NOTES: CH 7 part 2 - Transport Across the Cell Membrane (7.3-7.5)
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Transcript of NOTES: CH 7 part 2 - Transport Across the Cell Membrane (7.3-7.5)
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NOTES: CH 7 part 2 - Transport Across the Cell
Membrane (7.3-7.5)
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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
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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
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GLUCOSE
Binding
TransportRecovery
Dissociation
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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”
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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)
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Molecules of dye Membrane (cross section)
WATER
Net diffusion Net diffusion Equilibrium
Diffusion of one solute
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Net diffusion Net diffusion Equilibrium
Diffusion of two solutes
Net diffusion Net diffusion Equilibrium
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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
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INSIDE THE CELL
OUTSIDE THE CELL
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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 concentration to the region of higher solute concentration
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Lowerconcentrationof solute (sugar)
Higherconcentrationof sugar
Same concentrationof sugar
Selectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules can
H2O
Osmosis
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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!!!
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● 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
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Filling vacuole50 µm
50 µmContracting vacuole
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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
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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.
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Animalcell
Lysed
H2O H2O H2O
Normal
Hypotonic solution Isotonic solution Hypertonic solution
H2O
Shriveled
H2OH2OH2OH2OPlantcell
Turgid (normal) Flaccid Plasmolyzed
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7.3 - Passive Transport: FACILITATED DIFFUSION
● diffusion of solutes across a membrane, with the help of transport proteins;
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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
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EXTRACELLULARFLUID
Channel protein Solute
CYTOPLASM
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Carrier protein Solute
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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
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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
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Diffusion Facilitated diffusion
Passive transport
ATP
Active transport
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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!)
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Cytoplasmic Na+ bonds tothe sodium-potassium pump
CYTOPLASMNa+
[Na+] low[K+] high
Na+
Na+
EXTRACELLULARFLUID
[Na+] high[K+] low
Na+
Na+
Na+
ATP
ADP
P
Na+ binding stimulatesphosphorylation by ATP.
Na+
Na+
Na+
K+
Phosphorylation causesthe protein to change itsconformation, expelling Na+
to the outside.
P
Extracellular K+ bindsto the protein, triggeringrelease of the phosphategroup.
PP
Loss of the phosphaterestores the protein’soriginal conformation.
K+ is released and Na+
sites are receptive again;the cycle repeats.
K+
K+
K+
K+
K+
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OUTSIDE
INSIDE
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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)
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● 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
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**The Na+-K+ pump maintains the membrane potential…HOW?**
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● 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:
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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”:
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H+
ATP
CYTOPLASM
EXTRACELLULARFLUID
Proton pump
H+
H+
H+
H+
H+
+
+
+
+
+
–
–
–
–
–
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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
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H+
ATP
Proton pump
Sucrose-H+
cotransporter
Diffusionof H+
Sucrose
H+
H+
H+
H+
H+
H+
+
+
+
+
+
+
–
–
–
–
–
–
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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
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BULK TRANSPORT: EXOCYTOSIS & ENDOCYTOSIS
● transport of large molecules (e.g.
proteins and polysaccharides) across cell
membrane
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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)
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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)
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EXOCYTOSIS
● In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents
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ENDOCYTOSIS● In endocytosis, the cell takes in
macromolecules by forming vesicles from the plasma membrane
● Endocytosis is a reversal of exocytosis, involving different proteins
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Three types of Endocytosis:
1) Phagocytosis: part of the cell membrane engulfs
large particles or even entire cells (“cell eating”)
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Three types of Endocytosis:
2) Pinocytosis: part of the cell
membrane engulfs small
dissolved substances or fluid
droplets in vesicles (“cell
drinking”)
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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)
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Receptor
RECEPTOR-MEDIATED ENDOCYTOSIS
Ligand
Coatedpit
Coatedvesicle
Coat protein
Coat protein
Plasmamembrane
0.25 µm
A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).