ area\ \div number\ of\ cells\ observed}{magnification\ power}$$

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Example

You have prepared a microscope slide where your sample is $$25\ mm$$​ across and $$20\ mm$$​ down. In this area, you have observed $$56$$ nuclei when you use the $$\times100$$​ magnification lens. What is the size of a single cell in the sample?

The size of the sample area is:

 $$25\times 20 = 500\ mm$$

The size of each cell is:

 $$\dfrac{500\div56}{100} = 0.089\ mm$$

$$\underline{Therefore\ the\ size\ of\ a\ single\ cell\ is\ 0.089\ mm\ or\ 89\ \mu m}$$.