Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration. Heat provides the energy for molecules to move randomly via Brownian motion. While individual molecules cannot be seen moving, diffusion results in a net movement down the concentration gradient. Diffusion continues until equilibrium is reached and the concentration gradient is eliminated. In biological systems, concentration gradients of solutes and water drive diffusion across cell membranes.

1. Diffusion and Osmosis
Passive Movement into and out of cells
H2O
Solutes
Cell
Membrane
Membranes
1

2. Molecules can move long distances
2

3. Solutes
Solutes
Cell Membrane
Molecules diffuse in and out of cells.
3

4. Diffusion is spontaneous and passive
DIFFUSION
H2O
Energy is always required for movement in and
out
Active transport uses energy from the cell in the
form of ATP and facilitating proteins.
Passive transport does not require energy from the
cell. It is powered by spontaneous molecular
movement.
4

5. Why do molecules move at all?
Where do they get their energy?
DIFFUSION
H2O
Heat causes random movement of
molecules.
H2O
5

6. Brownian Movement … the movement of small
particles in response to collision with water
molecules.
Can we see molecules move?
Molecular Movement
H2O
The answer is no.
H2O
6

7. Defining Diffusion
Working Definition 1
Diffusion is … the net movement of molecules
from an area of high concentration, pressure,
and/or temperature to an area of low
concentration, pressure, and/or temperature.
H2OH2O
7

8. Defining Diffusion
Temperature + Concentration + Pressure = Free Energy
All three factors combine as the Free Energy of
molecules.
Molecules diffuse from high free energy to low
free energy.
H2OH2O
8

9. Defining Diffusion
Working Definition 2
Diffusion is … the net movement of molecules
from an area of high free energy to an area of
low free energy.
H2O
Final Definition of Diffusion
Diffusion is … the net movement of molecules
from an area of high concentration to an area of
low concentration.
H2O
9

10. Defining Diffusion
Final Definition of Diffusion
The NET movement of molecules from an area of
high concentration to an area of low concentration.
H2OH2O
10

11. Left Right
High Concentration Low Concentration
DIFFUSION
Concentration Gradient
11

12. High Concentration Low Concentration
DIFFUSION
Net Movement
12

13. Left Right
Equilibrium Concentrations
Diffusion Stops at Equilibrium
13

14. Concentration Gradient
Concentration Gradient
14

15. Concentration Gradient
Concentration Gradient
Diffusion
Diffusion
15

16. Equilibrium of and
Diffusion Stops at Equilibrium
16

17. H2O
Solutes
Cell
Membrane
• Diffusion is driven by heat.
• Diffusion is the net
movement of molecules
from an area of high free
energy to an area of low free
energy.
• Free energy gradients in
biological systems are usually
determined by concentration
differences.
• When diffusion of molecules
eliminates the concentration
gradient, then equilibrium is
achieved … diffusion stops.
In Summary:
17

18. 18
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Slide 1, 2-10, 17: Peter Raven, Biology, 9e © 2011 reproduced with permission of McGraw-Hill Education; Sylvia Mader, Biology 10e © 2010
reproduced with permission of McGraw-Hill Education