Plasma membranes are mainly made up of lipids with these mainky being phospholipids. Phospholipids are a lipid combined with a phosphate group therefore are made up of 2 fatty acid molecules, a glycerol molecule and a phosphate group. The hydrocarbon tail from 2 fatty acid molecules is hydrophobic. The phosphate group is hydrophilic.
The hydrophobic fatty acids and the hydrophilic phosphate group cause the phospholipids to arrange themselves into the plasma membrane shape. This is because the hydrophobic tails are repelled by water so face inwards and the hydrophilic end is attracted by water so therefore faces outwards. The phospholipid bilayer is a layer 2 phospholipid molecules thick. The phospholipid bilayer is fluid because the phospholipids are constantly moving. The phospholipid bilayer is also oily and flexible. The phospholipid bilayer allows the passage of certain lipid-soluble substances through a membrane.
Intrinsic protein is a membrane protein embedded in the inner layer of a cell memebrane. There are diiferent types of intrinsic proteins but they all act as a carrier molecule. The different types of intrinsic proteins are carrier proteins, channel proteins and co-transporter proteins.
One type of intrinsic protein is a carrier protein they allow the movement of differnet molecules either by facilitated diffusion or active transport. Carrier proteins work by a molecule going into its binding site this then causes the protein to change shape to allow the molecule to move through the membrane. A carrier protein can have one or more binding site but each binding site is specific to a certain molecule and has receptors on to notice this. The carrier protein can transport molecules into and out of a cell.
A channel protein is a type of intrinsic protein, meaning they span the width of the membrane. A channel protein moves molecules across the membrane by facilitated diffusion. The passage way of a channel protein is hydrophilic this acts as a pore in which charged particles (e.g sodium ions) move from an area of high concentration to an area of low concentration. The channel protein doesnt bind to the charged particles it just opens to allow the ions through. Most channel proteins are specific to certain ions. The movement through a channel protein is quicker than through a carrier protein.
Co-transporter proteins are a type of carrier protein which is a type of intrinsic protein. A co-transporter protein moves molecules by active transport against a concentration gradient. They work by the protein binding with two molecules. The concentration gradient of one of the molecules is used to move the other against its own concentration gradient. An example of a co-transporter protein is a sodium potassium pump in the epithelial cells of the small intestine.
Extrinsic proteins are membrane proteins embedded in the outer layer of a cell membrane. Many extrinsic proteins combine with carbohydrate groups to form glycoproteins. The carbohydrate groups usually extend from the cell surface like an antennae and act along with glycolipids (lipid molecules joined ti a carbohydrate group, as the chemical receptors of the cell. Some extrinsic proteins on the inner surface of the cell membrane attach to the cytoskeleton to act as an anchor.
The Fluid Mosaic Model
The fluid mosaic model was proposed in 1972 by Jonathan Singer and Garth Nicholson. The fluid mosaic modek of membrane strusture shoes that individual protein molecules mice along a fluid phospholipid bilayer.