Membrane Function (Module 2 Lecture 6)

Question 1

Contrast between the concentration of ions inside and outside of a cell.

Answer 1

1. Na+ (higher extracellular)
2. K+ (higher intracellular)
3. Mg2+ (higher extracellular; little of both)
4. Ca2+ (higher extracellular, little of both)
5. H+ (higher inside, very little of both)
6. Cl- (higher outside)

Question 2

Describe the permeability of the lipid bilayer.

Answer 2

1. Small nonpolar molecules such as O2 and CO2 readily diffuse across the lipid bilayer.
2. Uncharged polar molecules such as water and ethanol also diffuse (glycerol diffuses less rapidly and glucose hardly diffuses at all).
3. Lipid bilayers are highly impermeable to ions and charged molecules of all sizes.

Question 3

What is the function of transport proteins?

Answer 3

1. Move small water-soluble (hydrophilic) molecules across cell membranes.
2. Note: each transport protein is specific for a particular molecule or set of molecules.

Question 4

Give two examples of the specificity of transport proteins.

Answer 4

1. Inner mitochondrial membrane has transporters for substrates and for ATP.
2. Lysosomal membrane has proton pump to maintain acid environment of the organelle.

Question 5

Describe the acetylcholine receptor.

Answer 5

1. It’s a transmembrane ion channel with multiple subunits.
2. It’s a neurotransmitter.
3. When acetylcholine is bound, the receptor is opened and ions can flow across the membrane.

Question 6

Describe acetylcholine’s relationship with potassium and sodium ions as an ion pump.

Answer 6

1. When K+ concentrations

Question 7

What are carrier proteins also called?

Answer 7

transporter proteins or permeases

Question 8

Describe the structure and function of carrier proteins.

Answer 8

1. Multipass transmembrane proteins which alternate between two conformations.
2. Hydrophilic solute-binding site is sequentially accessible on one side of the membrane and then on the other side.
3. The solute never comes into contact with the hydrophobic interior of the bilayer.

Question 9

What is the difference between a symporter and an antiporter?

Answer 9

Symporters are carrier proteins that transport two different molecules in the same direction, and antiporters are carrier proteins that carry two different molecules in opposite directions.

Question 10

How do mammalian cells control their osmolarity?

Answer 10

They pump out ions (example: the Na+/K+ pump).

Question 11

Describe the common architecture of the ABC family of proteins.

Answer 11

1. MDR2 which confers multi drug resistance to tumor cells.

2. CFTR which acts as a chloride channel and is defective in cystic fibrosis.

Question 12

Describe receptor-mediated endocytosis and its function.

Answer 12

1. Used to transport large molecules into the cell

2. LDL binding –> internalization –> lysosomal hydrolysis