Glycolysis Pathway: Pathway, Steps & Products

Edited By Irshad Anwar | Updated on Jul 02, 2025 07:04 PM IST

What Is Glycolysis?

Glycolysis is a metabolic pathway that converts glucose, a six-carbon sugar molecule, into two molecules of pyruvate, each containing three carbons. The process takes place in the cytoplasm of the cell and leads to the formation of ATP and NADH.

Glycolysis is the first stage in cellular respiration and is very crucial for both aerobic and anaerobic production of energy. It is a rapid means to acquire ATP without entering the oxygen-dependent processes; this feature is especially important for the activity of muscles during high-intensity exercises and in anaerobic organisms.

Where Within The Cell Glycolysis Occurs?

It occurs in the cytosol of prokaryotic cells and the cytoplasm of eukaryotic cells.
Glycolysis is present in almost all the simple types of cells, like bacteria, to the most complex types, like human cells.

Role Of Glycolysis In Cellular Respiration

The process of glycolysis initiates the digestion process of the glucose for obtaining energy.
Pyruvate formed feeds into the Krebs cycle as well as the electron transport chain for further energy extraction inside the mitochondria.
Glycolysis also directly yields ATP using substrate-level phosphorylation.
It gives rise to intermediates for other kinds of metabolic pathways and leads to synthesising amino acids and fatty acids as well.

Steps In Glycolysis

Breakdown of the Steps

Step 1

Glucose phosphorylation by hexokinase/glucokinase:
Glucose is phosphorylated to glucose-6-phosphate with the use of ATP.

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Step 2

Isomerisation (Phosphoglucose Isomerase)
Glucose-6-phosphate produced is changed into fructose-6-phosphate.

Step 3

Second phosphorylation (Phosphofructokinase-1)
Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate with the usage of another ATP.

Step 4

Fructose-1,6-bisphosphate Cleavage (Aldolase)
Fructose-1,6-bisphosphate is cleaved into two three-carbon molecules.

Step 5

Isomerisation of DHAP to G3P (Triose Phosphate Isomerase)
DHAP is converted to G3P, resulting in two molecules of G3P.

Step 6

Oxidation and phosphorylation (Glyceraldehyde-3-Phosphate Dehydrogenase)
G3P is oxidised, producing NADH, and phosphorylated to 1,3-bisphosphoglycerate.

Step 7

ATP generation for the first time (Phosphoglycerate Kinase)
1,3-bisphosphoglycerate does a phosphate add to the ADP to create that ATP, with a by-product of 3-phosphoglycerate.

Step 8

Phosphate is shifted (Phosphoglycerate Mutase)
3-phosphoglycerate is converted to 2-phosphoglycerate.

Step 9

Dehydration (Enolase)
2-phosphoglycerate dehydrated to phosphoenolpyruvate (PEP).

Step 10

Second ATP generation (Pyruvate Kinase)
PEP donates a phosphate group to ADP, to form ATP and pyruvate.

Outputs Of Glycolysis

Glycolysis products

Pyruvate

Two molecules of pyruvate per glucose molecule.

ATP products

Net gain of 2 ATP molecules per glucose molecule, this is because whereby 4 is produced, 2 is consumed.

NADH products

Two molecules of NADH per glucose molecule.

Regulation In Glycolysis

Key regulatory enzymes:

Hexokinase

The commit regulatory enzyme: inhibited by ATP and citrate; activated by AMP.

Pyruvate Kinase

Catalyses the final step, is inhibited by ATP and alanine, and activated by fructose-1,6-bisphosphate.

Allosteric Regulation

Enzymes change shape and activity in response to the binding of regulatory molecules

Feedback Inhibition

Inhibition of the end products in early steps to prevent over-accumulation of intermediates and unnecessary use of resources.

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Frequently Asked Questions (FAQs)

1. What is glycolysis, and in which part of the cell does it take place?

Glycolysis is a metabolic pathway that breaks down glucose ultimately into pyruvate. This happens in the cytosol of the cells.

2. What are the major steps of Glycolysis?

There are ten steps in glycolysis, divided into two phases: an energy investment phase and an energy payoff phase.

3. What is the importance of glycolysis to cells?

Glycolysis produces a net amount of ATP and NADH both of which are very important for the energy metabolism of the cell, as well as other metabolic pathways.

4. How is glycolysis controlled?

A few key regulatory enzymes, hexokinase, phosphofructokinase-1 and pyruvate kinase control glycolysis via allosteric interactions, substrate inhibition, and end-product inhibition.

5. What is the net gain of ATP from glycolysis?

At the end of glycolysis, there will be a net gain of 2 ATP molecules per glucose molecule and an additional 2 NADH.

6. What is the net energy gain from glycolysis?

The net energy gain from glycolysis is 2 ATP molecules and 2 NADH molecules per glucose molecule. While 4 ATP are produced in the payoff phase, 2 ATP are consumed in the preparatory phase, resulting in a net gain of 2 ATP.

7. What is the significance of NADH production in glycolysis?

NADH production in glycolysis is significant because it represents the capture of energy from glucose in the form of electron carriers. These NADH molecules can later be used in the electron transport chain to produce more ATP through oxidative phosphorylation, or they can be used to drive other cellular reactions.

8. Why is glycolysis considered an anaerobic process?

Glycolysis is considered anaerobic because it does not require oxygen to occur. This allows cells to produce some ATP even in the absence of oxygen, which is crucial for many organisms and for human cells in low-oxygen conditions.

9. What is substrate-level phosphorylation and how does it occur in glycolysis?

Substrate-level phosphorylation is the direct transfer of a phosphate group from a substrate molecule to ADP, forming ATP. In glycolysis, this occurs twice in the payoff phase: first when 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate, and again when phosphoenolpyruvate is converted to pyruvate.

10. How does glycolysis differ from fermentation?

Glycolysis is the first step in both aerobic respiration and fermentation. The key difference is what happens to the pyruvate produced by glycolysis. In aerobic respiration, pyruvate enters the mitochondria for further oxidation. In fermentation, pyruvate is converted to other compounds (like lactate or ethanol) in the cytoplasm to regenerate NAD+ and continue glycolysis.

11. Why is phosphofructokinase considered a key regulatory enzyme in glycolysis?

Phosphofructokinase (PFK) is considered a key regulatory enzyme because it catalyzes one of the irreversible steps in glycolysis (the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate). PFK is allosterically regulated by ATP, ADP, and AMP levels, allowing the cell to control the rate of glycolysis based on its energy status.

12. How does the cell maintain the NAD+/NADH balance during glycolysis?

The cell maintains the NAD+/NADH balance during glycolysis through various mechanisms. In aerobic conditions, NADH is reoxidized to NAD+ in the electron transport chain. In anaerobic conditions, fermentation processes (like lactic acid fermentation or alcoholic fermentation) regenerate NAD+ from NADH, allowing glycolysis to continue.

13. How does feedback inhibition regulate glycolysis?

Feedback inhibition regulates glycolysis by controlling the activity of key enzymes based on the cell's energy status. For example, high levels of ATP inhibit phosphofructokinase, slowing down glycolysis when the cell has sufficient energy. Conversely, high AMP levels (indicating low energy) activate phosphofructokinase, speeding up glycolysis.

14. What is the Pasteur effect and how does it relate to glycolysis?

The Pasteur effect refers to the inhibition of fermentation by oxygen. In the presence of oxygen, cells shift from fermentation to aerobic respiration, which is more efficient. This relates to glycolysis because the presence of oxygen allows the pyruvate produced by glycolysis to enter the citric acid cycle and electron transport chain, rather than undergoing fermentation.

15. Why is the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate considered a committed step?

The conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, catalyzed by phosphofructokinase, is considered a committed step because it's essentially irreversible under cellular conditions. Once this step occurs, the glucose is committed to continuing through glycolysis rather than being used for other cellular processes.

16. How many steps are there in the glycolysis pathway?

The glycolysis pathway consists of 10 enzymatic steps. These steps can be divided into two phases: the preparatory phase (steps 1-5) where energy is invested, and the payoff phase (steps 6-10) where energy is harvested in the form of ATP and NADH.

17. What is the role of ATP in the early steps of glycolysis?

In the early steps of glycolysis (the preparatory phase), ATP is used to phosphorylate glucose and fructose-6-phosphate. This investment of energy is necessary to destabilize the glucose molecule, making it easier to split in later steps. This is why glycolysis is sometimes described as "spending energy to make energy."

18. How does the structure of glucose change during glycolysis?

During glycolysis, the 6-carbon glucose molecule is first phosphorylated, then split into two 3-carbon molecules. These 3-carbon molecules are then oxidized and rearranged, eventually forming two molecules of pyruvate. This process involves several structural changes, including the breaking and forming of chemical bonds.

19. What is the role of hexokinase in glycolysis?

Hexokinase catalyzes the first step of glycolysis, phosphorylating glucose to glucose-6-phosphate. This step is crucial as it "traps" glucose inside the cell (glucose-6-phosphate cannot pass through the cell membrane) and begins the energy investment phase of glycolysis.

20. How does glycolysis contribute to gluconeogenesis?

Glycolysis and gluconeogenesis are opposing pathways, with gluconeogenesis essentially reversing glycolysis to produce glucose. Many of the enzymes used in glycolysis also catalyze reactions in gluconeogenesis, but in the reverse direction. However, the irreversible steps of glycolysis (catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase) are bypassed in gluconeogenesis using different enzymes.

21. What is glycolysis and why is it important in cellular respiration?

Glycolysis is the first step of cellular respiration, occurring in the cytoplasm of cells. It's important because it breaks down glucose (a 6-carbon sugar) into two 3-carbon pyruvate molecules, producing a small amount of ATP and NADH in the process. This pathway is universal across all organisms, making it a crucial process for energy production in both aerobic and anaerobic conditions.

22. What is the significance of the phosphorylation steps in glycolysis?

Phosphorylation steps in glycolysis serve several purposes: they make molecules more reactive, prevent intermediates from diffusing out of the cell, and allow for energy storage and transfer. The early phosphorylations use ATP to activate glucose, while later phosphorylations are used to harvest energy in the form of ATP.

23. How does glycolysis relate to other metabolic pathways?

Glycolysis is central to metabolism, connecting to numerous other pathways. Its product, pyruvate, can enter the citric acid cycle in aerobic conditions. In anaerobic conditions, it can undergo fermentation. Glycolysis also connects to the pentose phosphate pathway, amino acid metabolism, and lipid synthesis through its various intermediates.

24. What is the role of isomerases in glycolysis?

Isomerases in glycolysis catalyze the rearrangement of molecules without changing their chemical composition. For example, phosphoglucose isomerase converts glucose-6-phosphate to fructose-6-phosphate. These steps are crucial for preparing molecules for subsequent reactions in the pathway.

25. How does the structure of glycolytic enzymes contribute to their function?

Glycolytic enzymes have specific three-dimensional structures that allow them to bind to their substrates and catalyze reactions efficiently. Many have active sites shaped to fit their specific substrates, and some have allosteric sites that allow for regulation. Their structures also often include domains for binding cofactors like NAD+.

26. How does the glycolysis pathway differ between prokaryotes and eukaryotes?

The glycolysis pathway is remarkably conserved between prokaryotes and eukaryotes, with the same basic steps and enzymes. The main difference lies in the cellular location: in prokaryotes, glycolysis occurs in the cytoplasm, while in eukaryotes, it occurs in the cytosol (the liquid portion of the cytoplasm). Some regulatory mechanisms may also differ between the two groups.

27. How does the compartmentalization of glycolysis in eukaryotes affect its regulation?

In eukaryotes, glycolysis occurs in the cytosol, separate from mitochondrial processes like the citric acid cycle and electron transport chain. This compartmentalization allows for more complex regulation, as the concentrations of metabolites and enzymes can be controlled independently in different cellular compartments.

28. How does the pH of the cellular environment affect glycolysis?

The pH of the cellular environment can significantly affect glycolysis as the enzymes involved have optimal pH ranges for their activity. Generally, a slightly alkaline pH (around 7.2-7.4) is optimal for most glycolytic enzymes. Extreme pH changes can denature enzymes and disrupt the pathway.

29. How does glycolysis contribute to thermogenesis in brown adipose tissue?

In brown adipose tissue, glycolysis contributes to thermogenesis by providing pyruvate and NADH. These products can enter the mitochondria, where they contribute to the electron transport chain. In brown adipose tissue, this process is uncoupled from ATP production, resulting in the generation of heat.

30. What is the relationship between glycolysis and the Cori cycle?

The Cori cycle involves the conversion of lactate (produced by anaerobic glycolysis in muscles) back to glucose in the liver. This cycle allows for the continued production of ATP in muscles during intense exercise, while the liver helps to remove the accumulating lactate and replenish glucose supplies.

31. What is the role of phosphoglycerate kinase in glycolysis?

Phosphoglycerate kinase catalyzes the first ATP-generating step in the payoff phase of glycolysis. It transfers a phosphate group from 1,3-bisphosphoglycerate to ADP, forming 3-phosphoglycerate and ATP. This step is an example of substrate-level phosphorylation.

32. How does the cell's energy state affect the rate of glycolysis?

The cell's energy state significantly affects the rate of glycolysis through allosteric regulation of key enzymes. High energy states (high ATP, low AMP) inhibit glycolysis, while low energy states (low ATP, high AMP) activate it. This allows the cell to adjust its energy production based on its current needs.

33. What is the importance of glyceraldehyde-3-phosphate dehydrogenase in glycolysis?

Glyceraldehyde-3-phosphate dehydrogenase catalyzes a key oxidation step in glycolysis, converting glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. This step is important because it's where NAD+ is reduced to NADH, capturing some of the energy from glucose in the form of electron carriers.

34. How does glycolysis contribute to the production of precursor molecules for biosynthesis?

Many intermediates of glycolysis serve as precursors for biosynthetic pathways. For example, glucose-6-phosphate can enter the pentose phosphate pathway to produce ribose for nucleotide synthesis. Dihydroxyacetone phosphate can be used for lipid synthesis, and 3-phosphoglycerate is a precursor for amino acid synthesis.

35. What is the role of mutase enzymes in glycolysis?

Mutase enzymes in glycolysis catalyze the transfer of a functional group from one position on a molecule to another. For example, phosphoglycerate mutase moves the phosphate group on 3-phosphoglycerate to create 2-phosphoglycerate. This rearrangement prepares the molecule for the next step in the pathway.

36. What is the significance of the irreversible steps in glycolysis?

The irreversible steps in glycolysis (catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase) are significant because they represent key control points in the pathway. These steps are highly regulated and help determine the overall direction and rate of glucose metabolism in the cell.

37. How does glycolysis contribute to maintaining cellular redox balance?

Glycolysis contributes to cellular redox balance primarily through the production of NADH. The NAD+/NADH ratio is crucial for many cellular processes, and glycolysis helps maintain this balance by reducing NAD+ to NADH. In anaerobic conditions, fermentation processes then reoxidize NADH to NAD+ to allow glycolysis to continue.

38. What is the role of enolase in glycolysis?

Enolase catalyzes the ninth step of glycolysis, converting 2-phosphoglycerate to phosphoenolpyruvate (PEP). This step is important because it creates a high-energy phosphate bond in PEP, which is used in the final step to generate ATP.

39. How does the structure of glucose affect its metabolism in glycolysis?

The structure of glucose, particularly its ring form, affects its metabolism in glycolysis. The first step of glycolysis involves phosphorylating the 6th carbon of glucose, which helps to destabilize the ring structure. This is necessary for the subsequent steps that eventually split the 6-carbon molecule into two 3-carbon molecules.

40. What is the role of aldolase in glycolysis?

Aldolase catalyzes the fourth step of glycolysis, splitting fructose-1,6-bisphosphate into two 3-carbon molecules: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. This is a key step in the pathway as it marks the transition from 6-carbon to 3-carbon sugar phosphates.

41. How does glycolysis differ in plant cells compared to animal cells?

The basic steps of glycolysis are the same in plant and animal cells. However, plant cells have additional pathways that can feed into or draw from glycolysis, such as the Calvin cycle in photosynthesis. Plants also have the ability to store large amounts of glucose as starch, which can be mobilized for glycolysis when needed.

42. What is the significance of the preparatory phase of glycolysis?

The preparatory phase of glycolysis (steps 1-5) is significant because it prepares the glucose molecule for splitting and subsequent energy extraction. Although this phase consumes ATP, it's necessary to activate the glucose and make it more reactive for the energy-yielding steps that follow.

43. What is the role of triose phosphate isomerase in glycolysis?

Triose phosphate isomerase catalyzes the rapid interconversion of dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. This is important because it allows all the carbon from the original glucose molecule to continue through the pathway as glyceraldehyde-3-phosphate.

44. How does the regulation of glycolysis differ in different cell types?

The regulation of glycolysis can vary significantly between cell types based on their specific metabolic needs. For example, in cancer cells, glycolysis is often upregulated even in the presence of oxygen (the Warburg effect). In contrast, in liver cells, glycolysis is tightly regulated to maintain blood glucose levels.

45. What is the role of pyruvate kinase in glycolysis?

Pyruvate kinase catalyzes the final step of glycolysis, transferring a phosphate group from phosphoenolpyruvate to ADP, forming pyruvate and ATP. This is one of the key ATP-generating steps in the pathway and is also an important regulatory point.

46. How does glycolysis contribute to the Warburg effect in cancer cells?

The Warburg effect refers to the observation that cancer cells tend to favor glycolysis followed by lactic acid fermentation over oxidative phosphorylation, even in the presence of oxygen. This increased glycolytic activity provides cancer cells with rapid ATP production and generates intermediates for biosynthetic pathways, supporting their rapid growth and proliferation.

47. What is the relationship between glycolysis and gluconeogenesis in terms of energy?

Glycolysis and gluconeogenesis are opposing pathways in terms of energy. Glycolysis breaks down glucose to produce ATP, while gluconeogenesis synthesizes glucose, consuming ATP in the process. The energy cost of gluconeogenesis is higher than the energy yield of glycolysis, reflecting the thermodynamic principle that synthesis requires more energy than breakdown.