Glucose is a hexose (6-carbon) sugar and the most common simple sugar. It is important in animal respiration and other processes in organisms. As a preliminary to respiration, more complex sugars, including disaccharides (e.g., sucrose) and polysaccharides (e.g. starch and cellulose), are broken down into molecules of the less chemically complex glucose. Glucose is also obtained from the deamination of amino acids. Then glucose enters the cell via special molecules, called glucose transporters, found in the cell membrane. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP) through two different pathways, glycolysis and the citric acid cycle (also known as the Kreb's cycle).
Glucose can take on several different structural forms which can be grouped into two distinct families of stereoisomers (mirror-images). Only one family of these isomers occurs naturally on Earth, and this is derived from the right-handed form of glucose, known as dextrorotatory glucose, D-glucose, or dextrose.