Process at Membranes

Question 1

Causes of increased diffusion? (4)

Answer 1

• Increased concentration gradient
• Increased molecule lipid solubility
• Decreased molecule size
• Presence of hydrophobic molecules – O2, CO2, N2, Benzene, Methanol, Water, Ethanol

Question 2

Carriers?

Answer 2

• Integral membrane proteins – usually single polypeptide, usually 12 transmembrane alpha helices, move molecules from a high to a low concentration
• Act very slowly – Medium velocity of 1000 molecules/s
• Uniporter
• Cotransporter – Symporter
• Cotransporter – Antiporter

Question 3

Uniporter?

Answer 3

Transports only one molecule species e.g. glucose uniporter (GLUT1 and GLUT4) – binding of glucose flips the transporter to a different conformation, this new conformation releases glucose on the other side of the membrane and this release allows it to flip back to repeat the cycle

Question 4

Cotransporter – Symporter?

Answer 4

Transports 2 different molecules in the same direction. When the molecules are bound the carrier changes conformation. All the ligands must be bound to initiate it

• e.g. glucose and sodium carrier in the intestine (very important in preventing dehydration and diarrhoea). Gradients from primary pumps power this secondary active transport. Glucose is pumped against its gradient to allow sodium to be transported out of the lumen

Question 5

Cotransporter – Antiporter?

Answer 5

Transports 2 different molecules in opposite directions, 1 out before the other can go in

• e.g. carbonic acid

Question 6

Pumps?

Answer 6

• Transmembrane ATPase hydrolyses ATP to form ADP and Pi - E.g. Na+ - K+ pump
• Pumps 3 sodium ions out and 2 potassium in
• Each cycle uses one ATP
• 100 cycle/s
• Process uses a quarter of the energy of most cells

If something blocks the pump e.g. the drug ouabain – more water enters and the cell bursts

Question 7

Channels?

Answer 7

• Around 1 million ions can pass through/s – much faster than any carrier
• Na+ channel and K+ channel

Question 8

Voltage-gated channels?

Answer 8

• Sodium VGCs

Question 9

Ligand gated channels?

Answer 9

• Ionotropic receptors
• *Anionic receptor** - GABA A, Glycine.
• Ionotropic glutamate receptors (anionic receptor) - AMPA, kainate, NMDA

Cationic receptors - serotonin, nicotinic acetylcholine.

• Metabotropic cell surface receptor

muscarinic cholinergic receptor – Ach binds to receptor which is linked to G protein, G proteins induce metabolic effect in other enzymes.

• GABA B, epinephrine receptors, histamine receptors, dopamine receptors

Question 10

Leak channels?

Answer 10

• Open all the time
• Best known are for potassium ions

Question 11

Resting membrane potential?

Answer 11

• Between -40 and -90 mV
• RMP strongly depends on external K+ concentration

Question 12

Nernst Equation?

Answer 12

• V = (RT/zF)ln(Co/Ci)
• V = equilbirum potential in volts
• Co/Ci = outside and inside concentration of the ion
• R = gas constant (2 cal mol-1 K-1)
• T = absolute temperature (K)
• F = Faraday’s constant (2.3 * 104 cal V-1 mol-1)
• z = valence (charge) of the ion

Assumptions: 1 ion at a time, membrane completely permeable to ion, ion must be in equilibrium