Several factors affect how fast a molecule will diffuse. The first of these is the kinetic energy of the molecule, which is most frequently measured as the temperature of the system. Molecules in a system at a higher temperature will have more energy and will move faster, and hence diffuse faster, than molecules of the same type in a low-temperature system. The size of the molecule also affects how rapidly it will diffuse. At the same temperature, smaller molecules will move more rapidly than larger molecules because it takes more energy to get the larger molecule moving.
Diffusion can occur through a cell membrane.
It is said to be permeable to these molecules. They will also move out of the cell at the same rate, and a dynamic equilibrium will exist. However, if the cell uses some of the oxygen as it comes into the cell, more oxygen will move into the cell than out of the cell. Likewise, as the chemical reactions in the cell use up oxygen they produce carbon dioxide. The concentration of carbon dioxide inside the cell increases so that more CO 2 molecules strike the inside of the cell and move out than strike the outside of the cell and move in.