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

Polymers-large molecules made up of many joined units of a more simple molecule. Examples are polysaccharides and polypeptides.

Polypeptides; -chains of amino acids joined by peptide bonds. They are not the size of proteins, but may be biologically very active. Some hormones are peptides, such as insulin which has 51 amino-acid residues. Peptides may also be powerful neurotransmitter substances.

Polyphyodont - continuous replacement of teeth with many generations (from "poly" =many; "phylo" = generation and "dont" =teeth). See also diphyodont

Polysaccharides - long molecules made of chains of sugars linked together. Examples are starch, glycogen and dextrans.

Predentine - the extracellular matrix produced by odontoblasts, which becomes mineralised to form dentine. Similar in structure to osteoid.

Procaryote - a cell in which the genetic material is dispersed throughout the cell in distinction to a eucaryote which has a nucleus and other organelles. Bacteria and blue-green algae are procaryotes

Prognathic- a prominent lower jaw which may bring the lower teeth ahead of the upper teeth

Proline- one of 20 amino acids common in proteins. It is a common amino acid of collagen and like lycine must be hydroxylated by ascorbic acid in order to allow the formation of hydrogen bonds which will hold the triple helix together.

Proline-rich proteins - a group of proteins in saliva which have the ability to bind to calcium. They provide the protective layer of pellicle on the tooth surfaces by binding to the calcium in enamel. They also bind onto microorganism, providing a link between organism and the tooth surface. Proline rich proteins help to de-toxify tanins, which are potentially poisonous plant substances found in tea, and unripe fruit.