(dextrose, grape sugar) An aldohexose sugar. In solution it forms a six-membered *pyranose ring
. Glucose is the major fuel source of nearly all organisms and the basic substrate from which starch, cellulose, sucrose, and other carbohydrates are synthesized. Glucose is synthesized by several routes; de novo synthesis from carbon dioxide is via the *Calvin cycle
, but synthesis from fatty acids via acetyl CoA and the *glyoxylate cycle
is also a major route in some cells (e.g. germinating oil-bearing seeds). It can be oxidized through *glycolysis
and the *TCA cycle
. Synthetic pathways from glucose involve activation, by phosphorylation or by formation of a nucleotide diphosphate sugar.