Diffusion
is the net movement of molecules from a region of high concentration to a region of low concentration down a concentration gradient
concentration gradient
the difference between the concentration of molecules in two places 
concentrated solution: has a large mass of solute dissolved in a given volume of solvent
Dilute solution: has a small mass of south dissolved in a given volume of solvent
concentration gradient
down the concentration gradient
movement of water molecules from the region of high water potential to lower water potential
against the concentration gradient
movement of water molecules from the region of lower water potential to higher water potential
Water potential : tendency of water to move from one are to another
Simple Diffusion
Cell membrane (semi-permeable) allow movement of small molecules - oxygen, carbon dioxide and water but not larger molecules
movement is passive movement. cells do not use energy to move molecules
the energy comes from random movement of molecules and ions
Factors that affect diffusion
distance molecules have to travel
concentration gradient
cell membranes are very thin
keep a low concentration gradient in the cell cytoplasm (keep diffusing in the cells)
the surface area
some folded to allow molecules to cross 9increase surface area)
Temperature
molecules move faster and collide more often as temperature increases
Size of molecule
small molecules diffuse taste than larger one
moisture
high moisture increases the rate of diffusion