Which of the following molecules is NOT classified as a lipid?

Introduction / Context:
Biological molecules are grouped into major classes such as carbohydrates, lipids, proteins, and nucleic acids. Lipids are generally hydrophobic or amphipathic molecules that play roles in membrane structure, energy storage, and signalling. Exam questions often present a list of biomolecules and ask you to identify the one that does not belong to a certain class. This question focuses on distinguishing lipids from nucleic acids by asking which option is not a lipid.

Given Data / Assumptions:

• We need to choose the molecule that is not a lipid.
• Options include wax, cholesterol, RNA, and phospholipid.
• We assume familiarity with the basic functions and classifications of these molecules.

Concept / Approach:
Lipids are a diverse group that includes fats, oils, waxes, phospholipids, and steroids. Waxes are long chain fatty acid esters that provide waterproofing in plants and animals. Cholesterol is a sterol lipid found in animal cell membranes and is a precursor for steroid hormones. Phospholipids consist of a glycerol backbone, fatty acid tails, and a phosphate containing head group; they are key components of biological membranes. RNA, or ribonucleic acid, by contrast, is a nucleic acid composed of ribose sugar, phosphate, and nitrogenous bases. It functions mainly in protein synthesis and regulation of gene expression, not as a lipid. Therefore, RNA is the molecule among the given options that is not a lipid.

Step-by-Step Solution:
Step 1: Identify wax as a hydrophobic lipid used in waterproofing leaves and in insect cuticles. Step 2: Recognise cholesterol as a sterol lipid integrated into animal cell membranes and important for fluidity. Step 3: Understand that phospholipids form the bilayer structure of cell membranes and are classic examples of lipids. Step 4: Recall that RNA is a nucleic acid, built from nucleotides, and acts in processes such as mRNA, tRNA, and rRNA. Step 5: Conclude that RNA is not a lipid and is therefore the correct answer to select.

Verification / Alternative check:
Biochemistry textbooks clearly divide biomolecules into four main categories and list examples in each. Waxes, cholesterol, and phospholipids are consistently grouped under lipids. RNA is always described as a nucleic acid alongside DNA. Chemical structure diagrams show that lipids are largely hydrocarbon based with relatively few polar groups, whereas nucleic acids consist of a sugar phosphate backbone and nitrogenous bases. These structural and functional differences support the classification of RNA as non lipid in this question.

Why Other Options Are Wrong:

• Wax, a hydrophobic substance used for waterproofing: Waxes are classic lipids composed of long chain fatty acids and alcohols.
• Cholesterol, a sterol component of cell membranes and precursor of hormones: Cholesterol is a key lipid involved in membrane structure and steroid hormone synthesis.
• Phospholipid, a major structural component of cell membranes: Phospholipids are an important class of lipids forming the lipid bilayer of membranes.

Common Pitfalls:
Students sometimes confuse classes because molecules like cholesterol and phospholipids look quite different from simple fats, leading them to think they might not be lipids. It is important to remember that lipids are defined more by physical properties and solubility than by one simple structure. Another mistake is to assume any large biological molecule is a lipid. A helpful strategy is to explicitly classify RNA and DNA as nucleic acids, and to remember that any molecule with a sugar phosphate backbone and bases is not a lipid.

Final Answer:
The molecule that is not a lipid is RNA, a nucleic acid involved in protein synthesis and gene regulation.