Chapter 10: Membrane Transport Flashcards
Nonmediated transport occurs through simple
diffusion
occurs through the action of specific carriers
mediated transport
The driving force for the nonmediated flow of a substance through a medium is what
chemical potential gradient
Consequently, nonpolar molecules such as steroids and O2 readily diffuse through biological membranes by
nonmediated transport
types of Mediated transport
Passive-mediated transport
Active transport
in which a specific molecule flows
from high concentration to low concentration.
Passive-mediated transport
or
facilitated diffusion
in which a specific molecule is transported from low concentration to high concentration, that is, against its concentration gradient. Such an endergonic process must be coupled to a sufficiently exergonic process to make it favorable
Active transport
Substances that are too large or too polar to diffuse across lipid bilayers on their own may be conveyed across membranes via proteins or other molecules that are variously
called
carriers, permeases, channels, and transporters.
are organic molecules of diverse
types, often of bacterial origin, that increase the permeability of membranes to ions. These molecules often exert an antibiotic
effect.
Ionophores
transports up to 104 K+ ions per second across a membrane. It has 10,000-fold greater binding affinity for K+ than for Na+
valinomycin
are normally shut and only open transiently to perform some specific task for the cell
Ion channels
Types of Ion channels
Mechanosensitive
Ligand-gated
Signal-gated
Voltage-gated
channels 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 open in response to an extracellular chemical stimulus such as a neurotransmitter
Ligand-gated
channels open on intracellularly binding a Ca 2+ ion or some other
signaling molecule.
Signal-gated
channels open in response to a change in membrane potential.
Voltage-gated
cell specialized for electrical signaling
neuron
causes Na+ channels to open so that Na+ ions spontaneously flow into the cell.
The stimulation of a neuron
induces neighboring voltage-gated Na+ channels to open causing the action potential to travel (10 m/s) in one direction along the length of the nerve cell.
increase in membrane potential
transient change in the membrane potential
action
potential
induces nearby voltage-gated K+
channels to open. This allows K+ ions to spontaneously flow out of the cell
local depolarization of the membrane
allowing the K+ ions to spontaneously flow out of the cell
repolarization
what determines what types of substances can pass through.
the size of the central aqueous channel and the residues that form its walls
Mediate the Transmembrane Movement of Water
Aquaporins
tissues that rapidly transport
water
kidneys, salivary glands, and lacrimal
glands
permit the passage of water molecules at an extremely high rate
but do not permit the transport of
solutes or ions, including, most
surprisingly, protons
Aquaporins
free passage would discharge the cell’s membrane potential.
protons
a nonequilibrium distribution of ions on either side of the plasma membrane
Mammalian cellsmaintain
Potassium ions passively diffuse from the cytoplasm to the extracellular space through transmembrane proteins known as
K+ channels
Transport Proteins Alternate between Two Conformations
erythrocyte glucose transporter
(also known as GLUT1)
It is triggered by the motion of a positively charged protein helix
Voltage Gating in K+ Channels
Types of mediated transport
uniport
symport
antiport
involves the movement of a single molecule at a time.
uniport
what kind of transport is Lactose permease
symporter
what kind of transport is GLUT1
uniport
what kind of transport is oxalate transporter
antiport
simultaneously transports two different molecules in opposite directions
antiport
is an endergonic process that, in most cases, is coupled to the
hydrolysis of ATP
Active transport
Families of A TP-dependent transporters
P-type ATPases
F-type ATPases
V-type ATPases
A-type ATPases
ABC transporters
undergo phosphorylation as they transport cations such as Na+,
K+, and Ca2+ across the membrane
P-type ATPases
are proton-transporting complexes located in mitochondria and
bacterial membranes. Instead of using the free energy of A TP to pump protons against their gradient, these proteins operate in reverse in order to synthesize ATP
F-type ATPases
resemble the F-typeA TPases and occur in plant vacuoles and
acidic vesicles such as animal lysosomes
V-type ATPases
are named for their A TP-binding cassette and transport a wide
variety of substances, including ions, small metabolites, and drug molecules
ABC transporters
The most thoroughly studied active transport systems is the
Na+–K+ –ATPase
or
(Na+–K+) pump
what kind of transport is Na+–K+ –ATPase
antiport
pumps Na+ out of and K+ into the cell
with the concomitant hydrolysis of intracellular ATP .
(Na+–K+) pump
is one the most commonly prescribed cardiac drugs which inhibits the (Na+–K+)–ATPase. This causes an increase in intracellular [Na+] which stimulates the cardiac (Na+–Ca2+) antiport system to pump Na+ out of and Ca2+ into the cell
Digitalin
triggers muscle contraction and increases the intensity of heart muscle
The release of Ca2+
A TP phosphorylates the transporter only in the presence of
Na+
resulting phosphorylated
aspartic residue hydrolyzes only in the presence of
K+
The [Ca2+] in the cytosol (~0.1 μM) is four orders of magnitude less than it is in the
extracellular spaces
This large concentration gradient is maintained by the active transport of Ca2+ across the plasma membrane and the endoplasmic reticulum by a
Ca2+–A TPase.
actively pumps two Ca2+ ions out of the cytosol at the expense of ATP hydrolysis, while countertransporting 2 or 3 protons
Ca2+ pump
trigger numerous cellular responses including muscle contraction, the release of neurotransmitters, and glycogen breakdown.
Transient increases in cytosolic [Ca2+]
pump ions, sugars, amino acids, and other polar and nonpolar substances.
ABC transporters
a member of the ABC class of
transporters, pumps a variety of amphiphilic substances—including many drugs—out of the cell.
P-glycoprotein
Active Transport May Be Driven by Ion what?
gradients
The immediate energy source for this “uphill” transport process is the what?
Na+ gradient
concentrates glucose inside the cell
The Na+–glucose transport system
