Introduction 

In humans the digestion system creates glucose. This glucose then travels in the bloodstream along with oxygen to all the mitochondria in each cell of the body. The process of respiration occurs in the mitochondria where the glucose is used to create an energy molecule called ATP for muscle movement, building other molecules, growth and keeping warm. 

Respiration in the mitochondria converts glucose with oxygen (called aerobic respiration) or without oxygen ( called anaerobic respiration) into energy (ATP). Without ATP cells will die. 

Aerobic respiration

Glucose + oxygen  →  carbon dioxide + energy (ATP) + water

Anaerobic respiration

Glucose  →  energy (ATP) + lactic acid

The digestion system creates glucose

(Note: Digestion is breaking down of food)

Mitochondria converts glucose into energy (ATP)