Sullivan General Human Physiology Lecture 5

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Movement across membranes - general: * plasma membrane is selectively permeable
* therefore concentration differences exist between extracellular and intracellular compartments
Movement across membranes: Mechanisms: 1. Diffusion
2. Mediated transport
a. facilitated diffusion
b. active transport
i. primary
ii. secondary
3. Osmosis
Diffusion: - general: redistribution of molecules from a region of high concentration to a region of low concentration
* molecules move
* random collisions
* FLUX is the amount of material crossing a surface in unit time
* NET FLUX is the DIFFERENCE between 2 opposing, 1way fluxes
-----A----->
<--B--
NET FLUX = A-B
In diffusion NET FLUX is always from ...: HIGH concentration to LOW
Diffusion: NET FLUX (proportional to): * concentration difference
* size of surface area
* membrane permeability constant (KP)
Diffusion: KP (membrane permeability constant) based on:: * type of molecule
* molecular weight
* temperature
* characteristics of membrane
DIFFUSION thru a CELL MEMBRANE can be thru:: * can be thru phospholipid bilayer
* can be thru channels (pores)
Diffusin THRU THE PHOSPHOLIPID BILAYER:: * polar molecules diffuse into cells slowly/or not at all
* nonpolar molecules (O2, CO2, fatty acids and steroids) diffuse rapidly because they readily dissolve in fatty acid chains (nonpolar portions) of membrane phospholipid
diffusion THRU PROTEINS CHANNELS: * Na+, K+, Cl-, Ca++
* integral proteins span lipid bilayer forming channels thru which these ions diffuse
* ion channels are slective for ion type (according to)
1. diameter
2. charged/polar surfaces of protein channel (attract/repel ions)
what FORCES also effect the diffusion of ions?: ELECTRICAL FORCES acting on ions also effect their diffusion
Membrane Potential: General: * a separation of electrical charge across a membrane
*** If the inside of a cell(-) there will be an electrical force attracting (+) ions into the cell and repelling (-) ions.
* membrane permeability to ions can be altered by channel gating (opening/closing channels)
Membrane Potential: * the DIRECTION AND MAGNITUDE of ion flux thru a channel, across a membrane will depend on:: 1. concentration difference
2. membrane potential (electrical difference)
Membrane Potential: * MAGNITUDE (number of ions) that pass thru channel will depend on:: 1. frequency of channel opening
2. duration of channel opening
Channel Conformation (whether open/closed) can be altered ...: 3 ways:
1. RECEPTOR OPERATED CHANNELS
(open/close when a specific CHEMICAL binds to receptor)
2. VOLTAGE-SENSITIVE CHANNELS
(open/close when membrane electrical potential is altered)
3. STRETCH-ACTIVATED CHANNELS
(open/close when smooth muscle cells are stretched)
Mediated Transport: * needed to move substances TOO POLAR to diffuse thru the lipid bilayer or TOO BIG to diffuse thru protein channels
* movement of these ions MEDIATED by CARRIERS
* carriers are integral proteins
General Mechanism of Mediated Transport: 1. Substance BINDS to specific site on a carrier (on membrane surface)
2. Carrier protein CHANGES SHAPE, exposing its binding site to opposite side of membrane
3. DISSOCIATION of substance from carrier binding site
Magnitude of mediated transport is proportional to:: 1. saturation of carrier
2. number of carrier proteins in membrane
Types of Carrier-Mediated Transport:: 2 Types:
1. Facilitated Diffusion
2. Active Transport
Facilitated Diffusion: * moves solute from HIGH TO LOW concentration across membrane
* does NOT involve diffusion
* does NOT require energy
* when carrier changes shape so that binding site goes from facing surface to facing other side binding properties of site DO NOT CHANGE
* binding determined by concentration difference
ie: glucose moves into cell (large/polar) across cell membrane by facilitated diffusion
- glucose metabolized immediately inside cell so .. concentration always LOW inside cell
Active Transport: * uses energy
* transport uphill
* called active transport "pumps"
* ion flux (depends on electrical difference/concentration)
* uses carrier-proteins
* maximum flux when binding sites are saturated
* some pump into cells
some pump out of cells
Active Transport: Carriers: * carrier CHANGES SHAPE so binding site accessible first on one side/then to molecules on other side.
* CARRIERS HAVE BINDING SITES THAT DIFFER IN AFFINITY ON OPPOSITE SIDES OF MEMBRANE
Active Transport: Affinity: * CARRIERS HAVE BINDING SITES THAT DIFFER IN AFFINITY ON OPPOSITE SIDES OF MEMBRANE
* direction of transport depends on which side has low affinity binding site (moves from high affinity side to low affinity side)
differnce in affinity produced by ENERGY (ATP)
PRIMARY Active Transport: * uses ATP directly
* chemical energy from ATP transferred to carrier
* carrier protein acts as an enzyme (ATPase) and catalyzes its own phosphorylation
(phosphorylation of carrier at one binding site alters affinity of solute binding site at different point on carrier)
* COVALENT MODULATION
* phosphorylation/dephosphorylation produces binding sites of differing affinities
primary active transport carriers: 4 primary active transport carriers
1. Na/K ATPase carrier
2. Ca ATPase carrier
3. H ATPase carrier
SECONDARY Active Transport: * uses ion concentration
* flow of some ion from high concentration to low concentration provides energy for uphill transport of solute
* binding of ion (Na+, HCO3-, Cl-, K+) to carrier alters affinity of other site for transporting solute or altering rate at which carrier moves binding site from one side to other (transport)
* carrier has 2 binding sites (1 for solute being transported/1 for ion)
* ALLOSTERIC MODULATION
ie: amino acids are transported this way
Components needed for OSMOSIS to occur:: 1. H2O
2. a membrane
3. a nonpenetrating solute
4. a concentration difference
Osmosis: * net diffusion of H2O from a region of high concentration of water to a region of low concentration of water
* adding solute to water lowers the concentration of H2O
* the more solute present... the less water (higher solute concentration/lower water concentration)
* water flows toward solutions of higher solute concentration, since such solutions have lower H2O concentration
** a molecule that IONIZES in solution decreases the water concentration in proportion to the number of ions formed
Osmosis illustration: 1 mole NaCl <=====> 1 mole Na+ + 1 mole Cl-
(2 moles of particles)

1 mole glucose <=====> 1 mole glucose
(1 mole of particles)
OSMOTIC PRESSURE: If a solution contains nonpenetrating solute, separated from pure H2O by a membrane, the pressure that must be applied to prevent a net flow of H2O across the membrane (from the pure H2O side to the nonpenetrating solute side) is called OSMOTIC PRESSURE
ISOTONIC SOLUTIONS: 2 solutions that each contain equal concentrations of nonpenetrating solute
(also equal concentrations of H2O)
HYPOTONIC SOLUTION: a solution that contains less nonpenetrating solute than the solution to which it is being compared (also more H2O)
HYPERTONIC SOLUTION: a solution that contains more nonpenetrating solute than the solution to which it is being compared
(also less H2O)

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