3. Role of Membranes as Permeability Barriers

Question 1

What is passive transport dependent on?

Answer 1

Permeability and concentration gradient.

Question 2

What are the important roles of transport processes?

Answer 2

Maintenance of ionic composition, maintenance of intracellular pH, regulation of cell volume, concentration of metabolic fuels and building blocks, extrusion of waste products of metabolism and toxic substances, the generation of ion gradients necessary for the electrical excitability of nerve and muscle.

Question 3

How are proteins transported across the membrane?

Answer 3

Via a gated pore. The protein binds, causes a conformational change so the gate releases the substrate on the other side of the membrane.

Question 4

How can transport be separated into active and passive?

Answer 4

Using the concentration ratio: passive transport goes down concentration gradient so releases free energy, active transport goes up concentration gradient so needs energy input.

Question 5

What does active or passive transport depend on?

Answer 5

Membrane potential,so the concentration gradient and the electrical charge.

Question 6

What is active transport?

Answer 6

Movement of ions or molecules against an unfavourable concentration and/or electrical gradient. Energy used directly or indirectly from ATP hydrolysis, uses 30-50% of the cell’s ATP?

Question 7

What are the concentrations of the following ions across the cell membrane? Na+, Cl-, Ca2+, and K+.

Answer 7

Na+ 12mM inside, 145mM outside.
Cl- 4.2mM inside, 123mM outside.
Ca2+ 10^-4mM inside, 1.5mM outside.
K+ 155mM inside, 4mM outside.

Question 8

What is an example of a primary active transporter?

Answer 8

Plasma membrane Ca2+ ATPase. Binding and hydrolysing ATP causes Ca2+ to be moved out.

Question 9

What is co-transport?

Answer 9

When more than one type of ion or molecule may be transport on a membrane transporter per reaction cycle.

Question 10

What is the difference between the he co-transporters symport and antiport?

Answer 10

Symport transport both molecule in the same direction, antiport transport them in different directions.

Question 11

How does the Na+-K+-ATPase transporter work?

Answer 11

It is an antiport - so it transports 3Na+ out for every 2K+ in. To transport them, ATP is hydrolysed to ADP and Pi. It is an active transporter.

Question 12

What is meant by Na+K+-ATPase being a P-type ATPase?

Answer 12

It transfers Pi to proteins in the reaction, this drives transport.

Question 13

What are the two subunits and their roles of the Na+K+-ATPase?

Answer 13

a-subunit: does the transporting

B-subunit: directs the pump to the surface.

Question 14

How can Ca2+ be transported across the membrane?

Answer 14

Ca2+Mg2+ATPase - transports Ca2+ out, high affinity, low capacity.
Na+Ca2+-exchanger - low affinity, high capacity, moves Ca2+ out and 3Na+ in.

Question 15

In what way is Na+H+exchanges a secondary active transporter?

Answer 15

It uses the gradient of Na+ that was established by another transporter, Na+K+ATPase. Na+K+APTase pumps 3Na+ out and 2K+ in, the gradient then means Na+ moves in via the Na+H+exchanger and H+ moves out.

Question 16

How does Na+-glucose co-transporter act?

Answer 16

Na+ gradient established by the Na+K+ATPase. Na+ then moves down concentration gradient into the cell, glucose also moves into the cell via this symport.

Question 17

List the following in order of increasing permeability through a synthetic lipid bilayer: hydrophobic molecules, ions, large uncharged polar molecules, small uncharged polar molecules.

Answer 17

Ions, large uncharged polar molecules, small uncharged polar uncharged molecules, hydrophobic molecules.