Transporting molecules
across cell membranes
Nutrients, wastes, hormones,
ions
Recall phospholipid bilayer
• It’s a barrier!
• Only a small subset of molecules may pass
through without help
Cell membrane
• Sense & respond to changes in environment
• Regulate exchange with environment
Cell membranes are
selectively permeable
•
Some compounds pass uninhibited through
membrane (passive diffusion), some require
assistance from membrane proteins (facilitated
diffusion), and some require assistance AND
energy expenditure (active transport)
1. Diffusion
–
–
Passive transport or diffusion
Carrier-mediated (facilitated) transport or diffusion
–
Pumps, bulk transport
2. Active Transport
PM + proteins mediate transport
Passive (Diffusion & Osmosis)…or Active
Simple
diffusion; P
Carriermediated; A
or P
Channelmediated; P
Osmosis; P
Proteins are responsible
• Some are receptors that regulate bulk
transport
– Induce endocytosis or exocytosis
• Cyto from “kyto” = hollow vessel
Concept Check
•
Membranes organize cell activities. The proteins
imbedded in the membranes are essential to their
function. These membrane proteins have
properties that allow them to “float” in the
membrane. Which of the following describe
those properties?
a) The surface region of the protein in the interior
of the membrane is mostly hydrophobic.
b) The surface region of the protein in the interior
of the membrane is mostly hydrophilic.
c) The surface region exposed to the exterior
environment is hydrophobic.
d) The surface region exposed to the interior
environment is hydrophobic.
Answer
•
Membranes organize cell activities.
The proteins imbedded in the
membranes are essential to their
function. These membrane proteins
have properties that allow them to
“float” in the membrane. Which of the
following describe those properties?
a) The surface region of the protein in the
interior of the membrane is mostly
hydrophobic.
So, what is diffusion?
• Solvent: The predominant liquid or gas in
a solution
• Solute: The stuff that is dissolved in a
solution
• Diffusion: The net movement of solute
from a higher to a lower concentration
(Concentration gradient), until
equilibrium is achieved. Uses intrinsic
Kinetic Energy (KE).
Everything has Energy
• Energy = The capacity to perform work;
to rearrange matter
• 2 forms:
– Potential Energy (PE): stored energy, due to
position or structure
– Kinetic Energy (KE): Energy of motion
• Heat is KE associated with the movement of
molecules/atoms
Passive Diffusion
Passive Diffusion
Passive diffusion
• Kinetic energy
causes particles
to move
• Diffusion occurs
due to random
collisions
between these
moving particles
Osmosis
• Passive diffusion of H20
down its concentration
gradient
– Due to solutes inside
flask, there’s more H20
outside than in
– Only H20 can pass
through pores
– H20 moves down its
concentration gradient
and INTO flask
Osmosis + Diffusion
• Both are happening all the time across
cell membranes
• Osmosis (H20) occurs RAPIDLY,
diffusion (solutes) occurs SLOWLY
• H20 moves into cells with high solute
concentration and out of cells with low
solute concentration
What happens?
•
If you place a cell into a solution whose
solute concentration is much lower
than that of the cell, what will happen
to the cell? Will it:
1. Shrink (crenate; as H20 leaves)
2. Swell (as H20 enters)
3. Remain unchanged
•
Concept Check
This diagram represents osmosis
of water across a semipermeable
membrane. The U-tube on the
right shows the results of the
osmosis. What could you do to
level the solutions in the two
sides of the right hand U-tube?
a)
Add more
side.
b) Add more
side.
c) Add more
side.
d) Add more
side.
water to the left hand
water to the right hand
solute to the left hand
solute to the right hand
Answer
•
This diagram represents osmosis
of water across a semipermeable
membrane. The U-tube on the
right shows the results of the
osmosis. What could you do to
level the solutions in the two
sides of the right hand U-tube?
c)
Add more solute to the left hand
side.
Tonicity
• Tonicity: measure of the solvent concentration
of a solution and it’s osmotic pressure
• Hypertonic: Solute concentration is greater
than the cytoplasm = net movement of H2O out
of cell
• Hypotonic: Solute concentration of solution is
less than the cytoplasm = net movement into
cell
• Isotonic: Solute concentration of solution is
equal to the cytoplasm = no net movement of
H20
Cells in different solutions
Facilitated diffusion
• Diffusion can be simple
or facilitated
• Facilitated is Passive
– No energy needed to
move molecules.
– Molecules move down
their concentration
gradient
Active transport
1. requires ATP (energy) to move
molecules.
2. Molecules move against their
concentration gradient.
Maintains gradient for
cotransport
Endo- & Exocytosis
• Bulk transport; A
• Requires ATP
– Phagocytosis: ingestion of solid particles
• “phagein” = to eat
– Pinocytosis: ingestion of liquid
Endocytosis
Active; Requires ATP
Exocytosis
Active; Requires ATP