Transporters and Channels

Question 1

What are 5 transport ‘channels’ types ?

Answer 1

• simple diffusion ( bilayer)
• aqueus diffusion ( channels )
• Facilitative transport ( may involve coupling of solute movements )
• active transport ( energy dependent) / against concentration( may involve coupling of solute movements )
• coupled transport -( eg exchange ) ( two solutes going opposite directions ) ( antiport !!!)

N.B. diffusion is independent of different molecules ( e.g. sodium and glucose )

Question 2

Transmembrane solute fluxes : - what can increase solute movement across a membrane/ cell layer

Answer 2

• increase in microvilli absorbance
• decrease in membrane thickness ( membrane is already thin !)
• increase in cell metabolism - helps generate gradients
• increase in pores ….

Question 3

What are two types of pores?

Answer 3

• water filled channels

- facilitative transporters

Question 4

cell membranes have …

Answer 4

low Diffusional permeability to many vital biological solutes including inorganic ions , sugars and amino acids.

Question 5

What are some important charachteristics of pores? ( transporters /channels )

Answer 5

• solute flux — predicted by passive diffusion ( avoids bilayer )
• substrate specific ( e.g. amino acids )
• transporters are saturable ( theoretiically - since diffusion is not saturable )
• specific inhibitors / inactivators

Question 6

what is the Transportome ?

Answer 6

it is the collection of all 1289 genes which are transporters and channels (406 ion channels / 883 transporters )
- it is 4% of human genome !

• these genes are classified into structurally related super- families and families !!!!

Question 7

how are ions moved ?

Answer 7

by an electrochemical gradient and concentration.

N.B. transporters(a.k.a carriers or permeases ) have much lower rate of transport than ion channels !!!

Question 8

A Gated Channel

Answer 8

voltage gated K+ channel

• various ligands open some gates
• large / fast solute flux when open !!!
• selectively related to size and charge of hydrated ion

e. g. Ca++ has bigger hyrdration in comparison to K+ and Na + since it has an increased charge !
- smaller ions have higher hydrated radius - since charge is higher in smaller ions !!!

Question 9

3 basic transporter mechanisms ?

Answer 9

• UNiport - one solute from one side to the other ( cis to trans direction- into cell )
• symport - two different solutes in same direction
• antiport - two different solutions in opposite directions ( quicker than Uniport !)

Question 10

what are some charachteristics of a basic transporter mechanism ? and what is this also called ?

Answer 10

• relatively small / slow solute flux
• selectivity ( tighter than channels ) - related to molecular interactions between solute ‘substrate’ and binding site on carrier protein.

the alternative access model

Question 11

differences between transporters and channels?

Answer 11

• transporters - work with solutes on both sides !
  channels- dont care if any solute is on any side - solute independent!

transport mechanisms are reversible ! ( N.B. facilitative transport )

substrate - loaded transporters change conformation quicker

therefore antiporter is quicker than uniport, leading to trans-stimulation and counter - transport ( TRANSPORTERS ONLY ,NOT CHANNELS !!)

counter transport - only occurs iwhen heteroexchange occurs
trans-simulation - only occurs when homoexchange occurs.

Question 12

there are two types of Antiport ?

Answer 12

• Antiport- Homoexchange - two solutes of same type

- Antiport - heteroexchange - two different solutes in different directions !

Question 13

Kinetic Characteristics of transporters ( using example of uniport )

Answer 13

(a) Cis - effects: -
- saturability :- substrate molecules ‘ compete ‘ for transporter binding sites
- stereospecificity - certain stereoisomers are better ‘ fit’ to binding site (e.g. D - sugars , L- amino acids ( biologically important forms )

(b) trans - effects
- exchange diffusion:- ( trans- substrate accelerates exchange)
- counter - transport :- trans-acceleration by a different substrate ( hetero- exchange )

Question 14

Channels vs Transporters ( Quantity vs Specificity )

Answer 14

Channels :

• selectively related to size and charge of hydrated ion
• conformational change ? ( but may be gated !!)
• diffusion - single file - through narrow point of channel ( saturable )

Transporters /Carriers:

• selectively related to molecular interaction between solute ‘ substrate ‘ and binding site on carrier protein (‘recognition criteria’)
• conformational change predicited
• carriage requires ‘ adsortpotion’ of solute from bulk fluid phases
• requirement for co-substrates in many cases
• exhibit counter transport

Question 15

What is the Michaelis - Menton ?

Answer 15

• Saturable Transport -

enzyme activity related to substrate concentration

Question 16

what is the michaelis menton equation?

Answer 16

v= vmax (S) /// km +(S)

v= velocity 
vmax= maximum velocity (mol/min) all binding sites occupied 
(S)= substrate concentration on CIS face of membrane (mol/l ) 
Km = michaelis constant (mol/l) - where 50% of sites occupied 

diffusion - shows linear line
transport - shows curve . which tails off.!

at low substrate concentration :

(S) < Km
V proportional to (S)
(v=Kx(S) )

At high substrate concentrations:

(S) > Km
V= Vmax

substrates show competitive inhibition with one another

( other molecules may exhibit non - competitive inhibition )

Question 17

Competitive vs non competitive competition ?

Answer 17

Competitive inhibitor :

• increase of substrate Km
• unchanged Vmax

non competitive inhibition :

• decreased Vmax
• unchanged Km

Question 18

in real cells….

Answer 18

• multiple transport components
• nature of dependence on any ‘co-substrate’ ( e.g. Na+ )
• binding of multiple substrates may be ordered
• diffusional components of flux

Question 19

there are three types of active transport ?

Answer 19

• primary
• secondary
• tertiary

coupling of solute transporter fluxes to create specific directional flows through facilitative transporters

(e. g. amino acids - eg glutamine (Gln) - non essential -
e. g. leucine (leu) essential - needed within the cell.

Question 20

What is endocytosis?

Answer 20

• may involved saturable receptor binding
• transmembrane movement
• no Trans - effects ( NO CIS and TRAN situation ! )