Glossary - P

peptide

The coavlent bond between the amino group of one amino acid and the carboxyl bond of another.

phantom

A system is described as a phantom model if self intersection is allowed. The opposite is a self avoiding system.

phase

A physically distinct state of a system.

Ice, water and steam are three different phases that water is commonly found in; they correspond to the solid, liquid and gaseous phase respectively.

phase transition

Transition between two phases of a system e.g. the melting of ice to water is a phase transition. The transition from a crumpled phase to the flat phase of a crystalline membrane is also a type of phase transition.

phospholipids

A class of molecules containing a polar head group that contain phosphorus atom and two non-polar hydrocarbon chains. They are the primary constitiuents of liposomes (lipid vesicles). There are many phospholipids due to the variuos possible types of head groups and hydrocarbon chains of different lengths.

polar

Exhibiting polarity: a molecule with opposite charges.

polar molecule

Capable of acquiring effective charge.

As a consequence of the nature of binding present amongst the atoms there is an intrinsic uneven distribution of charge- one part is positively charged whilst another part is negatively charged (even in the absence of an external field). Water is a polar molecule.

polymer

A molecule made up from many similar smaller molecules called monomers. Examples include PVC or poly-vinyl chloride and polyester.

polymerized

A system is said to be polmerized when the component parts bond together to form polymers.

potential energy

Potential energy is stored energy, energy that can be released or harnessed to do work. The energy of water held behind a dam is potential energy, the energy stored in the chemical bonds of coal is potential energy. In both cases the energy can be released, either by opening the dam or by burning the coal. See also: kinetic energy

plasma membrane

This is the name given to the external membrane of a cell. It arises because certain cells (such as human red blood cells) are surrounded by plasma (which is the largest constituent of blood by volume). However, the term is now used in preference to cell membrane to refer to the outer membrane of any cell.

p.p.m

Parts per million.

Is a commonly use method of quantifying concentration in chemistry.

pressure gradient:

Change of pressure across unit length .

Think of pressure gradient as force.

proton

A subatomic elementary particle.

Along with neutrons they constitute the nucleus of all atom. The charge of a proton is numerically equal but opposite to that of an electron.

protein

Long chains of amino acids, linked togehter by peptide bonds.

The shapes that the proteins fold into are crucial to their functions.