Lecture 16 Membrane Transport

Question 1

What is non-mediated transport?

Answer 1

When the substance does not directly use a transport protein

Question 2

What is mediated transport?

Answer 2

Substances are moved across the membrane with the help of the protein

Question 3

What is passive transport?

Answer 3

When substances move down their concentration/electrochemical gradients with only their kinetic energy

Question 4

What is active transport

Answer 4

When substances are moved across their electrochemical gradients using energy

Question 5

What is vesicular transport?

Answer 5

When materials are moved across membranes in small vesicles

Question 6

Describe how non mediated transport works and what is important for

Answer 6

Requires a concentration gradient, works for non-polar hydrophobic molecules such as O2, CO2, N, fatty acids, steroids, small alcohols, ammonia and fat soluble vitamins (A, E, D and K)

Question 7

Describe the structure of a ion channel and relate it to its purpose

Answer 7

Channel forms a water filled pore lined with hydrophilic amino acids that shields ions from hydrophobic core of the lipid bilayer. Outer portion of the channel within the hydrophobic core is made from hydrophobic amino acids.

Question 8

What are the properties of ion channels?

Answer 8

Has an ion selectivity filter which resembles an hourglass. This only allows certain ions through which means the channel can harness the energy stored in the different ion gradients
They have gates which can be opened or closed, usually a ball that blocks the pore. Different stimuli control the channel opening and closing: voltagem ligand binding, cell volume, pH and phosphorylation

Question 9

Describe the current generated by an ion channel

Answer 9

A measurable current of around 10^-12 amps is generated
Current fluctuations represents conformational changes of the channel and represents the opening and closing of single channels

Question 10

How can ion channel function be measured?

Answer 10

The patch clamp technique, clamps over one channel and shows its electrical current

Question 11

Describe carrier mediated transporters

Answer 11

Molecule to be transported directly binds to the transport protein and causes a conformational change which allows the molecule to be transported. Because a conformational change is made, this process is very slow

Question 12

What is the similarities between enzymes and carrier mediated transport?

Answer 12

They exhibit specificity, inhibition, competition, saturation. However they don’t catalyze chemical reactions they mediate transport across the cell membrane at a faster rate, and mediated transport can be passive or active

Question 13

What are glucose transporters called?

Answer 13

GLUT, this changes shape to allow glucose across cell membrane

Question 14

Why does the glucose concentration never go above 5mmol?

Answer 14

Because the kinase enzyme transforms glucose into glucose 6 phosphate, reducing its concentration. This conversion maintains the concentration for glucose entry

Question 15

Describe primary active transport

Answer 15

Energy derived from hydrolysis of ATP, and around 30% of energy is used

Question 16

Describe secondary active transport

Answer 16

Energy stored in an ionic concentration gradient is used to drive the active transport of a molecule against its gradient

Question 17

Describe the Na/KTPase

Answer 17

1. 3 Sodium binds to protein
2. ATP hydrolyzes, phosphate group attaches to the ion channel -> change in conformation
3. Na+ pushed out
4. 2 Potassium ions bind and phosphate is released -> change in conformation
5. Potassium pushed in
   This maintains a low concentration of Na+ and a high concentraton of K+

Question 18

Why are primary active transporters electrogenic?

Answer 18

because the pump generates a net current

Question 19

What is the difference in ion concentrations important for?

Answer 19

Maintaining resting membrane potential
Electrical excitability
Contraction of muscle
Maintenance of steady state cell volume
Uptake of nutrients via secondary active transporters
Maintenance of intracellular pH by secondary active transporters

Question 20

What is the pump leak hypothesis?

Answer 20

When Na and K continually leak back into the cell down their respective gradients so the pumps have to work continuously

Question 21

What is secondary active transport?

Answer 21

When energy stored in an ion gradient is created by primary active transporters to move other substances against their own concentration gradient, indirectly using the energy from hydrolysis of ATP

Question 22

What is an antiporters or exchangers

Answer 22

When ions rush inward and other ions are pushed outwards (Na+ pushed into cytosol)

Question 23

What is a symporter or cotransporter?

Answer 23

When ions rush inwards together (Na+ pushed into cytosol)

Question 24

What is the cycle of Na+ in the cell?

Answer 24

Na+ is pumped into the cell via active transport and leaves via secondary transport

Question 25

What determines the membrane permeability to water?

Answer 25

The P(d): Small, mercury sensitive, temperature dependent (lipid fluidity)
The P(f): Large, mercury sensitive, temperature independent
Because they express different aquaporin isoforms they have different P(w)