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Blood vessels

The tubes through which blood is carried around the body eg arteries, veins and capillaries


The hormone produced by the adrenal gland in response to stress or a 'fight or flight' situation.


A hormone produced by the pancreas. It causes the liver to convert glycogen back to glucose and to release glucose into the bloodstream.


Follicle stimulating hormone, as the name suggests, stimulates the follicles (of the ovary) to produce eggs. For this reason it is useful in fertility treatment of women whose ovaries are not producing eggs

Cell identification and communication

Cell ID

The glycoproteins and glycolipids on the cell surface membrane are key molecules in the process of cell recognition. These identifying molecules vary from organism to organism. The more closely related two organisms are, the more similar their identifying molecules will be. An identifying molecule that triggers an immune response in another organism is known as an antigen.

Identical twins have identical antigens on their cells. Siblings, parents and offspring have relatively similar antigens – but they are different enough for their cells to recognise each other as ‘non-self’. The cells from two organisms from the same species have more antigens in common than organisms from different species. The further apart organisms are in evolutionary terms, the more differences there will be in their cell surface antigens.

Receptor molecules

Identification of one cell by another involves specific receptor molecules in the cell surface membrane. Each cell has receptors that enable it to identify matching antigens. They also have receptors which recognise foreign antigens. The cells of the immune system are primed with millions of receptors that enable them to recognise and ultimately destroy material that they recognise as ‘non-self’, including pathogens, cells from other organisms of the same species, abnormal body cells and toxins.

antigens and receptors

The interaction between antigens and receptors on the surface of cell membranes is the key to cell recognition

Cell communication

Communication within cells is brought about in a number of ways. It often involves the production of a signalling molecule and the presence of specific receptors on the cell membrane.

Synaptic transmission between neurones

Neurotransmitter molecules made in the presynaptic knob are released into the synaptic cleft and picked up by receptors on the post-synaptic membrane. This triggers changes in membrane transport systems setting up a new action potential.

Animation of a synapse


Hormones act by binding to specific receptor sites on the membrane of their target cells. At this point the hormone needs to affect the target cell in some way to bring about the desired change in activity. There are two main ways in which hormones can affect a cell:

  • Direct messengers: The hormone may pass through the membrane and act as the internal messenger itself. Inside the cell the hormone binds to a receptor and the hormone-receptor complex passes through the pores of the nuclear membrane into the nucleus.

    The hormone-receptor complex acts as a transcription factor, regulating gene expression and switching sections of the DNA on or off. This is the mode of action of the lipid-soluble steroid hormones such as oestrogen and testosterone.

    Animation showing chemical communication via a transcription factor

  • Second messengers: Some hormones, for example adrenaline, glucagon and FSH, are not lipid-soluble and cannot pass through the cell membrane. The hormone molecule binds to a receptor in the cell membrane, triggering a series of membrane-bound reactions which result in the formation of a second chemical messenger inside the cell.

    This second messenger then activates enzymes within the cell, altering the metabolism. The most common second messenger is a substance called cyclic AMP (cAMP). This in turn triggers different responses in the cell that may include increased cellular respiration, increased contraction of muscle cells, relaxation of smooth muscle in blood vessels or other effects.

    Animation showing chemical communication via a second messenger in a cell