Lecture 2: Membrane Transport and Osmosis

Question 1

Transporters vs ion channels

Answer 1

Both involve membrane proteins and show specificity; however, ion channels move 1000X more ions per unit time as open ion channels can support continuous flow.

Question 2

Types of transport

Answer 2

1. Simple diffusion
2. Facilitated diffusion (carriers, channels)
3. Primary active transport (energy from ATP)
4. Secondary active transport (energy from electrochemical gradient)

Question 3

Mediated transport

Answer 3

Movement of solutes through transporters

Question 4

Transporter process

Answer 4

1. Solute binds
2. Conformational change exposes solute to other side
3. Solute dissociates

Question 5

Factors for magnitude of solute flux by mediated transport

Answer 5

1. Solute concentration
2. Affinity between solute and transporter
3. Number of transporters in membrane
4. Rate of transporter conformational change

Question 6

Facilitated diffusion

Answer 6

Downhill flux of a molecule that uses a transporter; does not require energy e.g. GLUTs.

Question 7

Active transport

Answer 7

Uses energy to move a substance uphill; often called pumps

Question 8

Process of primary active transport

Answer 8

1. ATPase binds solute and ATP
2. ATPase catalyzes ATP hydrolysis and self-phosphorylates, causing a conformational change and producing ADP
3. New conformation and change in binding site affinity releases solutes to opposite side
4. Return to original conformation as new ATP molecule binds.

Question 9

Na+/K+ ATPase pump

Answer 9

Primary active transporter that moves 3 Na+ out and 2 K+ in for 1 ATP. Maintains high EC sodium and high IC potassium. Also slightly electrogenic (1 net positive moved out)

Question 10

Major primary ATPases

Answer 10

1. Na+/K+ ATPase
2. Ca2+ ATPase
3. H+/K+ ATPase

Question 11

Secondary active transporters

Answer 11

These transporters couple uphill solute transport with moving an ion down its electrochemical gradient and are thus indirectly ATP dependent.

Question 12

Cotransporters and countertransporters

Answer 12

Secondary active transporters that move solutes in the same or opposite direction as an ion. Also called symporters and antiporters.

Question 13

Aquaporins

Answer 13

Membrane proteins that act as H2O channels for plasma membranes. Type and number varies by cells and number can be regulated by some cells (e.g. kidney ducts)

Question 14

Osmosis

Answer 14

The net movement of water across a membrane. Proceeds from low solute concentration to high, aka high water concentration to low.

Question 15

Concentration of pure water

Answer 15

[H2O] = 55.5 M. Adding solute “replaces” H2O molecules and depends on the number of particles (osmoles). New [H2O] = 55.5 - x Osm

Question 16

Osmotic pressure

Answer 16

The pressure that must be applied to prevent net flow of H2O into a compartment when separated by a semipermeable membrane. Prevents flow; does NOT drive flow.

Question 17

Semipermeable membrane

Answer 17

A membrane that is permeable to water but not to solutes

Question 18

Hydrostatic pressure

Answer 18

Pressure exerted by a fluid at equilibrium at a given point within the fluid due to gravity. At equilibrium, hydrostatic pressure will equal osmotic pressure and result in 0 net flow (P = RT[S] = Π)

Question 19

Penetrating/nonpenetrating solutes

Answer 19

Nonpenetrating solutes cannot cross the plasma membrane. Outside the cell, Na+ and Cl- are effectively nonpenetrating. Inside, K+ and organic solutes are nonpenetrating. Determines tonicity; nonpenetrating = effective solute.

Question 20

Isotonic solution

Answer 20

A solution with the same amount of nonpenetrating solutes as the intracellular space. 0 net H2O flow.

Question 21

Hypotonic solution

Answer 21

A solution with fewer nonpenetrating solutes than the intracellular space. Will cause cells to swell.

Question 22

Hypertonic solution

Answer 22

A solution with more nonpenetrating solutes than the intracellular space. Will cause cells to shrink.

Question 23

Osmolarity vs tonicity

Answer 23

Osmolarity includes penetrating and nonpenetrating solutes. Only the latter contributes to tonicity.

Question 24

Iso-, hypo-, hyperosmotic solutions

Answer 24

Osmolarity of a solution relative to the normal EC/IC fluid (300 mOsm).

Question 25

ABC transporters

Answer 25

ATP-binding cassette superfamily of primary active transporters. MDR1 and MRP2 are examples found on the apical membrane of renal tubular cells and biliary hepatocytes. Involved in anion excretion and mediate drug resistance in chemotherapy patients.

Question 26

Van’t Hoff equation

Answer 26

RT*[S] = Π
Equation to calculate osmotic pressure of a solution, where R = universal gas constant, [S] = solute concentration, T = absolute temperature, Π = osmotic pressure.

Question 27

Reflection coefficient σ

Answer 27

Dimensionless value from 0 to 1 that describes the ease with which a solute crosses a membrane, where 1 = total impermeability (100% reflected) and 0 is total permeability. Π effective = σRT[S].