Osmosis occurs within liquid solvents separated by semi-permeable membranes or materials. Energy known as osmotic pressure within smaller molecules in a solution may cause the solvent to move between membranes into the more highly saturated liquid on the other side of the membrane, while larger molecules cannot pass through.
Osmosis requires a liquid solvent in which molecules are suspended in order for it to occur. The substance in which molecules are suspended greatly affects the osmotic pressure between differing solvents, as the osmotic pressure fluctuates in relation between the molecules and the solvent. Molecules suspended within pure water as a solvent on either side of the membrane would not result in osmosis, as the osmotic pressure would not be great enough to cause the water or molecules to pass through the membrane.
Osmosis is essential to the transfer and dispersion of liquids on an organism's exterior, as well as molecular transfer in cells. A cell's semi-permeable cell membrane allows the intake of water molecules, nutrients and other substances necessary for the cell to perform biological processes from surrounding liquids. The cell membrane also serves as a barrier for the cell's interior from larger and potentially harmful substances attempting to enter the cell during osmosis.