Diffusion &
Osmosis
Define Diffusion
The movement of molecules from a
area in which they are highly
concentrated to a area in which they
are less concentrated.
Particles naturally travel from an area where they
are crowded to an area where they are less crowded.
http://lewis.eeb.uconn.edu/lewishom
e/applets/Diffusion/diffusion.html
An example of diffusion from class.
Diffusion of Water
 All
organisms need
water to live.
 The cells of living
organisms are
surrounded by and
filled with fluids that
are mostly water.
An amoeba
(image courtesy of BIODIDAC)
Cell
Membrane
 All
cells are surrounded by a cell
membrane made of phospholipids.
 This membrane acts as a barrier
between the inside of the cell and
the cell’s environment.
 It also controls the passage of
materials into and out of the cell.
Selectively Permeable Membrane
a membrane that permits the
passage of only certain molecules
•Many particles can slip through the
pores in the membrane because of
their small size.
•Not all particles that a cell needs can
pass through the membrane in this
way.
Define osmosis
The diffusion of
water across a
selectively
permeable
membrane.
Osmosis
Water
moves from a high
concentration of water (less
salt or sugar dissolved in it)
to a low concentration of
water (more salt or sugar
dissolved in it).
http://www.usd.edu/~bgoodman/Osmos.htm
Osmosis demo
http://www.usd.edu/~bgoodman/Osmos.htm
Osmosis in our body



In this picture a red blood cell is
put in a glass of distilled water (all
water with no salt or sugar in it).
Because there is a higher
concentration of water outside the
cell, water enters the cell by
OSMOSIS.
In this case too much water enters
and the cell swells to the point of
bursting open. In the end pieces
of cell membrane are left in the
water.
http://www.usd.edu/~bgoodman/Osmos.htm
See an animation of
osmosis here:
http://hydrodictyon.eeb.uconn.edu
/people/plewis/applets/Osmosis/os
mosis.html
Why are osmosis &
diffusion important?
Why are osmosis & diffusion
important?
All living things have certain requirements
they must satisfy in order to remain alive
– maintain homeostasis
 These include exchanging gases (usually
CO2 and O2), taking in water, minerals,
and food, and eliminating wastes.
 These tasks happen at the cellular level.
 Molecules move through the cell
membrane by diffusion

Passive Transport
 Passive
transport is the
movement of molecules
into or out of a cell by the
processes of diffusion and
osmosis (from an area of
high concentration to an
area of low concentration).
 Since the cell does no
work, the cell is passive.
Active Transport
 Active
transport is
when the cell moves
molecules against the
normal direction of
diffusion (from low
concentration to high
concentration).
 Since the cell must do
work and use energy
the cell is active.