This document discusses aerobic and anaerobic respiration. It defines respiration as the chemical reactions that take place in living cells to release energy. Respiration requires oxygen and food, and produces carbon dioxide, water and energy. Aerobic respiration fully breaks down glucose to carbon dioxide and water. Anaerobic respiration incompletely breaks down glucose without oxygen, producing carbon dioxide and alcohol or lactic acid. Anaerobic respiration is used by muscle cells during intense exercise when oxygen is limited, and by microbes like bacteria and yeast during fermentation.