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Otto Meyerhof. One of the main scientist involved in completing the puzzle of glycolysis. Public Domain,

OpenStax Biology 2e

Nearly all of the energy used by living cells comes to them in the bonds of the sugar glucose. Glycolysis is the first step in the breakdown of glucose to extract energy for cellular metabolism. In fact, nearly all living organisms carry out glycolysis as part of their metabolism. The process does not use oxygen directly and therefore is termed anaerobic. Glycolysis takes place in the cytoplasm of both prokaryotic and eukaryotic cells. Glucose enters heterotrophic cells in two ways. One method is through secondary active transport in which the transport takes place against the glucose concentration gradient. The other mechanism uses a group of integral proteins called GLUT proteins, also known as glucose transporter proteins. These transporters assist in the facilitated diffusion of glucose.

– What is a form of sugar and a primary source of energy for every cell in the body?

Glycolysis begins with the six-carbon ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar called pyruvate. Glycolysis consists of two distinct phases. The first part of the glycolysis pathway traps the glucose molecule in the cell and uses energy to modify it so that the six-carbon sugar molecule can be split evenly into the two three-carbon molecules. The second part of glycolysis extracts energy from the molecules and stores it in the form of ATP and NADH—remember: this is the reduced form of NAD.

– What is the aerobic catabolism of nutrients to carbon dioxide, water, and energy, and involves an electron transport system in which molecular oxygen is the final electron acceptor?

It should be noted that when there is a surplus of glucose in glycolysis, some of the glucose will be converted to glycogen. Later, when blood sugar begins to fall, glycogen can be broken down into monomers of glucose again, which will enter glycolysis. Glycogen synthesis is an important process to help us store sugar for use when we are not eating but need to maintain appropriate blood sugar levels.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: