Diffusion is the movement of molecules from an area where the molecule is in high concentration to an area where the molecule is in lower concentration. A diffusion animation is introduced here. In the animation, the molecules chosen represent a generic molecule (green balls) and carbon dioxide gas (small black and brown three ball structure). The generic molecules being in higher concentration outside of cells will freely diffuse into the cell and carbon dioxide being produced inside the cell due to cellular respiration will increase in concentration inside the cell and diffuse to the outside through the membrane. This allows the cell to obtain nutrients and dispose of carbon dioxide without any energy use. A very fortunate situation for the cell.

Facilitated diffusion is the movement of a molecule from an area of high concentration to an area of lower concentration with the help of a protein channel or carrier. The generic molecules in the previous animation used a channel protein to enter the cell. In the facilitated diffusion animation both amino acids and glucose are shown entering the cell facilitated by a protein carrier. In a cell membrane there would be proteins specific to each molecule and the carriers would not be shared in this way. The animation shows that movement occurs until the concentration of the molecules reaches equilibrium.

Osmosis is the diffusion of water through a semi-permeable membrane. Water moves from an area of high water molecule concentration (and lower solute concentration) to an area of lower water molecule concentration (and higher solute concentration). The osmosis animation shows water moving into a cell through a channel. Water molecules can be transported in this way, but can also diffuse directly through the membrane lipid bilayer.

Active transport is the movement of molecules from areas of low concentration to areas where the molecule is found in higher concentration. This movement is not spontaneous and requires ATP energy and a protein carrier. The ATP is used to drive conformational changes in the protein to pump molecules against their concentration gradient. This process occurs continuously in nerve cell membranes with sodium-potassium pumps. The active transport animation shows a simplified version of how such a pump operates.

Diffusion