Glycolysis and Krebs Cycle: Energy Production in Cells

Glycolysis and the Krebs Cycle

The Krebs cycle (also known as the citric acid cycle or tricarboxylic acid cycle) is named after its discoverer, Hans Krebs, who proposed the key elements of O₂ consumption in an amount disproportionate to the amounts added. Based on experimental observations, Hans Krebs proposed a cyclic pathway and cell reactions. The sequence, being a functional unit of all organisms, has a very coordinated and specific operation of all its organelles. However, within all this perfect machine, it is paramount that it takes energy for all this machinery to work to perfection. Cells use energy in the form of ATP (adenosine triphosphate). This is a molecule consisting of one molecule of adenine, a ribose, and three phosphate groups. The bonds between the phosphate groups are weak bonds that, when broken, release high amounts of energy, which is released in the form of calories. Finally, the ATP molecule is transformed into a molecule of ADP (adenosine monophosphate).

But the real question is, "What gives the cell these molecules of ATP?" Or, in other words, "How does the cell get its energy?" The answer to both questions is given in two fundamental processes for obtaining this energy molecule: glycolysis and the tricarboxylic acid cycle, or Krebs cycle. Acetyl-CoA (acetyl coenzyme A) is the main precursor of the cycle.

Glycolysis

Glycolysis is the pathway that transforms glucose, obtained from food, into two molecules of pyruvic acid through a series of transformations via an enzymatic pathway. This process occurs in the cytosol and has the property that it can happen in the presence of oxygen as well as in the absence of oxygen, but the latter implies an energy cost.

This process is divided into two phases and nine steps, which are detailed below:

First Stage: Energy Expenditure Stage

Step One: Here, the phosphorylation of glucose occurs. That is, its energy will increase, thus activating it for use in other processes when necessary. This occurs by the action of the enzyme hexokinase, which transfers a phosphate group from ATP to the glucose molecule. The result is a molecule of glucose-6-phosphate.