lecture 7

Question 1

what are the 3 grades of channels

Answer 1

• channels
• transporters
• pumps

Question 2

describe channels

Answer 2

• facilitated (passive) transport through a gated channel
• diffusion of ions down their concentration gradients
• quick process

Question 3

examples of transporters

Answer 3

• uniporter
• cotransporters: symporter and antiporter

Question 4

describe uniporters

Answer 4

• facilitated (passive) transport
• movement of a single molecule down its concentration gradient
• substrate binds to uniporter, triggering a conformational change in the protein to release it on the opposite side

Question 5

describe symporters

Answer 5

• secondary active transport
• couples a favourable reaction with an unfavourable reaction
• molecules enter in the same direction

Question 6

describe antiporters

Answer 6

• secondary active transport
• couples a favourable reaction with an unfavourable reaction
• molecules enter in the opposite direction

Question 7

is osmosis regulated

Answer 7

yes, by aquaporins

Question 8

what did peter agre discover

Answer 8

aquaporins

Question 9

how did peter agre discover aquaporins

Answer 9

1) isolated a suspected membrane protein
2) inserted it into frog eggs which have low water permeability
3) set up two groups: control (no protein) and experimental (with protein)
4) placed both groups in a hypotonic solution, so water rushes into the cell
5) control cells had little change (slow water entry)
6) experimental cells had rapid swelling and bursting, proving the protein increases water permeability

Question 10

describe the structure of an aquaphorin

Answer 10

4 subunits, each with a hydrophilic channel

Question 11

how does water move through an aquaporin

Answer 11

• each subunit has its own channel
• water molecules can only enter one at a time through a channel
• so 4 water molecules can move simultaneously through one channel

Question 12

what type of transporter transports glucose

Answer 12

uniporter

Question 13

describe the process of transporting glucose

Answer 13

1) higher glucose concentration outside than inside
2) glucose binds to the GLUT transporter
3) once glucose binds, the transporter protein changes shape
4) glucose is released inside the cell
5) the GLUT protein returns to its original shape

Question 14

what is Vmax

Answer 14

• the maximum rate of a molecule’s transport when all transporters are working at full capacity
• increasing the molecule’s concentration further does NOT increase rate

Question 15

what is Km

Answer 15

• the molecule concentration at which transport rate = ½ Vmax
• low Km = high affinity

Question 16

what GLUT transporter has a high affinity for glucose

Answer 16

GLUT 1

Question 17

what GLUT transporter has a low affinity for glucose

Answer 17

GLUT 2

Question 18

what can GLUT 2’s affinity for glucose help with

Answer 18

it can be used as a glucose sensor in the pancreas to release insulin when BGL get high

Question 19

how do we ensure that glucose can continue entering cells without ruining the concentration gradient

Answer 19

it is converted to G6P once it enters the cell so it is no longer recognised as glucose

Question 20

what organ has a low affinity for glucose

Answer 20

pancreas

Question 21

describe what happens when BGL get high

Answer 21

1) GLUT 2 transporters in the pancreas detect the high concentration and release insulin
2) insulin is exocytosed from the pancreatic cells and travels through the bloodstream
3) the insulin binds to insulin receptors on other cells
4) this binding triggers GLUT 4 transporters to travel through kinesin proteins in vesicles to be exocytosed from those cells
5) the GLUT 4 transporters are now present on the membranes of those cells, ready to take in glucose from the blood
6) once enough glucose has been cleared, the GLUT 4 transporters are endocytosed back into the cell

Question 22

describe how the Na+/glucose symporter works

Answer 22

1) sodium–potassium pump pumps Na⁺ out of the cell using ATP
2) so there is high Na⁺ outside, low Na⁺ inside
3) Na⁺ binds to the symporter outside the membrane
4) now glucose can bind to the transporter outside the membrane
5) the symporter changes shape and opens toward the inside of the cell
6) Na⁺ and glucose are released inside
- Na⁺ moves down its gradient, glucose moves against it
7) the symporter returns to original outward-facing shape

Question 23

what are the 2 types of pumps

Answer 23

• P-class
• V-class

Question 24

examples of P-class pumps

Answer 24

in:
- plasma membranes of higher eukaryotes (Na+/K+ pumps)
- sarcoplasmic reticulum membrane in muscle cells (Ca2+ pump)

Question 25

examples of V-class pumps

Answer 25

endosomal and lysosomal membranes in animal cells

Question 26

is the lysosome more acidic than the ER and why

Answer 26

- yes it is - V-type pump uses ATP to move H⁺ from cytosol → into lysosome - positive charge builds up inside so Cl⁻ ions move into the lysosome (via ion channels) - we end up with acidic HCl in the lysosome

Question 27

what are P-class pumps responsible for

Answer 27

- maintaining the salt gradients across the memrbanes - e.g. high K+ inside cell, high Na+ outside cell, high Cl- outside cell

Question 28

example of a P-class pump and how it works

Answer 28

- Na+/K+ ATPase 1) 3 Na⁺ bind inside the cell and ATP is hydrolysed 2) 3 Na⁺ are released outside the cell 3) 2 K⁺ now bind outside the cell and the pump opens back to the inside 4) the 2 K⁺ released inside the cell

Question 29

what sets up the membrane potential

Answer 29

the Na+/K+ ATPase