Permeability - the ability of a substance to pass through ......Membrane Transport Cell membranes are selectively permeable Permeability is determined by A. the phospholipid bilayer

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


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


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


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


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”)


5


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


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


7

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


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


9

Vesicular Transport

• Phagocytosis

– Vesicles created by the cytoskeleton

– Cell engulfs bacterium or other particle into

phagosome

© 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


Slide 2

Phagocyte


Bacterium

Lysosome


10



Slide 3

Phagocyte



Bacterium

Lysosome


Figure 5.18 Phagocytosis Slide 4

Phagocyte




Bacterium

Lysosome


Slide 5Figure 5.18 Phagocytosis

Phagocyte





Bacterium

Lysosome



Slide 6

Phagocyte





The bacterium is killedand digested within thevesicle.

Bacterium

Lysosome


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


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 .


Receptor

Extracellular fluid

Intracellular fluid




Receptor

Clathrin-coatedpit

Extracellular fluid

Intracellular fluid

Clathrin



12



Receptor

Clathrin-coatedpit

Clathrin

Endocytosis

Extracellular fluid

Intracellular fluid© 2016 Pearson Education, Inc.



Receptor

Clathrin-coatedpit

Clathrin

Endocytosis

Extracellular fluid





Receptor

Clathrin-coatedpit

Clathrin

Endocytosis

Extracellular fluid



Endosome




Receptor

Clathrin-coatedpit

Clathrin

Endocytosis

Extracellular fluid



Endosome

Intracellular fluid





13



Receptor

Clathrin-coatedpit

Clathrin

Endocytosis

Extracellular fluid



Endosome

Intracellular fluid





Transport vesicleand cell membranefuse (membranerecycling).



Receptor

Clathrin-coatedpit

Clathrin

Endocytosis

Extracellular fluid



Endosome

Intracellular fluid





Epithelial Transport

• Apical (mucosal) membrane• Basal membrane• Paracellular transport

– Through junctions between adjacent cells• Transcellular transport

– Through cells themselves– Transcytosis with vesicular transport



14

Epithelial Transport

• Absorption from lumen to extracellular fluid (ECF)

• Secretion from ECF to lumen• Transcellular transport of glucose uses

membrane proteins


Figure 5.20 Transporting epithelia are polarized


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


[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

Documents

Permeability - the ability of a substance to pass through ......Membrane Transport Cell membranes are selectively permeable Permeability is determined by A. the phospholipid bilayer