The Fluid Phospholipids: The Plasma Membrane

The plasma membrane is a fluid phospholipid bilayer. The outside of the bilayer is

hydrophilic portions of porteins and phospholipids because they are exposed to water.

This results in a stable membrane structure. Inside the bilayer hydrophobic portions of

proteins and phospholipids because the inside of the bilayer is nonaqueous. the proteins

bob in the fluid bilayer of phospholipids. The proteins also vary in structure and function.

There are teh integral proteins; transmembrane proteins with hydrophobic regions that

cover the hydrophobic interior of the plasma membrane. There is also the peripheral

proteins, they are not embedded in the membrane, but they are attached to the membrane's

surface.

One way in which
the plasma membrane permits interactions with the outside

environment is the permeability of the lipid bilayer. Permeability is the ability of

substances to cross the hydrophobic core of the plasma membrane. Some substances pass

through the plasma membrane better then others. The nonpolar hydrophobic molecules

will dissolve in the membrane and pass through with no difficulty.
The smaller polar

hydrophilic molecules, uncharged, are small enough to pass through the membrane lipids.

The larger polar hydrophilic molecules, uncharged, will not easily pass through the

membrane.

A second way in which the plasma membrane permits interactions with the

outside environment is passive transport. Passive transport is the diffusion of a substance

across a bilogical membrane. Diffusion is the net movement of a substance down a

concentration gradient. Net movement is the overall movement away from the center of

concentration. Concentration gradient is the graded concentration change over a distance

in a particular direction. Diffusion continues until equilibrium is reached.

A third way in which a plasma membrane permits interactions with the outside

environment is Osmosis. Osmosis is the diffusion of water across a selectively permeable

membrane. The direction of osmosis is determined by the difference in total solute

concentration. However, sometimes water cannot freely diffuse across a membrane

because some solute molecules can reduce the proportion of water molecules that
freely

diffuse.

A fourth way in which a plasma membrane permits interactions with the outside

environment is active transport. Active transport is an energy-requiring process during

which a transport protein pumps a molecule across a membrane, against it's concentration

gradient. This helps cells maintain steep ionic gradients across the cell membrane.