Substances can pass through the cell membrane in three ways. If a favorable
concentration gradient is present, substances such as water or those soluble in lipids can
pass through the membrane via simple diffusion. Substances, such as simple sugars or
amino acids, that are too big to move through the membrane's pores and are non-lipid
soluble, can pass through the membrane through a process called facilitated diffusion or
passive transport. If a favorable concentration gradient is present, proteins, called
permeases, can bind to substances and transport them across the membrane without the
expenditure of energy. Substances can even pass through the membrane against the
concentration gradient via a process called active transport. In active transport, the cell
must expend energy in order for the permeases to bind with the substance and pump it
through the membrane. Cells must use active transport to maintain the proper
concentration of sodium and potassium ions in their cytoplasm, for example, since the
concentration gradients favor too much sodium and too little potassium entering the cell.
If two solutions of different concentration are separated by a semi-permeable membrane
which is permeable to the smaller solvent molecules but not to the larger solute
molecules, then the solvent will tend to diffuse across the membrane from the less
concentrated to the more concentrated solution. This process is called osmosis.
Selectively Permeable - membranes that allow some things through, the cell membrane is
selectively permeable, water and oxygen move freely across the cell's membrane, by
Osmosis - the diffusion of water (across a membrane)
Water will move in the direction where there is a high concentration of solute (and hence
a lower concentration of water.
A simple rule to remember is:
Salt is a solute, when it is concentrated inside or outside the cell, it will draw the water in
its direction. This is also why you get thirsty after eating something salty.