molecular transport across the cell membrane Flashcards
what type of trans-membrane (intrinsic) cell membrane protein is involved in the transport of water molecules?
aquaporin
who many molecules of water can pass through an aquaporin channel at one time and why?
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.
how many different types of aquaporins are found in the body which differ in time/demand/number thus controlling differential water permeability/function/time?
13
what is the general structure of a aquaporin monomer?
6 transmembrane segments, 5 extra membrane loops.
how many monomers make up one aquaporin molecule?
4 (they exist as tetramers with each monomer acting as a single water channel
how does ADH increase water reabsorption in the kidneys?
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)
what features of a diffusing substance influences its specificity to a certain type of membrane protein?
pore diameter (eg aquaporins), charge/chemical property (e.g. facilitated carrier proteins)
how does facilitated diffusion work (also called carrier-mediated diffusion)?
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.
how is the rate of diffusion limited in facilitated diffusion?
by the Vmax of the carrier protein (the carrier protein requires time to open and close)
what are the two different ion channels in the plasma membrane?
un-gated channels (always open) and gated channels (open and close)
what are the characteristics of un-gated ion channels?
these channels are always open. selectivity is determined by size, shape and charge of chanel and ion.
what are the characteristics of gated channels?
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)
how does sodium pass across the plasma membrane? (by voltage gated channel)
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.
how do potassium ions diffuse across the plasma membrane?
carbonyl oxygens in the selectivity channel strip the water molecules from the potassium molecule which allows them to pass through.
Do these substances require active transport across the membrane: Na, K, Ca, Fe, H, Cl, iodide, urates, some sugars, most amino acids
yes!
what is primary active transport?
the transport of molecules against an electrochemical gradient at the expense of energy in the form of ATP
what is secondary active transport?
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
how big is the alpha sub unit of the sodium/potassium pump?
100kda
how big is the beta sub unit of the sodium/potassium pump?
55kda
what is the structure of the alpha sub unit like?
it has 2 external potassium binding sites, 3 internal sodium sites and ATPase activity occurs near the sodium binding sites.
how is sodium extruded and potassium introduced to the cell?
by the phosphorylation of the sodium/potassium pump which causes a conformational change of the protein
how much percent does the sodium/potassium pump contribute to the membrane potential?
10%
what are other examples of primary active transport?
Ca2+ ATPase transporter and H+ ATPase transporter
where would the Ca2+ ATPase transporter be found and what is its purpose (another primary active transport protein)?
present on the cell membrane and the sarcoplasmic reticulum in muscle fibres; maintains a low cytosolic Ca2+ concentration
where would the H+ ATPase transporter be found and what is its purpose? (another primary active transport protein)
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
what is a sodium symporter?
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.
what is a sodium anti porter?
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
