Mechanisms for Crossing the Membrane Barrier

The three main catagories of mechanisms are:

Simple (or passive) diffusion
Facilitated
Active Transport

See diagram that compares all three types of diffusion
.

Passive Diffusion

Simple diffusion is the movement of a substance from a place of high concentration to one of lower concentration. Here is a sample diagram of diffusion across a membrane.
A special case of passive transport occurs with osmosis. Osmosis is the diffusion of water across a selectively permeable membrane. A spontaneous transport of solvent occurs from a dilute solute or salt solution to a concentrated solute or salt solution across a semipermeable membrane which allows the passage of the solvent but impedes passage of the salt solutes. This is very important with cells. All cells have some amount of dissolved salts,proteins and such, the flow of water across the plasma membrane depends upon the concentraton of water in the liquid that the cells are in. In humans the concentrations are equal inside and outside the cell. Therefore, water will neither enter or exit the cell. However, if the cells are removed from the body annd placed in a solution with a lower water content, the water will leave the cells thru osmosis. If the solution has a higher water concentration, the water will enter the cell causing swelling of the cell. Osmosis across plasma membranes is crucial to the functioning of many biological systems, including water uptake by plant roots, adsorbtion of dieting water from intestions and the readsorption of water and minerals in the kidneys. A diagram of osmosis
Some helpful osmosis definitions:

Isotonic - same salt concentration on either side of membrane
Hypertonic - salt concetration in solution is higher and water leaves cell
Hypotonic - salt concentration in solution is lower and water enters cell

Facilitated diffusion

Carrier proteins or permeases allow specific molecules to cross the bilayer.
Movement is along a concentration gradient, so no energy is required.

For more information on carrier (transport) proteins, see Functions of Membrane Proteins
Also, see co-transport example at bottom of page!

Active transport

Active transport is yet another form of diffusion. This occurs when a cell uses it's own energy to push a molecule across a membrane against the concentration gradient. Active transport is a major factor in the ability of a cell to maintain internal concentrations of small molecules while being exposed to different concentrations in the surrounding environment. The work of active transport is performed by specific proteins that are inserted in the membranes. These proteins use cell energy to accomplish their work. ATP is the protein that is usually used to supply the energy for most active transport. One way ATP can power active transport is by transfering it's terminal phosphate group directly to the transport protein. This may encourage the protein to change its configuration in a way that translucates a solute bound to the protein across the membrane.
Example of a P-type ion motive ATPase, where Three Na+ are pumped out of a cell for every two K+ pumped in. Other types include a V-type ATPase, where H+ are pumped into vacuoles, and F-type ATPases where a proton gradient is used to synthesize ATP.

Check out an example of a co-transport system involving epithelial cells!!