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Medicines which prevent the immune response of the body from destroying a transplanted organ


A distinct part of the cell, such as the nucleus, ribosome or mitochondrion, which has structure and function.


A thin, flexible sheet-like structure that acts as a lining or a boundary in an organism.


A bacterium, virus, or other microorganism, capable of causing disease.


Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus


A list of often difficult or specialised words with their definitions.


Reusable protein molecules which act as biological catalysts, changing the rate of chemical reactions in the body without being affected themselves


The basic unit from which all living organisms are built up, consisting of a cell membrane surrounding cytoplasm and a nucleus.

Cells - the fundamental unit of life

Living organisms are made up of cells. Understanding the structures and functions of cells is the key to understanding how whole organisms work and interact with the world around them.


How does your body recognise an invading pathogen? How do cells communicate so they can act as a coordinated whole? Why are transplanted organs rejected without immunosuppressant drugs? What are the differences between your cells and those of bacteria?

The size and structure of cells varies enormously, but there are many common features. The cell surface membrane forms a barrier between the contents of the cell and the environment. Membranes also surround all the organelles inside the cell. The structure of these membranes affects the functioning of almost every aspect of the cell.

Cells act as biological factories, producing many different substances which need to be exported to different regions of the cell or the body. Often these substances are produced as a result of signals from inside or outside the cell itself.

Cells display complex identification systems on their surfaces, and these act as part of overall cell communication. These systems are sometimes used by pathogens to gain entry to cells – and both surface identification systems and internal communication cascades can be used as targets for drugs in the battle against disease.

The fundamental unit of life The fundamental unit of life

A cell contains a hundred or more chemical reactions, each contained by membranes and controlled by enzymes.

How to use this site

There are a number of interactive features in this e-source:

  • A glossary of terms: any word with a glossary entry is highlighted like this. Moving the mouse over the highlighted word will show a definition of that word.
  • Quick questions: at the end of most pages or sections there is a question or set of quick questions to test your understanding.
  • Animations: most of the animations can be expanded to full screen size, ideal for showing on an interactive whiteboard. The animations will play all the way through or can be viewed one section at a time.
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