Osmosis is diffusion of water across a biological membrane. Water is usually the solvent, hence the term aqueous solution. 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. 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. Passive form of transport caused by the help of carrier proteins specific for the molecules they transport.
Movement is always down the concentration gradient.
Polar molecules and ions are usually moved in this fashion. The cell must expend its own energy to move the substances. It takes 3 sodium ions out of the cell for every 2 potassium ions it takes into the cell. Since sodium is more abundant outside the cell and potassium inside the cell, the cell is working against the concentration gradient. As this occurs the cytoplasm generates an over all negative charge 2K as opposed to 3Na on the outside, giving the extracellular fluid a positive charge.