What is the most basic source of energy for living things?

Glucose is the most basic source of energy for living things because it is a simple sugar that is readily used by cells during cellular respiration to produce ATP (adenosine triphosphate), which is the primary energy currency of the cell. When organisms metabolize glucose, they can efficiently convert its chemical energy into a form that is usable for various biological functions, including growth, repair, and maintaining homeostasis.

While fructose, cellulose, and sucrose are important carbohydrates, they do not serve as the primary energy source in the same manner as glucose. Fructose is a simple sugar found in many plants but is often converted to glucose in the body before being utilized for energy. Cellulose, on the other hand, is a complex carbohydrate that provides structural support to plant cell walls and is not directly usable as an energy source by most organisms due to its indigestibility. Sucrose is a disaccharide made up of glucose and fructose, and while it can provide energy, it ultimately needs to be broken down into its monosaccharide components before it can be utilized, while glucose can be used directly. Thus, glucose remains the primary and most direct source of energy for living organisms.