Mechanisms of Membrane Transport

What is the ABC superfamily primary active transport?

ATP-binding cassette (ABC) superfamily of pumps transport a wide variety of molecules across the membrane. These are transmembrane proteins that function in the transport of a wide variety of substrates across extra- and intracellular membranes, including metabolic products, lipids, and sterols, and drugs. ABC transporters are involved in tumor resistance, cystic fibrosis, bacterial and multidrug resistance.

what is osmosis?

Diffusion of water 1. defined as the flow of water across a semi-permeable membrane from a compartment with lower solute concentration to a compartment with higher solute concentration 2. flow of water through membranes by osmosis described by: Flow = (σ)*L*(π1-π2) σ= reflection coefficient (index of membrane's permeability to the solute and varies between 0 and 1; if particles are impermeant, the have a σ= 1, and freely permeable then σ= 0. L=hydraulic conductivity is the the ease at which water can move π = osmotic pressure is pressure necessary to prevent solute migration (in mmHg) 3. Osmotic pressure is calculated using van't Hoff equation

How does the Michaelis-Menten equation work with facilitated diffusion?

Flux = (Fluxmax * C)/(Km + C) Fluxmax = maximal rate of transport (same as Vmax) C is the concentration of the transported substance Km (michaelis constant) is the concentration required for half-maximal transport of the substance

What are the 4 types of primary active transport?

P-type V-type F-type ABC superfamily

What are the V- and F-type primary active transport

Proton specific. They do not utilize a phosphoprotein intermediate. P-type pumps requires direct input of metabolic energy; linked to terminal phosphate bond of ATP

GLUT4 is what kind of transporter?

a glucose uniporter - facilitated diffusion located in adipose tissue and muscle upregulated in these cells in response to the hormone insulin

what is osmotic pressure

a hydrostatic pressure produced by a difference in concentration between solutions on the two sides of a surface such as a membrane that is semi-permeable

what are transporters (carriers)?

bind specific solute to be transported and undergo conformational changes to transfer the solute across the membrane. Some transporters carry out passive transport while others mediate active transport.

what are the two main classes of membrane transport proteins?

channels transporters

describe active transport

coupled to a source of metabolic energy that allows transport "uphill" against an electrochemical gradient

what are channels?

form aqueous pores that extend through lipid bilayer and allow solutes to pass through and cross membrane (always passive movement)

what is osmolality?

number of osmoles/kg of solvent

what is osmolarity?

number of osmoles/liter of solution, ie plasma

explain the Multidrug resistance (MDR) protein and how ABC transporters affect it

over-expression of MDR can make cells resistant to a variety of chemically unrelated cytotoxic drugs and make cells resistant to anticancer drugs. Studies indicate that up to 40% of human cancers develop mutlidrug resistance -cystic fibrosis is caused by mutation in a gene encoding an ABC transporter (Cystic Fibrosis Transmembrane Regulator protein; CFTR). In this case, the CFTR protein functions as a Cl- channel in plasma membrane of epithelial cells and regulates ion concentration in the extracellular fluid and mutations block salt and water movement out of the cells. (~1/27 caucasians carries a gene encoding a mutant of this protein)

what are the types of transport?

passive active

what are aquaporins?

protein channels in the plasma membrane that conduct water molecules. Although water can diffuse across lipid bilayers, the plasma membrane of cells contain proteins called aquaporins that further facilitate water transport.

what is the Ptype Primary active transport?

pumps are directly phosphorylated by ATP

how does the cell establish the -70mV resting potential?

sodium-potassium ATPase pumps sodium out and potassium in. There are leak channels that allow the potassium to escape back out down its concentration gradient, but the membrane is impermeable to sodium. This forms an electrochemical gradient *the electrical gradient and chemical gradient have opposite effects on potassium ions*

what are inhibitors of the sodium/potassium ATPase

the cardiac glycoside drugs ouabain and digitalis

What is Fick's law?

the rate of net diffusion for small uncharged molecules across a membrane D= [(ΔC*P*A)/(√MW*d)] D = the rate of diffusion (flux) ΔC= C1-C2 is the concentration difference A = the area of the membrane P = the permeability coefficient MW = molecular weight d = the membrane thickness

how can an ion channel be selective for potassium (not take in sodium)

uses the water "cloud" around the potassium. There are methods of ripping off the water molecules of potassium, but because sodium is too small, it's water molecules cannot be ripped off, inhibiting its passage through the channel.

what is the van't Hoff equation

π = RT (φic) R = ideal gas constant T = absolute temperature φ = osmotic coefficient (value of 1 typically used) i = number of ions formed by dissociation of a molecule c = molar concentration of solute ... thus φic is the osmolarity of the solution

what molecules can diffuse across the lipid bilayer?

-Water, gases (O2, CO2, N2) and some small uncharged molecules can diffuse across the lipid bilayer (urea, glycerol, steroid hormones) -Most charged molecules and large molecules do not readily diffuse across the lipid bilayer (glucose, sucrose, H+, Na+, HCO-3, K+, Ca2+, Cl-, Mg2+)

describe passive transport

-also known as simple and facilitated diffusion -driven by concentration gradient "downhill" with electrochemical gradient -Driving forces for facilitated diffusion: 1. chemical (concentration) gradient 2. electrical gradient

describe the Na+/Ca2+ exchanger

-example of an antiporter -transports calcium uphill from low intracellular [Ca2+] to high extracellular [Ca2+] -energy derived from downhill sodium transport -the inward gradient is maintained by the sodium-potassium-ATPase so blocking th epump inhibits sodium-calcium-exchange thus elevating intracellular calcium

describe the Na+/glucose transporter

-example of symporter -Na+ moves down electrochemical gradient and drags in glucose -the greater the gradient for Na+, the greater the rate of solute entry -found in the epithelial cells in the intestine and kidney

what is channel conductance?

-the rate of ion travel through the channel 1. conductance depends on probability that the channel is open. Thus the higher the probability the channel is open, the higher the conductance or permeability 2. channels are gated; thus channels may be open or closed

what is an Osmol?

1 mol of solute particles

what are some other ATP-dependent pump proteins and what are their purposes?

1. H+/K+ ATPase (P-type pump) - maintain acidity of the stomach 2. H+ pumps (V-type) - maintain acidity of some organelles such as lysosomes 3. Ca2+ ATPase (P-type) - pump calcium out of cells; in muscle cells pumps Ca2+ into the sarcoplasmic reticulum

what are characteristics of ion channels?

1. charged molecules are unable to cross the lipid bilayer by simple diffusion 2. channels are integral membrane proteins that span the membrane and permit passage of ions 3. channels distinguished from membrane carrier proteins by the high velocity of ion flux 4. channels influence the rate, not the direction, of ion flow across membranes 5. ion selectivity is based on pore size, charge, and differences in hydration of ion in aqueous solution 6. structure of selectivity filter - carbonyl oxygens interact with dehydrated ion

what are the types of transporters?

1. coupled transporters - couple uphill transport of one solute across the membrane to the downhill transport of another (secondary) 2. ATP-driven pumps couple uphill transport to hydrolysis of ATP (primary)

How do small molecules cross the plasma membrane?

1. diffusion across the lipid bilayer 2. protein-facilitated diffusion 3. active transport

what are the characteristics of active transport?

1. occurs against an electrochemical gradient (uphill) 2. requires metabolic energy (directly or indirectly) 3. carrier mediated characteristics (saturable, stereospecific) 4. time course slower than ion channels

how does osmosis/osmotic pressure work in biological aspects?

1. plasma membranes are relatively impermeable to many solutes so movement of water by osmosis leads to swelling or shrinking of cells 2. term tonicity is used to describe the osmolarity of a solution relative to plasma 3. solutions with same osmolarity as plasma (300 mOsm/L) are isotonic 4. solutions are isoosmotic if they contain 300 mOsm/L (hyperosmotic if >300; hypoosmotic if <300)

What are the key characteristics of facilitated diffusion?

1. requires no metabolic energy (passive transport) 2. occurs down an electrochemical gradient (downhill) 3. more rapid than simple diffusion for that particular solute

describe facilitated diffusion (properties of uniporters)

1. stereospecific 2. saturable a. the transport rate increases as concentration of solute increases, until carriers saturate b. maximum flux (Fluxmax) is analogous to maximum velocity (Vmax) in enzyme kinetics 3. competition - structurally related compounds can compete for transport site on carriers

describe secondary active transport

1. transport is coupled 2. one solute is transported downhill and provides energy for uphill transport of another solute(s) 3. metabolic energy is indirect using the energy store in the electrochemical gradient of one solute (typically an ion, and usually Na+) 4. if the solute moves in the same direction across the membrane, it is called symport 5. if the solute moves int he opposite direction across the membrane, it is called antiport (also called counter transport or exchange)

describe the sodium/potassium ATPase (pump)

1. uses energy of ATP hydrolysis to pump 3 Na+ ions from the intracellular to the extracellular fluid and 2 K+ from the extracellular to the intracellular fluid; thus maintaining low intracellular [Na+] and high intracellular [K+] 2. Both Na+ and K+ are transported against their electrochemical gradient 3. Usual stoichiometry is 3 Na+/2 K+; thus pump is electrogenic 4. binding of intracellular Na+ ions and phosphorylation by ATP induces conformational change that transfers the Na+ across the membrane and releases it on the outside 5. binding sites for 2 extracellular K+ ions induces subsequent dephosphorylation and returns the pump to its original conformation, transferring the K+ ions across the membrane into the cell

what are different types of channels, based on their stimuli?

1. voltage-gated channels - open and close in response to changes in membrane potential 2. ligand-gated channels open or close in response to extracellular or intracellular ligands (e.g. neurotransmitters and signaling second messengers) 3. mechanically-gated channels - open and close in response to movement of cell membrane; due cytoskeletal filaments linked to the ion channel protein * there are hundreds of different types of ion channels expressed in humans; they aid in regulating membrane potential