Cell membranes.

This is when a substance enter or leaves a cell against its concentration gradient, usually from area of low concentration to area of high concentration.

It requires energy in the form of ATP (Adenoside Triphosphate) which is released by the mitochondria in the cell.   

ANY MOLECULE THAT MOVES BY ACTIVE TRANSPORT WILL MOVE THROUGH CARRIER PROTEINS IN THE CELL MEMBRANE.

Carrier proteins allow the access of polar, hydrophillic and large molecules. 

They have receptor sites which have a specific shape - only molecules that have complementry shape can 'fit' the receptor site - molecule attatches on one side -carrier protein changes shape - releases molecule onto other side of cell. 

Facillitated diffusion: Large molecules and charged ions pass through the phospholipid bilayer with the help of CHANNEL PROTEINS.

Channel proteins are lined with charged particles, which help the passage of ions such as Na, K, HCO and Cl. 

This type of transport DOES NOT REQUIRE ENERGY. 

Diffusion: net movement of substances from a region of higher concentration to region of lower concentration. 

It is a passive process because again, it DOES NOT REQUIRE ENERGY.

Small, lipid-soluble molecules can diffuse accross the phospholipid bilayer.

This is the movement of water from a region of higher water potential to a region of lower water potential across a selectively permeable membrane. 

It is basically:"the tendancy for a solution to gain or lose water."

ONLY WATER MOLECULES USE OSMOSIS.

A solution with a lower water potential than the cell is called HYPERTONIC.

A solution with a higher water potential than the cell is called HYPOTONIC. 

WITHIN: 1) surrounds organelles, 2) prevents disruption of reactions, 3) reactions take place on membranes (photosynthesis on grana membrane) 4) helps with the attatchment of ribosomes on the RER, 5) increases the surface area of the organelle.

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SURFACE: 1) seperates cell from the environment, 2) controls entry and exit of molecules (selectively permeable) 3) allows active transport, facillitated diffusion, diffusion and osmosis, 4) phagocytosis and exocytosis, 5) increases the surface area of the cell.