Question

The simplest hydrophilic moiety present in the membrane lipid is

• A. Phosphate group
• B. Hydroxyl group
• C. Amino group
• D. Glucose

Hint:

Cholesterol is amphipathic but has a tiny polar head.
Its single, small polar hydroxyl group ($-\text{OH}$) orientates toward the aqueous phase, while the rest of its bulky hydrophobic steroid ring sits parallel to the fatty acid chains of neighboring phospholipids.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Membrane lipids are amphipathic molecules, meaning they possess both a hydrophobic (water-fearing) tail and a hydrophilic (water-loving) head group.
These polar head groups interact with the aqueous environment on both sides of the lipid bilayer.

Step 2: Detailed Explanation:
Let us analyze the chemical structure of the polar head groups in different membrane lipids:
- Phospholipids: These contain a complex polar head group composed of a negatively charged phosphate group ($\text{PO}_4^{3-}$) attached to other polar or charged organic moieties (such as choline, ethanolamine, serine, or glycerol).
- Glycolipids: These contain carbohydrate moieties (such as glucose or galactose) as their hydrophilic head groups, which are relatively large, multi-carbon ring structures.
- Cholesterol: This is an essential sterol lipid in animal cell membranes. It consists of a rigid hydrophobic steroid ring core with a very small polar head group, which is a single hydroxyl group ($-\text{OH}$).
The hydroxyl group consists of only one oxygen and one hydrogen atom.
Structurally, this is the smallest and simplest possible hydrophilic moiety found among all classes of membrane lipids.

Step 3: Final Answer:
Thus, the simplest hydrophilic group in membrane lipids is the hydroxyl group, making option (B) the correct choice.