Process of glycolysis

Almost every living organism we see around us breathes and respires for survival. Despite the diversity in biological characteristics, the molecular reactions involved in respiration are largely the same.Simply put, the process of glycolysis is the breakdown of glucose, which can occur with or without the presence of oxygen. In the presence of oxygen, glycolysis is the first stage of cellular respiration. Glycolysis allows cells to create ATP in the absence of oxygen through a fermentation process.

Definition of Glycolysis

Considered the first stage of cellular respiration, glycolysis is the process of splitting glucose (sugar, six carbon molecules) into two molecules of pyruvate (three-carbon molecules). Prior to the initiation of glycolysis, the transportation of glucose into the cell takes place, where it is phosphorylated. It occurs in the cytosol in most organisms. 

Krebs cycle

The Krebs cycle in prokaryotic organisms occurs in the cytosol of the cell. The process of glycolysis forms the pyruvate within the cytoplasm. Other reactions occur in the mitochondria. Thus, pyruvate is transported there. There are different enzymes present both in the inner membrane and matrix of mitochondria.

The steps of the Krebs cycle are as follows:

Step 1- The first step of the Krebs cycle involves condensation. The acetyl CoA condenses with oxaloacetate (4C) to form citrate (6C), and coenzyme A releases. The enzyme Citrate synthase is responsible for catalysing the reaction. 

Step 2: Step two of the Krebs cycle involves the conversion of citrate to isocitrate, its isomer. The aconitase enzyme catalyses the second step of the Krebs cycle, i.e., this reaction. 

Step 3: Step 3 of the Krebs cycle involves the dehydrogenation and also the decarboxylation of isocitrate forming 𝝰-ketoglutarate (5C). CO2 molecules are released in this reaction. The catalyst of this reaction is isocitrate dehydrogenase. Step 3 of the Krebs cycle converts NAD+ into NADH.

Step 4: The oxidative decarboxylation of α-ketoglutarate (5C) produces succinyl CoA (4C). α-ketoglutarate dehydrogenase enzyme catalyses the reaction. NAD+ converts to NADH.

Step 5: In step 5 of Krebs cycles, the Succinyl CoA converts to succinate. Succinyl CoA synthetase carries out this reaction. This enzyme couples with substrate-level phosphorylation of GDP and causes the formation of GTP. This GTP is responsible for transferring its phosphate to ADP. It results in the formation of ATP.

Step 6: In step 6 of the Krebs cycle, the succinate (produced in step 5) oxidised to fumarate. It occurs by the action of the enzyme succinate dehydrogenase. In step 6, the FAD converts to FADH2.

Step 7: Now, in step 7, the fumarate conversion occurs. It gets converted to malate by the addition of one water molecule. The enzyme fumarase catalyses this reaction. 

Step 8: Step 8, or the last step of the Krebs cycle, causes the dehydrogenation of Malate, forming oxaloacetate. This oxaloacetate now combines with another molecule of acetyl CoA. And the cycle starts all over again. The enzyme malate dehydrogenase catalyses the reaction of the last step of the Krebs cycle.

Cellular Respiration

Cellular respiration is a process in which an organism acquires energy to maintain various functions, such as growth, movement, repair, reproduction, and defence against danger. The process is carried out in both autotrophic and heterotrophic organisms and involves a series of metabolic processes within a cell. Cellular respiration is the most critical process that occurs in any organism, but it does not happen in the same manner in every organism. There are different types of cellular respiration. 

Both prokaryotic and eukaryotic cells carry out this process. In prokaryotic cells, it takes place in the cytoplasm, and in eukaryotic cells, it initiates in the cytosol and then continues in the mitochondria. The process mainly entails converting adenosine diphosphate (ADP) to adenosine triphosphate (ATP).

In eukaryotes, the process takes place in the following stages:

1. Glycolysis
2. Krebs cycle or citric acid cycle
3. Electron transport chain or oxidative phosphorylation

Where Does Cellular Respiration Take Place? 

Cellular respiration occurs in the cytosol and mitochondria of a cell in an organism.

The first stage, i.e. glycolysis, occurs in the cytosol, and the rest of the stages occur in the mitochondrion. 

Conclusion

Glycolysis is vital in the cell since glucose is the body’s primary source of energy. For example, glucose is the brain’s sole source of energy. The body needs to maintain a consistent supply of glucose in the blood to sustain appropriate brain function. The process of glycolysis forms the pyruvate within the cytoplasm. Other reactions occur in the mitochondria. Thus, pyruvate is transported there. There are different enzymes present both in the inner membrane and matrix of mitochondria.