What are the main components and properties of a plasma membrane?

A plasma membrane is composed mainly of an unordered phospholipid bilayer (double layer) embedded with membrane proteins which forms a thin flexible sheet. This gives the membrane several general properties including: semi permeability to different molecules, stability and the ability to form compartments which create different environments for biochemical reactions. The membrane is usually described by the ‘fluid mosaic model’ as the components can move around freely and all fit together (like a mosaic).Phospholipids are amphipathic molecules meaning they have a hydrophobic (‘water hating’) end which is 2 fatty acid chain tails which face inwards and a hydrophilic ‘water loving’ end which is a phosphate containing head group which face outwards towards the water. When in a polar medium (e.g. water) this causes them to spontaneously form a lipid bilayer which usually contains a 3-4nm hydrophobic core in cellular membranes. The hydrophobic layer is impermeable to most molecules and thus acts as a barrier and effectively isolating each side of the membrane.The membrane has a high level of fluidity and is stabilised by van der Waals forces between the hydrocarbon chains, and hydrophobic and ionic interactions between hydrophobic head groups and with water. The structure of phospholipids can alter membranes properties. For example, unsaturated fatty acids have a double bond which ‘kinks’ the tail and so the phospholipids can’t compact tightly together giving a lower melting point. Chain length can also affect this as a shorter chain means less van der Waals forces and so again giving a lower melting point. These properties are important as they change the fluidity of membranes at different temperatures.Other components of a plasma membrane include other lipid types such as cholesterol. Cholesterol is another amphipathic molecule which keeps the membrane fluidity consistent.Proteins are also a major part in most membranes composing of around 50% in plasma membranes. Protein types include integral proteins which span from one side of the membrane to the other and peripheral proteins which lie on the surface of the membrane. Peripheral proteins usually have a temporary and weaker association with the membrane and can just sit on the surface or dip into the hydrophobic layer slightly. Integral proteins include: channel proteins, pumps, enzymes, anchors and receptors. A final group is lipid anchored membrane proteins which are attached covalently to the lipid which hydrophobic tail is in the hydrophobic layer. All these proteins are able to move around each side of the membrane feely but are unable to flip from one side to the other.Carbohydrate groups are also attached to some lipids (glycolipid) and proteins (glycoprotein) on the outer surface of the membrane.