Question

1. Make a diagram of the fluid mosaic model of the structure of cell membranes. Label: phospholipid molecules, cholesterol, hydrophilic and hydrophobic portions, proteins, and carbohydrates.
2. Describe the structure, arrangement and functions of membrane lipids, membrane proteins and membrane carbohydrates.

Answer 1

Membrane Lipids-

Membrane lipids are a group of compounds (structurally similar to fats and oils) which form the double-layered surface of all cells (lipid bilayer). The three major classes of membrane lipids are phospholipids, glycolipids, and cholesterol.

Being cylindrical, phospholipid molecules spontaneously form bilayers in aqueous environments. In this energetically most-favorable arrangement, the hydrophilic heads face the water at each surface of the bilayer, and the hydrophobic tails are shielded from the water in the interior.

Membrane lipids also form a matrix in which membrane proteins reside. Historically lipids were thought to merely serve a structural role. Functional roles of lipids are in fact many: They serve as regulatory agents in cell growth and adhesion. They participate in the biosynthesis of other biomolecules.

Membrane Proteins-

Membrane proteins can be made of alpha helices or beta strands,or the combination of both alpha helices or beta strands. Proteins can span the membrane with alpha helices. A typical membrane-spanning segment consists of 20-25 hydrophobic amino acids arranged in an alpha helix, although not all transmembrane proteins fit this model. Peripheral membrane proteins are found on the outside and inside surfaces of membranes, attached either to integral proteins or to phospholipids.

Membrane proteins perform a variety of functions vital to the survival of organisms: Membrane receptor proteins relay signals between the cell's internal and external environments. Transport proteins move molecules and ions across the membrane.

Membrane carbohydrates-

Carbohydrates present in the plasma membrane as short sometimes branched chains of sugars attached either to exterior peripheral proteins (forming glycoproteins) or to the polar ends of phospholipid molecules in the outer lipid layer (forming glycolipids)

Biological membranes usually involve two layers of phospholipids with their tails pointing inward, an arrangement called a phospholipid bilayer.Carbohydrate groups are present only on the outer surface of the plasma membrane and are attached to proteins, forming glycoproteins, or lipids, forming glycolipids.

Membrane carbohydrates perform two main functions: participate in cell recognition and adhesion, either cell-cell signaling or cell-pathogen interactions, and they have a structural role as a physical barrier.