What is the structure of an animal cell?

At GCSE level, you probably looked at the basic structure of an animal cell comprising the cell membrane, cytoplasm and nucleus. These are all that is normally visible of a cell when you look at it under a light microscope.

At AS and A level, we look more at the ultrastructure of a cell which can be seen using an electron microscope. This comprises several internal parts - called organelles - including mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus and vesicles and lysosomes.

The mitochondria are rod-shaped organelles often described as the power stations of the cell. They are involved in converting glucose and other energy sources into several smaller energy molecules called ATP (adenosine triphosphate). ATP contains a small enough amount of energy to be useful in many cellular chemical reactions without wasting much energy.

The ribosomes are small cytoplasmic granules involved in manufacturing proteins. The endoplasmic reticulum is divided into the rough endoplasmic reticulum (rER) which synthesises, stores and transports proteins and the smooth endoplasmic reticulum (sER) which synthesises, stores and transports lipids and carbohydrates.

The Golgi apparatus is a structure of membranes which make up flattened stacks, similar to the sER. It is often described as the "post office" of the cell as its main function is to label proteins with carbohydrates in order to send them to their correct destinations within the cell.

The Golgi apparatus produces lysosomes which are vesicles of low pH containing proteases and liases. These can be used to break down material ingested by phagocytosis, digest worn out organelles so their parts can be reused or to break down the cell itself when it dies. In secretory cells, they are also used to release enzymes to the outside of the cell in a process called exocytosis.