Every living organism that you can see around breathes and respires to live. Despite the vast diversity of organisms, the biochemical reactions that are part of the respiratory process are more or less the same in every organism.
If you were to hand out a molecule of glucose to the friendly Lactobacillus acidophilus, the bacterium that converts milk into curd, the metabolism would be quite different from that of yours. Nevertheless, the first steps to the process would remain pretty much the same. Both you and L. acidophilus would need to first split the glucose molecule into two parts through the process of glycolysis.
This process of glycolysis expends energy. It consists of two phases, namely the energy-absorbing and energy-releasing phases. The energy is released in the form of ATP.
Rack Your Brains: Stay ahead of your class and find out how many ATP molecules are produced in glycolysis. Then discuss your answers with your friends.
What is Glycolysis?
Glycolysis is a series of reactions in which glucose molecules split into two 3-Carbon molecules called pyruvates, and release energy in the form of ATP. It is a very ancient pathway and is the first stage of cellular respiration.
Like all metabolic processes, glycolysis too follows a distinct pathway which is catalysed by various enzymes. It starts in the cytosol of a cell and consists of two phases. These are:
- The energy-releasing phase
- The energy-requiring phase
Here are the detailed steps of the entire energy-requiring phase of glycolysis.
- The ATP transfers its phosphate group to glucose, to release ADP. The glucose now changes into glucose-6-phosphate.
- Glucose-6-phosphate transforms into the isomer fructose-6-phosphate.
- ATP transfers a phosphate group to fructose-6-phosphate, thus producing fructose-1,6,-biphosphate. This step of the pathway is catalysed by phosphofructokinase. It is an enzyme that regulates the speed of the glycolytic pathway.
- The fructose-1,6-bisphosphate then splits into two 3-Carbon sugars. These are the isomers dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate.
- DHAP changes into glyceraldehyde-3-phosphate. These two then exist in equilibrium.
Pop Quiz 1
- Where does glycolysis take place?
- Cell wall
In the next section, you will learn about the various steps in the energy-releasing phase of the glycolytic pathway and find out how many ATP are made in glycolysis.
The Energy-Releasing Phase
We have already learnt what happens in the energy-requiring phase of glycolysis. We shall now learn about the second half of the process and how many ATP molecules are produced in glycolysis.
- Glyceraldehyde-3-phosphate oxidises, and the NAD+ reduces to NADH and H+. This process is an exergonic or energy-releasing one. This energy then phosphorylates the molecule, leading to the formation of 1,3-diphosphoglycerate.
- 1,3-diphosphoglycerate releases one phosphate group which is then added to ADP to make a molecule of ATP. Subsequently, a molecule of 3-phosphoglycerate is formed.
- 3-phosphoglycerate changes into 2-phosphoglycerate.
- 2-phosphoglycerate releases a molecule of H2O and changes into phosphoenolpyruvate (PEP).
- PEP now releases a phosphate group which adds to ADP, making ATP. PEP changes into pyruvate, which is the final product of glycolysis.
Now answer the following question.
Pop Quiz 2
- How many ATP are produced in glycolysis?
Overall the process of glycolysis is an oxidation reaction. Moreover, in glycolysis, glyceraldehyde-3-phosphate changes into 1,3-bisphosphoglycerate. In this, an aldehyde group undergoes oxidation to form a carboxylic acid group.
We saw how NADH was formed in the glycolytic pathway. What happens to it? Depending on whether the respiration is aerobic or anaerobic, the NADH can oxidise to NAD+ and be utilised in glycolysis again.
This process occurs in the absence of oxygen and is marked by lactic acid production.
- The NADH releases its electrons to pyruvate, which is the product of glycolysis. It then changes to NAD+. As a result, pyruvate reduces to lactic acid or lactate. This is precisely what happens when bacteria act upon milk and change it into curd.
- Pyruvate also loses a carbon molecule in order to form acetaldehyde. It then adds electrons from NADH to change to ethanol.
FAQ (Frequently Asked Questions)
- How many ATP molecules are produced in glycolysis?
Two molecules of ATP are produced in glycolysis.
- What is the function of glycolysis?
Glycolysis is a series of reactions in which glucose molecules split into two 3-Carbon molecules called pyruvates and release energy in the form of ATP. It is a very ancient pathway and is the first stage of cellular respiration. The process of glycolysis expends energy. It consists of two phases, namely the energy-absorbing and energy-releasing phases. The energy is released in the form of ATP.
- Where does glycolysis take place?
Glycolysis occurs in the cytosol.