Passive Transport
Unit 4, Lesson 2
Passive Transport
 Passive Transport is the movement of molecules across a
membrane that does not require energy
 No energy is required because molecules are moving down a
concentration gradient from areas of higher concentration to
areas of lower concentration
 Types of passive transport:
 Diffusion
 Facilitated Diffusion
 Osmosis
Diffusion
 The net movement of particles from an area
of high concentration to an area of low
concentration
 Diffusion results from the random
movement of particles
 http://highered.mheducation.com/sites/9834092339/stud
ent_view0/chapter38/how_diffusion_works.html
Concentration Gradient
 The difference in the concentration of a substance over
distance
 The bigger the difference between “high” and “low” areas of
concentration, the steeper the gradient
Results of Diffusion
 As substances continue to diffuse, they will
eventually become evenly distributed.
 This is called dynamic equilibrium
 Molecules will continue to move across a membrane
in both directions, since there is no “high” or “low”
anymore
Facilitated Diffusion
 Movement of molecules down a concentration gradient from
areas of HIGH concentration to areas of LOW concentration
with the help of transport proteins.
Facilitated Diffusion
 Still passive transport because no energy is required (still
moving from high concentration to low concentration)
 Larger molecules or charged molecules move via facilitated
diffusion
 They can’t slip through the lipid bilayer directly, but they can
go through these transport proteins
 Channel protein – like tunnels
 Carrier proteins – bind to molecules and carry them across
 http://highered.mheducation.com/sites/9834092339/student_v
iew0/chapter38/how_facilitated_diffusion_works.html
OSMOSIS
 Diffusion of water across a semipermeable
membrane is called osmosis
 Water will move towards the areas where there
are more solutes (and therefore less water)
 Water flow helps maintain homeostasis
Osmosis
 Water moves TOWARDS the solutes
 Solutes = dissolved particles like salts and sugars
Osmosis
 http://highered.mheducation.com/sites/9834092339/stud
ent_view0/chapter38/how_osmosis_works.html
Types of Solutions
 Isotonic = balanced concentration inside and outside the cell
 Water will move in and out equally
 Hypertonic =greater concentration of solutes outside the
cell
 Water will move out of the cell (toward the solutes
outside)
 Hypotonic = lower concentration of solutes outside the cell
 Water will move into the cell (toward the solutes inside)
WATER MOVES TOWARDS THE SALTS/SUGARS
Red Blood
Cells
Plant
cells
Water
moves
towards
the
solutes
Types of Solutions
 http://highered.mheducation.com/sites/9834092339/stud
ent_view0/chapter38/animation_-_osmosis.html