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.
Active transport is 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.
Diffusion across cell membranes play a vital role in our world. In nature nutrients can enter the cells while wastes are allowed to leave. Synthetic membranes can be made for many uses. The smaller pores will allow only gases or water to pass, medium ones can sort out different size proteins while the larger ones may keep out microorganisms. The knowledge obtained by studing cell diffusion can be applied in many ways. Purification and seperation processess are some of the methods that have been advanced with this knowledge.