11 - Transport across membranes Flashcards
3 types of active transport
- ATP-driven. A solute is transported across the membrane against its gradient by using energy from ATP hydrolysis
- coupled transporters - the downhill transport of one solute is used to simultaneously transport another solute against its own gradient - can be symport or antiport.
- Light or redox-driven. uses energy from light or redox (bacteriorhodopsin and cytochrome c, respectively).
Transporters generally have their solute-binding sites midway through the membrane. In inward-open and outward-open conformations, these binding sites are accessible through passageways from one side of the membrane but not the other. When neither passage is open it is called occluded.
ATP-dependent transporters
P-type pumps are often involved in maintaining ion gradients
ABC transporters (ATP-Binding cassettes) transport small molecules across membranes
V-type ATPases: pump protons into vacuoles or lysosomes (acidification)
F-type ATPases work in reverse, using passage of H+ with their gradient to make ATP
the Na+ K+ pump
P-type ATPase
mainly responsible for high [K+] in cells.
the graident made by this pump is used for nutrient transport and pH maintanence
pumps 3 Na+ out and 2 K+ in (antiport)
high [Na+] outside cell, high [K+] inside cell
Ca 2+ pump
in muscle cells, there is a sarcoplasmic retikulum (SR). The SR is a specialized type of ER that forms a network of tubular sacs in the muscle cell cytoplasm, and it serves as an intracellular store of Ca2+. When an action potential depolarizes the muscle cell plasma membrane, Ca2+ is released into the cytosol from the SR through Ca2+ release channels, stimulating the muscle to contract. The Ca2+ pump moves Ca2+ from the cytosol back into the SR
ABC transporters structure and function
2 cytosolic ATP-binding domains, and 2 hydrophobic domains with transmembrane alpha-helices.
unidirectional transport of amino acids, oligo/polysaccharides, peptides and proteins. in euks, it mostly transports stuff out of the cytosol (to extracellular matrix or ER), in proks they transport both directions
fixed anions / cations
anions/cations that are confined to the inside of the cell
channel proteins
passive transport, typically gated, selective (typically narrow)!!
channel proteins allow passage of inorganic ions but not water (Na+, K+, Ca++, Cl-), selectivity achieved by selectivity filter
Can be voltage gated, ligand-gated (intra- or extracellular ligand), or mechaninically gated
membrane potential maintenance
membrane potential = difference in electrical charge on two sides of the membrane, due to a slight excess of positive/negative ions on one side and a deficience on the other.
Aquaporins
Aquaporins must allow water to pass, but not ions (dont want to disrupt the ion gradients). This is accomplished by a narrow pore thar allows water molecules to traverse the membrane in a singla file, following the path of carbonyl oxygens that line one side of the pore. Hydrophobic AAs line the other side, the pore is too narrow for any hydrated ions (energy cost of dehydrating is not worth it).
selectivity filter in ion channels
narrow channels that do not allow hydrated ions to pass, they must often pass in a single file and only those of appropriate charge can do so.
K+ leak channels
K+ leak channels can open without stimulus, and allow K+ to pass through. The purpose is to make the membrane more permable to K+ than other ions, and are therefore important for maintaining the membrane potential.
the K+ channel structure and function
size is not a critera, as Na+ are just as small. - Cytosolic side contains negatively charged amino acids which attract cations (here: K+) and repel anions.
K+ must lose associated H2O molecules to pass through the channel.
Dehydration requires energy which is regained by interaction with CO.
Na+ can not interact with CO (too small) so cannot regain energy from dehydration.
Voltage-gated Na+ channel
Voltage-gated cation (+) channels are responsible for
generation of action potentials
Important components:
- central channel
- selectivity filter
- voltage sensor (gating) - detects a change in membrane potential, the outside becomes more negative which will make the voltage sensor (positively charged) be draw towards it, and the channel will open due to the change in conformation
- inactivation gate, a flexible tail that works as a plug
- Na+ enters the cell along the concentration gradient which depolarizes the membrane thus
opening adjacent channels, propagating the wave (positive feedback.) - Na+ channels are automatically inactivated after a randomly variable period in the open
state, which prevents depolarization moving backwards.
signaling through neurons (input/output)
input signals recevied at synapses, outputs are changes in electrical potentials in the membrane
How are action potentials propagated?
voltage-gated Na+ channels open when there is depolarization of the membrane. This makes Na+ flow into the cell, activating nearby Na+ channels (positive feedback). After opening, there is automatic inactivating of the channel. Before the channel can open again, it must return to the closed (not inactivated, but closed) conformation. This prevents the signal from moving back again, pushing it forward.
Voltage-gated K+ channels normalize membrane potential again.
Myelin
specialized cell membrane made up of multiple tightly wrapped layers- facilitates AP propagation (electrical insulation)
Myelin sheath has small breaks (nodes of ranvier) where almost all the Na+ channels are located. This allows the jumping of depolarization between the nodes, and thus quicker travel of the signal. this is called salatory conduction.
How are chemical signals converted into electrical ones in synapses?
cells are electrically separated, but when an action potential reaches the presynaptic site, the deolarization of the membrane opens voltage-gated Ca++ channels. Ca++ influx triggers the release nto the cleft of small signal molecules known as neurotransmitters, which can trigger an electrical change (depolarization) in the postsynaptic cell by opening transmitter-gated ion channels.
Chemical synapses
excitatory neurotransmitters open cation channels (Ca2+ / Na+
influx)
• inhibitory neurotransmitters open either Cl- or K+ channels
(buffering the membrane potential)
• Acetylcholine, glutamate, serotonin normally excitatory
• Glycine and γ- aminobutyric acid (GABA) normally inhibitory
Ionotropic receptors are ion channels e.g. skeletal muscle
acetylcholine receptor. Fast, simple, brief..
• Metabotropic receptors are G-protein coupled receptors which
regulate ion channels indirectly via small intracellular signaling
molecules e.g. neuropeptide GPCRs. Slow, long lasting
modulation of neurotransmission
Neuromuscular junctions - how mucles contract
Five things happen when neuromuscular junctions are activated:
- Membrane depolarization opens voltage-gated Ca++ channel, which causes local release of acetylcholine
- acetylcholine binds ligand-gated cation channels, causing an influc og Na+ and local membrane depolarization
- local depolarization opens voltage-gated Na+ channels in local propagation
- local propagation activates voltage-gated Ca++ release channels in the transverse tubules (specialized structures)
- coupled to activation of Ca++ release channels in the SR that open transiently and release Ca++ into the cytosol leading to contraction of myofibrils in the muscle cell.
Long term potentiation
long term potentiation and long term depression are involved in learning and memory.
