All living organisms are composed of cells. The structure of a cell relates directly to the function of that cell. In order for a dissolved substance to enter or leave a cell it needs to cross the cell membrane.

By the end of this section you should understand:

• how the structure of a cell relates to it function

Cells and cell structure

Nearly all animal and plant cells contain the following parts:

Nucleus – controls the activities that occur in the cell.

Cytoplasm – the substance in which most chemical reactions take place.

Cell Membrane – the barrier which holds the cell parts together and controls what enters and leaves the cell.

Mitochondria – where the majority of energy is produced through respiration.

Ribosomes – where protein synthesis takes place.

Plant and algal cells sometimes also contain a few of other parts:

Cell Wall – strengthens the cell

Chloroplast – absorbs light energy and produces food by photosynthesis

Vacuoles – are filled with cell sap to keep the cell turgid (or rigid)

Single-celled organisms

Not all cells follow this structure, however. A bacterium is a single-celled organism. It consists of cell wall which contains a membrane and cytoplasm. It does contain genetic material but not in the form of a nucleus.

Yeast is also a single-celled organism. This also consists of cytoplasm and a membrane surrounded by a cell wall but they do contain a nucleus.