DIFFUSION, OSMOSIS AND
CELLULAR TRANSPORT
• Explain what you see in the following
pictures:
Directional Movement of Water
Across Membranes
• Osmosis
– Diffusion of water due to a water concentration
gradient between two regions that are separated
by a selectively permeable membrane
• Osmotic movement
– Dependent on concentration of solutes in the
water
– Side with more solutes has a lower concentration
of water
Osmosis
NET WATER MOVEMENT
Effect of solute concentration on water movement
Effects of Tonicity
• The solute concentrations of
two fluids
– Hypotonic: fewer solutes
• Water diffuses in
• Cell swells
– Hypertonic: more solutes
• Water diffuses out
• Cell shrinks
– Isotonic
• No net change
2M sucrose
solution
1 liter of
distilled water
10M sucrose
solution
2M sucrose
solution
Fig. 5.14, p. 88
HYPOTONIC
CONDITIONS
HYPERTONIC
CONDITIONS
ISOTONIC
CONDITIONS
compartment
1
compartment
2
membrane permeable to
water but not to solutes
fluid volume increases
In compartment 2
Fig. 5.15, p. 89
when salt water
is added to plant
cells, water
moves
out of the cell due
to the increase of
solutes outside of
the cell
(hypertonic)
Fig. 5.16, p. 89
Effects of Fluid Pressure
• Volume of fluid exerts hydrostatic pressure
– Force against a wall or membrane
Diffusion Through the Plasma Membrane
Extracellular fluid
Lipidsoluble
solutes
Lipid-insoluble
solutes
Small lipidinsoluble
solutes
Water
molecules
Lipid
bilayer
Cytoplasm
(a) Simple diffusion
directly through the
phospholipid bilayer
(b) Carrier-mediated facilitated
diffusion via protein carrier
specific for one chemical; binding
of substrate causes shape change
in transport protein
(c) Channel-mediated
facilitated diffusion
through a channel
protein; mostly ions
selected on basis of
size and charge
(d) Osmosis, diffusion
through a specific
channel protein
(aquaporin) or
through the lipid
bilayer
Figure 3.7
Active Transport
• ATP required
• Movement is
against the
concentration
gradient
– Sodium-potassium
pump
– Calcium pump
Exocytosis and Endocytosis
• Exocytosis
– Vesicle moves to cell
surface and fuses
with plasma
membrane
• Endocytosis
– Substances move in
when plasma
membrane balloons
inward
concentration gradient
high
P energy
input
low
DIFFUSION ACROSS
LIPID BILAYERS
lipid-soluble
substances as
well as water
diffuse across
PASSIVE
TRANSPORT
Water-soluble substances, and water,
diffuse through interior of transport
proteins. No energy boost required.
Also called facilitated diffusion
ACTIVE
TRANSPORT
Specific solutes are
pumped through interior
of transport proteins.
Requires energy boost
Fig. 5.9a, p. 85
Passive Membrane Transport:
Diffusion
• Simple diffusion – nonpolar and lipidsoluble substances
– Diffuse directly through the lipid bilayer
– Diffuse through channel proteins
Passive Membrane Transport:
Diffusion
• Facilitated diffusion
– Transport of glucose, amino acids, and ions
– Transported substances bind carrier proteins
or pass through protein channels
Carrier Proteins
• Are integral transmembrane proteins
• Show specificity for certain polar molecules
including sugars and amino acids
Passive Transport
• Solute transport
through transport
protein
• Movement
– From higher to lower
concentration
• No ATP required
Passive Membrane Transport –
Review
Process
Energy Source
Example
Simple diffusion
Kinetic energy
Movement of O2 through
membrane
Facilitated diffusion
Kinetic energy
Movement of glucose into cells
Osmosis
Kinetic energy
Movement of H2O in & out of cells
Filtration
Hydrostatic pressure
Formation of kidney filtrate
Active Membrane Transport –
Review
Process
Energy Source
Example
Active transport of solutes
ATP
Movement of ions across
membranes
Exocytosis
ATP
Neurotransmitter secretion
Endocytosis
ATP
White blood cell
phagocytosis
Fluid-phase endocytosis
ATP
Absorption by intestinal cells
Receptor-mediated
endocytosis
ATP
Hormone and cholesterol
uptake
Endocytosis via caveoli
ATP
Cholesterol regulation
Endocytosis via coatomer
vesicles
ATP
Intracellular trafficking of
molecules
Membrane Potential
• Voltage across a membrane
• Resting membrane potential – the point
where K+ potential is balanced by the
membrane potential
– Ranges from –20 to –200 mV
– Results from Na+ and K+ concentration
gradients across the membrane
– Differential permeability of the plasma
membrane to Na+ and K+
• Steady state – potential maintained by
active transport of ions
Generation and Maintenance of
Membrane Potential
PLAY
InterActive Physiology ®:
Nervous System I: The Membrane Potential
Figure 3.15