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Characteristics of Krebs cycle

1. The Krebs cycle is the third stage of the cycle.
2. The cycle occurs in the mitochondria of cells.
3. This process requires oxygen.
4. The amount of energy generated in this process is high.
5. CO2 is produced in this phase.
6. In this process complete oxidation of respiratory material takes place.
7. The first compound produced in this cycle is citric acid.
8. This process is called TCA cycle or amphibolic pathway.

Krebs cycle, citric acid cycle, TCA cycle

The process in which acetyl CoA reacts with oxaloacetic acid to produce citric acid and finally oxaloacetic acid is called the Krebs cycle. In 1937, the British biochemist Sir Hans Krebs first observed these reactions in the breast meat of pigeons, so it is called the Krebs cycle. Oxaloacetic acid is a stable compound called a resident molecule. Acetyl CoA is the chemical link between glycolysis and the Krebs cycle.
The first compound produced in this cycle is citric acid, also known as the citric acid cycle. Again, some compounds produced in this cycle have three carboxylic (-COOH) groups, so it is called Tricarboxylic Acid cycle. It is also called amphibolic pathway.

Formation of Acetyl Co-A

Pyruvic acid is oxidized to acetyl Co, producing NADPH+H+ and CO2. It is an oxidative decarboxylation process. Pyruvic acid produces acetyl Co-A stepwise in oxidative decarboxylation process. The enzyme pyruvic dehydrogenase plays a role in this reaction. The process is –
1. In the presence of decarboxylase enzyme, pyruvic acid and thiamine pyrophosphate (TPP) react to form thiamine pyrophosphate compound (TPP).
2. In the presence of trans acetylase enzyme, TPP compounds and lipoic acid react to produce acetyl lipoic acid. At this time TPP was released.
3. Acetyl lipoic acid reacts with Co-A to form Acetyl Co-A. At this time lipoic acid is oxidized.

Glucogenolysis

The process by which glucose is produced from glycogen is called glucogenolysis. This process takes place in the liver and kidneys.

Gluconeogenesis

Glucose production in the reverse process of glycolysis is called gluconeogenesis. It occurs more in animal body. Less occurs in plant cells. The oil stored in the seeds of plants like rapeseed, sunflower etc. is broken down in the process of gluconeogenesis to produce sucrose or glucose. It helps in the growth of sprouts.

Regulation of glycolysis process

1. The rate of glycolysis increases when the use of ATP is faster. The rate of this process slows down when the use of ATP is reduced.
2. The amount and supply of glucose regulates this process.
3. Allosteric enzyme phosphofructokinase which catalyzes the formation of fructose 1, 6-bisphosphate from fructose 6-phosphate. Glycolysis process is dependent on its speed.

Glycolysis, Importance or significance of glycolysis

1. A variety of substances produced in the glycolysis process have metabolic functions.
2. The process of glycolysis occurs in the cytoplasm of the cell (Salisbury,1996). Hence it is called cytoplasmic respiration.
3. In this process, glucose is oxidized in the organism.
4. The key to this process is the formation of pyruvic acid. Two molecules of pyruvic acid are produced from each molecule of glucose.
5. Pyruvic acid produced in this cycle is used as raw material in Krebs cycle.
6. Glycolysis produces ATP and NADPH+H+.
7. This cycle does not require oxygen.
8. The main source of energy from food is glycolysis.
9. If pyruvic acid is not produced in this process, respiration will stop.
10. The main pathway of fructose and galactose metabolism is glycolysis.
11. ATP and NADPH+H+ generated in glycolysis account for 17% of total latent energy. Only 4% of this is expended and about 80% of the energy is stored in pyruvic acid.

Process of glycolysis cycle

Out of 9 reactions in glycolysis process, 1st, 3rd and 9th reactions are unidirectional, rest are bidirectional. The reactions are:
1. Glucose 6-phosphate is produced from glucose by the action of hexokinase enzyme. During this reaction, ADP takes part and produces ATP.
2. Fructose 6 phosphate is produced from glucose 6 phosphate under the influence of phosphoglucoisomerase enzyme. The reaction is bidirectional.
3. In the presence of the enzyme phosphofructokinase, fructose 6-phosphate is converted to fructose 1, 6-bisphosphate. At this time, ATP takes part in the reaction and produces ADP.
4. 3-phosphoglyceraldehyde and dihydroxyacetone phosphate are produced from fructose 1,6 bisphosphate by the action of aldolase enzyme. The reaction is bidirectional.
5. 1, 3 bisphosphoglyceric acid is produced from 3 phosphoglyceraldehyde under the influence of phosphoglyceral dehydrogenase enzyme. At this time, NADP participates in the reaction to form NADPH+H+. The reaction is bidirectional.
6. Phosphoglyceric acid is converted to 1, 3 bisphosphoglyceric acid, 3-phosphoglyceric acid with the help of kinase enzyme. During this reaction, ADP takes part and produces ATP. The reaction is bidirectional.
7. 2-phosphoglyceric acid is formed from 3-phosphoglyceric acid in the presence of phosphoglyceromutase enzyme. The reaction is bidirectional.
8. 2-phosphoenol pyruvic acid is produced from 2-phosphoglyceric acid by the action of inolase enzyme. The reaction is bidirectional.
9. Pyruvic acid is produced from 2-phosphoenol pyruvic acid under the action of pyruvic acid kinase enzyme. During this reaction, ADP takes part and generates ATP.

Steps of glycolysis cycle

Characteristics of glycolysis cycle

1. Glycolysis is the first stage of respiration.
2. It occurs in the cytoplasm of the cell. Hence it is also called cytoplasmic respiration.
3. This process does not require oxygen.
4. The amount of energy produced in this process is less.
5. CO2 is not produced at this stage.
6. In this process partial oxidation of respiratory material (glucose) takes place.
7. This process is called EMP trajectory.