Which must an animal do in order for cellular respiration to begin? 1) turn energy into sugars 2)eat food 3)gain energy 4)change sugars into food

The correct answer is option 2, that is, eat food.  

Cellular respiration refers to an array of metabolic reactions and procedures, which occurs in the cells of organisms to transform biochemical energy from nutrients (food) into ATP and then discharge waste components.  

The reactions taking part in respiration are catabolic, that is, in the process dissociation of bigger molecules into smaller ones, and discharging energy in the process. The nutrients, which are generally used by animals in respiration include amino acids (proteins), sugar (carbohydrates) and fatty acids (fats).