Sugar is a structural component of which of the following classes of biomolecules?

Introduction / Context:
Sugars are simple carbohydrates that play many roles in biology. They are well known as energy sources, but they also serve as important structural components in larger biomolecules. This question tests your understanding of which biomolecular classes contain sugar as a key structural part.

Given Data / Assumptions:

Carbohydrates include simple sugars and complex polysaccharides.
Nucleic acids such as DNA and RNA are composed of nucleotides.
Nucleotides include sugar as one of their components.

Concept / Approach:
Carbohydrates by definition are made of sugar units, whether as simple monosaccharides like glucose or as complex polysaccharides like starch and cellulose. Nucleic acids such as DNA and RNA are polymers of nucleotides, and each nucleotide consists of three parts: a sugar, a phosphate group, and a nitrogenous base. In DNA, the sugar is deoxyribose, and in RNA it is ribose. Therefore, sugar is clearly a structural component of both carbohydrates and nucleic acids. While sugar can sometimes be attached to lipids or proteins as side groups, the core structure of standard lipids does not always include sugar, and the question focuses on classes where sugar is a defining component.

Step-by-Step Solution:
Step 1: Recall that carbohydrates themselves are composed of sugar units, making sugar a primary component. Step 2: Remember that each nucleotide in DNA and RNA contains a pentose sugar as part of its backbone. Step 3: Recognize that nucleic acids depend on sugar phosphate backbones for their structure. Step 4: Choose the option that includes both nucleic acids and carbohydrates.

Verification / Alternative check:
In DNA, the backbone is often described as alternating deoxyribose and phosphate groups, attached to nitrogenous bases. This confirms that sugar is a core structural element. Carbohydrates, by their very name and definition, are made of sugar units. This leaves no doubt that sugar is fundamental to both of these biomolecule classes.

Why Other Options Are Wrong:
Lipids only: Many lipids, such as triglycerides, do not contain sugar; some glycolipids do, but it is not universal for all lipids.
Nucleic acids only: This ignores the obvious fact that carbohydrates are sugars and their polymers.
Carbohydrates only: This misses the important sugar component of nucleic acids.

Common Pitfalls:
Students sometimes focus only on the role of sugar as an energy source and forget its structural roles. Another common confusion is to overlook the sugar phosphate backbone of DNA and RNA. Keeping a clear picture of nucleotide structure helps avoid this mistake.

Final Answer:
Sugar is a key structural component of both nucleic acids and carbohydrates.