Question

1- All of the following characteristics directly influence the fluidity of a biological membrane EXCEPT:

a. Amount of cholesterol molecules

b. Length of fatty acid chains in phospholipids

c. Amount of unsaturated fatty acid chains in phospholipids

d. Type of phospholipid head group

e. Temperature

2- How can polar proteins be inserted into a hydrophobic phospholipid bilayer?

a. There is no way a polar protein can be inserted into a phospholipid bilayer

b. Any polar protein can be inserted without special folding or modification requirements

c. Polar proteins must be modified with a carbohydrate to be inserted

d. Polar proteins can have the hydrophilic portions inserted into the bilayer

e. Polar proteins can be inserted if they have α-helices made of nonpolar amino acids

3- Which of the following statements about membrane transport is FALSE?

a. Hydrogen ions cannot freely diffuse across a lipid bilayer

b. Channel proteins selectively transport molecules based on size

c. Channel proteins are constantly open and cannot be regulated

d. Carrier proteins passively transport specific molecules

e. Transporters can move molecules against a concentration gradient

Answer 1

1. Factors that affect the membrane fluidity are:

a. Amount of cholesterol: It act as a buffer to maintain membrane fluidity within an optimum range.

b. Amount of unsaturated fat: Unsaturated fatty acids increase the membrane fluidity due to kinked hydrocarbon tail.

c. Length of fatty acid chain: short chain fatty acids are less viscous.

Hence the answer is D. Type of phospholipid head.

2. Polar proteins that are embedded in a hydrophobic environment of lipid bilayer has amino acid residues with non polar side chains. Since peptide bond itself is polar and water molecules are absent hence the peptide bonds are forced to form hydrogen with one another.Thy hydrogen bonding would be maximum if the polypeptide chain forms alpha helix while spanning the membrane.

Ans E is correct

3. Many channel proteins are not permanently open and are also regulated by various factors like change in membrane potential, binding of ligand etc. e.g. voltage gated channels, ligand gated channels.

correct answer is c