TRANSPORT PROTEINS
• Make lipid bilayer permeable to hydrophilic
substances: ions and polar molecules
• SPAN entire membrane – “transmembrane”
• TWO general classes
– Channel proteins
– Carrier proteins
Channel Proteins
• A hydrophilic “tunnel”
• Example: aquaporins
– 3 billion water molecules per
second per aquaporin
– Water molecules move single
file/channel holds 10
– Tremendous increase in rate!
Carrier Proteins
• Hold on to molecules/ions;
change conformation; shuttle
across
• Specific for substance translocated
• Ex. glucose transporter; sodium
potassium pump
This is a receptor
protein
• The signal DOES NOT
come in
• There is NO channel
• There is a binding site for
the signal.
• The message is
transmitted to the inside
of the cell but THE
MESSENGER DOES NOT
COME IN
Selective Permeability
• What determines direction of transport?
• What determines whether a substance will
enter or leave the cell?
• To answer these questions we need to
understand 2 modes of transport:
– Passive transport
– Active transport
PASSIVE TRANSPORT
ACTIVE TRANSPORT
Solutes move DOWN the
concentration gradient
Solutes move AGAINST the
concentration gradient
Does not require cell energy –
uses the potential energy of the
chemical gradient
Requires cell energy – ATP directly
or indirectly (cotransport)
Solutes move between
phospholipids or through
transport protein (channels or
carriers)
Solutes bind carriers and carrier
changes conformation and
translocates solute across the
membrane
Passive Transport
• Passive transport - diffusion of a substance
across a biological membrane.
• Diffusion - net movement of molecules from high
concentration to low concentration (down the
concentration gradient)
• Results from kinetic energy of molecules. Does
not require energy input.
• Diffusion continues until a dynamic equilibrium is
reached (no net directional movement)
• Diffusion – movement of molecules of a substance so that they
spread out evenly into the available space
• Why do molecules move?
• Is the movement here random or directional (net movement)?
Explain.
• Describe the movement of the molecules at equilibrium.
• Is the movement in the pictures “down the concentration gradient”
or “against the concentration gradient”?
• What is the free energy change for this process?
• What is different in this picture?
• Where is the concentration gradient highest initially?
• Does the diffusion to the purple dye influence the diffusion of the
yellow dye?
• Would you describe the membrane between the two compartments
as permeable to the dye or selectively permeable to the dye?
Osmosis
• Osmosis - passive transport (diffusion!) of water
across a selectively permeable membrane.
• What is the role of water in cells?
• Biological fluids are usually dilute solutions.
• Some water within the cell is not “free” to move
down it’s concentration gradient. Why?
• Cell membranes are not barriers to diffusion of
water.
– Aquaporins
Osmosis
• Relative concentration terms:
a) Hypertonic - solution with greater solute
concentration than inside the cell.
b) Hypotonic - solution with lower solute
concentration than inside the cell.
c) Isotonic - solution with equal solute
concentration compared to that inside a cell.
• The prefixes refer to solute concentration.
What would the water concentration be in each
example above?
• What are some units that could be used to
express the amount of solute in an aqueous
solution?
HYPO = LESS HYPER = MORE ISO = EQUAL
•
TIP!
WATER FOLLOWS SOLUTES
WHAT ARE THESE THINGS ANYWAY?
Movement of Water
• If two solutions are separated by a selectively
permeable membrane that is permeable to
water but not to solute, water will diffuse
from the hypoosmotic solution to the
hyperoosmotic solution.
• “Water moves to DILUTE!”
• Direction of osmosis is determined by the
difference in total solute concentration,
regardless of the types of solutes in the
solutions.
• At equilibrium, water molecules move in
both directions at the same rate. (True for
isotonic solutions also)
Cell Survival
• Balance of water between the cell and its
environment are crucial to organisms.
• Osmoregulation - control of water balance.
• Animal cells (no cell walls) have adaptations for
osmoregulation. Ex) Paramecium have
contractile vacuoles
• Plants can take in water and become turgid
(firm), while in isotonic solutions cells become
flaccid (limp). In hypertonic solutions, the
plant cell shrivels and plasmolysis occurs.
•
cytolysis
plasmolysis
Facilitated Diffusion
• Diffusion of polar molecules and ions across a
membrane with the aid of transport proteins.
• Proteins have a specialized binding site for
the solute they transport.
• Some proteins have gated channels that open
or close in response to a stimulus.
Facilitated Diffusion
Active Transport
• Pumping of solutes against the concentration
gradient, requiring energy from the cell.
• Sodium-potassium pump allows cells to
exchange Na+ and K+ across animal cell
membranes.
•
•
Ion Pumps
• Because anions and cations are unequally
distributed across plasma membranes, all
cells have voltages across their plasma
membrane (membrane potential).
• Forces that drive passive transport of ions
across membranes include: concentration
gradient of the ion (chemical force), and
effect of membrane potential (electrical force)
on the ion.
• The combination of these forces is called the
electrochemical gradient.
VOLTAGE OCCURS WHEN ELECTICAL CHARGES ARE SEPARATED.
Electrogenic Pumps
• Transport proteins that generate voltage across a
membrane.
• Na+/K+ ATPase is the major electrogenic
pump in animal cells
• A proton pump (H+) is the major electrogenic
pump in plants, bacteria, and fungi.
• Mitochondria and chloroplasts use a proton
pump to drive ATP synthesis.
• Voltages created by electrogenic pumps are
sources of potential energy available to do
cellular work.
•
Cotransport
• Process where a single ATP-powered pump
actively transports one solute and indirectly
drives the transport of other solutes against
their concentration gradients.
• Example: Plants use the mechanism of
sucrose/H+ cotransport to load sucrose
produced by photosynthesis into specialized
cells in the veins of leaves. Transport proteins
can move sucrose into the cell against the
concentration gradient only if it travels with
the H+ ion.
•
Movement of Large Molecules or Cells
• Large molecules such as proteins or
polysaccharides and cells cross
membranes by the processes of
endocytosis and exocytosis.
• Material to be transported is placed in
membrane-bound sacs
• Formation and transport of these vesicles
requires energy
Exocytosis
• Process of exporting macromolecules from a cell
by fusion of vesicles with the plasma membrane.
• Vesicle usually budded from the ER or Golgi and
migrates to plasma membrane
• Used by secretory cells to export products, such
as insulin in the pancreas or neurotransmitters
from neurons.
Endocytosis
• Process of importing macromolecules into
a cell forming vesicles derived from the
plasma membrane.
• Vesicle forms from a localized region of
plasma membrane that sinks inward,
pinches off into the cytoplasm.
• Used by cells to incorporate extracellular
substances.
Types of Endocytosis: Phagocytosis
• Endocytosis of solid particles.
• Cell engulfs particle with pseudopodia,
and pinches off a food vacuole.
• Vacuole fuses with a lysosome containing
hydrolytic (digestive) enzymes that break
down the particle.
• Ex. unicellular protists like amoeba and
some immune system cells
Phagocytosis of a
bacterial cell by an
amoeba
Types of Endocytosis: Pinocytosis
• Endocytosis of fluid droplets
• Extracellular fluid is engulfed in small
vesicles
• Nonspecific in the substances it
transports. Cell takes in all solutes
dissolved in the droplet.
Pinocytosis of
fluid into cell
lining a blood
vessel
Types of Endocytosis:
Receptor-mediated Endocytosis
• Very specific – molecule to be transported
(ligand) must bind to receptor of cell surface
• After ingested material is released from the
vesicle for metabolism, the receptors are
recycled to the plasma membrane.
• Ex. uptake of cholesterol
•
Receptor-mediated endocytosis