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Glycolysis: Overview, Steps, Energetics and Fate of Pyruvic Acid

Glycolysis: Overview, Steps, Energetics and Fate of Pyruvic Acid

Wait a minute!! Are you having your favourite food right now?
What do you think it is going to do to your body? Yes, it is going to provide energy to the body.
But how? Here comes the process of cellular respiration into play.
Food that we eat reaches your cells in the form of glucose where it undergoes cellular respiration to produce energy out of it.

Did you know that all living beings, whether aerobes or anaerobes, start their respiration mechanism using glucose as a substrate? Thus the breakdown of glucose is the first step of respiration. The process in which glucose is broken down into pyruvic acid without utilising oxygen is known as Glycolysis’. Another name of glycolysis is Embden Meyerhof Parnas pathway (EMP pathway), named after the name of the scientists involved in its discovery.

1

What is the site for cellular respiration within the cell? We will all think of the mitochondria, right? But did you know that the first step of cellular respiration, i.e, glycolysis, does not occur in the mitochondria? Then where and how does it occur? Come let’s venture deeper into this amazing pathway.

Table of Contents

Location of Glycolysis

Glycolysis occurs in the cytoplasm of every living cell. This is because this step does not involve oxygen and is common for both aerobic and anaerobic respiration. The cells like mature RBCs do not have any cell organelle but they can still produce energy for their activity because they can perform glycolysis in their cytoplasm.

Overview of Glycolysis

Glycos’ means sugar and ‘lysis’ means splitting. In glycolysis glucose is partially broken down to pyruvic acid or pyruvate in the absence of oxygen and in the process energy is released in the form of adenosine triphosphate (ATP). 

overview of glycolysis

Glycolysis is like a business. Initially, the cell needs to invest in some ATPs but at the end, more ATPs are generated in return. Glycolysis occurs in all kinds of living beings like prokaryotes, eukaryotes, aerobic or anaerobic organisms.

Steps Involved in Glycolysis

Glycolysis process is divided into a total of 10 steps, which are kept in two phases-

1. Preparatory phase - This phase involves five steps in which glucose is broken down to glyceraldehyde-3-phosphate. During this phase, two molecules of energy are invested.
2. Pay off phase - This phase involves five steps in which glyceraldehyde-3-phosphate is converted to pyruvate and four molecules of ATP are generated during this phase.

phase of glycolysis

Preparatory phase

Step-1

The first step involves the conversion of glucose to glucose-6-phosphate with the help of enzyme hexokinase. ATP is used in this reaction in which the third phosphate bond breaks from ATP to join the sixth carbon of the glucose molecule forming glucose-6-phosphate (phosphorylation).

formation of glucose 6 phosphate

Step-2 

This step involves the conversion of glucose-6-phosphate to fructose-6-phosphate through isomerisation (conversion of one isomer to another). The enzyme involved is phosphohexose isomerase.

formation of fructose 6 phosphate

Step-3

This step involves the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. The last phosphate group breaks off from ATP to join the first carbon of fructose-6-phosphate. The enzyme involved is phosphofructokinase.

formation of fructose-1,6-bisphosphate

Step-4 

In this step fructose-1,6-bisphosphate breaks into two compounds- dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (GAP) with the help of the aldolase enzyme.

Step-5

Dihydroxyacetone phosphate is then converted into glyceraldehyde-3-phosphate with the help of the triosephosphate isomerase enzyme.

After conversion, there are two molecules of glyceraldehyde-3-phosphate formed.

splitting of fructose-1,6 bisphate

These 5 steps show the complete preparatory phase.

summary of preparatory phase

Pay off phase

This phase starts with two molecules of glyceraldehyde-3-phosphate produced from one molecule of glucose during the preparatory phase.

Step-6

Two molecules of glyceraldehyde-3-phosphate (GAP) are oxidised in the presence of the enzyme glyceraldehyde phosphate dehydrogenase to form two molecules of 1,3-bisphosphoglyceric acid (BPGA). NAD+ acts as the electron acceptor in the oxidation process and accepts two electrons and 2H+ ions from each molecule of glyceraldehyde-3-phosphate and gets reduced to NADH+ + H+.

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Step-7

This is the very first reaction in glycolysis where ATP is released through substrate level phosphorylation. In this reaction 1,3-bisphosphoglyceric acid (BPGA) is converted to 3-phosphoglyceric acid(PGA) with the help of the enzyme phosphoglycerate kinase. The phosphate released from the 1st carbon of one molecules of 1,3-bisphosphoglyceric acid is taken up but ADP to form ATP. As there are two 1,3-bisphosphoglyceric acids prod, two ATPs are produced.

formation of 3pga

Step-8

In this step, two molecules 3-phosphoglyceric acid is isomerised to two molecules of 2-phosphoglycerate or 2-phosphoglyceric acid by the enzyme phosphoglyceromutase.

formation of 2pga

Step-9

In this step each of the two molecules of 2-phosphoglycerate loses a water molecule to form two molecules of phosphoenolpyruvate. The reaction is catalysed by the enolase enzyme.

formation of 2 phosphoenolpyruvate

Step -10

This is the last step of glycolysis.The energy is released in the form of two molecules of ATP when each of the two molecules of phosphoenolpyruvate lose their phosphate group to form two molecules of pyruvate. The phosphate groups are taken up by two molecules of ADP and ATP is generated. The enzyme involved is pyruvate kinase.

formation of pyruvate

These 5 steps (step 6-10) belong to the payoff phase of glycolysis. In this way one glucose molecule breaks to form two molecules of pyruvic acid per glycolytic pathway.

payoff phase

Energetics of Glycolysis

Four ATP molecules are produced in the payoff phase whereas 2 ATP molecules are consumed during the preparatory phase. Two molecules of NADH are formed in the pay-off phase.Thus the net yield in the process of glycolysis is 2 ATP and 2 NADH molecules.

 

Investment/Return

Step

Number of NADH gained

Number of NADH gained per glucose molecule

Return

Glyceraldehyde-3-

phosphate

1,3-bisphospho

glyceric acid

1

2

Total

2

Fate of Pyruvic acid

There are three fates of pyruvic acid produced by glycolysis. In the presence of oxygen it travels to the mitochondria, undergoes oxidative decarboxylation to form acetyl coA and enters the Krebs cycle.

In the absence of oxygen, the pyruvic acid is either fermented to form alcohol, as seen in yeast, or is fermented to form lactic acid, as seen in lactic acid bacteria and muscle cells.

fate of  pyruvate

Practice problems of Glycolysis

Question 1. How can you calculate the net gain of ATP from the glycolytic pathway?

Answer: In the preparatory phase of glycolysis, for every molecule of glucose,

  • One molecule of ATP is consumed during conversion of glucose to glucose-6-phosphate
  • One molecule of ATP is consumed during conversion of fructose-6-phosphate to fructose-1,6-bisphosphate

At the end of the preparatory phase, the fructose-1,6-bisphosphate is broken to one molecule of glyceraldehyde-3-phosphate (GAP) and one molecule of dihydroxyacetone phosphate (DHAP). The DHAP also gets converted to GAP so from one molecule of glucose, two molecules of GAP are produced which enter the payoff phase.

In the payoff phase, 

  • Two molecules of ATP are generated during conversion of two molecules of 1,3-bisphosphoglycerate to two molecules of 3-phosphoglycerate
  • Two molecules of ATP are generated during conversion of two molecules of phosphoenolpyruvate to pyruvate.

Thus, the net gain of ATP = ATP generated during payoff phase - ATP consumed in preparatory phase = 4-2 = 2.

Thus, net gain of ATP in the glycolytic pathway is 2.

Question 2. Which of the following enzymes participate in the first step of glycolysis?

A. Hexokinase
B. Phospho-glucose isomerase
C. Aldolase
D. All of the above

Answer: The first step of glycolysis involves the conversion of glucose to glucose-6-phosphate with the help of enzyme hexokinase.  Hence ,Option A is correct

Question 3. Which of the following statements about glycolysis is false?

A. It occurs in cytosol of the cell
B. It also helps in fructose metabolism
C. Glycolysis generates ATP
D. Glycolysis generates CO2

Answer: Glycolysis is the process in which glucose is broken down into pyruvic acid without utilising oxygen. Fructose can also be metabolised using the glycolytic pathway by converting fructose to fructose-6-phosphate. The net gain of ATP during glycolysis is 2 ATP. It does not also involve the production of CO2. Hence ,Option D is correct

Question 4. Which of the following products are likely to be formed by the oxidation of GAP?

A. Dihydroxyacetone phosphate
B. Fructose-1,6-bisphosphate
C. 3-phosphoglyceric acid
D. 1,3-bisphosphoglyceric acid

Answer: Two molecules of glyceraldehyde-3-phosphate (GAP) are oxidised in the presence of the enzyme glyceraldehyde phosphate dehydrogenase to form two molecules of 1,3-bisphosphoglyceric acid using two molecules of inorganic phosphate and NAD+.

FAQs of Glycolysis

Question 1. What will be the effect if no glycolysis occurs in the cell?

Answer: All the cells need energy to carry out basic life functions. Energy is obtained from the process of cellular respiration and glycolysis is the first step of cellular respiration, whether it is occurring in the presence of oxygen or in its absence. Thus, without the occurrence of glycolysis cellular respiration will not occur and eventually the cell will die.

Question 2. Does glycolysis require oxygen?

Answer: Glycolysis does not need oxygen in any step. It is an anaerobic process that occurs in the cytoplasm and results in formation of 2 molecules of pyruvic acid by the partial breakdown of one molecule of glucose.

Question 3. Can glycolysis occur in cells which have no mitochondria?

Answer: Glycolysis occurs in the cytoplasm of the cell. Hence, it can occur in cells such as the reb blood cells which do not possess mitochondria to avoid the consumption of oxygen that they transport. Thus, these cells generate energy by the process of glycolysis.

Question 4. Is glycolysis catabolic or anabolic in nature?

Answer: Glycolysis is a catabolic pathway because it involves the breakdown of glucose into pyruvic acid.


 

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