All living organisms are composed of cells.
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:
- 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.
Organisms that have a nucleus and other membrane-based organelles inside their cells are called Eukaryotes
and they include all animals and plants, fungi and protoctistans.
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.