A white crystalline sugar, C₆H₁₂O₆, 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. It is a major energy source, being transported around the body in blood, lymph, and cerebrospinal fluid to the cells, where energy is released in the process of glycolysis. Glucose is present in the sap of plants, in fruits, and in honey and is also a constituent of many polysaccharides, most notably of starch and cellulose. These yield glucose when broken down, for example by enzymes during digestion. Maintenance of a constant glucose concentration in the blood, primarily through the actions of the hormones insulin and glucagon, is vital for cell metabolism. In particular, cells of the nervous system depend almost exclusively on glucose as an energy source, whereas most other cells can switch to fatty acids when levels of glucose fall.