4 - Membrane Transport Flashcards
- restricts passage of most polar molecules
- different concentration of solutes in the cytosol and extracellular fluid
- membrane transport proteins
- transporters and channels
Membrane transport
2 main classes of membrane proteins
- Transporter (carriers)
- Channels
3 Solute carrier (SLC) transporters
- Uniporter
- Symporter
- Antiporter
An ABC transporters
pump
Principles of membrane transport
transport by carriers can be either active or passive, solute flow through channel proteins is always passive.
Moevement of solute in uniporter
facilitated diffusion
Symporter, antiporter, and pump move solute using active or passive transport?
Active transport
uses primary active transport
pump
Use secondary active transport
Symporter and antiporter
lipid bilayers are impermeable to ___
ions
The passage of molecules across membrane depends on ___ and _____
size and hydrophobicity
Example of hydrophic molecules
- O2
- CO2
- N2
- Steroid hormones
Example of small uncharged polar molecules
- H2O
- Urea
3.Glycerol - NH3
Example of large uncharged polar molecules
- glucose
- Sucrose
Example of ions
- H+
- Na+
- HCO3-
- K+
- Ca2+
- Cl-
- Mg2+
transfer solutes across cell
membranes
membrane transport proteins
- transfer specific molecular
species or a class of molecules - highly specific
transport proteins
Example genetic disorder that is due to single-gene mutation in transport proteins
cystinuria
- single-gene mutation
- autosomal-recessive defect in the reabsorptive transport of cystine from the urine or intestine into the bloodstream.
- Csytine accumulates in the urine. This build up leads to the formation of cystine stones in the kidney
cystinuria
All membrane proteins that have been studied in detail are ____ ____ ___
multipass transmembrane
proteins
means that their polypeptide chain cross the lipid bilayer multiple times
multipass transmembrane protein
pathway formed by membrane proteins which allows specific hydrophobic solutes to cross
protein-lipid pathway
transporters are also known as
carriers or permeases
- bind
- conformational changes
transporters
form continuous pores
channels
“downhill” transfer
passive transport
Movement of uncharged molecule across membrane is driven by
concentration gradient
Movement of charged molecule aross membrane is driven by
membrane potential
- uphill
- against their electrochemical
gradients
active transport
In most biological cells, the inside of the cell has ___ ___ ___
Negative electrical potential
negative
insided
electrical potential
Tightly coupled with energy
transporter
Process by which transporter transfer a solute resembles an ___-___ reaction
enzyme-substrate react
transporters have specific ___ sites for its solute
binding
transporters undergo a ___ ___ ___ after binding to their substrate molecules.
reversible conformational change
In some transporter proteins, an intermediate state called the ___ state exists during the transport cycle
occluded
represents a conformation where the binding site for the substrate is inaccessible from either side of the membrane.
Occluded state
3 Confirmational changes in transporters
- Outward-open
- Occluded
- Inward-open
3 main ways for cells to perform active transport
- Coupled transport
- ATP -driven pumps
- Light-or-redox-driven pumps
use energy stored in concentration gradients
Coupled transport
used energy from hydrolysis of ATP
ATP-driven pumps
Use energy from light
light- or redox-driven pumps
2 main type of transporter
- Uniporter
- Coupled transporters
- passive transport
- move one type of solute across membrane
uniporters
- transfer of one solute depends on the
transport of a second - move 2 different solute at the same time
Coupled transporters
2 types of coupled transporters
- Symporters
- Antiporters
- transfer 2 different solute in the same direction
Symporters
transfer 2 different solute in the opposite direction
Antiporters
harvest energy stored in the electrochemical gradient
Coupled transport
Example of coupled transporters
- Na+ - co-transported ion
- Na+-K+ pump
- Na+-driven symporters
- Na+ and glucose, and neurotransmitters
Example of symporter
Na+ - co-transported ion
Example of primary active transporter
Na+-K+ pump
Transporters have 10 or more ___ ____ that span
the membrane
a helices
are located midway through
the membrane
solute- and ion-binding sites
Allows the transporter to alternately open and close on other side of membrane
pseudosymmetric or inverted repeats
structure
Inverted repeats are also known as
pseudosymmetric
2 channels that evolved from coupled transporter in which gating mechanism were lost, allowing the channels to open simultaneously on both sides of the membrane
aquaporin water channel and
the Sec61 channel (ER)
3 classes of ATP-driven pumps
- P-type pumps
- ABC (ATP-binding cassette) transporters
- V-type pumps
-phosphorylates
-phosphorylate themselves during the transport cycle
P-type pumps
-pump small molecules
ABC (ATP-binding cassette) transporters
- turbine-like
- made from multiple different subunits
- pumps H+ ions into organelles like lysosomes, vacuoles, and synaptic vesicle
V-type pumps
structurally related to V-type pumps but function in reverse manner
F- type (ATP synthase)
use the H+ gradient across the membrane to drive synthesis of ATP
F-type (ATP synthase)
Pumps Ca2+ into the sarcoplasmic reticulum in muscle cells
P-type ATPase
pumps Ca2+ out of the cell
Ca2+ ATPase
intracellular storage of Ca2+ in
muscle cells
sarcoplasmic reticulum
release of Ca2+ into the cytosol (Ca+-release channels) can trigger ____ _____
muscle contraction
P-type ATPase structure
10 transmembrane α helices
connected to three cytosolic
domains
3 cytosolic domains
- nucleotide-binding domain
(N) - phosphorylation domain (P)
- activator domain (A)
Binds and hydrolyzes ATP, providing the energy needed for ion transport.
nucleotide-binding domain
(N)
Becomes phosphorylated during the transport cycle, which is essential for the conformational changes that drive ion transport
phosphorylation domain (P)
Regulates the dephosphorylation of the P-domain and resets the pump after each cycle.
activator domain (A)]
established Na+
and K+ gradient across the
plasma membrane
Na+-K+ pump/Na+-K+ ATPase
Has higher concentration in the cell
K+
Has low concentration in the cell
Na+
Na+-K+ pump has ___-____ antiporter
ATP-driven antiporter
___ out, ____ in
Na+
K+
Na+-K+ pump regulate ____ __
cytosolic pH
The pump creates an ____ because it moves more positive charges (Na⁺) out than it brings in (K⁺), contributing to the cell’s negative internal charge.
electrogenic
create an
electric potential
electrogenic
-contains highly conserved ATPase domains (ATP-binding
“cassettes)
ABC transporters
ABC transporters contains highly conserved ______ ______
ATPase domains (ATP-binding cassettes)
brings together two domains
ATP binding
leads to domain dissociation
ATP hydrolysis
alternately expose solute-binding sites
ABC transporters
ABC transporters expose their ___-___ sites alternately
solute-binding sites
ABC transporters were first found in ____
bacteria
In bacteria, it captures nutrients and deliver them to ABC transporters
auxiliary mechanism
-pump hydrophobic drugs out of the cytosol
ABC transporter
What do ABC transporters pump out of the cytosol?
hydrophobic drugs
- A member of ABC transporter family that pump hydrophobic drugs out of
the cytosol - present at elevated levels in human cancer cells and makes the cell simultaneously resistant to a variety of chemically unrelted cytotoxic drugs that are widely used in cancer chemotherapy
multidrug resistance (MDR) protein
or P-glycoprotein
causes malaria, pumps out chloroquine
Plasmodium falciparum
P. falciparum have amplified a gene encoding an ABC transporter that pumps out ___
chloroquine
TAP transporter
transporter associated with
antigen processing
-pumps peptides from the cytosol to ER lumen
transporter associated with
antigen processing (TAP transporter)
peptides in cytosol are prodced by?
protein degradation in proteasomes
peptides are carried from ER to ___ ___, where they are displayed for scrutiny by cytotoxic T lymphocytes
cell surface
-scrutinize the peptides on the cell surface
- kills the cell if the peprides are derived from virus or other microorganism
cytotoxic T lymphocytes
- member of the ABC transporter family which was discovered through studies of cystic fibrosis
-Cl- transport protein - regulates ion concentrations in the extracellular fluid, esp. in lung
cystic fibrosis transmembrane conductance regulator protein (CFTR)
caused by mutation in the gene encoding CFTR
cystic fibrosis
- form pores across membrane
-one class found in virtually all animals forms gap junctions between adjacent cell - not energy coupled, “downhill”
- diffuse down electrochemical
gradients
Channels
– connects the cytoplasm of two cells
gap junctions
- inorganic ion transport
ion channels
- water channels
- allow water to move more rapidly
- narrow pore that allows water molecules to traverse the membrane in single file
aquaporins
2 properties of ion channels that distinguish them from aqueous pores
- Ion selectivity
- gated
-allow inorganic ions to pass
- has selectivity filters
ion selectivity
the permeating ions have to shed most or all of their water molecules to pass through the narrowest part of the channel called____
selectivity filters
4 Main types of stimuli known to cause ion channels to open
- Voltage-gated
- Ligand-gated (extracellular ligand)
- Mechanically gated
open briefly and then close again
gated
2 typesnof ligand-gated channel
- Extracellular mediator
- Intracellular mediator
extracellular mediator
transmitter-gated channel
intracellular mediator
ion-gated channels or
nucleotide-gated channels
- voltage-gated
- membrane potential
- K+ leak channels
- active electrogenic pumping and
from passive ion diffusion
ion channels
Opens even in an unstimulated or “resting” cell
K+ leak channels
purpose of k+ leak channels
making plasma membrane much mkre permeable to K+ than to other ions.
bacterial K+ channels are made from
4 identical transmembrane subunits
Together form a central pore through the membrane
four identical transmembrane
subunits
selectivity filter
pore helix
each four identical transmembrane
subunits contributes two transmembrane __ ____
a helices
the polypeptide chain that connects the 2 transmembrane helices forms a short __ __ and ___ ___
a helix (pore helix) and crucial loop
pore helix and crucial loop that protrudes into the wide section of cone to form ___
selectivity filter
To enter selective filter, K+ ions must?
K+ ion must lose almost all of its
bound water molecules
K+ ions interact with ___ ___ lining the filter
Carbonyl oxygen
oxygen are rigidly spaced at the exact distance to accomodate a __
K+ ion
Cannot enter selectivity filter because the carbonyl oxygen are too far away from them to compensate for the energy expense associated with the loss of water molecules required for entry
Na+ ions
What do the structure of a closed K+ channel shows
tilting the inner helices
structural and functional diversity of voltage-gated Ca2+ channels is due to
alternative splicing of RNA transcripts
Voltage-gated Ca2+ (also K+ and Na+) channels belong to a __ superfamily
large
Cause of nerve, muscle, brain, or heart disease
mutant genes encoding ion
channels
mutation in genes that encode voltage-gated Na+ channels in skeletal muscle cells can cause ___
myotonia
A condition in which there is a delay in muscle cell relaxation after voluntary contraction, causing painful muscle spasms
myotonia
mutation in genes that encode Na+ or K+ channels in the brain can cause ___
epilepsy
-mechanically gated ion channels
-capable of responding to mechanical forces
- extremely rare channel
mechanosensitive channels
How many mechanically gated ion
channels are found in hair cells of human cochlea
50-100
Open in response to mechanical stretching of the lipid bilayer
mechanosensitive channels found in bacterial plasma membrane
mechanical stretching of the
lipid bilayer
hypotonic
When bacterium experience hypotonic condition, what happens tomthe cell
Cell swells as water seeps in due to an increase in the osmotic pressure
It will open when pressure rises to dangerous levels to allow small molecules to leak out
mechanosensitive channels
receive, conduct, and transmit signals
neurons
Fundamental task of neurons
receive, conduct, and transmit signals
Despite the varied significance of signaks carried by different classes of neurons, the form of signal is always the same, consisting ?
changes in the electrical potential across the neuron’s plasma membrane
-traveling wave of electrical excitation
- carry a message without attenuation from one end of a neuron to the other
- consequence of the properties of voltage-gated cation channels
action potential or nerve impulse
- electrically excitable cells
- responsible for generating the action potentials
voltage-gated cation channels
triggers an actional potential
depolarization of the plasma membrane
In nerve and skeletal cells, a stimulus that causes sufficient depolarization promotly open ___
open Na+ channels
open Na+ channels, Na+ enters the cell
self-amplification
process
The electrical potential in the local region of membrane has shifted from its resting value of about
-40mV to +50mV
- photosensitive ion channels
- contains covalently bound retinal group
- light-driven cation channel
- revolutionized the study of neural circuits
channelrhodopsins
- increase the rate at which the axon can conduct an action potential
- formed by glial cell
myelination
myelin is formed by
glial cells
glial cells that myelinte axons in peripheral nerves
Schwann cells
glial cells that myelinte axons in central nervous system
oligodendrocytes
little current can leak across it
myelin
- interrupt myelin sheath
- where almost all the Na+ channelsnin the axon are concentrated
-allows for saltatory conduction
nodes of Ranvier
- demyelinating disease
- immune system destroys myelin sheaths in some region of CNS
multiple sclerosis
where neuronal signals are transmitted
synapses
____ cell – ____cleft – ____cell
presynaptic cell – synaptic cleft –
postsynaptic cell
Release of neurotransmitter is from
presynaptic cell
neurotransmitter bins to and opens
transmitter-gated ion channel in the postsynaptic cell
transmitter-gated ion channel as receptor
highly selective binding sites
-highly selective binding sites
- selective in the type of ions that they let pass across the plasma membrane
-neurotransmitter receptors
transmitter-gated ion channel
- open cation channels
- influx of Na+ or Ca2+
- firing an action potential
excitatory neurotransmitters
- open Cl- or K+ channels
- suppresses firing
inhibitory neurotransmitters
Used as excitatory neurotransmitters
acetylcholine, glutamate, serotonin
Used as inhibitory neurotransmitters
γ-aminobutyric acid (GABA) and glycine
2 major classes of neurotransmitter receptors
ionotropic receptors
metabotropic receptors
ion channels and feature at fast chemical
synapses
ionotropic receptors
– G-protein coupled receptors that bind to all
other neurotransmitte
metabotropic receptors
- well-studied transmitter-gated ion channel
- composed five transmembrane polypeptides
acetylcholine receptors
__ acetylcholine binds →___change → ___
channe
two
conformational
open
involves the sequential activation of five
different sets of ion channels
neuromuscular transmission
