The type of organic molecule that serves as the main energy source for living things is carbohydrates. Carbohydrates are composed of carbon, hydrogen, and oxygen, typically following a basic formula of \[C_n(H_2O)_n\]. They are categorized into simple sugars, like glucose and fructose, and complex carbohydrates, such as starch and cellulose. In cellular metabolism, carbohydrates are broken down during cellular respiration to release energy stored in their bonds for cellular activities. They play critical roles in energy storage, structural functions, and signaling processes within organisms.