Explain how the structure of the plasma membrane prevents the diffusion of many of the biological molecules?

Lipids or fatty structures that surround the cellular wall.

The plasma membrane consists of a bilayer of lipids (including phospholipids, cholesterol and other modified lipids). The structure of lipids is such that they are what is called "amphiphilic", that is they have two distinct chemical characters. The fatty acid tail is hydrophobic (hates being in water) and the polar head group is hydrophilic (loves water). When the bilayer is formed, the charged head groups align to face away from the lipid tails in a double leaflet. Thus the inside of this bilayer has no desire to see water and the outside of the bilayer is exposed to aqueous conditions.

Since the molecules in the cytoplasm of a cell are dissolved in the aqueous environment and can freely diffuse, they are stopped from diffusing out of the cell by the bilayer which acts as a barrier to free diffusion by dissolved solutes.

My answer sorta repeats Guy's, but this is from my A&P 1 notes:

Plasma membrane-forms the cell?s flexible outer surface, separating the cell?s internal environment from the external environment.  It is a selective barrier that regulates the flow of materials into and out of a cell.  It also plays a key role in communication among cells and between cells and their external environment.

The basic structural framework of the plasma membrane is the lipid bilayer, two b2b layers made up of 3 types of lipid molecules:  phospholipids, cholesterol, and glycolipids (appears only in the membrane that faces the ECF).  The bilayer arrangement occurs because the lipids are amphipathic which means that they have both polar and nonpolar parts.  The polar part is hydrophilic, liking water, facing outward to the ECF; the nonpolar parts are the two long fatty acid tails which are hydrophobic, fearing water, facing inward.