Cell Membranes:

The Fluid Mosaic Model has been developed to describe cell membranes. The "fluid" portion of the cell membrane is made of phospholipids. In the last lecture, we learned that a phospholipid molecule is composed of a hydrophilic head and hydrophobic tail. If phospholipid molecules are surrounded by water, their hydrophobic fatty acid tails cluster and a bilayer results. The hydrophilic heads are at the outer faces of a two-layer sheet. Bilayers of phospholipids are the structural foundation for all cell membranes.

A variety of different proteins are embedded in the bilayer or positioned at its two surfaces. Membrane proteins serve as transport proteins, communication proteins, receptor proteins, recognition proteins, and adhesion proteins.

This animation (Audio - Important) describes the structure of cell membranes and associated proteins.

Because of their small size, most cells can only be seen by using light and electron microscopes. Cell size is determined by the surface-to-volume ratio. If a cell expands in diameter during growth, its volume will increase more rapidly than its surface area will. A cell that is too large will not be able to move materials into and out of the cell interior.

This animation (Audio - Important) will help you visualize how cell volume changes as the diameter increases.

REVIEW: Cell membranes consist mainly of a _____ .
Answer lipid bilayer and proteins