What is Osmosis?
'''Osmosis''' (/ɒzˈmoʊ.sɪs/) is the movement of [[solvent]] (liquid) molecules through a [[membrane]], from one [[solution]] to another, which happens without an outside force. Solvent will move to the side that has a ''higher concentration'' of ''[[solute]]s'', or dissolved particles — and thus a ''lower concentration'' of ''solvent''. This happens because the membrane is selectively permeable: the solvent can pass through, but the solute cannot. Solvent molecules move randomly, and so the concentrations on both sides become more equal. Osmosis can be made to do [[Work (physics)|work]]. '''Osmotic pressure''' is the external [[pressure]] that must be applied so that there is no net movement of solvent across the membrane. Osmotic pressure depends on the [[molar concentration]] of the solute. Osmosis is important in biological (living) systems, as biological membranes are semipermeable. In general, these membranes are impermeable to large molecules such as ions, proteins, and polysaccharides. They are permeable to non-polar or hydrophobic molecules like lipids and small molecules like oxygen, carbon dioxide, nitrogen, and nitric oxide. Permeability depends on solubility, charge, or chemistry, as well as solute size. Water molecules travel through the plasma membrane, vacuole, or protoplast by diffusing across the phospholipid bilayer. Osmosis provides the main way water is gets in and out of cells. The turgor pressure of a cell is largely maintained by osmosis across the cell membrane between the cell interior and its environment. == Hypotonic, isotonic, and hypertonic == Solutions may have more or less solute per unit of solvent. The one with less is called ''hypotonic.'' When the two solutions have equal concentration, they are ''isotonic.'' The one with more is ''hypertonic''. When the hypotonic solution is outside the cell, and hypertonic solution inside, the cell becomes swollen and distorted. == Cell membranes == The plasma membrane of a [[cell]] is semi-permeable, which means it allows the entry