Osmosis: the diffusion of water molecules across a selectively premeable membrane.

Diffusion: the movement of molecules from an area of higher concentration to an area of lower concentration.

Facilitated Diffusion: protein carrier molecules speed up the movement of molecules already moving across the cell membrane. Cell energy is not used.

Protein Channels and Facilitated Diffusion

Some proteins in a cell membrane act as channels for specific ions or molecules. These channel proteins don't use energy at all. The simple allow the materials to naturally diffuse from the side with more solutes to the side with less. Whether the direction is out or in depends on where concentration is higher for each different solute.

Effect of Salt Water on Plants

When the salt is added, water stops diffusing into cells. The cells loose water and their internal pressure drops. The central vacuole and cytoplasn shrink and the plasma membrane pulls away from the cells wall.

Concentration Gradient

Concentration Gradient: a difference in the number of molecules or ions of a substance between adjoining regions. Without the addition of energy, molecules tend to diffuse from the area of higher concentration to the area of lower concentration.

Osmosis

Osmosis: the diffusion of water molecules across a selectively premeable membrane.

Equilibrium:[Stable Enviroment] a condition in which all acting influences are balanced, resulting in a stable enviroment.

Isotonic Solution:[Equal Concentration] a solution where the concentration of solute molecules outside a cell is equal to the concentration of solute molecules inside the cell.

Homeostasis:[Maintained Enviroment] a process by which a constant internal enviroment is maintained despite changes in the internal and external enviroment.

Hypotonic Solution:[Concentration is lower outside the cell] a solution where the concentration of solutes outside a cell is lower than that found inside the cell

Hypertonic Solution:[Concentration is higher outside the cell] a solution where the concentration of solutes outside the cell is higher than that found inside the cell.

Contractile Vacuole

Contractile Vacuole: a membrane-bound organelle found in certain protists that pumps in a cyclical manner from within the cell to the outside by alternately filling and then contracting to release it's contents at various points on the surface of the cell. It functions in maintaing osmotic equilibrium

Contractile Vacuole Expelling Water

The contractile vacuoles of cilia are permanent structures that periodicaly squeeze excess water out through a tiny pore. In some paramecium species the contractile vacuoles have conspicious feeder arms. But with most cilia the canal system is less aparant and the contractile vacuole appears as a simple sphere.

Paramecium: Usualy oval and having an oral groove for feeding.