The internal environment of a cell remains relatively constant at all times.  This condition known as homeostasis means that factors such as pH and concentration of cell substances are held constant over the life of the cell.  The ability of cells to establish and maintain homeostasis is the result of the special properties of the cell membrane.

What is selective permeability?  How does it help to maintain homeostasis? See pg 84 also
· Ability of a cell to regulate what materials may pass through a cell. (Size is the key most times)
· By controlling what substances can pass through the membrane a cell can help maintain a constant internal environment.

What is diffusion?
· Movement of molecules from an area of high concentration to an area of low concentration. 
· This is due to the constant motion of molecules.  Molecules randomly move through an area until they are spread out evenly.

How does the concentration gradient affect diffusion?
· Is the difference in concentration between an area of low concentration and an area of high concentration.
· The greater the concentration gradient the more a substance will diffuse through a membrane.

What is equilibrium?
· Eventually particles will become evenly distributed throughout a given area. 
· Particles will still be in motion but as one particle leaves an area it is replaced by another particle.

What is facilitated diffusion?
· Some substances maybe moved across a membrane at a faster rate with the aid of transport proteins. 
· Just as with enzymes transport proteins are specific for only certain kinds of substances.
· Still works in the direction of the concentration gradient.

What is osmosis?
· The diffusion of water molecules through a membrane.
· Water will continue to move through a membrane until equilibrium is reached.
· Adding dissolved particles lowers the concentration of water. (There are fewer particles of water in a given space)
· The concentration of water is highest in pure (distilled) water. Water diffuses according to the conc. grad.