Week 6 (membrane Transport)

Question 1

What are the two types of transmembrane proteins?

Answer 1

Transporters- binding of solute causes a conformational chance (outwards open and inwards open)

and

channels (aqeous pore that can be gated and that lets hydrophilic molecules through) - define which substrates are allowed into the compartment

Question 2

What is a concentration gradient?

Answer 2

• Difference in distribution of a solute across a partially permeable membrane
• A difference in concentration (gradient) represents a chemical potential
• Laws of thermodynamics only allow movement in one direction

Question 3

Net flow of molecules across a membrane will occur until:

Answer 3

Concentration outside=concentration inside

Question 4

What if you need to transport something against its concentration gradient?

Answer 4

Requires an input of energy-> active transport

Question 5

What are the two types of active transporters?

Answer 5

• primary- ATP driven pumps - secondary- coupled transporters (energy has been used to set up concentration gradient beforehand)

Question 6

What are the 3 classes of primary ATP driven pumps?

Answer 6

• P type pump (phosphorylation- ATPases) transports ions across membranes
• F type found on mitochondrial inner membrane- synthesises ATP from ADP (ATP Synthases) and driving protons across a membrane (and V type which transports protons across membranes)proton pump but does NOT synthesise energy
• ABC transporter (small molecules transported) uses 1 or 2 molecules of ATP (which is hydrolysed to ADP)

Question 7

What are the main classes of secondary coupled transporters

Answer 7

Concentration gradient pre-established

• uniport (only one molecule/ion is transported)
• symport (one molecule transported down concentration gradient and the other is transported against its concentration gradient)- coupled transport
• antiport (molecules transported in different directions) - coupled transport

Question 8

How is energy released from ATP?

Answer 8

Through hydrolysis of phosphodiester/ phosphoanhydride bonds Approx -32 KJ/mol-1

Question 9

Give an example of transport against a concentration gradient that uses ATP hydrolysis

Answer 9

Example: Removal of toxins by the ABC transporter P-glycoprotein

Question 10

Symporters: example is co transporters of glucose and Na+

Answer 10

• electrochemical gradient is used to transport:
• glucose against its concentration gradient (chemical potential)
• And sodium down its concentration gradient (active transport of glucose)
• Symporter has binding site for both glucose and sodium -Both molecules bind and are transported in the same direction
• conformational change occurs - open inside - release both molecules

Question 11

Antiporters: example of antiport of Na+ and Ca2+

Answer 11

• electrochemical gradient of Na+ is used to rapidly expel Ca2+ from the cytosol
• active transport of calcium
• 3 Na+ is transported into the cell which forces Ca2+ out the cell
  1. Na+ outside the cell binds and is transported into the cell
  2. inwards conformation occurs
  3. Inwards conformation has a higher affinity for Ca2+
  4. Calcium released (actively transported) outside

Question 12

Define membrane potential

Answer 12

unequal distribuion of ions accross the membrane which gives it an electric charge

Net flow of ions across the membrane induces an electric potential (membrane potential)

Question 13

How do electrochemical potentials occur?

Answer 13

The combination of the chemical and electrical potentials results in electrochemical potential, Chemical and electrical potential work against each other so net transport stops when they are equal

Question 14

Active membrane transport: pumps (Na+/K+pump)

Answer 14

• coupling ion transport to ATP hydrolysis slows the build up of an electrochemical gradient
  1. Inwards open conformation- binds 3 Na+ ions and hydrolyses ATP and uses phosphate to phosphorylate itself
  2. Changes conformation to outside open conformation and released Na + ions
  3. Now has a high affinity for K+ ions and 2 binds and cause dephosphorylation and goes back to inwards open conformation

Question 15

What does ATP synthase rely on?

Answer 15

Electrochemical gradients - proton motive force (gradient)

Question 16

properties of Gated ion channels:

Answer 16

• conduct ions rapidly
• has aqueous pore
• passive transport along electrochemical gradient
• exquisite specificity to a particular ion
• used to manipulate membrane potential
• number of control mechanisms

Question 17

What are some mechanisms of gating?

Answer 17

• voltage gated
• ligand gated (extracellular ligand)
• ligand gated (intracellular ligand)
• mechanically gated

Question 18

Control of membrane potential is determined by movement of ions:

Answer 18

• depolarisation (influx of positive ions) - hyper-polarisation (efflux of positive ions)

Question 19

What can the Nernst equation be used to calculate?

Answer 19

• electric potential/ voltage induced by a concentration gradient - all you need to know is the temperature, charge of the ion and concentrations

Question 20

How is resting potential restored?

Answer 20

By the sodium potassium pump

Question 21

List molecules from most to least permeable through the plasma membrane?

Answer 21

Gases (O2, CO2)

Hydrophobic molecules (Benzene)

Small polar molecules (H2O, ethanol)

Large polar molecules (Glucose)

Charged molecules/ ions (Amino acids, Cl-)

Question 22

For huge changes in membrane potential to occur, what relative amount of ions crossing the membrane must take place?

Answer 22

Only a few

Question 23

describe transporter interplay

Answer 23