Membrane Transporters

Question 1

What kinds of molecules can freely diffuse across the plasma membrane?

Answer 1

• hydrophobic molecules
  
  • some gases
  • steroid hormones

Question 2

Ion Channels

Specificity/Speed/Direction

Answer 2

• low specificity (will let different ions through)
• ions move downhill gradient
• NO ENERGY REQUIRED

Question 3

Ion Carriers

Specificity/Speed/Direction

Answer 3

• intermediate specificity
• ions move downhill gradient
• NO ENERGY REQUIRED

Question 4

Ion Pumps

Specificity/Speed/Direction

Answer 4

• absolute specificity
• ions move uphill gradient
• ATP REQUIRED

Question 5

Ion Channels are:

Answer 5

• ion-specific pores that open and close in a regulated manner
• passive transport (no energy required)

Question 6

Ion carriers are:

Answer 6

• enzyme-like proteins that mediate passive transport down concentration gradients without chemical change
• passive transport (no energy)

Question 7

Ion pumps are:

Answer 7

• enzymes that need energy to move ions and other solutes across the membrane against a concentration gradient
• energy provided via ATP-mediated phosphorylation or ATP binding and hydrolysis

Question 8

Membrane potential and ion transport:

Answer 8

• combines with concentration gradient to form the electrochemical gradient
  
  • membrane potential can either support or oppose ions moving with their concentration gradient.

Question 9

Ion channels are critical to the function of what types of tissues?

Answer 9

• nerve and muscle
  
  • dependent on K⁺, Na⁺, Ca²⁺ channels

Question 10

Ion channels can reject ions on the basis of:

Answer 10

• wrong charge
• wrong size

note: ion channels have low specificity overall

Question 11

Toxins that can block ion channels:

Answer 11

• tetrodotoxin (Na+ channels)
• lidocaine (Na+ channels)
• scorpion/snail poison (K+ channels)
• Nicotine (acetylcholine channels)

Question 12

What are the three major “carrier-type” transporters?

Answer 12

1. uniport
2. symport (coupled transport)
3. antiport (coupled transport)

Question 13

Uniport Ion Carriers:

Answer 13

• transport a single ion down its concentration gradient.
  
  • passive transport (no energy)
  • rate dependent on concentration gradient
  • binding affinity-similar to enzyme kinetics

Question 14

Na⁺/glucose symport ion carrier:

Answer 14

• co-operative transport.
• transports one ion down its concentration gradient along with another ion against its concentration gradient.
  
  • passive transport (no energy)
  • co-operative binding
  • will not function unless both substrates bound.

Question 15

Epithelial cell polarity can be maintained due to:

Answer 15

• the presence of tight junctions
  
  • do not allow membrane proteins to diffuse around membrane.
  • cause non-random distribution of membrane transporter proteins.

Question 16

Steps in transcellular glucose transport from digestive system through epithelial cell:

Answer 16

1. Symporter moves Na+ with its gradient and glucose against its gradient from the intestinal lumen through the apical region of epithelial cell.
2. Glucose moves down its gradient through a uniporter on the basal side of the epithelial cell.
3. Na+/K+ pump on basal side of epithelial cell maintains Na+ levels in epithelial cell.

Question 17

P-type ion pumps:

Answer 17

• ATP-driven pumps
  
  • autophosphorylate themselves with ATP, which results in a conformational change
• multi-pass transmembrane domains
• pumps ions

Question 18

ABC transporter:

Answer 18

• ATP-driven pump
• pumps small molecules
  
  • ​such as drugs

Question 19

Ca²⁺ ATPase:

Answer 19

• p-type ATP-driven ion pump
• 90% of membrane protein in muscle cells
• moves calcium against its concentration gradient

Question 20

Steps in Ca²⁺ ATPase activity:

Answer 20

• Cavity in non-phosphorylated transporter binds calcium.
• ATP binds and contributes phosphate for phosphorylation.
• When aspartate phosphorylated, conformational change occurs.
• Opens channel to lumen of sarcoplasmic reticulum and releases Ca⁺²

Question 21

Na+/K+ ATPase:

Answer 21

• p-type ATP-driven ion pump
• 1/3 of cellular energy used to maintain this pump
• ATP causes a conformation change in the transporter that moves Na+/K+ both against their concentration gradients.

Question 22

ABC Transporters (General Information):

Answer 22

• ATP-binding casettes
• Two ATPase domains on each transporter
• abundant in bacteria
• present in eukaryotes

Question 23

Steps in ABC Transport:

Answer 23

1. small molecule binds to non-ATP bound state.
2. ATP binds to both ATPase binding domains.
3. ATPase binding domains dimerize to produce a conformational change that exposes substrate to opposite side of membrane for release.
4. ATP hydrolysis releases substrate, then prepares transporter for another round of transport.

Question 24

ABC transporters in humans are important for:

Answer 24

• multiple drug resistance
  
  • tumor cells overexpress ABC transporter that pumps chemo-drug out.
• chloroquine resistance
  
  • malaria induces cells to overexpress ABC transporter that pumps malaria-drug out.
• cystic fibrosis
  
  • ATP binding to ABC transporter can cause it to become a channel for chlorine