Science Entrance Practice Exam 2026 - Free Science Practice Questions and Study Guide

Carbohydrates are primarily recognized as a source of stored energy within biological systems. They are composed of carbon, hydrogen, and oxygen in a typically ratio of 1:2:1, allowing them to be easily converted into energy through metabolic pathways. In particular, polysaccharides like glycogen and starch serve as significant energy reserves. Glycogen, found in animals, is stored mainly in the liver and muscles, while starch, found in plants, serves a similar purpose. When energy is needed, these carbohydrates can be broken down into glucose, which provides ATP (adenosine triphosphate)—the primary energy currency of cells—through processes like glycolysis and cellular respiration.

Nucleic acids, proteins, and minerals do not primarily serve as energy sources. Nucleic acids, such as DNA and RNA, are primarily involved in genetic information storage and transmission. Proteins serve numerous functions, including building and repairing tissues, acting as enzymes, and supporting immune responses, but they are not the first choice for energy. Minerals are essential for various biochemical functions but do not provide energy themselves. Thus, carbohydrates stand out as the macromolecule category primarily responsible for energy storage.