ch. 8: transport across the cell membrane · 7.5 - bulk transport across the plasma membrane occurs...
TRANSCRIPT
NOTES: CH 7 part 2 -
Transport Across the Cell
Membrane (7.3-7.5)
The Permeability of the Lipid Bilayer
● Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly
● Polar molecules, such as sugars, do not cross the membrane easily
Transport proteins:
● membrane proteins that transport specific
molecules or ions across biological
membranes:
-may provide hydrophilic tunnel thru
membrane
-may bind to a substance and physically
move it across the membrane
-are specific for the substance they move
GLUCOSE
Binding
TransportRecovery
Dissociation
Movement across the cell membrane can be:
1) PASSIVE
● cell does not have
to expend energy
2) ACTIVE
● energy-requiring
process during which a
transport protein pumps a
molecule across a
membrane, against its
conc. gradient; is
energetically “uphill”
7.3 - Passive Transport:
DIFFUSION
● net movement of a substance down a concentration gradient
-results from KE of molecules
-results from random molecular movement
-continues until equilibrium is reached (molecules continue to move but there is no net directional movement)
Molecules of dye Membrane (cross section)
WATER
Net diffusion Net diffusion Equilibrium
Diffusion of one solute
Net diffusion Net diffusion Equilibrium
Diffusion of two solutes
Net diffusion Net diffusion Equilibrium
7.3 - Passive Transport:
OSMOSIS
● diffusion of water across a selectively
permeable membrane; water moves down
its concentration gradient
-continues until equil. is reached
-at equil. water molecules
move in both directions
at same rate
INSIDE
THE CELL
OUTSIDE
THE CELL
Effects of Osmosis on Water Balance
● The direction of osmosis is determined
only by a difference in total solute
concentration
● Water diffuses across a membrane from the region of lower solute concentrationto the region of higher solute concentration
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
Selectively
permeable mem-
brane: sugar mole-
cules cannot pass
through pores, but
water molecules can
H2O
Osmosis
Water Balance of Cells Without Walls
● Isotonic solution: solute concentration is the
same as that inside the cell; no net water
movement across the plasma membrane
● Hypertonic solution: solute concentration is
greater than that inside the cell; cell loses
water
● Hypotonic solution: solute concentration is
less than that inside the cell; cell gains water
WATER MOVES FROM HYPO TO HYPERTONIC!!!
● Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment
● To maintain their internal environment, such organisms must have adaptations for osmoregulation, the control of water balance
● The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
Filling vacuole50 µm
50 µmContracting vacuole
Water Balance of Cells with Walls
● Cell walls help maintain water balance
● A plant cell in a hypotonic solution swells until the
wall opposes uptake; the cell is now turgid (firm)
● If a plant cell and its surroundings are isotonic,
there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt
● In a hypertonic environment, plant cells lose
water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
RECAP: In cells with cell walls:
● in a HYPERTONIC environment,
PLASMOLYSIS occurs; cells shrivel
and usually die
● in a HYPOTONIC environment, water
moves into cell, causing it to swell; cell
becomes more TURGID.
Animalcell
Lysed
H2O H2O H2O
Normal
Hypotonic solution Isotonic solution Hypertonic solution
H2O
Shriveled
H2OH2OH2OH2OPlantcell
Turgid (normal) Flaccid Plasmolyzed
7.3 - Passive Transport:
FACILITATED DIFFUSION
● diffusion of solutes across a membrane, with
the help of transport proteins;
Facilitated Diffusion:
Passive Transport Aided by Proteins
● Channel proteins provide corridors that
allow a specific molecule or ion to cross
the membrane
● Carrier proteins undergo a subtle
change in shape that translocates the
solute-binding site across the membrane
EXTRACELLULAR
FLUID
Channel protein Solute
CYTOPLASM
Carrier protein Solute
7.4 - Active transport uses energy to
move solutes against their gradients
● Facilitated diffusion is still passive because the solute moves down its concentration gradient
● Some transport proteins, however, can move solutes against their concentration gradients
The Need for Energy in Active Transport
● Active transport moves substances against
their concentration gradient
● Active transport requires energy, usually in the
form of ATP
● Active transport is
performed by specific
proteins embedded
in the membranes
Diffusion Facilitated diffusion
Passive transport
ATP
Active transport
Examples of Active Transport protein
“pumps”:
1) Sodium-Potassium Pump:
-actively pumps Na+ ions out / K+ ions in
-in every pump cycle, 3 Na+ leave and 2
K+ enter cell
-Na+ and K+ are moved against their
gradients (both concentration and electric
potential!)
Cytoplasmic Na+ bonds to
the sodium-potassium pump
CYTOPLASMNa+
[Na+] low
[K+] high
Na+
Na+
EXTRACELLULAR
FLUID[Na+] high
[K+] low
Na+
Na+
Na+
ATP
ADP
P
Na+ binding stimulates
phosphorylation by ATP.
Na+
Na+
Na+
Phosphorylation causes
the protein to change its
conformation, expelling Na+
to the outside.
P
Extracellular K+ binds
to the protein, triggering
release of the phosphate
group.
PP
Loss of the phosphate
restores the protein’s
original conformation.
K+ is released and Na+
sites are receptive again;
the cycle repeats.
OUTSIDE
INSIDE
Maintenance of Membrane Potential
by Ion Pumps
● Membrane potential is the voltage
difference across a membrane
● Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane:
-A chemical force (the ion’s concentration gradient)
-An electrical force (the effect of the membrane potential on the ion’s movement)
● Membrane Potential: voltage across
membrane; in most cells the interior is
negatively charged w/respect to outside
-favors diffusion of cations into
cell and anions out of cell
● Electrochemical Gradient: diffusion
gradient resulting from the combined
effects of membrane potential and conc.
gradient
**The Na+-K+ pump maintains the
membrane potential…HOW?**
● An electrogenic pump is a transport
protein that generates the voltage across
a membrane
● The main electrogenic pump of plants,
fungi, and bacteria is a PROTON PUMP.
ELECTROGENIC PUMPS:
2) Proton Pump: pumps protons (H+ ions) out
of the cell, creating a proton gradient (protons
are more concentrated outside the
membrane than inside)…this is an
energetically “uphill” process!
-protons then diffuse back into cell
-the force of the proton pushing back through
the membrane is used to power the
production of ATP
Examples of Active Transport protein
“pumps”:
H+
ATP
CYTOPLASM
EXTRACELLULAR
FLUID
Proton pump
H+
H+
H+
H+
H+
+
+
+
+
+
–
–
–
–
–
3) Cotransport / Coupled Channels:
process where a single ATP-powered pump
actively transports one solute and indirectly
drives the transport of other solutes against
their conc. gradients.
-Example: plants use a proton pump
coupled with sucrose-H+ transport to
load sucrose into specialized cells
H+
ATP
Proton pump
Sucrose-H+
cotransporter
Diffusion
of H+
Sucrose
H+
H+
H+
H+
H+
H+
+
+
+
+
+
+
–
–
–
–
–
–
7.5 - Bulk transport across the plasma
membrane occurs by exocytosis and
endocytosis
● Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins
● Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles
BULK TRANSPORT: EXOCYTOSIS & ENDOCYTOSIS
● transport of large molecules (e.g.
proteins and polysaccharides) across cell
membrane
Exocytosis Endocytosis
*exporting macromolecules
by fusion of vesicles w/the
plasma membrane
*vesicle buds from ER or
Golgi and migrates to plasma
membrane
*used by secretory cells to
export products (e.g. insulin in
pancreas)
Exocytosis Endocytosis
*exporting macromolecules
by fusion of vesicles w/the
plasma membrane
*vesicle buds from ER or
Golgi and migrates to plasma
membrane
*used by secretory cells to
export products (e.g. insulin in
pancreas)
*importing macromolecules
by forming vesicles derived
from plasma membrane
*vesicle forms in localized
region of plasma membrane
*used by cells to incorporate
extracellular substances (e.g.
macrophage engulfs a
bacterial cell)
EXOCYTOSIS
● In exocytosis, transport vesicles
migrate to the membrane, fuse with it,
and release their contents
ENDOCYTOSIS
● In endocytosis, the cell takes in
macromolecules by forming vesicles from
the plasma membrane
● Endocytosis is a reversal of exocytosis, involving different proteins
Three types of Endocytosis:
1) Phagocytosis: part of the cell membrane engulfs
large particles or even entire cells (“cell eating”)
Three types of Endocytosis:
2) Pinocytosis: part of the cell
membrane engulfs small
dissolved substances or fluid
droplets in vesicles (“cell
drinking”)
Three types of Endocytosis:
3) Receptor-Mediated Endocytosis: importing of specific
macromolecules by receptor proteins bind to a specific
substance which triggers the inward budding of vesicles
formed from COATED PITS (how mammalian cells take up
cholesterol)
Receptor
RECEPTOR-MEDIATED ENDOCYTOSIS
Ligand
Coated
pit
Coated
vesicle
Coat protein
Coat
protein
Plasma
membrane
0.25 µm
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs).