Planktonic - a form of life style in which an organisms floats freely in a fluid without significant attachment or association with other living forms.

    Plaque is a film of bacteria in a matrix of salivary and bacterial polymers. It can be called a biofilm as it has a complex population of organisms which when mature, reach an ecological balance with one another.

    Plasma - the fluid part of blood, containing proteins and salts, from which the blood cells have been removed. see alsoserum.

    Plasmid- a circular piece of DNA found in the cell cy lasm of bacteria which is able to reproduce itself independently of it host. Plasmids may transmit a resistance to antibiotics from one bacteria to another. They are of great importance in techniques using for recombinant DNA.

    Platelet- derived growth factor - a cytokine found especially in platelets. It stimulates cell proliferation and encourages wound healing.

    Platelets - are small colourless discs of cy lasm found in blood. When platelets come into contact with a damaged vessels surface they change in several important ways. They begin to swell, their shape becomes irregular with protruding processes, they become sticky and they release an enzyme which causes the formation of thromboxane, one of the precursors of thrombin. Thromboxane also activates nearby platelets, thus starting a positive feedback which rapidly increases the mass of sticky platelets which form a platelet plug. This process accounts for daily damage to capillary walls. Damage on a larger scale requires other mechanism for haemostasis. Platelets also release serotonin which acts as a powerful vasoconstrictor