molecular transport across the cell membrane

Question 1

what type of trans-membrane (intrinsic) cell membrane protein is involved in the transport of water molecules?

Answer 1

aquaporin

Question 2

who many molecules of water can pass through an aquaporin channel at one time and why?

Answer 2

one: the channel is only one molecule of water wide. (this also means that aquaporins do not permit any other ions to pass through them.

Question 3

how many different types of aquaporins are found in the body which differ in time/demand/number thus controlling differential water permeability/function/time?

Answer 3

13

Question 4

what is the general structure of a aquaporin monomer?

Answer 4

6 transmembrane segments, 5 extra membrane loops.

Question 5

how many monomers make up one aquaporin molecule?

Answer 5

4 (they exist as tetramers with each monomer acting as a single water channel

Question 6

how does ADH increase water reabsorption in the kidneys?

Answer 6

by up-rgulating AQP-2 channels in the epithelial cells of the late distal tubules and collecting ducts. (increase in AQP-2 channels allow increased water up-take. AQP-3 channels constant)

Question 7

what features of a diffusing substance influences its specificity to a certain type of membrane protein?

Answer 7

pore diameter (eg aquaporins), charge/chemical property (e.g. facilitated carrier proteins)

Question 8

how does facilitated diffusion work (also called carrier-mediated diffusion)?

Answer 8

the molecule being transported binds to a spot in the channel protein causing a conformational change in the protein which releases the molecule to the other side of the plasma membrane.

Question 9

how is the rate of diffusion limited in facilitated diffusion?

Answer 9

by the Vmax of the carrier protein (the carrier protein requires time to open and close)

Question 10

what are the two different ion channels in the plasma membrane?

Answer 10

un-gated channels (always open) and gated channels (open and close)

Question 11

what are the characteristics of un-gated ion channels?

Answer 11

these channels are always open. selectivity is determined by size, shape and charge of chanel and ion.

Question 12

what are the characteristics of gated channels?

Answer 12

these channel can open and close. Their selectivity is determined by voltage (e.g. voltage-gated sodium channels), chemical properties (e.g. nicotinic acetylcholine receptor channels)

Question 13

how does sodium pass across the plasma membrane? (by voltage gated channel)

Answer 13

the sodium channels are lined with amino acids with negatively charged side groups which pull the sodium ion away from its water shell, releasing the smaller unhydrated sodium ion which can then diffuse through the channel.

Question 14

how do potassium ions diffuse across the plasma membrane?

Answer 14

carbonyl oxygens in the selectivity channel strip the water molecules from the potassium molecule which allows them to pass through.

Question 15

Do these substances require active transport across the membrane: Na, K, Ca, Fe, H, Cl, iodide, urates, some sugars, most amino acids

Answer 15

yes!

Question 16

what is primary active transport?

Answer 16

the transport of molecules against an electrochemical gradient at the expense of energy in the form of ATP

Question 17

what is secondary active transport?

Answer 17

transport is driven by the energy stored in the electrochemical gradient of another molecule (usually sodium) which was already pumped into/out of the cell using active transport

Question 18

how big is the alpha sub unit of the sodium/potassium pump?

Answer 18

100kda

Question 19

how big is the beta sub unit of the sodium/potassium pump?

Answer 19

55kda

Question 20

what is the structure of the alpha sub unit like?

Answer 20

it has 2 external potassium binding sites, 3 internal sodium sites and ATPase activity occurs near the sodium binding sites.

Question 21

how is sodium extruded and potassium introduced to the cell?

Answer 21

by the phosphorylation of the sodium/potassium pump which causes a conformational change of the protein

Question 22

how much percent does the sodium/potassium pump contribute to the membrane potential?

Answer 22

10%

Question 23

what are other examples of primary active transport?

Answer 23

Ca2+ ATPase transporter and H+ ATPase transporter

Question 24

where would the Ca2+ ATPase transporter be found and what is its purpose (another primary active transport protein)?

Answer 24

present on the cell membrane and the sarcoplasmic reticulum in muscle fibres; maintains a low cytosolic Ca2+ concentration

Question 25

where would the H+ ATPase transporter be found and what is its purpose? (another primary active transport protein)

Answer 25

found in parietal cells of gastric glands (HCl secretion) and intercalated cells of renal tubules (controls blood pH); concentrates H+ ions up to 1 million fold

Question 26

what is a sodium symporter?

Answer 26

a mechanism of secondary active transport where a molecule binds to the symporter along with sodium ions and both enter the cell by the high electrochemical pressure for sodium to move into the cell.

Question 27

what is a sodium anti porter?

Answer 27

a trans membrane protein which transports molecules in the opposite direction of sodium (sodium goes into the cell, other molecule moves out) by the electrochemical energy sodium has created across the membrane