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What is Osmosis?
Osmosis is the diffusion of water through a plasma membrane (lipid bilayer). It is a form of active transport. The water molecules diffuse through a semi-permeable membrane, moving from areas of lower concentration of water molecules to areas of higher concentration of water molecules. It is the main way water is transported in and out of cells.
Hypotonic Vs. Hypertonic
The areas of high concentration of water molecules are called hypotonic. Hypotonic solution has a low concentration of dissolved substances. Areas of low concentration of water molecules are called hypertonic. Hypertonic solution has a high concentration of dissolved substances. During osmosis, water molecules naturally travel from hypotonic areas to hypertonic areas. This process equalizes the concentrations of water and dissolved substances.
When ions and molecules move from lower to higher concentration areas, it is called active transport. Active transport requires energy and help from proteins to move substances against the concentration gradient in and out of a cell. Active transport is basically the opposite of simple diffusion.
Osmosis in the Small Intestine
The small intestine is one place where osmosis is constantly occurring. The liquids you drink, as well as saliva and other liquids, are always being absorbed and secreted in the small intestine via osmosis. Sodium is what sets the gradient that water counters in the small intestine.
Facilitated diffusion speeds up osmosis but isn't required for it. Facilitated diffusion occurs when transfer proteins make watery holes that molecules and ions can pass through. The cell can open or close those holes depending upon its needs. In osmosis, a semi-permeable membrane will not allow unlimited molecules and ions to pass through, only unlimited water molecules. Facilitated diffusion allows more molecules and ions to pass through the semi-permeable membrane than osmosis alone.