The Fluid Mosaic Model: Plasma Membrane

· lipids and proteins are the main ingredients of membranes, although other chemical compounds are present.

· lipids are a group or family of compounds such as fats, phospholipids, and steroids, that are insoluble in water.

· proteins are a 3-D biological polymer constructed from a set of 20 different monomers called amino acids.

· the phospholipid bilayer has 2 layers of phospholipids with the hydrophobic tails pointing towards one another and the hydrophilic heads face the water.

· the proteins are spread throughout the membrane and are not on the surface of the membrane but are in fact embedded in it.

· it has also been found that the membrane is not fixed in place but actually the membrane is moving as if it were a fluid, hence the word fluid in the name.

Why call it fluid?

· experiments with human and mice cells fused together have shown that proteins specific to humans and those specific to mice eventually spread around the entire surface of the new fused cell.

· It was also noted that certain proteins do not move because they are attached to the cytoskeleton, a network of microfilaments and microtubules that maintain the shape of the cell.

· the plasma membrane remains fluid as the temperature decreases, until finally, at some critical temperature, it solidifies, much like bacon grease when it cools.

· the plasma membrane is as fluid as salad oil.

Why is it mosaic?

· a membrane is a collage of many different proteins embedded in the fluid matrix of the lipid bilayer.

· the plasma membrane and the membrane of the various organelles each have their unique collection of proteins.

· more than 50 kinds of proteins have been found in the plasma membrane of red blood cells, for example, and more have not yet been detected.

Some functions of the membrane proteins

· Transport proteins:

i) a protein may act as a tunnel through the membrane

ii) some proteins use ATP to pump substances across the membrane

· Enzymes: a protein built into the membrane that can be used to control reaction rates of substances that it comes in contact.

· Proteins as receptor sites: a protein with a specific shape that allows the connection of a messenger molecule known as a hormone. Hormones binding to this site cause specific chemical reactions to occur in the cell.

· Cell Adhesion: membrane proteins connect with similar proteins of adjacent cells.

· Attachment to the cytoskeleton: microfilaments of other elements of the cell structure may be bonded to membrane proteins to maintain the shape of the cell or to lock down certain proteins to a certain area.

Other important molecules of the plasma membrane

· glycoproteins - proteins that carry special sugars on the surface that act as identifiers. These sugar molecules differ between individuals of the same species. In humans they help to distinguish blood types. It will also help your immune system to identify foreign invaders. This also helps to explain why transplant organs are often rejected by recipients.

· glycolipids - are sugars that are in place of the phospholipids. These may include things like cholesterol. May also aid in identification or recognition.