Lecture 3

Question 1

Movement Across Membranes

Answer 1

Passive diffusion: directly through membrane

Facilitated diffusion: channels, uniporters

Secondary active transport: co-transporters (symporters), counter-transporters (antiporters)

Active transport: hydrolyse ATP, transport against concentration gradients

Question 2

P-type ATPases

Answer 2

Phosphorylate themselves during pumping cycles:

Includes Na+-K+ pumps

Ca2+ pumps in the SR of skeletal/cardiac muscle

Question 3

Na+/K+ Pump

Answer 3

Generally, Na+/K+ pump responsible for:

Intracellular K+ 10-30 times higher

Extracellular Na+ 10-30 times higher
1/3 of cells’ energy is used to power this

Electrogenic: drives 3 Na+ out for 2 K+ in

Contributes 10% electrical potential across cell membrane

Inhibited by Ouabain (strophantus) and cardiac glycosides from digitalis spp

Question 4

Secondary Active Transport

Answer 4

Secondary active transporters use the kinetic energy provided by electrochemical gradients

• Transport one solute down a concentration gradient
• Coupled to the transport of another against concentration gradient

Question 5

Secondary Active Transport - Symporters

Answer 5

Symporters (co-transporters) are integral membrane proteins involved in the transport of two or more molecules or ions in the same direction

*Example: Na+/glucose transporter (SGLT)

Question 6

Counter Transporters – Antiporters

Answer 6

Antiporters (counter transporters) are integral membrane proteins involved in the transport of two or more molecules or ions in opposite directions

• Examples: Na+/H+ (NHE), Na+/Ca2+ (NCX)
• NHE: maintains cytosolic pH
• NCX: maintains low cytosolic [Ca2+]