Glycolysis is the 10-step metabolic respiration of the sugar glucose that yields chemical energy for use by a cell. Scientists consider glycolysis an ancient respiration pathway because it can occur in the absence of oxygen, which is how it could allow the survival of primitive anaerobic bacteria that predated Earth’s oxygen atmosphere. The ingredient list for glycolysis includes a living cell, enzymes, glucose and the energy transfer molecules nicotinamide adenine dinucleotide (NAD+) and adenosine triphosphate (ATP).
The basic input for glycolysis is sugar. Normally the sugar used is glucose, but enzymes can convert other six-carbon sugars, such as galactose and fructose, into intermediate substances that enter the glycolysis pathway downstream of the starting point for glucose. Plants create glucose during photosynthesis, and the sugar is available in a wide variety of foods directly or as starch and cellulose, which break down into glucose. Glucose dissolves in water and, with the help of enzymes, can easily be transported into or out of a cell, depending on its relative concentrations on either side of a cell membrane.
Enzymes are proteins that act as catalysts for biochemical reactions. Enzymes lower the energy needed to drive a reaction without being used up by the process. Glucose transporter enzymes help cells import glucose, but the first enzyme within the glycolysis pathway is hexokinase, which converts glucose to glucose-6-phosphate (G6P). This first step depletes the glucose concentration of the cell, thereby helping additional glucose to diffuse into the cell. The G6P product doesn't readily diffuse out of the cell, so hexokinase in effect locks up a glucose molecule for use by the cell. Nine other enzymes participate in glycolysis.
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ATP is a coenzyme that stores, transports and releases chemical energy within cells. An ATP molecule contains three phosphate groups, each held by a high-energy bond. ATP yields chemical energy when enzymes remove one or more phosphate groups, In the reverse reaction, enzymes use energy when adding phosphates to precursors, resulting in the production of ATP. Glycolysis uses two ATP molecules to get underway, but produces four ATPs by the last step, giving a net yield of two ATPs.
NAD+ is an oxidizing coenzyme that accepts electrons and protons from other molecules, creating the reduced form NADH. In the reverse reaction, NADH acts as a reducing agent that donates electrons and protons when it’s oxidized back into NAD+. NAD+ and NADH are used in a variety of biochemical pathways, including glycolysis, that require an oxidizing or reducing agent. Glycolysis uses two molecules of NAD+ per glucose molecule, producing two NADHs as well as two hydrogen ions and two molecules of water. The end product of glycolysis is pyruvate, which the cell can further metabolize to yield a large amount of additional energy.