Composition of Membranes

transformed into branched chain fatty acids. There are many steps necessary in order to make branched-chain fatty acids that can be between 12 and 17 carbons in total length. The reaction involves branched-chain fatty acid synthase.

COMPOSITION OF MEMBRANES

Fatty acids can be made into phospholipids in order to create the plasma membrane. In total, the plasma membrane is made from lipids and proteins. The phospholipid bilayer forms a stable barrier between the two aqueous compartments (inside and outside the cell). There are several different types of proteins that perform different functions in and out of the cell, including the all-important functions of selective molecular transport and cell-to-cell recognition.

There are four major phospholipids in the plasma membrane of animal cells: sphingomyelin, phosphatidyl serine, phosphatidylethanolamine, and phosphatidylcholine. These together make up more than half of the total lipids in the membranes.

Interestingly, the makeup of the outer layer is slightly different than the makeup of the inner layer. The outer layer consists of sphingomyelin, phosphatidylcholine, and glycolipids. The inner layer consists of phosphatidylserine and phosphatidylethanolamine. Phosphatidylinositol is also a major part of the inner layer but is not seen much in the outer layer. The head groups of the inner layer are more negatively charged so the inner part of the cell is more negatively charged when compared to the outer layer.

Glycolipids and cholesterol are also seen as part of the cell membrane. The glycolipids are found only on the outer layer, with their carbohydrate moieties sticking out of the cell. They represent just about 2 percent of the lipids in the plasma membrane. Cholesterol is a major part of the cell membrane—it makes up about half of the total lipids seen in animal cells.

The membrane is relatively impermeable to water-soluble molecules, including ions and most small and large biological molecules. Some fatty acids in the phospholipid molecule are unsaturated and have double bonds that make them hard to pack together.

The structure of these membranes makes lateral movement possible of proteins and lipids. Cholesterol can move freely within the membrane but cannot form a membrane by itself.

Most plasma membranes of animal cells have 50 percent lipids and 50 percent protein by weight. Carbohydrates make up about 5 to 10 percent of the mass of the membrane. Because proteins are heavier, there is only about 1 protein molecule per 50 to 100 molecules of lipids in the membrane. The fluidity of the membrane creates a fluid mosaic model—a raft of sorts where things can move relatively freely throughout the membrane. Figure 26 shows these free-floating proteins in the cell membrane:

Figure 26.

There are peripheral proteins and integral membrane proteins that are identified in the laboratory by their ability or lack of ability to be dissociated from the membrane after being treated with polar reagents. Peripheral proteins lie on the surface of the membrane, while integral proteins are within the membrane itself. Integral proteins cannot be isolated unless the lipid bilayer is disrupted.

Most integral proteins are transmembrane proteins that span the entirety of the lipid bilayer with parts of the protein exposed on both sides of this membrane. Electron microscopy will show these proteins as they stick out on either side of the membrane