Glycolysis, Key Reactions, and Examples

Glycolysis is a basic and important cellular metabolic pathway that happens in the cytoplasm of cells. It is the first stage of cellular respiration where one molecule of glucose (a six-carbon sugar) breaks down into two molecules of pyruvate (a three-carbon compound).

What is Glycolysis?

Glycolysis is a basic and important cellular metabolic pathway that happens in the cytoplasm of cells. It is the first stage of cellular respiration where one molecule of glucose (a six-carbon sugar) breaks down into two molecules of pyruvate (a three-carbon compound). Glycolysis consists of a series of chemical reactions, each classified by specific enzymes.

Image of Glycolysis
Image of Glycolysis /credit

Key Reactions and Examples:

Details of the breakdown of the glycolytic pathway, including key reactions and examples are detailed below.

1. Glucose Phosphorylation:

The first step of glycolysis involves the phosphorylation of glucose. In this process, it forms glucose-6-phosphate, and it requires the enzyme hexokinase.

Equation: Glucose + ATP → Glucose-6-phosphate + ADP

2. Isomerization:

Glucose-6-phosphate is isomerized to fructose-6-phosphate by the enzyme phosphoglucose isomerase.

3. Phosphorylation:

Fructose-6-phosphate is then phosphorylated to fructose-1,6-bisphosphate using ATP, and this reaction is caused by the enzyme phosphofructokinase-1.

Equation: Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP

4. Cleavage:

Fructose-1,6-bisphosphate is split into two three-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P). Only G3P continues in the glycolytic pathway.

5. Isomerization:

DHAP is isomerized into another molecule of G3P, which ensures that both molecules can proceed through the next steps.

 Now, two molecules of G3P are available for the next set of reactions.

6. Energy Harvesting and ATP Formation:

Each G3P molecule undergoes a series of reactions, leading to the production of NADH and ATP.

G3P is oxidized, and NAD+ is reduced to NADH.

The energy released in these reactions is used to phosphorylate ADP to ATP.

Equation: 1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP + NADH

7. Phosphoenolpyruvate Formation:

In the next set of reactions 3-phosphoglycerate converted to phosphoenolpyruvate (PEP), with the production of ATP.

 Equation: Phosphoglycerate + ADP → Phosphoenolpyruvate + ATP

8. Pyruvate Formation:

In the final step, PEP is converted to pyruvate, resulting in the production of ATP.

Equation: Phosphoenolpyruvate + ADP → Pyruvate + ATP

At the end of glycolysis, one molecule of glucose has been converted into two molecules of pyruvate, with a net gain of 2 ATP molecules (four ATP produced, but two used in the initial steps) and 2 NADH molecules have occurred. The pyruvate generated can further enter the citric acid cycle (Krebs cycle) if oxygen is available, leading to the complete oxidation of glucose and the production of more ATP. If oxygen is limited, pyruvate can undergo fermentation to generate ATP in the absence of oxygen.


Glycolysis is rigidly regulated by various factors. They include ATP levels, AMP levels, and the availability of glucose ensuring that the pathway only runs when necessary and production maximizes ATP.

Different Types of Glycolysis:

Embden-Meyerhof pathway is the most common type of glycolysis, however, other variants of glycolysis are found in different organisms or under specific conditions, such as the Entner-Doudoroff pathway and the pentose phosphate pathway.


Glycolysis is a fundamental metabolic pathway present in almost all living organisms. It plays a crucial role in energy production, providing ATP for essential cellular functions, even in the absence of oxygen. It also serves as an antecedent for other metabolic pathways, such as gluconeogenesis and the pentose phosphate pathway.


Q1. What is glycolysis?

Answer: Glycolysis is a fundamental metabolic pathway that breaks down glucose into pyruvate. During the process energy in the form of ATP is produced and reducing power in the form of NADH.

Q2. Where does glycolysis occur?

Answer: Glycolysis occurs in the cytoplasm of the cell.

Q3. What is the purpose of glycolysis?

Answer:  The fundamental purpose of glycolysis is to generate ATP and NADH from glucose, to provide a quick source of energy to cells.

Q4. How many stages are there in glycolysis?

Answer: It can be broadly divided into three stages: the preparatory phase, the payoff phase, and the energy-releasing phase.

Q5. What is the net ATP yield of glycolysis?

Answer: The net ATP yield of glycolysis is 2 ATP molecules per molecule of glucose. However, in due process, four ATP molecules are produced, but two ATP molecules are initially consumed in the preparatory phase.

Q6. What happens to pyruvate after glycolysis?

Answer: The destiny of pyruvate depends on the presence or absence of oxygen. If oxygen, is present pyruvate can enter the citric acid cycle. However, pyruvate may be converted to lactate or undergo alcoholic fermentation in the absence of oxygen depending on the organism.

Q7. What role does NADH play in glycolysis?

Answer: NADH is produced during the glycolytic pathway. It acts as a carrier of reducing equivalents and plays a crucial role in transferring electrons to the electron transport chain (if oxygen is available) or in other cellular processes.

Q8. Is glycolysis an aerobic or anaerobic process?

Answer: Glycolysis itself is an anaerobic process, meaning it does not require oxygen. But, the fate of pyruvate, the end product of glycolysis, can be aerobic or anaerobic.

Q9. Which organisms use glycolysis?

Answer: Glycolysis is a universal pathway found in nearly all organisms, from bacteria to human beings.

Q10. Can glycolysis occur in the absence of glucose?

Answer: While glucose is the most common substrate for glycolysis, other hexose sugars, such as fructose and mannose, can also enter the pathway at various points.

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