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Cell structure 1 - animal (and other) cells

This unit contains some information to assist you in relation to other units, and some supplementary details - pass your mouse cursor over the green text (and any pictures).

All living organisms are composed of cells.

You can easily see the nucleus, several crinkles in the cell membrane, and some small dots in the cytoplasm which might be mitochondria (see below), but ribosomes definitely cannot be seen at this magnification
Now try clicking!

This is a single cheek cell that has been stained,
as seen under a light microscope.
Click the diagram for a simple labelled version.

Most animals and plants are composed of many millions of cells, but some organisms such as bacteria and protoctistans consist of single cells.
Cells are generally microscopic, so you need a microscope to see the parts they are made of.
Most cells contain the following parts:

nucleus
cytoplasm
cell membrane

- all of which can be fairly easily seen with a light microscope.

The nucleus is said to control the activities of the cell, because the DNA it contains acts as the recipe for the proteins the cell can make. There is generally only one nucleus in a cell. On the outside of the nucleus is a membrane, which gives it a rounded or ovoid shape. Inside the nucleus there are are structures called chromosomes, which are mostly made of DNA. These cannot be seen very clearly unless the cell is about to divide.

The cell membrane is on the outside of the cell, so it is partly a barrier preventing larger structures inside from leaving the cell, and also the point of contact with the liquid outside the cell, and it to some extent controls what enters or leaves the cell. It is elastic - stretchy - so the cell contents can increase or decrease in size.

The cytoplasm is the main volume of the cell, and it is here that most of the activities - biochemical reactions - in the cell take place. In particular there are smaller structures here, including:

mitochondria
ribosomes.

Mitochondria - which can just be seen using the light microscope - are the site of (aerobic) respiration, which is the release of energy which powers the cell's activities: chemical synthesis, movement etc. There can be many mitochondria in a cell.

Ribosomes - which cannot be seen with a light microscope, but only using an electron microscope - are the site of protein synthesis. Even at very high magnification, they look like (pairs of) dots.

This is a rather grainy diagram of a section through an animal cell as seen with the electron microscope, showing several structures which are only of interest at a higher level. Click for labels on the mitochondria and (some of) the ribosomes

Click for labels

Approximate sizes:

in micrometres (痠) also known as microns
[1痠 = 0.001mm = 0.000001m (10^-6m)]

cell/component	size / 痠	note
cheek cell	35	some variation as these cells are fairly flat
nucleus	4	ovoid
mitochondrion	2	ovoid to sausage-shaped
ribosome	0.025 (25nm)	very small indeed!

What you should know

Most animal cells have the following parts:

a nucleus, which controls the activities of the cell

cytoplasm, in which most of the chemical reactions take place

a cell membrane, which controls the passage of substances into and out of the cell

mitochondria, which is where most energy is released in respiration

ribosomes, which is where protein synthesis occurs.

Most human cells are like most other animal cells.

Notes on words:

Nucleus - plural nuclei - is a Latin word meaning kernel, the inside of a nut. The adjective nuclear is derived from it.

In physics and chemistry, nucleus means the centre of an atom.

The word cytoplasm is built up from cyto- for cell and -plasm for moulding - so it means 'giving shape to the cell'.

The cytoplasm is surrounded by the cell membrane which is a very thin and flexible layer - not to be confused with a cell wall (not found in animal cells, but in plant cells and in bacteria and fungi).

Mitochondria is a plural word. One is called a mitochondrion.

The word ribosome is a compound of ribo- from ribonucleic acid (RNA) and -some which means body (i.e. a structure within the body).

chromosome: chromo- means coloured and -some means body: these absorb stain

DNA: DeoxyriboNucleic Acid

RNA: RiboNucleic Acid

Some background research

This man introduced the term cell:
Robert Hooke

Why did he use the word cell?
What other physical phenomenon did he give his name to?
How long ago did he live?
What else did you find interesting about him?

This man introduced the term nucleus:
Robert Brown (botanist)
What other physical phenomenon did he give his name to?
How long ago did he live?
What else did you find interesting about him?

This man made several discoveries with early microscopes:
Antonie van Leeuwenhoek
How did his microscope differ from modern ones?
What did he discover?
What else did you find interesting about him?

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