# Osmosis --- **Osmosis** is the spontaneous net movement of a solvent like [[water]], through a semipermeable [[plasma membrane]], which moves to equalize a [[concentration gradient]]. [[solution|Solutes]] by nature [[brownian motion|move randomly]] in all directions. When a concentration gradient exists, there is a net movement *away* from regions of high concentration *towards* regions of low concentration. In other words this results in an increase in [[entropy]]. Recall that even at [[equilibrium]] movement does not stop, but continues to move back and forth, resulting in a lack of *net* change. If there is a difference in concentrations of a particular solute on either side of a [[plasma membrane#Semipermeable Membrane|semipermeable membrane]], and that solute *cannot* diffuse across, but water *can*, then in order to even out the concentrations water *will* move in the direction of the higher solute concentration. Equilibrium (and by extension, osmosis) is a very very powerful natural force. **Osmotic pressure** is the force that would be needed to oppose the osmosis in a closed system. ![[osmosis.png]] ## Tonicity **Tonicity** describes the osmotic pressure gradient across a semipermeable membrane. It is determined by comparing the concentration of nonpenetrating solutes in a solution to those inside the cell. Tonicity describes the change in a cell volume at equilibrium tonicity (of the solution)|concentration on the outside|concentration on the inside|water moves |the cell... :---------:| :-----:| :----: | :------------: | --- hypertonic | higher | lower | out | ...shrinks and dies hypotonic | lower | higher | in | ...swells or bursts and dies isotonic | equal | equal | doesn't change | ...stays the same ![[tonicity.png]] Note: Tonicity is always relative. A solution outside a vesicle that has a higher concentration of solutes than the interior has is said to be hypertonic *relative to the inside of the vesicle*. ## Osmolarity The number of solute molecules per solution volume or weight is known as **osmolarity**. Salt water has a higher osmolarity than fresh water. A solution is **hyperosmotic** if it exerts more pressure, or has a higher concentration of particles. A hyperosmotic solution has a greater concentration of solute. If a cell from a salt water fish is placed in a beaker of fresh water, the cell is said to be hyperosmotic to the water A solution is **hypoosmotic** if it exerts less pressure, or has a lower concentration of particles. Hypoosmotic solution has a lesser concentration of solute. If a cell from a fresh water fish is placed into a beaker of salt water, the cell is said to be hypoosmotic to the water. A solution is **isosmotic** if two solutions have the same concentration of particles. ## Tonicity vs Osmolarity (I think...the difference is subtle, don't worry about it too hard.) Tonicity is a measure of the *osmotic pressure gradient* between two solutions. Osmolarity is a measure of the *solute concentration* (in osmoles/volume) of a single solution**. ## Special Types of Osmosis Sometimes it is useful to specify a specific type of osmosis, usually differentiated by what type of molecule is generating the osmotic pressure. - **Chemiosmosis** is a type of osmosis where the concentration gradient is comprised of positively charged protons. This is used by [[ATP synthase]] to generate ATP. - **Oncotic pressure** or **colloid osmotic pressure** is a type of osmosis where the pressure is generated specifically by proteins. For example, [[albumin]] generates oncotic pressure within the blood vessels. ___