Cell membrane transport

Question 1

What are the types of transport across cell membranes?

Answer 1

Active
Passive

Question 2

What are the types of passive transport?

Answer 2

Simple
Facilitated
No energy input (ATP) is required for these

Question 3

What is simple diffusion?

Answer 3

Movement down a concentration gradient.
No membrane proteins are involved.

Question 4

What is simple diffusion dependent on?

Answer 4

Concentration gradient
Hydrophobicity / charge
Size

Question 5

What is the relative ease of compounds simply diffusing across membranes?

Answer 5

Easy: Hydrophobic molecules - O2, CO2, N2 steroid homones
Small uncharged polar molecules - H2O, urea, glycerol
Large uncharged polar molecules - glucose, sucrose
Impossible: Ions - H+, Na+, HCO3-, K+ etc

Question 6

What is transport of inorganic ions required for?

Answer 6

Regulation of intracellular ion concentrations.
Uptake of nutrients - e.g. glucose and amino acids.
Excretion of metabolic waste products.

Question 7

What are the classes of facilitated diffusion?

Answer 7

Channels - discriminates on size and charge
Uniporter carrier proteins - involves a binding site for solutes.

Question 8

What is facilitated diffusion?

Answer 8

Transports inorganic ions or small molecules across the membrane passively along their concentration / electrochemical gradients.

Question 9

What is an electrochemical gradient?

Answer 9

Combines the concentration gradient and membrane potential.
This force drives a charged solute across a membrane.

Question 10

What are ion channels?

Answer 10

Membranes are impermeable to ions so ion channels are used to transport ions across membranes.

Question 11

What are the features of ion channels?

Answer 11

Exhibit ion selectivity.
Driven by concentration/electrochemical gradient.
Fast - transport 10^7 molecules per sec
May be regulated (open in response to stimulus)

Question 12

How are ion channels regulated?

Answer 12

Voltage gated
Ligand-gated - extracellular ligand
Ligand-gated - intracellular ligand
Mechanically gated

Question 13

What are K+ channels?

Answer 13

Most common ion channels.
Continuously open
Selective.
Quickly moves K+ out of the cell.

Question 14

What are uniporter carrier proteins?

Answer 14

E.g. the Glucose transporter (Glut2) in gut epithelia.
Highly selective - transported molecule is bound to carrier
Relatively slow - <1000 molecules per second - because it requires conformational change.

Question 15

What does a uniporter carrier protein look like?

Question 16

What are glucose transporters?

Answer 16

They only transport glucose.
Expressed by most cell types
12 pass membrane spanning proteins
Alternate between 2 conformations.

Question 17

How is glucose transported into erythrocytes by Glut1?

Answer 17

Glucose concentration is higher in blood than in erythrocyte.
Transported into cell by Glut1 along concentration gradient.
Glut1 works in both directions so gradient needs maintaining - glucose is converted to glucose-6-phosphate.
Glucose-6-phosphate not recognised by Glut1 so one-directional transport.

Question 18

Why do cells maintain electrochemical gradients?

Answer 18

To drive transport across membranes.
To maintain osmotic balance
Electrical forces inside and outside of the cell must be balanced.

Question 19

What happens if there is no active transport?

Answer 19

Without active transport to maintain electrochemical gradients, ions would flow down their gradients through channels, disturbing osmotic balance.

Question 20

What is active transport?

Answer 20

It moves solutes against their electrochemical gradients.
This requires energy.

Question 21

How do cells carry out active transport?

Answer 21

ATP-driven pumps
Coupled transporters
Light-driven pumps

Question 22

What are ATP-driven pumps?

Answer 22

Couple the transport of a solute against its gradient to the hydrolysis of ATP - primary active transport.

Question 23

What are coupled transporters?

Answer 23

Couple the transport of one solute with the gradient to another solute against the gradient - secondary active transport.

Question 24

What are light-driven pumps?

Answer 24

Couple the transport of a solute against its gradient to the input of energy from light.

Question 25

What are the concentrations of Na+ and K+?

Answer 25

Na+ concentration is high on the outside of the cell. K+ concentration is high on the inside of the cell.

Question 26

How is the Na+ electrochemical gradient maintained?

Answer 26

Na+/K+ ATPase: Pump operates continuously to expel Na+ that enters through other carrier proteins and channels. Hydrolyses ATP to ADP - it is an enzyme and carrier protein. Couples the export of Na+ to the import of K+.

Question 27

What is the mechanism of Na+/K+ ATPase?

Answer 27

3 Na+ ions bind. Pump hydrolyses ATP and is phosphorylated. Na+ dependent phosophorylation causes pump to undergo conformational change, and Na+ is transferred across membrane. 2 K+ ions bind and the pump is dephosphorylated. K+ dependent phosphorylation causes the pump to return to its original conformation and K+ is transferred across the membrane.

Question 28

What are the types of coupled transporters?

Answer 28

Symport - transports ions or molecules in the same direction Antiport - transports ions or molecules in opposite directions.

Question 29

What is the Na+/glucose symporter?

Answer 29

Na+ electrochemical gradient is used to drive the movement of glucose against its gradient. [Na+] is high in the gut. [glucose] is high in the cell.

Question 30

What is the Na+/glucose coTransporter (SGLT1)?

Answer 30

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

Question 31

How is glucose absorbed?

Answer 31

Na+ gradient is created by Na+/K+ ATPase (primary). Na+/glucose symporter transports glucose into epithelial cells (secondary). Glut2 transports glucose out of the cells along concentration gradient and into blood supply (facilitated diffusion).

Question 32

What is the Na+/Ca2+ antiporter?

Answer 32

Cardiac muscle contraction is triggered by a rise in intracellular [Ca2+]. Na+/Ca2+ antiporter reduces intracellular [Ca2+] and reduces the strength of cardiac muscle contraction.

Question 33

How is cardiac failure treated?

Answer 33

Ouabain and digoxin inhibit Na+/K+ ATPase. This increases intracellular [Na+] and reduces the Na+ gradient. The effectiveness of the Na+/Ca2+ antiporter is reduced, [Ca2+] is maintained and cardiac contraction is strengthened.

Question 34

What is the configuration of the Na+/glucose symporter?

Answer 34

Transporter can exist as different confugurations: Either open to the inside of the cell, or to the outside, and it can flip between these forms.

Question 35

Why are glucose symporters required for the absorption of glucose from the GI tract?

Answer 35

There are tight junctions between epithelial cells so small molecules such as glucose cannot travel by paracellular movement. They must travel through the cells - transcellular movement.