2.1 - Membrane Transport

Question 1

components permeable to the cell membrane

Answer 1

CO2, N2, O2, ethanol

Question 2

Types of Molecules Crossing Plasma Membrane

Most Permeable → Least Permeable

Answer 2

gases (nonpolar molecules)

small uncharged polar molecules

large uncharged polar molecules

ions

charged polar molecules

Question 3

water flow in hypotonic and hypertonic solutions

Answer 3

hypotonic: water flows into cell

hypertonic: water flows out of cell into solution

Question 4

contractile vacuole

Answer 4

• evoluationary adaptation in paramecium for osmoregulation
• can expell fluid from the cell when the vacuole fills up

Question 5

aquaporin

Answer 5

• a “water channel”
• alpha-helices form central pore
• channel diameter just large enough for water molecule
• hydrophlic amino acids in channel to help attract water molecules through
• no conformation change or energy used

Question 6

aquaporin frog egg experiment

Answer 6

control:

• natural frog eggs – frog eggs do not normally contain aquaporins and are impermeable to water

experiment:

• mRNA encoding aquaporin protein injected into frog eggs
• frog eggs expressing aquaporin protein placed into water
• frog eggs burst within minutes

conclusion

• aquaporin = water channel protein

Question 7

selective transport / facilitated transport

Answer 7

• a specific protein embedded in the membrane is the transporter
• solute moves down gradient
• no ATP needed
• example: channel

Question 8

cotransport proteins

Answer 8

symporters and antiporters

involve the movement of two molecules across the membrane simultaneously

Question 9

transport rates of transporters: fastest to slowest

Answer 9

Fastest

• channels
• transporters
• ATP-powered pumps

Slowest

Question 10

uniport transporter protein kinetics

Answer 10

• kinetics of uniporter similar to that of enzyme kinetics demonstrating Michaelis-Menten kinetics
• can use Michaelis-Menten model to describe transporter activity

Question 11

Michaelis-Menten Constant

Answer 11

Km = conc. of substance at 1/2 Vmax

Question 12

Types of GLUT transporters

Answer 12

GLUT 1 = erythrocytes

GLUT 2 = liver cells

GLUT 4 = adipose / muscle cells

Question 13

process of removing integral membrane protein for study

Answer 13

• disrupt membrane
• add detergen to solubilize protein
• mix detergent-covered proteins with phospholipids
• dialyze or dilute to remove detergent molecules
• result: liposome with transport protein in membrane

Question 14

non-gated potasium channels

Answer 14

• specific to K+
• always open
• channel environment similar in nature and size to when an K+ is surrounded by water molecules
• water molecules stripped from K+ when K+ passes through the channel

Question 15

membrane electric potential with the membrane is impermeable

Answer 15

generation of a transmembrane electric potential (voltage) depends on the selective movement of ions across a semipermeable membrane – if the membrane is impermeable then there will be no generation of membrane potential

Question 16

Process of Gate Ion Channel Opening/Closing

Answer 16

1. resting state – positively charged alpha-helices atttracted to the negative charges on the cytosolic side of the resting membrane, which keeps gate closed
2. depolarization – voltage-sensing helices move through the phospholipid bilayer toward the outer membrane surface ⇒ conformational change and gate opens
3. short after depolarization – ball and chain (inactivating segment) moves into open channel and prevents further flow
4. repolarization – voltage-sensing helices return to the resting position, channel inactivating segment replaced, and gate closes

Question 17

Structure of Voltage Gated K+ Ion Channel

Answer 17

• N-terminus is globular domain
  
  • ball of the ball and chain inactivating segment
  • cytosolic face
• 6 membrane-spanning alpha helices

Question 18

Types of ATPases

Answer 18

P-class, V-class, F-class, and ABC

Question 19

P-class Pump

Answer 19

• has two identical alpha subunits, each with an ATP-binding site
• during transport, at least one of the subunits becomes phosphorylated; transported ions move through

Question 20

P-Class Pump Examples

Answer 20

• Na+/K+ pump
• H+/K+ pump of apical membrane of stomach
• Ca2+ pumps (i.e. ER, SR)

Question 21

Sodium-Potassium Pump Process

Answer 21

1. 3 Na+ and 1 ATP attach (cyt. side)
2. phosphorylation of aspartate
3. conformational change
4. 2 K+ attach, 3 Na+ release (extra. side)
5. dephosphorylation and conformational change
6. 2 K+ release (cyt. side)

Question 22

Typical Intracellular and Extracellular Ion concentrations (high/low)

Answer 22

Intracellular: K+

Extracellular: Na+, Ca2+, Cl-

Question 23

Calcium ATPase Structure

Answer 23

Cytosolic face has:

• actuator domain
• phosphorylation domain
• nucleotide-binding domain

Question 24

Ca²⁺ ATPase in SR: Process

Answer 24

1. calcium and ATP-binding on cytosolic side (E1)
2. phosphorylation of aspartate (E1)
3. conformational change (E1)
4. calcium release (E2)
5. dephosphorylation (E2)
6. conformational change (E2)

Question 25

V-Class Proton Pumps

Answer 25

* transport only protons (H+) and does so in a process that doesn't require phosphoprotein intermediate * often used to lower pH of an environment

Question 26

Examples of V-Class Proton Pumps

Answer 26

* endosomal and lysosomal membranes in animal cells * plasma membrane of osteoclasts

Question 27

getting an acidic environment with V-class pumps

Answer 27

V-class pumps pump H+ into the cell, but this generates an electric potential --\> need to pair with import of a (-1) molecule like Cl^- to keep the charge neutral so that more protons can get into the cell

Question 28

F-class proton pumps

Answer 28

* H+ flow through to lead down concentration gradient in order to produce * ATP synthases

Question 29

ABC Superfamily

Answer 29

* "ATP-binding casette" * two transmembrane domains * two nucleotide binding domains * can act as a flippase * known as multi-drug resistant transporter

Question 30

glucose transporters in hepatocytes

Answer 30

1. the sodium-potassion pump in the basolateral surface membrane generates Na+ and K+ concentration gradients 2. the outward movement of K+ ions through non gated K+ channels generates an inside-negative membrane potential across the entire plasma membrane. both the Na+ conc. gradient and the membrane potential are used to drive uptake of glucose from the intestinal lumen by the two-Na+/one-glucose symporter located in the apical surface membrane 3. glucose leaves the cell via facilitated diffusion catalyzed by GLUT 2, a glucose uniporter located in the basolateral membrane

Question 31

calcium transport out of a myocardial cell

Answer 31

To relax muscle → get calcium out of the myocardial cell Calcium Antiporter; gets energy from Na+ gradient which was created by the Sodium Potassium pump (one Ca++ pumped out for every 2 Na+ pumped in)

Question 32

acidification of the stomach lumen by parietal cells in the gastric lining

Answer 32

Function → Acidifying the Stomach (pH=1) → High H+ Concentration apical membrane has H+/K+ ATPase (P-class pump); K+ and Cl- channels leading from cytosol to stomach lumen, H+ from H20 — OH- combines with Co2 to give HCO3- that is transported to blood through HCO3-/Cl- antiporter

Question 33

ways to inhibit calcium antiporter

Answer 33

digoxin and oubin