Diffusion

Diffusion is the net movement (the difference between the number of molecules either entering or leaving the cell) of particles from a region of higher concentration to a region of lower concentration. It continues until the concentration of substances is unified throughout the cell. Oxygen can diffuse through cell membranes. And since there difference between the number of oxygen molecules entering the cell is higher the oxygen molecules leaving the cell, the net movement is into the cell.

Cellular diffusion is similar to chemical diffusion, the only difference is that the atoms move from high chemical potential to low chemical potential.

Diffusion is  more efficient in small single-celled microorganisms. Diffusion becomes less efficient as the surface to volume ratio decreases, so diffusion is less effective in larger animals. To overcome the limitations of diffusion, multi-cellular organisms have developed specialized tissues and systems that are responsible for completing a limited number of nutrient and waste tasks.

The rate of diffusion increases as the temperature increases. At higher temperatures, particles move faster because more energy is available to diffuse them. The rate of diffusion increases as the concentration difference increases. The rate of diffusion is directly proportional to a particle’s distance.

let’s consider a cubical cell 10 microns on each side. The volume is 1000 cubic microns which is served by 600 sq microns of surface to allow nutrients an oxygen in and waste out, and mainly to allow diffusion. Double cell size and volume goes up to 8000 cubic microns, but the surface only increases to 2400 sq microns. The surface to volume ratio is now half compared to the smaller cell. At some point the ratio drops to the point where not enough material can cross the membrane to supply the cell.