CBS - Membrane Transport

Question 1

What is meant by the cell membranes having selective permeability?

Answer 1

Membranes are selective permeability barriers which block the passage of almost all hydrophilic molecules (into cells and organelles).

Small uncharged or hydrophobic molecules can freely cross the membrane by simple diffusion along their concentration gradients.

Charged polar molecules require specialist proteins (pumps, transporters, pores) to allow them to across the membrane.

Question 2

List examples and the permeability of these molecules:

• hydrophobic molecules
• small, uncharged polar molecules
• large uncharged polar molecules
• ions
• charged polar molecules

Answer 2

Hydrophobic molecules: O2, N2, CO2, benzene, short chain fatty acids
PERMEABLE

Small, uncharged polar molecules: H2O, urea, glycerol
PERMEABLE (slightly less than above)

Large uncharged polar molecules: glucose, sucrose
NON-PERMEABLE (mostly, very small amounts can get through)

Ions: H+, Na+, Mg2+, HCO3- K+, Ca2+, Cl-
NON-PERMEABLE

Charged polar molecules: amino acids, ATP
NON-PERMEABLE

Question 3

List the mechanisms of transport of molecules across the membrane.

Answer 3

• simple passive transport / Diffusion
• facilitated diffusion
• gated ion channels
• primary active transport
• secondary active transport

Question 4

Describe passive transport (and an example of a molecule that utilizes it to cross membranes).

Answer 4

The solutes are able to move down a concentration gradient, crossing the membrane.

At Equilibrium, the [inside cell] = [outside cell]. The rate of diffusion depends on the Partition Coefficient of the solute.

Some solutes that are more hydrophobic have a higher Partition Coefficient and equilibrate more quickly.

Example: H2O

Question 5

Describe facilitated/ carrieri=-mediated transport (and examples of molecules that utilize it to cross membranes).

Answer 5

The solutes move down a concentration gradient crossing membrane.

At Equilibrium, the [inside cell] = [outside cell]. However, it requires a membrane protein (ion channel)

Examples:

• Cl-/HCO3 channel in erythrocytes
• aquaporin: water channel
• GLUT glucose transporters

Question 6

How do passive transport and facilitated diffusion compare kinetically?

Answer 6

Facilitated diffusion is much more efficient, and allows an equilibrium to be reached quicker compared to the passive transport.

Question 7

How is transporter affinity exemplified?

Answer 7

The transporter affinity for a solute is given by the Km.

The lower the Km, the higher the affinity.

Question 8

List the location and function of the GLUT1 transporter.

Answer 8

LOCATION:
ubiquitous, abundant in erythrocytes, low in skeletal muscle

FUNCTION:

• has a low Km ~1.8 mM (high affinity)
• mediates constitutive glucose uptake in many tissues

Question 9

List the location and function of the GLUT2 transporter.

Answer 9

LOCATION:
liver, pancreatic beta-cells

FUNCTION:

• has a high Km ~ 20mM (low affinity) and large Jmax (high capacity)
• transports glucose into hepatocytes and pancreatic ß-cells when [glucose] in blood is high, to regulate blood glucose levels

Question 10

List the location and function of the GLUT3 transporter.

Answer 10

LOCATION:
neurones

FUNCTION:

• has a low Km (high affinity)
• allows glucose to pass

Question 11

List the location and function of the GLUT4 transporter.

Answer 11

LOCATION:
muscle, adipocytes

FUNCTION:

• has a Km ~ 5mM
• similar to fed state blood glucose concentration
• regulated by insulin

Question 12

Describe the insulin-stimulated uptake of glucose.

Answer 12

• insulin stimulates the uptake of glucose in muscle and adipose
• the insulin increases the amount of the GLUT4 in the plasma membrane: the GLUT4 are present on membrane bound vesicles in cytoplasm - insulin triggers the movement of these vesicles to plasma membrane
• vesicles merge with the plasma membrane and increase level on cell surface
• the increase in glucose transporters increases the uptake of glucose into the cell

Question 13

Describe gated ion-channels.

Answer 13

They are ion channels that allow facilitated diffusion selective for different ions (K+, Na +, Ca 2+)

They open or close in response to a stimulus. They can be either ligand gated or voltage gated.

• ligand-gated e.g. acetylcholine and acetylcholine-gated Na+/K+ channel (acetylcholine receptor) on postsynaptic membranes
• voltage-gated e.g. Na+ and K+ channels in axons involved in nerve transduction in axons

Question 14

Describe active transport.

Answer 14

The solutes move against a concentration gradient. This requires a membrane protein and energy – hydrolysis of ATP.

It is primary active transport if the ATP hydrolysis directly causes the movement of solute (uniport).

Example:
- Na+/K+ pump (Na+/K+ ATPase) in plasma membrane

Question 15

Describe the ion gradient of [Na+] and [K+] across the membrane.

Answer 15

[internal] of cell:
Na+: ~12 mM
K+: ~140 mM

[external] in blood plasma:
Na+: ~145 mM
K+: ~4 mM

The different of these ions inside and outside the cell facilitates nerve transmission.

Question 16

Describe the Na+/K+ pump.

Answer 16

The Na+/K+ pump consists of a tetramer (α2β2).

The phosphorylation from ATP at cytoplasmic site causes conformational change.

Question 17

Briefly describe how the Na+/K+ pump works.

Answer 17

Sodium enters inside aqueous pocket from the internal cytoplasmic access.

ATP closes the internal side, and opens the external side so that the sodium ions leave and potassium enter.

Hydrolysis of the ATP returns it back to its original orientation, and the potassium can enter the cell, and sodium go back in.

Question 18

How do co-transport systems work?

Answer 18

A pre-established gradient is used to drive the transport of solutes across membrane against a gradient.

ATP hydrolysis is used to establish the primary gradient.

Symport : transport of two solutes in the same direction
Antiport : transport of two solutes in opposite direction

Question 19

Describe how the Na+-glucose cotransporter (SGLUT) works.

Answer 19

SGLUT is a symport, working through secondary active transport. Glucose absorption occurs from intestine, against the gradient.

An Na+ gradient is established by the Na+/K+ pump and ATP hydrolysis is used to drive the uptake of glucose into the cells.

A similar system operates for the uptake of amino acids.

Question 20

Describe the entire picture of glucose transporters in the gut.

Answer 20

We have SGLUT on the apical membrane, and GLUT2 and the Na+/K+ pump on the basal membrane.

The SGLUT brings in glucose and Na+ from the gut into the cell.

The glucose is transported via GLUT2 into the blood stream.

The Na+ is used in the Na+/K+ pump, using ATP to drive Na+ out into the blood stream, and bring K+ into the cell.

Question 21

Describe the Na+/Ca2+ cotransporter.

Answer 21

It works via secondary active transport. The Ca2+ export from the muscle cells is against the gradient

The Na+ / Ca2+ cotransporter (sodium-calcium exchanger) is an antiport.

THe Na+ gradient established by the Na+/K+ pump and ATP hydrolysis is used to drive the export of Ca2+ from cells.

Question 22

What are some clinical considerations of digoxin and digitoxin on the Na+/K+ pump?

Answer 22

Cardiac glycosides, such as digitoxin and digoxin, are originally made from fox-gloves.

They inhibit the Na+/K+ pump by blocking the dephosphorylation step.

The [Na+] inside heart muscle increases and the Na+ gradient is lost.

The export of Ca2+ via the antiport does not happen. Thus, the [Ca2+ ] inside heart muscle increases and contraction increases.

The plant extract used in poison arrows, oubain, works in a similar manner.

Question 23

Describe the CFTR protein.

Answer 23

It is a transmembrane protein, and an ATP-ated ion channel. It is part of the
ABC (ATP-binding cassette) transporter family.

The chloride ions move by facilitated diffusion down concentration gradient – out of cell.

The cytoplasmic regulatory domain is phosphorylated by cyclic AMP dependent protein kinase (PKA).

The PKA activates and opens the channel.

Question 24

How does affecting the chloride pump relate to CFTR?

Answer 24

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator protein (CFTR).

CFTR is a chloride ion channel in cells responsible for producing mucus, sweat, saliva and tears.

Movement of chloride ions in turn regulates movement of water.

Patients have reduced chloride transport that results in the production of thick mucus. This leads to blocked lungs and infections.

Question 25

What is a clinical consideration of cholera treatment?

Answer 25

Vibrio cholerae causes electrolyte and fluid secretion. The cholera toxin stimulates an increase in cAMP level that activates CFTR and secretion of chloride ions.

Na+ and water follow into the lumen via osmosis and the paracellular route.

Oral rehydration therapy includes high glucose concentration (~110 mM) which drives Na+ (and consequently Cl- and H2O) uptake into cells via SGLUT.

Question 26

Describe how insulin is secreted by beta cells.

Answer 26

1. Glucose is transported into beta cells through facilitated diffusion by GLUT2.
2. Glucose is metabolized, increasing the level of ATP.
3. Elevation in the ATP/ADP ratio induces the closure of cell-surface ATP-sensitive K+ (K-ATP) channels, leading to cell membrane depolarization.
4. Voltage-gated Ca2+ channels are opened, and intracellular [Ca2+] increases.
5. The increase in [Ca2+] triggers the exocytosis of insulin stored in vesicles.