Osmosis and Tonicity Notes

Osmosis is the net diffusion, or flow, of water molecules through a semi-permeable membrane into an area of higher solute concentration.
This difference in concentration is called the concentration gradient.
The net flow of water molecules will go to zero when the solute concentration is the same on both sides of the membrane.
Even though the net flow is in one direction or the other or zero, individual water molecules will continue to move in both directions.
Osmosis is a type of passive transport; it requires no energy.

The movement of water is based on the concentration of the solute because of the semi-permeable membrane.
The membrane does not allow the passage of the solute.
Water molecules diffuse through the membrane, but solute molecules do not.
Osmosis applies to the movement of a solvent (water) based on the concentration of the solute.

Water is polar, so it cannot easily cross the non-polar, lipid bilayer, even if it is a small molecule.
Biological membranes have aquaporins, protein water channels that allow the diffusion of water molecules across the membrane.
Aquaporins are highly specific and exclude other molecules.
The alpha helices of aquaporins create a hydrophilic water channel.

Tonicity describes the three conditions of solute concentration outside the cell membrane relative to the solute concentration within the cell.
- Hypertonic: Solute concentration is greater outside the cell.
- Hypotonic: Solute concentration is smaller outside the cell.
- Isotonic: Solute concentration is equal inside and outside the cell.

A solution outside the cell is hypertonic when it has a higher solute concentration than the solution inside the cell.
The net flow of water is out of the cell.
If too much water leaves the cell (due to being in a hypertonic solution), it can shrink or shrivel up.

The solution outside the cell is hypotonic when it has a lower solute concentration than the solution inside the cell.
The net flow of water is into the cell.
If too much water enters a cell due to it being in a hypotonic solution, it can swell, and if it swells too much, it can burst.

The solution outside the cell is isotonic when it has the same solute concentration as the solution inside the cell.
There is no net flow of water.
However, water will continue to flow in both directions.

If a solution A has a higher solute concentration than solution B, solution A is hypertonic relative to solution B.
- Hyper = more, tonic = solute
If a solution A has a lower solute concentration than solution B, solution A is hypotonic relative to solution B.
- Hypo = less, tonic = solute
Isotonic solutions have equal solute concentrations.
- Iso = the same, tonic = solute