Lecture 16

Question 1

Describe the structure of the transmembrane domain

Answer 1

Alpha helical peptide sequence
Hydrophobic
Spans membrane
Amino acids with hydrophobic side chains
Permanently attaches protein to plasma membrane (hydrophobic fatty acid chain interacts with hydrophobic TMD)
Facilitates protein to protein interactions

Question 2

How are substance moved across cell membranes?

Answer 2

Compounds do not move pass freely (for the most part)

Small uncharged molecules move relatively easily (water, O2, CO2, NO)

Large, polar, charged CANNOT easily crop

Specific mechanisms are required for transport

Question 3

What are the 4 basic mechanisms that move molecules across membranes

Answer 3

Simple diffusion
Diffusion through a channel
Facilitated diffusion
Active transport

Question 4

Which mechanisms of transport are passive

Answer 4

Simple diffusion
Diffusion through a channel
Facilitated diffusion

Question 5

What is passive transport

Answer 5

Transportation using molecular concentrations of the substance across the membrane

No energy required

Usually moves from high to low concentration

Question 6

What type of molecules are transported during simple diffusion?

What are aquaporins?

Answer 6

Only works for small uncharged molecules (H2O, O2, CO2)

Aquaporins: water channels that moves H2O through it in “single file” down the concentration gradient

Question 7

What is the structure of channels (ion channels) and what is their role in transport
(Diffusion through a channel)

Answer 7

Formed by integral membrane proteins (multiple small units) that line a aqueous port

Only effective at transporting small charged molecules (ions)

Are selective, only lets through certain types of ions (Na+, K+, Ca2+, Cl- etc)

Question 8

What are the two types of gated ion channels? What are their functions

Answer 8

1) voltage-gated channels: channels that respond to charges across a membrane
(Ex. Action potentials in neurons)

2) ligand-gated channels: the channel responds to binding of specific molecule on it’s surface
Lock and key
(Ex. Acetylcholine receptor)

Question 9

What is tetrodotoxin (TTX)

Answer 9

Very potent neurotoxin found in pufferfish

A Na+ channel blocker, inhibits firing of action potentials, bind to the voltage-gated sodium channel

Question 10

What basic mechanisms use non-mediated transportation?

Answer 10

Simple diffusion
Diffusion through a channel

Question 11

What basic mechanisms used transporter mediated transportation

Answer 11

Facilitated diffusion
Active transport

Question 12

What basic mechanism use active transport

Answer 12

Active transport

Question 13

What occurs during facilitated diffusion

Answer 13

Compound bind to intergalactic membrane protein called a facilitative transporter

Change in transporter conformation (shape) that allows compound to move across the membrane

Question 14

How does a glucose transporter function

Answer 14

Imports glucose from blood into cells down the concentration gradient (via facilitative transport)

1) transporter is ready to accept a glucose molecule

2) glucose is accepted by transporter

3) intracellular side of transporter opens

4) glucose is released and cycle repeats

Question 15

How does a symporter carrier function

Answer 15

Moves substances from low to high concentration (against the gradient)

Uses a chemical gradient of a second molecule (concentration gradient and glucose)

Both molecules are transported in the same direction

Question 16

How does the Na+ - glucose symporter fucntion

Answer 16

1) binding of 2 Na+ and 1 glucose to the transporter at the same time

2) conformational (shape) change of the transporter

3) transport forms a inward-facing conformation

4) 2 Na+ dissociate in the cytosol, glucose is pushed in also

5) transporter takes on an outward facing conformation and cycle repeats

Question 17

How does a antiporter function?

Answer 17

The concentration gradient of one molecule is used to transfer a second molecule in opposite directions
(Ex. Na+/H+ exchanger, Na+ goes into the cell and H+ goes out of the cell

Question 18

How does active transport occur?

Answer 18

Compound bind to the active transporter (integral membrane protein)

Conformation change of transporter due to hydrolysis of an ATP molecule (allows molecule to be released on the other side of the membrane

Compounds move against concentration gradient

Requires input of energy in the form of ATP

Question 19

Give an example of active transport

Answer 19

Na+/K+ ATPase - maintain cellular concentrations of Na+ and K+

3 Na+ molecules exist the cell and 2 K+ enter the cell for each hydrolyzed ATP molecule

Question 20

Why is the Na+/K+ pump important

Answer 20

Maintains higher concentration of Na+ outside of the cell

ATP molecules (energy) is spent to sustain Na+ chemical gradient for non-stop activity of Na+-glucose symporter