Question

phospholipid diversity, cholesterol, and membrane structure
now lets analyze the phospholipid bilayer itself. depending on the species you are studying, you may encounter a wide array of phospholipids in cell membranes. and you wont be surprised to learn that variation in the structure of the lipids found in membranes correlates with variation in how those membranes function.
to understand how a particular lipid bilayer is put together, start by asking yourself three questions:

1. how long are the hydrocarbon tails in the phospholipids? the answer is important, because longer tails support stronger hydrophobic interactions.
2. are the hydrocarbon tails saturated or unsaturated? this has consequences, because saturated tails are straighter, meaning that they can pack more closely and sustain stronger hydrophobic interactions.
3. does the membrane contain the steroid cholesterol and if so, how much? recall that all steroids have a bulky \four fused - ring\ structure, and that cholesterol also has a hydrocarbon tail on one end and a polar - oh group on the other end. because it is amphipathic, cholesterol is a common component of some cell membranes. when cholesterol is present in a membrane, the bulky steroid rings can change the way the surrounding phospholipids pack together. typically, cholesterol strengthens the hydrophobic interactions in a membrane.

in the diagram below, which bilayer has the strongest hydrophobic interactions?
a
b
c
saturated phospholipid
unsaturated phospholipid
cholesterol

Explanation:

Step1: Recall membrane - interaction factors

Longer tails and saturated tails support stronger hydrophobic interactions, and cholesterol strengthens hydrophobic interactions.

Step2: Analyze each option

• Option A: Has some cholesterol and a mix of saturated and unsaturated phospholipids.
• Option B: Has more cholesterol and a mix of saturated and unsaturated phospholipids. Cholesterol strengthens hydrophobic interactions, and more cholesterol means stronger interactions compared to A.
• Option C: Has only phospholipids, no cholesterol, and a mix of saturated and unsaturated phospholipids. It has the least strengthening factor among the three.

Answer:

B.