11 - Transport across membranes

Question 1

3 types of active transport

Answer 1

1. ATP-driven. A solute is transported across the membrane against its gradient by using energy from ATP hydrolysis
2. coupled transporters - the downhill transport of one solute is used to simultaneously transport another solute against its own gradient - can be symport or antiport.
3. 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.

Question 2

ATP-dependent transporters

Answer 2

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

Question 3

the Na+ K+ pump

Answer 3

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

Question 4

Ca 2+ pump

Answer 4

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

Question 5

ABC transporters structure and function

Answer 5

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

Question 6

fixed anions / cations

Answer 6

anions/cations that are confined to the inside of the cell

Question 7

channel proteins

Answer 7

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

Question 8

membrane potential maintenance

Answer 8

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.

Question 9

Aquaporins

Answer 9

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).

Question 10

selectivity filter in ion channels

Answer 10

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.

Question 11

K+ leak channels

Answer 11

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.

Question 12

the K+ channel structure and function

Answer 12

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.

Question 13

Voltage-gated Na+ channel

Answer 13

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

1. Na+ enters the cell along the concentration gradient which depolarizes the membrane thus
   opening adjacent channels, propagating the wave (positive feedback.)
2. Na+ channels are automatically inactivated after a randomly variable period in the open
   state, which prevents depolarization moving backwards.

Question 14

signaling through neurons (input/output)

Answer 14

input signals recevied at synapses, outputs are changes in electrical potentials in the membrane

Question 15

How are action potentials propagated?

Answer 15

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.

Question 16

Myelin

Answer 16

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.

Question 17

How are chemical signals converted into electrical ones in synapses?

Answer 17

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.

Question 18

Chemical synapses

Answer 18

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

Question 19

Neuromuscular junctions - how mucles contract

Answer 19

Five things happen when neuromuscular junctions are activated:

1. Membrane depolarization opens voltage-gated Ca++ channel, which causes local release of acetylcholine
2. acetylcholine binds ligand-gated cation channels, causing an influc og Na+ and local membrane depolarization
3. local depolarization opens voltage-gated Na+ channels in local propagation
4. local propagation activates voltage-gated Ca++ release channels in the transverse tubules (specialized structures)
5. 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.

Question 20

Long term potentiation

Answer 20

long term potentiation and long term depression are involved in learning and memory.