Membrane transport

Question 1

Name the two types of passive transport

Answer 1

Simple diffusion

Facilitated diffusion

Question 2

Define passive diffusion

Answer 2

Requires no energy input

Question 3

What is a chemical gradient?

Answer 3

Difference in concentrations of an ion inside and outside the cell

Question 4

What is an electrochemical gradient?

Answer 4

Combines the chemical gradient and the membrane potential

Question 5

What is the membrane potential?

Answer 5

The difference in charge between the inside and outside of cells

Question 6

The ability of a solute to cross the membrane depends on what 4 things?

Answer 6

Concentration gradient
Electrochemical gradient
Hydrophobicity
Size

Question 7

Membranes are highly ________ to ions

Answer 7

Impermeable

Question 8

Name the two classes of membrane protein involved in facilitated diffusion

Answer 8

Channels

Transporters

Question 9

Describe ion channels

Answer 9

They exhibit ion selectivity
Driven by concentration or chemical gradient
Work quickly
Regulated

Question 10

Describe K+ channels

Answer 10

Most common ion channel
Subdivided into leak and gated
10,000 more effective for K+ than Na+

Question 11

What is present at the entrance of the K+ channel?

Answer 11

Negatively charged amino acids to repel negatively charged ions

Question 12

What happens to K+ ions at the exit of the K+ channel?

Answer 12

K+ ions in hydrated state are too big to pass through
K+ is dehydrated
Energy lost in dehydration = energy regained when K+ interacts with carbonyl oxygens
K+ is rehydrated

Question 13

Why doesn’t Na+ go through the K+ channel?

Answer 13

The dehydration of Na+ occurs in the channel
The Na+ is too small to be rehydrated by the carbonyl oxygens
Energetically unfavourable

Question 14

Describe transporters

Answer 14

Exhibit selectivity
Slow transport
Oscillate between two conformations

Question 15

Describe glucose transporters

Answer 15

Uniporters (only glucose)
Many isoforms
12 pass membrane spanning proteins
Present in all cells
Glut1 deficiency syndrome = microcephaly, seizures, retarded development

Question 16

How do we ensure glucose concentration is maintained?

Answer 16

Once transported into the cell, glucose is converted into glucose-6 phosphate
The glucose concentration remains higher outside the cell

Question 17

What is active transport?

Answer 17

Transport against the concentration or electrochemical gradient that requires ATP

Question 18

What are the two types of membrane proteins involved in active transport?

Answer 18

ATP-driven pumps

Coupled transporters

Question 19

What do ATP-driven pumps do?

Answer 19

Move solutes against the concentration/electrochemical gradient by expending energy

Question 20

Descrive Na+K+ATPase

Answer 20

Expressed by all cell types
Some cells have 30 million
Hydrolyses ATP
Uses 30% of the cells energy (more in nerve cells)
Continually expels Na+ and brings K+ in

Question 21

How does Na+K+ATPase work?

Answer 21

3Na+ bind and the pump is phosphorylated by ATP
Pump undergoes conformational change
Na+ transferred across the membrane and released
2K+ ions bind and the pump is dephosphorylated
K+ transferred across the membrane and released

Question 22

Describe coupled transporters

Answer 22

Move solutes against their concentration/electrochemical gradient by coupling their transport to the Na+ gradient created by Na+K+ATPase

Question 23

What makes coupled transporters an example of secondary active transport?

Answer 23

They don’t rely directly on the hydrolysis of ATP

Question 24

What is a symporter?

Answer 24

Moves both molecules in the same direction

Question 25

What is an anti porter?

Answer 25

Moves the two molecules in opposite directions

Question 26

Describe the Na+Glucose symporter

Answer 26

Expressed by epithelial cells in the GI tract
Glucose concentration is higher in epithelial cells than in the lumen
Symporter utilise the Na+ electrochemical gradient to transport glucose into the cells against its concentration gradient

Question 27

How does the Na+Glucose symporter work?

Answer 27

Oscillates between two conformations
Binding is cooperative
Na+ binding increases the affinity for glucose
Glucose is more likely to bind on the extracellular side where Na+ concentration is high
Glucose more likely to be moved in than out

Question 28

What prevents paracellular movement?

Answer 28

Tight junctions

Question 29

What proteins make up tight junctions?

Answer 29

Claudin

Occludin

Question 30

What does Glut2 do?

Answer 30

Transports glucose out of the cell via facilitated diffusion

Question 31

What is asymmetric distribution?

Answer 31

The arrangement of transporters in cells so that concentrations are maintained

Question 32

Describe the Na+Ca2+ anti porter

Answer 32

Expressed by cardiomyocytes in heart muscle
Ca2+ concentration is greater in the interstitial fluid than in cardiomyocytes
Antiporter uses Na+ to move Ca2+ out of the cells
This relaxes muscle contraction

Question 33

How do you strengthen muscle contraction?

Answer 33

By inhibiting Na+K+ATPase
Reduce Na+ gradient
Ca2+ not moved out of the cell
Reduces muscle relaxation

Question 34

Describe an aquaporin

Answer 34

A tetramic complex made of four monomers
Each monomer is a six pass membrane spanning protein
Each monomer is a channel itself