Glucose (dextrose; grape sugar) A white crystalline sugar, C₂H₂O₂, occurring widely in nature. Like other monosaccharides, glucose is optically active: most naturally occurring glucose is dextrorotatory. Glucose and its derivatives are crucially important in the energy metabolism of living organisms. Glucose is also a constituent of many polysaccharides, most notably starch and cellulose. These yield glucose when broken down, for example by enzymes during digestion.