Which category of substance is an essential structural component of living cells and a source of energy for animals?

The correct answer is carbohydrate. Carbohydrates are vital for living organisms as they serve both as a structural component of cells and as a primary energy source, especially for animals. They are made up of carbon, hydrogen, and oxygen atoms and include sugars and starches.

In addition to providing energy, carbohydrates can also play a structural role, particularly in plants, where they form the cell wall component known as cellulose. This structural aspect is crucial for maintaining cell integrity and supporting plant structures.

While proteins play numerous roles in the body, including enzymatic functions and structural support, they are not primarily considered an energy source. Lipids, on the other hand, are indeed a significant energy reserve and also contribute to cellular structures, but they do not fulfill the same role in energy availability as carbohydrates in most animals’ diets. Nucleic acids are essential for genetic information storage and transfer but do not serve as energy sources or structural components in the same way carbohydrates do.