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Biol219 Lecture 9 Fall 2016 Dr Scott
1
Membrane Transport
Cell membranes are selectively permeablePermeability is determined by
A. the phospholipid bilayer and B. transport proteins in the membrane
Permeability- the ability of a substance to pass through a membrane
A. Permeability through the Lipid Bilayer1. molecular size - smaller molecules are more permeable2. lipid solubility - lipid-soluble (non-polar) molecules are more permeable
- polar molecules and ions do not easily cross the lipid bilayer
B. Membrane Transport Proteins (channels, carriers, and pumps)- enable certain ions and polar molecules to cross the membrane
• protein channels form passageways for certain ions
Biol219 Lecture 9 Fall 2016 Dr Scott
2
• carrier proteins are used for certain polar molecules such as glucose
Highly permeableO2 & CO2
fatty acidssteroidsH2O (variable: pores)
Less permeableNa+, K+, Cl– (via channels)glucose, a.a.’s (via carriers)
Impermeableproteins*DNA & RNAATP
* except viamembrane-boundvesicles
Permeability of the Plasma Membrane
Transport Across Membranes
1. Simple Diffusion2. Osmosis3. Diffusion through channels4. Facilitated diffusion5. Primary active transport6. Secondary active transport7. Transport via membrane-bound vesicles
Transport Across Membranes
1. Simple Diffusion
2. Osmosis
3. Diffusion through channels
4. Facilitated diffusion
5. Primary active transport
6. Secondary active transport
Passive Transport- no energy required
Active Transport- requires energy
Protein-mediatedTransport
Biol219 Lecture 9 Fall 2016 Dr Scott
3
Diffusion in a Solution Diffusion Across a Membrane
Diffusion occurs from high to low concentration(down a concentration gradient)
Rate of Diffusion Across a MembraneFick’s Law of Diffusion
Osmosis – passive movement of water across a membrane in response to a solute concentration gradient
Selectively-permeablemembrane:
permeable to water,impermeable to solute
Water follows solutes;“solutes suck”
Osmotic pressure:force that results from thedifference in concentration of solutes across the membrane
Biol219 Lecture 9 Fall 2016 Dr Scott
4
Osmolarity and Osmotic Pressure
Ø solute concentration (osmolarity) difference results in a large osmotic pressure difference (π = CRT)
Ø osmotic pressure difference is the driving force for osmosis
Ø osmosis depends on osmolarity = total concentration of all solute particles in solution
Ø 1 Osm (= 1,000 mOsm) = 1M total solute concentration
(0 mOsm)
Hypotonic Solution - water enters thecell by osmosis
- cell gains volume(swells)
Hypertonic Solution - water leaves thecell by osmosis
- cell loses volume(shrinks)
Tonicity• effect of an extracellular solution on cell volume • due to osmosis across the cell membrane • depends on concentration of non-penetrating solutes only
Image: Copy right © 2004Dennis Kunk el Mic ros c opy , Inc.
Crenated RBCs in a Hypertonic SolutionIsotonic Solution - concentration of non-penetrating solutes is equal in the ECF and ICF
- no net water movementby osmosis
- cell volume remainsconstant
ICF
300 mOsm
ECF
300 mOsm
Example: 0.9% NaCl (0.9 g/dL) (“normal saline”)
Biol219 Lecture 9 Fall 2016 Dr Scott
5
Transport Across Membranes
1. Simple Diffusion
2. Osmosis
3. Diffusion through channels
4. Facilitated diffusion
5. Primary active transport
6. Secondary active transport
Passive Transport- no energy required
Active Transport- requires energy
Protein-mediatedTransport
Protein-Mediated Transportchannels, carriers, and pumps
Diffusion Through Channels
Biol219 Lecture 9 Fall 2016 Dr Scott
6
Diffusion of ions is influenced by both chemical and electrical driving forces.
The combined force is the electrochemical gradient.
No membrane potential –chemical force only(outward for K+, inward for Na+)
Membrane potential present –chemical and electrical forces = electrochemical gradient
Electrical force on ions is due to membrane potentialwhich results from slight imbalance of + and – charges between the ICF and ECF (more negative inside).
For K+, chemical and electrical gradients are in the opposite direction (chem. outward, elec. inward), so the electrochemical gradient is small.
For Na+, chemical and electrical gradients are in the same direction (both inward), so the electrochemical gradient is large
Facilitated Diffusion
Biol219 Lecture 9 Fall 2016 Dr Scott
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Facilitated Diffusion
GLUT transporters
- mediate transport of glucose into most cells
- the GLUT4 transporterin skeletal muscle andadipose tissue isactivated by insulin
Carrier-mediated transport exhibits saturation.The transport maximum depends on the number of carrier proteins present in the membrane.
- Transports molecules against a gradient
- Uses energy from ATP directly
Primary Active TransportThe sodium-potassium pump
(Na+-K+-ATPase)
Mechanism of the Na+-K+ ATPase
Biol219 Lecture 9 Fall 2016 Dr Scott
8
Secondary Active Transport- Transports molecules against their gradient
- Uses potential energy stored in ionic gradients(energy supplied indirectly by ATP )
- Involves coupled transport with an ion moving “downhill”
The sodium-glucose (SGLT) transporter
Mechanism of the SGLT Transporter
Biol219 Lecture 9 Fall 2016 Dr Scott
9
Vesicular Transport
• Phagocytosis
– Vesicles created by the cytoskeleton
– Cell engulfs bacterium or other particle into
phagosome
© 2016 Pearson Education, Inc.
© 2016 Pearson Education, Inc.
Slide 1Figure 5.18 Phagocytosis
Phagocyte
The phagocytic white bloodcell encounters a bacteriumthat binds to the cellmembrane.
The phagocyte uses itscytoskeleton to push itscell membrane around thebacterium, creating a largevesicle, the phagosome.
The phagosome containingthe bacterium separatesfrom the cell membrane andmoves into the cytoplasm.
The phagosome fuses withlysosomes containingdigestive enz ymes.
The bacterium is killedand digested within thevesicle.
Bacterium
Lysosome
Figure 5.18 Phagocytosis
© 2016 Pearson Education, Inc.
Slide 2
Phagocyte
The phagocytic white bloodcell encounters a bacteriumthat binds to the cellmembrane.
Bacterium
Lysosome
Biol219 Lecture 9 Fall 2016 Dr Scott
10
Figure 5.18 Phagocytosis
© 2016 Pearson Education, Inc.
Slide 3
Phagocyte
The phagocytic white bloodcell encounters a bacteriumthat binds to the cellmembrane.
The phagocyte uses itscytoskeleton to push itscell membrane around thebacterium, creating a largevesicle, the phagosome.
Bacterium
Lysosome
© 2016 Pearson Education, Inc.
Figure 5.18 Phagocytosis Slide 4
Phagocyte
The phagocytic white bloodcell encounters a bacteriumthat binds to the cellmembrane.
The phagocyte uses itscytoskeleton to push itscell membrane around thebacterium, creating a largevesicle, the phagosome.
The phagosome containingthe bacterium separatesfrom the cell membrane andmoves into the cytoplasm.
Bacterium
Lysosome
© 2016 Pearson Education, Inc.
Slide 5Figure 5.18 Phagocytosis
Phagocyte
The phagocytic white bloodcell encounters a bacteriumthat binds to the cellmembrane.
The phagocyte uses itscytoskeleton to push itscell membrane around thebacterium, creating a largevesicle, the phagosome.
The phagosome containingthe bacterium separatesfrom the cell membrane andmoves into the cytoplasm.
The phagosome fuses withlysosomes containingdigestive enz ymes.
Bacterium
Lysosome
Figure 5.18 Phagocytosis
© 2016 Pearson Education, Inc.
Slide 6
Phagocyte
The phagocytic white bloodcell encounters a bacteriumthat binds to the cellmembrane.
The phagocyte uses itscytoskeleton to push itscell membrane around thebacterium, creating a largevesicle, the phagosome.
The phagosome containingthe bacterium separatesfrom the cell membrane andmoves into the cytoplasm.
The phagosome fuses withlysosomes containingdigestive enz ymes.
The bacterium is killedand digested within thevesicle.
Bacterium
Lysosome
Biol219 Lecture 9 Fall 2016 Dr Scott
11
Vesicular Transport• Endocytosis
– Membrane surface indents and forms vesicles
– Active process that can be nonselective
(pinocytosis) or highly selective
– Receptor-mediated endocytosis uses coated pits• Clathrin most common protein in coated pits
– Membrane recycling
© 2016 Pearson Education, Inc.
Transport vesicleand cell membranefuse (membranerecycling).
Exocytosis
Ligand binds to membrane receptor.
Receptor-ligand migrates to clathrin-coated pit.
Receptor
Clathrin-coatedpit
Clathrin
Endocytosis
Extracellular fluid
Vesicle losesclathrin coat.
Receptorsand ligandsseparate.
Endosome
Intracellular fluid
Transport vesiclewith receptors movesto the cell membrane.
Ligands go to lysosomesor Golgi for processing.
To lysosome orGolgi complex
© 2016 Pears on Education, Inc .
Ligand binds to membrane receptor.
Receptor
Extracellular fluid
Intracellular fluid
© 2016 Pearson Education, Inc.
Ligand binds to membrane receptor.
Receptor-ligand migrates to clathrin-coated pit.
Receptor
Clathrin-coatedpit
Extracellular fluid
Intracellular fluid
Clathrin
© 2016 Pearson Education, Inc.
Biol219 Lecture 9 Fall 2016 Dr Scott
12
Ligand binds to membrane receptor.
Receptor-ligand migrates to clathrin-coated pit.
Receptor
Clathrin-coatedpit
Clathrin
Endocytosis
Extracellular fluid
Intracellular fluid© 2016 Pearson Education, Inc.
Ligand binds to membrane receptor.
Receptor-ligand migrates to clathrin-coated pit.
Receptor
Clathrin-coatedpit
Clathrin
Endocytosis
Extracellular fluid
Vesicle losesclathrin coat.
Intracellular fluid© 2016 Pearson Education, Inc.
Ligand binds to membrane receptor.
Receptor-ligand migrates to clathrin-coated pit.
Receptor
Clathrin-coatedpit
Clathrin
Endocytosis
Extracellular fluid
Vesicle losesclathrin coat.
Receptorsand ligandsseparate.
Endosome
Intracellular fluid© 2016 Pearson Education, Inc.
Ligand binds to membrane receptor.
Receptor-ligand migrates to clathrin-coated pit.
Receptor
Clathrin-coatedpit
Clathrin
Endocytosis
Extracellular fluid
Vesicle losesclathrin coat.
Receptorsand ligandsseparate.
Endosome
Intracellular fluid
Ligands go to lysosomesor Golgi for processing.
To lysosome orGolgi complex
© 2016 Pearson Education, Inc.
Biol219 Lecture 9 Fall 2016 Dr Scott
13
Ligand binds to membrane receptor.
Receptor-ligand migrates to clathrin-coated pit.
Receptor
Clathrin-coatedpit
Clathrin
Endocytosis
Extracellular fluid
Vesicle losesclathrin coat.
Receptorsand ligandsseparate.
Endosome
Intracellular fluid
Transport vesiclewith receptors movesto the cell membrane.
Ligands go to lysosomesor Golgi for processing.
To lysosome orGolgi complex
© 2016 Pearson Education, Inc.
Transport vesicleand cell membranefuse (membranerecycling).
Ligand binds to membrane receptor.
Receptor-ligand migrates to clathrin-coated pit.
Receptor
Clathrin-coatedpit
Clathrin
Endocytosis
Extracellular fluid
Vesicle losesclathrin coat.
Receptorsand ligandsseparate.
Endosome
Intracellular fluid
Transport vesiclewith receptors movesto the cell membrane.
Ligands go to lysosomesor Golgi for processing.
To lysosome orGolgi complex
© 2016 Pearson Education, Inc.
Epithelial Transport
• Apical (mucosal) membrane• Basal membrane• Paracellular transport
– Through junctions between adjacent cells• Transcellular transport
– Through cells themselves– Transcytosis with vesicular transport
© 2016 Pearson Education, Inc.
Biol219 Lecture 9 Fall 2016 Dr Scott
14
Epithelial Transport
• Absorption from lumen to extracellular fluid (ECF)
• Secretion from ECF to lumen• Transcellular transport of glucose uses
membrane proteins
© 2016 Pearson Education, Inc.
Figure 5.20 Transporting epithelia are polarized
© 2016 Pearson Education, Inc.
Apical membranewith microvilli
faces the lumen.
Tight junction limitsmovement of substances
between the cells .
Transportingepithelia l cell
Absorption (paracellular)
Extracellular fluid
Absorption (transcellular)
Secretion
Lumen of intestineor kidney
Basolateral membranefaces the ECF.
Transport proteins
Figure 5.21 Transepithelial absorption of glucose
© 2016 Pearson Education, Inc.
[Glucose]low Glu
[Glucose]high
[Glucose]low Glu
Na +
Na +
[Na +]high
Apicalmembrane
Glu Na + K+
K+
Basolateralmembrane
Extracellularfluid
Lumen of k idney or intestine
Glu
GLUT transporter transfers glucose to ECFby facilitated diffusion.
Na+-K+-ATPase pumps Na + out of the cell, keepingICF Na+ concentration low.
Na+-glucose symporterbrings glucose into cell against its gradient usingenergy stored in the Na +
concentration gradient.
ATP
Epithelia lcell
FIGURE QUESTIONS
3. Why doesn't Na+ movement at the apicalmembrane requireATP?
(a)apical membrane(b)basolateral
membrane
Choose either
[Na +]l ow
2. Is glucose movement across thebasolateral membrane active orpassive? Explain.
1.Match each transporter to itslocation.1. GLUT2. Na +-glucose symporter3. Na +-K+-ATPase
[Na +]high
Na +
Osmosis:H2O follow solute movement
Active transportof solutes into lateral intercellular space
Transepithelial Transport of Water