Osmosis

This is a shot from a three dimensional computer simulation of the process of osmosis. The blue mesh is impermeable to the larger balls, but the smaller balls can get through. All the balls are bouncing about.

Osmosis is the movement of water through a plasma membrane from a region of low solute concentration to a region of high concentration. It is a type of diffusion. Osmosis is passive transport, meaning it does not require energy to be applied. What causes osmotic pressure is different concentrations of solutes on the two sides of the membrane.

When osmosis happens, water moves from the side of the membrane with a lower amount of osmotic pressure to the side of the membrane with the higher amount. An important example of osmosis is the movement of liquid (solvent) molecules across a cell membrane into a cell with a higher solute concentration. Osmosis is a fundamental part of cell biochemistry, but also has mechanical applications and usages.

Hypotonic, isotonic, and hypertonic

Effect of different solutions on blood cells

Plant cell under different environments

Hypotonic describes the less concentrated solution. An isotonic solution is when the concentration is equal on both sides of the membrane. Hypertonic describes the more concentrated solution. The usual cell biology situation is: hypotonic solution outside the cell, and hypertonic solution inside.

Cell membranes

The plasma membrane of a cell is semi-permeable, which means it lets small molecules pass through, but blocks larger molecules. However, the membrane has special gateways which allow certain kinds of macromolecules to get though. This is not passive transport, but requires energy, and is selective.