In cell membrane structure, a phospholipid is best described as which type of amphipathic molecule that forms the basic framework of biological membranes?

Introduction / Context:
Biological membranes such as the plasma membrane and organelle membranes consist mainly of a phospholipid bilayer with proteins and other lipids. Understanding what a phospholipid is helps explain membrane structure, fluidity, and selective permeability. This question asks for the best description of a phospholipid molecule.

Given Data / Assumptions:

• Cell membranes are mainly composed of phospholipids and proteins.
• Phospholipids are amphipathic, meaning they contain both hydrophilic and hydrophobic parts.
• We assume the typical phospholipid found in eukaryotic cell membranes.
• The question focuses on structural features of phospholipids.

Concept / Approach:
A typical phospholipid consists of a glycerol backbone. Two of the glycerol hydroxyl groups are esterified with fatty acid chains that are hydrophobic. The third hydroxyl group is linked to a phosphate group that often has additional polar groups attached, forming a hydrophilic head. Because of this dual nature, phospholipids arrange themselves into bilayers in aqueous environments, with tails facing inward and heads facing outward. Therefore, the correct option must describe this amphipathic structure.

Step-by-Step Solution:
Step 1: Recall that phospholipids have both hydrophilic and hydrophobic regions, making them amphipathic. Step 2: Remember that two fatty acid tails are attached to a glycerol molecule, providing the hydrophobic part. Step 3: Note that a phosphate group attached to the third position of glycerol, often with additional small polar groups, provides the hydrophilic head. Step 4: Compare this description with option A, which mentions a glycerol backbone, two fatty acid tails, and a phosphate containing head. This matches the correct structure. Step 5: Check other options and see that they describe sugars, proteins, DNA bases, or steroid hormones, none of which correctly define a phospholipid.

Verification / Alternative check:
Textbook diagrams of the phospholipid bilayer show molecules with two nonpolar hydrocarbon tails and a polar head group. Experimental studies using X ray diffraction and electron microscopy support the bilayer arrangement driven by hydrophobic interactions of the tails and exposure of hydrophilic heads to water. This arrangement explains many membrane properties such as self sealing, fluidity, and the ability to form vesicles.

Why Other Options Are Wrong:
Option B describes a simple sugar like glucose, which is not a lipid and does not form membranes. Option C describes a polypeptide chain of amino acids, which is a protein, not a phospholipid. Option D describes a nitrogenous base component of nucleic acids, which again is unrelated to membrane lipids. Option E describes a steroid hormone that is fully water soluble without a hydrophobic region, which does not match typical steroid or phospholipid properties.

Common Pitfalls:
Students sometimes confuse phospholipids with triglycerides, which have three fatty acid tails and no phosphate group, and therefore are not amphipathic in the same way. Another error is to forget the importance of amphipathic nature and think any lipid could form a bilayer. Remember that the phosphate containing head and hydrophobic tails together create the bilayer structure characteristic of cell membranes.

Final Answer:
A phospholipid is best described as a lipid molecule with a glycerol backbone attached to two fatty acid tails and a phosphate containing polar head group.