11.3 Solute Transport Across Membranes Flashcards
summary of transport types
-nonpolar compounds can dissolve in the lipid bilayer and cross a membrane unassisted
-polar compounds and ions require a specific membrane protein carrier
simple diffusion
movement of a solute from the region of higher concentration to the region of lower concentration
chemical gradient
membrane potential
Vm
a transmembrane electrical gradient that occurs when ions of opposite charge are separated by a permeable membrane
produces a force that:
-opposes ion movements that increase Vm
-drives movements that reduce Vm
electrical gradient
electrochemical gradient (electrochemical potential)
determines the direction in which a charged solute moves across a membrane
composed of:
-the chemical gradient
-the electrical gradient (Vm)
passive transporters
facilitate movement down a concentration gradient, increasing the transport rate
process is called passive transport or facilitated diffusion
active transporters
move substrates across a membrane against a concentration gradient or an electrical potential
process is called active transport
primary active transporters
use energy provided DIRECTLY by a chemical reaction
secondary active transporters
couple uphill transporter of one substrate with downhilll transport of another
transporter proteins reduce the ________ for diffusion by:
energy of activation by:
1. forming noncovalent interactions with the dehydrated solute
2. providing a hydrophilic transmembrane pathway
ion channels
provide an aqueous path across the membrane through which inorganic ions can diffuse at very high rates
-most have a “gate” regulated by a biological signal
-typically show some specificity for an ion
-are not saturable with their ion substrate
-flow stops either when the gate is closed or when there is no longer an electrochemical gradient
differences between ion channels and transporters
ion channels: single gate while a transporter (pump) ha alternating gates
the glucose transporter of erythrocytes mediates
passive transport
describe glucose transporter mechanism
enters the erythrocyte by passive transport via GLUT1
analogous with an enzymatic reaction where:
-glucose (“substrate”) outside is Sout
-glucose (“product”) inside in Sin
-transporter (“enzyme”) is T
GLUT1
integral membrane protein with 12 hydrophobic segments that form 12 membrane-spanning helices
-helices are amphipathic (residues are nonpolar on one side and polar on the other side)
the model of glucose transport into erythrocytes cycles between two extreme conformations:
T1 form
T2 form
T1 form
glucose-binding site exposed on the outer membrane surface
T2 form
glucose-binding site exposed on the inner surface
transport of glucose into a myocyte by GLUT4 is regulated by
insulin
know pathway
chloride-bicarbonate exchanger
anion exchanger essential in CO2 transport to the lungs from tissues
-passive transport system
-electroneutral exchange= no net transfer of charge
cotransport systems
simultaneously transport two solutes across a membrane
1. antiport
2. symport
antiport
moves in opposite directions
symport
moves in the same direction
uniport systems
carry only one substrate
active transport results in
the accumulation of a solute above the equilibrium point
thermodynamically unfavorable (endergonic)
-must be directly or indirectly coupled to an exergonic process
primary active transport
solute accumulation is coupled directly to an exergonic chemical reaction
secondary active transport
endergonic transport of one solute is coupled to the exergonic flow of a different solute that was originally pumped uphill by primary active transport
free-energy change for transport of an uncharged solute
no bonds are broken and delta G is zero
free-energy change for transport of an ion
without movement of an accompanying counterion, the process is electrogenic (produces an electrical potential)
P-type ATPases
family of cation transporters that are reversibly phosphorylated by ATP as part of the transport cycle
-integral proteins with 8 or 10 predicted membrane-spanning regions
-sensitive to inhibition by the transition-state analog vanadate
examples of P-Type ATPases include
Na+K+ATPase= animal cell antiporter for Na+ and K+
H+ATPase= plant and fungi transporter
SERCA pump
SERCA pump
sarcoplasmic/ endoplasmic reticulum Ca2+ ATPase pump
uniporter for calcium ions
general structure of the P-type ATPases
critical Asp residue in the P domain undergoes phosphorylation and dephosphorylation
postulated mechanism of the SERCA pump
each catalytic cycle moves two calcium ions across the membrane and converts an ATP to ADP and Pi
two conformations of the transporter, E1 an E2 interconvert
Na+K+ATPase
couples phosphorylation-dephosphorylation of the critical Asp resiue to the movement of Na+ and K+ against their electrochemical gradients
-maintains low sodium concentration and high potassium concentration in the cell
-essential to the conduction of action potentials in neurons
V-Type and F-Type ATPases are
ATP-Driven proton pumps
V-type ATPases
class of proton-transporting ATPases responsible for acidifying intracellular compartments
-V0 domain serves as a proton channel
-V1 domain contains the ATP-binding site and the ATPase activity
F-Type ATPases
catalyze the uphill transmembrane passage of protons, driven by ATP hydrolysis
-F0 integral membrane protein complex provides a pathway for protons
-F1 protein uses energy of ATP to drive proteins uphill
F-Type ATPases catalyze reactions in
both direction
a large proton gradient can supply the energy to drive ATP synthesis
when functioning in this direction, F-type ATPases are named ATP synthases
ABC transporters
family of ATP-driven transporters that pump substrates across a membrane against a concentration gradient
they have 2 ATP-binding domains (“cassettes”) and two transmembrane domains
example substrates: amino acids, peptides, proteins, metal ions, various lipids, bile salts, and hydrophobic compounds including drugs
mechanism of ABC transporters
substrates move across the membrane when two forms of the transporter intercovert
-driven by ATP hydrolysis
ion gradients provide the energy for
secondary active transport
Na+-glucose symporter
takes up glucose from the intestine in a process driven by the downhill flow of Na+
strong thermodynamic tendency for Na+ to move into the cell provides the energy needed for the transport of glucose into the cell
ionophores
“ion bearers”
compounds that shuttle ions across membranes in this way
acts as a shuttle, carrying K= across the membrane down its concentration gradient and deflating that gradient
aquaporins (AQPs)
provide channels for movement of water molecules across plasma membranes
-each protein has a specific location and role
-low activation energy suggests that water moves in a continuous stream
-do not allow passage of protons (hydronium ions)
ion-selective channels
move inorganic ions across membranes
-the rate of flux can be orders of magnitude greater than the turnover number for a transporter
-not saturable
-gated in response to cellular event
ligand-gated channels
binding of an extracellular or intracellular small molecules forces an allosteric transition in the protein, which opens or closes the channel
-generally oligomeric
voltage-gated ion channels
a change in transmembrane electrical potential (Vm) causes a charged protein domain to move relative to the membrane, opening or closing the channel
patch-clamping
currents are measured through a region of the membrane surface containing only one or a few ion-channel molecules
