Glu-ca-gly-co-ly-gen-sis? Keeping the terminology straight

Metabolism is a complex and often confusing topic. To make matters worse, a lot of the words for some of its molecules and processes sound a little too much alike.

Here are some definitions that may help you keep it all straight.


Glucose (GLOO-kose) is a simple sugar made of 6 carbon atoms, 6 oxygen atoms, and 12 hydrogen atoms. Any cell in the body can burn it for energy. The suffix -ose is a clue that this molecule is a type of sugar. Some other sugars are fructose, sucrose, and lactose.

Glycerol (GLISS-er-oll), also called glycerin, is a 3-carbon molecule that is chemically similar to sugar. You could think of it as half of a glucose molecule. A fat molecule, also called a triglyceride, is made up of three fatty acids attached to a molecule of glycerol.

Glycogen (GLY-koh-jen) is sometimes called "animal starch." It is a large molecule made up of hundreds or thousands of glucose molecules. Packed away inside of liver and muscle cells, it is one way that nutrients are temporarily stored in the body.

Glucagon (GLOO-kah-gone) is a protein hormone that signals the body to go into "mobilization" mode: stored nutrients are released for cells to burn for energy. Glucagon is made in the pancreas, which releases it between meals when blood sugar levels are low.


All of these processes are metabolic pathways that involve multiple protein enzymes working like an assembly line to carry out the necessary chemical reactions. The suffix -lysis indicates that the process breaks something apart, and the suffix -genesis indicates that the process puts something together.

Glycolysis (gly-KOL-ih-sis), which means "sugar splitting," is the name for the metabolic pathway that breaks a 6-carbon glucose molecule into two 3-carbon pyruvate molecules. The glycolysis pathway involves 10 separate protein enzymes, and it is present in most of the body's cells.

Glycogenesis (gly-koh-JEN-ih-sis) is the name for the metabolic pathway that puts glucose molecules together to make the storage molecule glycogen.

Glycogenolysis (GLY-koh-jen-OLL-ih-sis), which means "glycogen breaking," is the name for the metabolic pathway that breaks glycogen apart into individual glucose molecules.

Gluconeogenesis (gloo-koh-nee-oh-JEN-ih-sis) is the metabolic pathway that builds glucose from other small molecules, including amino acids (which come from protein breakdown) and glycerol (described above). Most gluconeogenesis is carried out in the liver.