Diffusion and Osmosis Diffusion and Osmosis Movement of molecules in biological systems involve the processes of diffusion and osmosis. Diffusion is defined as the movement of molecules from an area of greater concentration to an area of lesser concentration. A biological example was the diffusion of O2 and CO2 between blood vessels and tissues in the body. Phermones and their role in mating behaviors were also mentioned. Osmosis is the diffusion of water across a semi-permeable membrane.
Tonicity is a measurement of salt concentration in solution.
If the salt concentration is the same, inside and outside of a cell, the system is said to isotonic. A hypotonic system would have less salt outside of the cell which would mean that the concentration of water would be higher and osmotic pressure would draw water into the cell resulting in lysis. In the case of a hypertonic system the salt concentration would be higher outside of the cells, the water concentration would be higher inside of the cell and osmotic pressure would pull water out of the cell, resulting in crenation or shrinkage. These concepts can be used to explain why one can die of dehydration while drinking salt water. The energy used by biological systems to counteract entropy ultimately comes from energy in sunlight, a form of electromagnetic radiation.