How is the energy from glucose harvested in the process of glycolysis?

The energy from glucose is harvested in glycolysis through the production of ATP, NADH, and pyruvate.

Glycolysis is a metabolic pathway that takes place in the cytoplasm of cells. It is the first stage of cellular respiration and is both anaerobic (does not require oxygen) and exergonic (releases energy). The process involves the breakdown of glucose, a six-carbon sugar, into two molecules of pyruvate, a three-carbon compound.

The process of glycolysis is divided into two phases: the energy investment phase and the energy payoff phase. In the energy investment phase, two molecules of ATP are used to phosphorylate glucose and convert it into fructose 1,6-bisphosphate. This is then split into two three-carbon compounds, glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). DHAP is quickly converted into G3P, so two molecules of G3P are produced for every glucose molecule that enters glycolysis.

In the energy payoff phase, the two G3P molecules are oxidised to form two molecules of pyruvate. During this oxidation, two molecules of NAD+ are reduced to form two molecules of NADH, a high-energy electron carrier. Additionally, four molecules of ATP are produced by substrate-level phosphorylation, where a phosphate group is transferred from a high-energy reactant to ADP. Since two ATP molecules were used in the energy investment phase, this results in a net gain of two ATP molecules per glucose molecule.

The energy harvested from glucose in glycolysis is therefore stored in the form of ATP and NADH. These molecules can be used in other metabolic pathways to produce more ATP, or they can be used directly by the cell to perform work. The pyruvate produced in glycolysis can also be further metabolised in the absence or presence of oxygen, leading to the production of more ATP. Thus, glycolysis is a crucial process for energy production in cells.