Chapter 6
Page 118
Diffusion
 Diffusion is the movement of molecules from an area
of higher concentration to an area of lower
concentration
 Molecules move down their concentration gradient
Diffusion
 Diffusion is caused by random movement of molecules
 Given time, molecules will move randomly into all the
available space
 Diffusion is a spontaneous process, and requires no
energy input
 When equilibrium is reached, molecules continue to
move
 Dynamic Equilibrium
Diffusion
 Diffusion of two molecules can occur simultaneously
in one solution
 Molecules move down their concentration gradient
independently of one another
Rates of Diffusion
 Heat makes molecules move faster, so heating
something up will speed diffusion
 Currents can speed up the movement and distribution
of molecules
 Gases have the fastest rate of diffusion, then liquids,
then solids
 How Diffusion Works
Limits of Diffusion
 Molecules on the outside of a cell need to get to every
region within the cell. In many cases, molecules
cannot pass through every part of the membrane, so
diffusion takes a long time.
 Why can’t we (humans) be one giant cell?
 Diffusion limits cell size!
Osmosis
 Osmosis is a specific type of diffusion
 Osmosis is the diffusion of water through a selectively
permeable membrane (solutes cannot pass through
the membrane)
 Water moves down it’s own concentration gradient
 Osmosis Animation
In what direction will water move?
 Will water move into the
bag or out of the bag?
 *Hint- Water moves down
it’s own concentration
gradient. Where is there a
higher concentration of
water?
5% sucrose
20% sucrose
Osmosis
 A solution with a higher concentration of solute is
hypertonic to its surroundings.
 A solution with a lower concentration of solute is said
to be hypotonic to its surroundings.
 Solutions in which the concentration of solutes are
equal are isotonic to one another
Osmosis
Gummy Bear Mini-Lab
 In your lab groups, predict what will happen to gummy
bears if you let them sit in different types of solutions
until the next class
 Pure water
 Salt water
 Sugar water
 Mass your bears before and after they
soak