Massachusetts Comprehensive Assessment System (MCAS) Biology Practice Test

Carbohydrates serve as the primary energy source for living organisms because they are easily broken down by the body to release glucose, which is a simple sugar and crucial for cellular respiration. During this process, glucose is metabolized to produce adenosine triphosphate (ATP), the energy currency of the cell. Carbohydrates are often stored in the form of glycogen in animals and starch in plants, providing readily available energy when needed.

While proteins and lipids also provide energy, they serve other main functions in the body; proteins play crucial roles in structure, function, and regulation of the body's tissues and organs, and lipids are important for storing energy and forming cell membranes. Nucleic acids, such as DNA and RNA, are primarily involved in storage and transmission of genetic information, rather than energy provision. Thus, carbohydrates are uniquely positioned as the go-to source for quick energy in living organisms.