What are Plants and Animals Built From?

Are living things are built from cells. The structures of different types of cells are related to their functions. 

Cells are the membrane-bounded units that make up all living organisms. Some very small organisms e.g. bacteria may consist of a single cell, whereas humans consist of millions of cells.

Organisms that consist of more than one cell are called multicellular. In multicellular organisms, the cells specialise so that different types of cell perform different tasks. We call this division of labour; a similar division of labour is found in many factories. A specialised cell will do its job better than a generalised cell. The structure of a cell is related to its function, so cells with different jobs have different structures. 

Using a microscope, you can examine the epithelial cells that line the inside of your cheek. To do this, first, gently scrape the inside of your cheek with a spoon to remove some cells. Place them on a microscope slide, cover with a drop of saliva and spread the cells out. Put a coverslip on top of the cells and examine them using the microscope. You may see something similar to;

When we look at animal cells with a light microscope, we can see that they have a simple structure:

• Cell Membrane - The outer surface of the cell is the cell membrane. This is a barrier between the cell and the outside world. It's a selective barrier, able to regulate what may enter or leave the cell.
• Cytoplasm - The cytoplasm is a jelly-like fluid, which makes up the bulk of the cell. It is the site of much of the cell's metabolism. It contains small organelles called mitochondria where aerobic respiration takes place. Also, ribosomes will be present, but only visible under a power electron microscope. Ribosomes are the site of protein synthesis.
• Nucleus - The nucleus controls the cell's activists. It contains thread-like structures called chromosomes. These are visible only when the cell is dividing. Chromosomes carry the genes which contain the information that controls the cell. The shape of cheek epithelial cells enables them to fit together as a continuous lining.