membrane transport

Question 1

what does a signal tranduction cacade lead to?

Answer 1

amplification of orignal signal

Question 2

briefly describe active transport

Answer 2

Process where you can pump ions across a membrane using ATP. Can use a variety of transporters+channels
- co transporter
- ligand gated
- voltage gated
- mechanically gated

Question 3

how are Na2+, Ca2+, K+ channels structurally similar?

Answer 3

have 6 transmembrane domain helixes
all have S4 voltage sensor subunit made of many +amino acids
around 70kD

Question 4

what molecule is calcium (signalling) regulated by?

Answer 4

calmodulin!
which exposes the hydrophobic residues of calcium =changes conformation so signal cascade can continue

active site of receptors become active and can target effector protein (PKA)

Question 5

where is calcium stored?

Answer 5

biggest store is ER
also cytosolic, golgi, secretory vesicles
mitch temporarily

Question 6

describe structure of calmodulin

Answer 6

has 2 domains joined by a flexible linker and each domain has 2 EF hands that can bind to one calcium molecule each

EF hands found on many binding proteins

Question 7

Describe the ‘‘off mechanisms’’ in calcium signalling

Answer 7

SERCA pump
NCX changer
Ca buffers and chaperone like calsequestrin

problems in these can lead to pathologies!

Question 8

What does the SERCA pump do?

Answer 8

pumps the Ca2+ from cytosol to the ER/SR lumen using ATP

Question 9

what does calsequestrin do?

Answer 9

buffer/chelatator protein within SR and binds to calcium
helps with the SERCA pump so it doesn’t have to pump against a v high concentration gradient

Question 10

How can SERCA be regulated?
> both +/- control

Answer 10

phospholambin inhibit SERCA
Under b-adrenergic stimulation it gets phosphorylated so this is reversed

calsequestrin activates

Question 11

how does the sodium-calcium exchanger work? (NCX)

Answer 11

3 sodium IN CELL, 1 calcium out of cell so helps with calcium efflux

this exchanger is electrogenic and creates a current/ membrane potential

Question 12

why is NCX better than ATPases?

Answer 12

NCX works quicker than ATPases and at higher concentrations

Question 13

what is the normal physiological range cytosolic and extracellular calcium

Answer 13

extracellular - 1.4mMol
intracellular 100nMol at rest
so calciuim has one of the largest ioninic gradients

Cells use transient changes in gradient to drive cellular processes

Question 14

how can calcium be put into its stores?

Answer 14

SERCA pumps are located on the membrane of endo/sarcoplasmic reticulum to store calcium

Question 15

what can too much mitch uptake of calcium lead to?

Answer 15

too much ROS generation and eventual apoptosis

and sig reduction in energy production and eventual necrosis

Question 15

How do calcium spikes stay local to PAC?

Answer 15

secretory granules
are surrounded by mitch which have mitochondrial uniporters to uptake calcium/buffering a global rise of Ca2+

Ca+ used for energy production for secretion

Question 16

how does calcium maintain its low intracellular/cytosolic conc? (100nmol)

Answer 16

1. using pumps and exchangers
2. sequester/store calcium in SR, ER, mitch stores
3. chelating calcium, having it bound to proteins like calbindin sequestrin

Question 17

which receptors are present on the ER?

Answer 17

Ip3 receptors and ryanodine receptors which mediate calcium release

Question 18

how do non excitable cells become activated?

e.g Pancreatic Acinar Cells

Answer 18

ey do not have voltage gated ion channels so cannot generate an AP

rely on a second messenger allowing
calcium release from intracellular stores which can then activate processes like granular secretion as seen in PAC

Question 19

how do excitable cells become activated?

Answer 19

through voltage gated calcium channels where membrane potential changes lead to depolarisation and conformational change of RyR and release of calcium from store

pulses of calcium

Question 20

how can we replenish the ER store ?

Answer 20

via store operated calcium entry
use the STIM1 ER membrane proteins which respond to low calcium in ER and interact with plasma membrane to TO ACTIVATE ORAI proteins and activate the CRAC channels
calcium entry then moves through the ER pump to remain in ER

to terminate SOCE, de-oligomerisation of STIM1 occurs when cytosolic Ca2+ is increased

Question 21

generally speaking how would elevated calcium levels result in pathology in AP?

Answer 21

high cytosolic calcium can detabilize zymogen granules and expand and rupture

this releases digestive enzymes like trypsin and result in cell death -> may spread to neighbouring cells = inflammation

Question 22

what could be some causes of elevated calcium channels

Answer 22

bile acids
alcohol metabolites
drugs like aspariganases

Aspariganases used to treat leukemia

Question 23

what is the issue with CRAC channels in AP?

Answer 23

CRAC channels on plasma membrane open to replenish the ER store

but this can EXACERBATE THE Problem and further contribute to Ca+ overload

Question 24

How are cAMP concentrations established?

Answer 24

Can be compartmentalised in cell and slowly degraded by phosphodiesterase

Can be immobile at low concentrations and upon stimulus there is fast diffusion rate of cAMP in cell so we see an effect

Question 25

Why do cell membranes require so many ion channels, transporters and atpase?

Answer 25

Because the lipid bilayer is impermeable to charged ions so transport mechanisms other than diffusion need to be developed

Question 26

where can EF hands be found?

Answer 26

they able to sense calcium
> calmodulin has 2 EF hands so 4 binding sites
> STIM1/2 have EF too to sense ER store of calcium