Chapter 10 Flashcards
there are two types of transport processes: … transport and … transport
nonmediated; mediated
nonmediated transport occurs through …
simple diffusion
mediated transport occurs through the action of
specific carrier proteins
the driving force for the nonmediated flow of a substance through a medium is its …. thus, the substance diffuses in the direction that eliminates its concentration gradient, at a rate proportional to the … of the gradient. the rate of diffusion of a substance also depends on its … in the membrane’s nonpolar core
concentration gradient; magnitude; solubility
mediated transport is classified into two categories depending on the thermodynamics of the system:
…, or …, in which a specific molecule flows from high concentration to low concentration
…, in which a specific molecule is transported from low [] to high [] that is, against its gradient. such an endergonic process must be coupled to a sufficiently exergonic process to make it favorable
passive-mediated transport; facilitated diffusion; active transport
… are organic molecules of diverse types, usually of bacterial origin, the increase the permeability of membranes to ions
ionophores
…, which increase the permeabilities of membranes to their selected ion by binding it, diffusing through the membrane, and releasing the ion on the other side. for net transport to occur, the uncomplexed ionophore must then return to the original side of the membrane ready to repeat the process. carriers therefore share the common property that their ionic complexes are soluble in … solvents
carrier ionophores; nonpolar
…, which form transmembrane channels or pores through which their selected ions can diffuse
channel-forming ionophores
since ionophores passively permit ions to diffuse across a membrane in either direction, their effect can only be to … the concentrations of their selected ions across the membrane
equilibrate
valinomycin, one of the best characterized ionophores, specifically binds ..
K+
maltoporin demonstrates solute selectivity: bacterial outer membrane protein facilitates the diffusion of …, which are alpha 1,4 linked glucose oligosaccharide degradation products of starch
maltodextrins
maltoporin contd:
translocation along the helical channel requires the maltodextrin to follow a screwlike path that maintains the helical structure of the oligosaccharide, much like the movement of a bolt through a nut, thereby excluding molecules of comparable …that have different …
size; shapes
maltoporin contd: The translocation process is unlikely to encounter any large energy barrier due to the smooth surface of the greasy slide and the multiple polar groups at the channel constriction that would permit the essentially continuous exchange of …as a maltodextrin moves through the constriction.
hydrogen bonds
potassium ions passively diffuse from the cytoplasm to the extracellular space through transmembrane proteins known as …
K+ channels
…: rate of transport per unit area
flux
passive-mediated transport is carried out by …, …, …, …, and …
ionophores; porins; ion channels; aquaporins; transport proteins
ionophores may carry … or form …
ions; channels
porins provide a passageway for … or …
ions; nonpolar solutes
ion channels are highly … and may be …
selective; gated
the coordinated opening and closing of ion channels generates an … in nerve cells
action potential
aquaporins mediate the transmembrane passage of …
water molecules
transport proteins may mediate …, …, and … transport
uniport; symport; antiport
ion channels are normally shut and only open … to perform some specific task for the cell–> gating
transiently
the opening and closing of ion channels, a process known as gating, can occur in response to a variety of stimuli:
… open in response to local deformations in the lipid bilayer. consequently, they respond to direct physical stimuli such as touch, sound, and changes in osmotic pressure
mechanosensitive channels
the opening and closing of ion channels, a process known as gating, can occur in response to a variety of stimuli:
… open in response to an extracellular chemical stimulus such as a neurotransmitter
ligand-gated channels
the opening and closing of ion channels, a process known as gating, can occur in response to a variety of stimuli:
… open on intracellularly binding a Ca2+ ion or some other signaling molecule
signal-gated channels
the opening and closing of ion channels, a process known as gating, can occur in response to a variety of stimuli:
… open in response to a change in membrane potential. multicellular organisms contain numerous varieties of voltage-gated channels, including those responsible for generating nerve impulses
voltage-gated channels
The release of neurotransmitters across a synaptic junction causes Na+ channels on the postsynaptic membrane to open so that Na+ ions spontaneously flow into the downstream neuron. The consequent local increase in membrane potential induces neighboring … to open. The resulting local … of the membrane induces nearby … to open. This allows K+ ions to spontaneously fl ow out of the cell in a process called …
voltage-gated Na+ channels; depolarization; voltage-gated K+ channels; hyperpolarization
…: a wave of transient change in the membrane potential travels along the length of the nerve cell
action potential
the action potential, which travels at about 10 m/s, propagates in only one direction bc, after the ion channels have spontaneously closed, they resist reopening until the membrane potential has …, which takes a few milliseconds
regained its resting value
since action potentials all have the same amplitude, the magnitude of a stimulus is conveyed by the rate at which a ..
nerve fires
the Kv channel contains two voltage-sensitive gates, one to … the channel on an increase in membrane potential and one to … it a short time later
open; inactivate
This structural homology suggests that voltage-gated ion channels share a common architecture in which differences in ion selectivity arise from precise …variations within the central pore.
stereochemical
aquaporins:
much of the pore is lined with … groups whose lack of strong interactions with water molecules hastens their passage through the pore.
hydrophobic
if water were to pass through aquaporin as an uninterrupted chain of hydrogen bonded molecules, then protons would pass even more rapidly through the channel via … aquaporin interrupts this process by forming H bonds from the side chain NH2 groups of two highly conserved Asn residues to a water molecule centrally located in the pore
.proton jumping
membrane proteins known as … form channels, in the form of gap junctions between cells
connexins
some proteins undergo … changes to move substances from one side of the membrane to the other, such as erythrocyte glucose transporter
conformational
transport through GLUT1 occurs like:
- glucose binds to the protein on one face of the membrane
- a … closes the first binding binding site and exposes the binding site on the other side of the membrane
- glucose dissociates fromt he protein
- the transport cycle is completed by the reversion of GLUT1 t its … in the absence of bound glucose (recovery)
conformational change; initial conformation
GLUT1 provides a means of equilibrating the glucose concentration across the erythrocyte membrane without any accompanying …of small molecules or ions
leakage
All known transport proteins appear to be …situated … proteins that alternate between two conformational states in which the ligand binding sites are exposed, in turn, to opposite sides of the membrane
asymmetrically; transmembrane
a … involves the movement of a single molecule at a time. GLUT1 is a uniport system
uniport
a … simultaneously transports two different molecules in the same direction
symport
an … simultaneously transports two different molecules in opposite directions
antiport
(types of ATP-dependent transporters)
… undergo phosphorylation as they transport cations such as Na+, K+, and Ca2+ across the membrane
P-type ATPases
(types of ATP-dependent transporters) … are proton-transporting complexes located in mitochondria and bacterial membranes. instead of using the free energy of ATP to pump protons against their gradient, these proteins usually operate in reverse to synthesize ATP
F-type ATPases
(types of ATP-dependent transporters) … resemble the F-type ATPases and occur in plant vacuoles and acidic vesicles such as animal lysosomes
V-type ATPases
(types of ATP-dependent transporters) … transport anions across membranes
A-type ATPases
(types of ATP-dependent transporters) … are named for their ATP-binding cassette and transport
ABC transporters
P-type ATPases and ABC transporters carry out … active transport
primary
in … active transport, the free energy of the electrochemical gradient generated by another mechanism, such as an ion-pumping ATPase, is used to transport a molecule against its concentration gradient
secondary
without a functioning (Na+–K+)–ATPase to maintain a low internal [Na+], water would osmotically leak in to such an extent that animal cells, which lack cell walls, would …
swell and burst
The key to the (Na+–K+)–ATPase is the …of a specifi c Asp residue of the α subunit. ATP phosphorylates this Asp residue only in the presence of …, whereas the resulting aspartyl phosphate residue (at right) is subject to hydrolysis only in the presence of …
phosphorylation; Na+; K+
This suggests that the (Na+–K+)–ATPase has two …, called E1 and E2, with different structures, different catalytic activities, and different ligand specificities. In particular, E1’s ion-binding sites are exposed to the cytoplasm, and it has high affinity for Na+ but low affinity for K+, whereas E2’s ion binding sites are exposed to the exterior of the cell and it has low affi nity for Na+ but high affinity for K+
conformational states
for the Na+-K+ ATPase, … and … are coupled unidirectional (vectorial) processes. Thus, neither rxn can go to completion unless the other one also does
ATP hydrolysis; ion transport
. This Ca2+ pump actively pumps …Ca2+ ions out of the cytosol at the expense of ATP hydrolysis, while countertransporting two or three protons.
two;
…: This member of the ABC family of transporters pumps a variety of amphiphilic substances— including many drugs—out of the cell, so that it is also called a multidrug resistance (MDR) transporter.
P-glycoprotein
The ABC transporters, which pump ions, sugars, amino acids, and other polar and nonpolar substances, are built from four modules: two highly conserved cytoplasmic …domains, and two …domains that typically contain six transmembrane helices each.
nucleotide-binding; transmembrane
Bacterial ABC transporters mediate the uptake as well as the effl ux of a variety of compounds, whereas their eukaryotic counterparts apparently operate only as …that transport material out of the cell or into …. such as the endoplasmic reticulum.
exporters; intracellular compartments
Only one of the thousands of known ABC transporters (48 in humans) functions as an ion channel rather than a pump: the….
cystic fi brosis transmembrane conductance regulator (CFTR).
Systems such as the (Na+–K+)–ATPase generate …across membranes. The free energy stored in an electrochemical gradient (Eq. 10-3) can be harnessed to power various endergonic physiological processes.
electrochemical gradients;
The Na+–glucose transport system concentrates …inside the cell. Glucose is then transported into the capillaries through a passive-mediated glucose uniport
glucose
… (also known as galactoside permease), utilizes the proton gradient across the bacterial cell membrane to cotransport H+ and lactose. The
, lactose permease
lactose permease has two major conformational states:
- E-1, which has a low-affi nity lactose-binding site facing the …
- E-2, which has a high-affi nity lactose-binding site facing the …(the space between the plasma and outer membranes;
cytoplasm; periplasm
Ronald Kaback established that E-1 and E-2 can interconvert only when their H+- and lactose-binding sites are either both … or both … This prevents dissipation of the…without cotransport of lactose into the cell
fi lled; empty; H+ gradient
