Cellular Respiration


Glycolysis is the first pathway used by the cell to break down glucose for energy.
Glycolysis is a metabolic pathway in which glucose is broken down into pyruvate in the cytoplasm of the cell. It takes place outside the mitochondria, organelles found in the cytoplasm of cells in large numbers and in which cellular respiration and energy production occur. This reaction splits glucose (a 6-carbon sugar molecule) into two molecules of pyruvate (a 3-carbon molecule). Four ATP molecules, two water molecules, and two molecules of NADH are also produced. However, two ATP are used during the energy-investing phase, so this results in a net gain of only two ATP molecules from glycolysis. This pathway involves 10 reactions, and each reaction has its own unique enzyme. It can be helpful to divide this pathway into an energy-investing and energy-harvesting phase. ATP is used during the energy-investing phase to power the glycolysis reaction; ATP is produced during the energy-harvesting phase. Glycolysis is the first step in both aerobic and anaerobic respiration. If oxygen is present, the reaction is called aerobic. If oxygen is not present, the following reactions are called anaerobic. The other main pathways of cellular respiration occur inside the mitochondria, where oxygen is a final acceptor of electrons. Therefore, if anaerobic conditions exist, alternative pathways are followed to produce energy for the organism.
The reaction of glycolysis involves the production of two pyruvate sugar molecules, two molecules of NADH, two molecules of water, and two molecules of ATP.

The Energy-Investing Phase of Glycolysis

The first half of glycolysis is the energy-investing phase.
The glycolysis pathway, the process by which glucose is broken down releasing energy, consists of 10 steps. The first five are the energy-investing phase of the pathway. ATP is used during the energy-investing phase to power the glycolysis reaction. Two molecules of glyceraldehyde 3-phosphate are generated from each glucose molecule.
The first half of glycolysis, the energy-investing phase, involves the following five steps.

Step 1. The enzyme hexokinase is used to phosphorylate glucose using ATP. Phosphorylation is the addition of a phosphate group to a molecule in a chemical reaction. In this case, ATP is the source of the phosphate. Hexokinase causes the phosphorylation reaction of glucose to produce glucose 6-phosphate.

Step 2. Phosphoglucose isomerase is the enzyme used to convert glucose 6-phosphate to fructose 6-phosphate. Fructose 6-phosphate is an isomer (a molecule with the same molecular formula as another molecule but with a different structure) of glucose 6-phosphate. An isomerase is an enzyme that drives the conversion of a molecule into its isomer form.

Step 3. Phosphofructokinase is the enzyme that uses ATP to phosphorylate the molecule on one end of fructose 6-phosphate. Here phosphofructokinase is a rate-limiting enzyme. There is less activity from this enzyme when ADP levels are low and the concentration of ATP is high. It is more active when ADP concentration is high. When there is sufficient ATP in the system, this step of the pathway slows down. At this point along the glycolysis pathway, two ATP molecules have been used, resulting the production of fructose 1,6-bisphosphate.

Step 4. Aldolase breaks fructose 1,6-bisphosphate into two different 3-carbon sugars. These 3-carbon sugars are dihydroxyacetone-phosphate (DAP) and glyceraldehyde 3-phosphate (G3P).

Step 5. Triose phosphate isomerase converts DAP to G3P, which is an isomer of DAP. This reaction is fully reversible but proceeds in this direction because the G3P is immediately used as a reactant for Step 6, the beginning of the energy-harvesting phase.

The Energy-Harvesting Phase of Glycolysis

The second half of glycolysis is the energy-harvesting phase.
The glycolysis pathway consists of 10 steps, the last five of which are the energy-harvesting phase of the pathway. A net of two ATP molecules are generated during this process.
The second half of glycolysis, the energy-harvesting phase, involves the following five steps.

Step 6. Glyceraldehyde 3-phosphate dehydrogenase catalyzes a two-step reaction. The first reaction oxidizes G3P with the coenzyme NAD+ to make NADH. Energy is produced from this reaction to power the second reaction where a phosphate group is oxidized to form 1,3-bisphosphoglycerate.

Step 7. Phosphoglycerate kinase catalyzes the transfer of a phosphate from 1,3-bisphosphoglycerate to ADP. This phosphorylation reaction is done to make ATP and the product 3-phosphoglycerate.

Step 8. Phosphoglycerate mutase rearranges the phosphate in 3-phosphoglycerate to make 2-phosphoglycerate.

Step 9. Enolase removes a water molecule from 2-phosphoglycerate. This hydrolysis reaction, the process by which the water molecule is removed, results in the formation of a double bond between two carbon atoms. Phosphoenolpyruvate is also produced.

Step 10. Pyruvate kinase catalyzes the transfer of a phosphate from phosphoenolpyruvate to ADP. This results in the formation of pyruvate and ATP.