Passive transport

[1][2][4] Fundamentally, substances follow Fick's first law, and move from an area of high concentration to an area of low concentration because this movement increases the entropy of the overall system.

[6] It is also true for supersaturated solutions which are more and more important owing to the spreading of the application of amorphous solid dispersions for drug bioavailability enhancement.

Osmosis is much like simple diffusion but it specifically describes the movement of water (not the solute) across a selectively permeable membrane until there is an equal concentration of water and solute on both sides of the membrane.

Simple diffusion and osmosis are both forms of passive transport and require none of the cell's ATP energy.

Prokaryotes typically have small bodies, allowing diffusion to suffice for material transport within the cell.

Larger cells like eukaryotes would either have very low metabolic rate to accommodate the slowness of diffusion, or invest in complex cellular machinery to allow active transport within the cell, such as kinesin walking along microtubules.

A biological example of diffusion is the gas exchange that occurs during respiration within the human body.

[8] Simultaneously, carbon dioxide moves in the opposite direction, diffusing across the membrane of the capillaries and entering into the alveoli, where it can be exhaled.

Facilitated diffusion, also called carrier-mediated osmosis, is the movement of molecules across the cell membrane via special transport proteins that are embedded in the plasma membrane by actively taking up or excluding ions [14].

Through facilitated diffusion, energy is not required in order for molecules to pass through the cell membrane.

[13] Since glucose is a large molecule, it requires a specific channel to facilitate its entry across plasma membranes and into cells.

[12][13] Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system.

For example, the membrane pores of the Bowman's capsule in the kidneys are very small, and only albumins, the smallest of the proteins, have any chance of being filtered through.

On the other hand, the membrane pores of liver cells are extremely large, but not forgetting cells are extremely small to allow a variety of solutes to pass through and be metabolized.

In hypertonic solution, the water will move out, causing the cell to shrink.