3. Diffusion
Diffusion – movement of molecules
from an area of higher concentration to
an area of lower concentration
◦ Concentration Gradient – difference in
the concentration of molecules across a
distance
◦ Molecules tend to move from where they are
more concentrated to where they are less
concentrated (“down” their concentration
gradient)
4. Diffusion
Diffusion is driven entirely by the molecules’
kinetic energy
◦ Molecules are in constant motion
Diffusion will eventually cause the molecules
to be in equilibrium – the concentration of
molecules will be the same throughout the
space the molecules occupy
◦ At equilibrium, molecules continue to move
How Diffusion Works
6. Diffusion Across Membranes
Diffusion Across Membranes
◦ Simple diffusion – diffusion across a cell
membrane
◦ Depends on:
Size of molecule
Type of molecule
Chemical nature of the membrane
Diffusion Across Membranes
8. Direction of Osmosis
Hypotonic solution – concentration of
solute outside the cell is lower than the
concentration inside the cell
◦ Water diffuses into the cell
9. Direction of Osmosis
Hypertonic solution – concentration of
solute outside the cell is higher than the
concentration inside the cell
◦ Water diffuses out of the cell
10. Direction of Osmosis
Isotonic solution – concentration of solutes
is equal outside and inside of the cell
◦ Water diffuses into and out of the cell at
equal rates
12. How Cells Deal With Osmosis
Contractilevacuoles –
organelles that
remove water
Solute pumps – pump
solutes out of the
cell
13. How Cells Deal With Osmosis
Cellwall in plants resists the pressure exerted by
water inside of a cell
◦ Turgor pressure – pressure that water molecules exert
against a cell wall
◦ Plasmolysis – cells shrink away from cell walls and turgor
pressure is lost
14. How Cells Deal With Osmosis
Some cells cannot compensate for changes
in solute concentration
◦ Ex: red blood cells can swell and burst (cytolysis)
15. Facilitated Diffusion
For molecules that cannot diffuse across
cell membranes, even when there is a
concentration gradient
Movement of molecules is assisted by
carrier proteins
17. Important Properties of Facilitated
Diffusion
Can help substances move either into or
out of the cell, depending on the
concentration gradient
Carrier proteins involved in facilitated
diffusion are each specific for one type of
molecule
18. Diffusion Through Ion Channels
◦ Ion channels – transport ions such as
sodium (Na+), potassium (K+), calcium (Ca2+),
and chloride (Cl-)
◦ Some ion channels are always open
◦ Some have “gates” that open and close in
response to:
Stretching of the cell membrane
Electrical signals
Chemicals in the cell or external environment
20. Active Transport
Movement of materials from lower
concentration to higher concentration
(“up” their concentration gradient)
Requires energy from the cell
23. Steps of the Sodium-Potassium
Pump
1. Three Na+
ions from the
inside of the
cell bind to
the carrier
protein
24. Steps of the Sodium-Potassium
Pump
2. A phosphate
group is
removed from
ATP and bound
to the carrier
protein
25. Steps of the Sodium-Potassium
Pump
3. The carrier
protein changes
shape, allowing
three Na+ ions to
be released to
the outside of
the cell
26. Steps of the Sodium-Potassium
Pump
4. Two K+ ions
from the
outside of the
cell bind to
the carrier
protein
27. Steps of the Sodium-Potassium
Pump
5. The phosphate
group is
released and
the carrier
protein goes
back to its
original shape
28. Steps of the Sodium-Potassium
Pump
6. The two K+ ions
are released to
the inside of the
cell and the
cycle is ready to
repeat
29. Importance of the Sodium-
Potassium Pump
The ion exchange creates an electrical
gradient across the cell membrane
◦ Outside becomes positively charged
◦ Inside becomes negatively charged
This difference in charge is important for
the conduction of electrical impulses
along nerve cells
30. Movement in Vesicles
Used for:
◦ Substances that are too large to pass through
the cell membrane
◦ Transporting large amounts of small
molecules into or out of cells at the same
time
32. Endocytosis
Process in which cells ingest external
fluid, macromolecules, and large particles,
including other cells
Two types of endocytosis:
◦ Pinocytosis – transport of solutes or fluids
◦ Phagocytosis – transport of large particles or
whole cells
Endocytosis
34. Exocytosis
Process by which a substance is released from
the cell through a vesicle that transports the
substance to the cell surface and then fuses
with the membrane to let the substance out of
the cell
Exocytosis
