Which system is responsible for breaking down food into substances usable by cells?

The digestive system is responsible for breaking down food into smaller, usable substances that can be absorbed and utilized by cells in the body. This complex system includes organs such as the mouth, stomach, and intestines that work together to mechanically and chemically process food.

When food is consumed, the digestive system facilitates the breakdown of macronutrients like carbohydrates, proteins, and fats into their smaller components—simple sugars, amino acids, and fatty acids, respectively. This process allows the nutrient molecules to pass through the intestinal walls into the bloodstream, where they can be transported to cells and tissues for energy production, growth, and repair.

By understanding the role of the digestive system in nutrient processing, one can appreciate how essential it is for overall health and functioning. The other systems mentioned, such as the excretory system, circulatory system, and nervous system, have different primary functions that do not focus directly on the breakdown of food for nutrient absorption.