Glycolysis: Steps, Enzymes, Products, and Regulation

Almost every cell on Earth, from a Streptococcus to a human neuron, begins extracting energy from glucose the same way: glycolysis. It is the oldest and most universal energy pathway we know, which is why students in medicine, nursing, pharmacy, biochemistry, and microbiology all have to learn it.

The pathway takes one six-carbon glucose molecule and, in ten enzyme-controlled steps, splits it into two three-carbon pyruvate molecules, capturing a little energy as ATP and NADH along the way. This article walks through all ten steps and their enzymes in one table, shows exactly how the famous "net 2 ATP" is counted, and explains how glycolysis is controlled and what happens to pyruvate next.

Glucose is the most preferred source of energy for many cells. A common pathway for the catabolism of glucose is glycolysis, which breaks down glucose into pyruvate. Glycolysis is also called Embden-Meyerhof-Parnas (EMP) pathway for its major discoverer.

Glycolysis is the pathway that breaks down glucose into two pyruvates (product) and produces 4 ATP (net gain of 2 ATP) and 2 NADH (nicotinamide adenine dinucleotide + hydrogen) in the process. The pathway occurs in the cytoplasm of the cell.

It can take place in the presence (oxidation) and absence of oxygen (fermentation). In the presence of oxygen, the pyruvate is changed into Acetyl CoA. It enters the citric acid (TCA or Krebs) cycle, whereas, in anaerobic conditions, pyruvate is converted into lactate or ethanol.

The overall (net) reaction of glycolysis is:

Glucose + 2 NAD⁺ + 2 ADP + 2 Pi → 2 pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O

Note that 4 ATP are actually made but 2 are spent earlier in the pathway, so the net gain is 2 ATP. The next sections show exactly where each is spent and made.

The two phases of glycolysis

Glycolysis has two halves, and keeping them separate is the key to understanding the energy count:

• Energy-investment phase (steps 1–5): the cell spends 2 ATP to phosphorylate and destabilize glucose, then splits the six-carbon sugar into two three-carbon molecules (glyceraldehyde-3-phosphate). No energy is captured yet; this phase costs ATP.
• Energy-payoff phase (steps 6–10): the two three-carbon molecules are converted to pyruvate, and this phase captures energy: 4 ATP and 2 NADH in total.

The crucial point for counting: from step 6 onward, everything happens twice, once for each of the two three-carbon molecules produced in step 5. That is why the payoff numbers are doubled.

The 10 steps of glycolysis with enzymes

Steps 1–5 are the investment phase (spending 2 ATP and splitting glucose into two G3P). Steps 6–10 are the payoff phase, and because there are now two G3P molecules, steps 6–10 each run twice, giving the doubled yields shown.

The three phosphorylation points to remember: ATP is spent at steps 1 and 3 (hexokinase, PFK-1) and made at steps 7 and 10 (phosphoglycerate kinase, pyruvate kinase). NADH is made at step 6.

Note: Isomers are those compounds that have the same molecular formula but different atomic arrangements.

Did you know: The suffix “-ase” is used to signify an enzymes.

Products of glycolysis: how the net yield is counted

Per molecule of glucose, glycolysis produces:

• 2 pyruvate (the two three-carbon end products)
• 2 NADH (made at step 6, once for each of the two G3P molecules)
• 2 ATP net (this is the number students most often get confused about)

How the net 2 ATP is counted:

So 4 ATP are produced, but 2 were spent up front, leaving a net of 2 ATP. When a question asks for "ATP produced," the answer is 4 gross or 2 net; "net ATP" is 2. This distinction is a very common exam point.

Figure: Diagram: Glycolysis Pathway

Regulation of glycolysis

Glycolysis is controlled at its three irreversible steps, the ones catalyzed by hexokinase, PFK-1, and pyruvate kinase. These are the only steps that cannot easily run backward, so they are the natural control points.

• Hexokinase (step 1): inhibited by its own product, glucose-6-phosphate. When G6P builds up, hexokinase slows, preventing wasteful glucose trapping.
• Phosphofructokinase-1 (step 3): the rate-limiting and committed step, and the most important control point. It is the "gatekeeper" of glycolysis. It is inhibited by high energy signals (ATP and citrate) and activated by low energy signals (AMP and ADP), so glycolysis speeds up when the cell needs energy and slows when energy is plentiful. It is also activated by fructose-2,6-bisphosphate. This step is called "committed" because, once fructose-1,6-bisphosphate is made, the molecule is destined to continue through glycolysis (glucose-6-phosphate, earlier, could still be diverted to other pathways).
• Pyruvate kinase (step 10): inhibited by ATP and acetyl-CoA (signals of energy sufficiency) and activated by fructose-1,6-bisphosphate (an example of feed-forward activation, where an earlier intermediate switches on a later step).

Why PFK-1, not hexokinase, is the rate-limiting step: glucose-6-phosphate (hexokinase's product) can be pulled into other pathways such as the pentose phosphate pathway or glycogen synthesis, so step 1 does not commit glucose to glycolysis. PFK-1 does. That is why PFK-1, not hexokinase, is considered the true rate-limiting and committed step.

What happens to pyruvate after glycolysis?

Glycolysis ends at pyruvate, but pyruvate's fate depends on whether oxygen is available:

• With oxygen (aerobic): pyruvate enters the mitochondrion and is converted by the pyruvate dehydrogenase complex to acetyl-CoA, which enters the citric acid (Krebs/TCA) cycle for full oxidation and much more ATP.
• Without oxygen (anaerobic): pyruvate is converted to a fermentation product to regenerate NAD⁺ (which glycolysis needs to keep running). In human muscle and many bacteria, this gives lactate (via lactate dehydrogenase); in yeast and some bacteria, it gives ethanol and CO₂ (alcoholic fermentation).

Regenerating NAD⁺ is the key reason fermentation exists: without it, glycolysis would stall once all the cell's NAD⁺ was used up at step 6.

Glycolysis in microbial metabolism: the EMP pathway and its alternatives

In microbiology, the glycolysis described above is usually called the Embden-Meyerhof-Parnas (EMP) pathway, after its discoverers. It is the most common route bacteria use to break down glucose, and it works the same way in microbes as in human cells, ending in two pyruvate, 2 net ATP, and 2 NADH.

But microbes are more versatile than human cells, and the EMP pathway is not their only way to catabolize glucose. Two important alternatives are worth knowing:

• Pentose phosphate pathway (PPP), also called the hexose monophosphate (HMP) shunt. This route does not aim to make ATP. Instead it produces NADPH (for biosynthesis) and five-carbon sugars (pentoses) such as ribose-5-phosphate, needed to build nucleotides. Many organisms run it alongside glycolysis.
• Entner-Doudoroff (ED) pathway. Used by some bacteria (such as Pseudomonas and Zymomonas) in place of glycolysis. It reaches pyruvate by a different route and yields only 1 net ATP per glucose (plus 1 NADPH and 1 NADH), less than the EMP pathway's 2 ATP, but it uses fewer enzymes.

One more microbial detail: many bacteria do not use ATP-dependent hexokinase to bring glucose in. They use the phosphotransferase system (PTS), which phosphorylates glucose as it is transported into the cell, using phosphoenolpyruvate (PEP) as the phosphate donor rather than ATP.

For most students, the take-home is: the EMP pathway is the standard glycolysis; the pentose phosphate pathway makes NADPH and pentoses rather than energy; and the Entner-Doudoroff pathway is an alternative, lower-yield route found in certain bacteria.

How to remember

Invest 2, make 4, net 2. The whole energy story in five words. Spend 2 ATP in the first half, make 4 in the second half, keep 2. NADH: 2. Pyruvate: 2. If you remember "invest 2, make 4, net 2," you have the products.

Everything after the split happens twice. Glucose splits into two three-carbon molecules at step 5. From step 6 on, every reaction runs twice, which is why the payoff numbers are doubled. This single idea explains where the "×2" comes from.

The three gatekeeper enzymes: Hexokinase, PFK-1, Pyruvate kinase. The three irreversible, regulated steps, in order (1, 3, 10). They are the only ones that spend or make ATP at the entry and exit, and they are the control points. PFK-1 is the boss (rate-limiting and committed).

EMP is glycolysis; ED and PPP are the microbial alternatives. For microbiology: EMP = standard glycolysis (2 ATP), Entner-Doudoroff = alternative route (1 ATP), pentose phosphate = makes NADPH and pentoses, not ATP.

Key exam facts

Where students get confused

"Is the net ATP 2 or 4?" Both numbers are correct for different questions. Four ATP are produced in the payoff phase, but 2 were spent in the investment phase, so the net gain is 2. If a question says "ATP produced," it may want 4 (gross) or 2 (net); if it says "net ATP," the answer is 2. Read the wording carefully.

"Which is the rate-limiting step, hexokinase or PFK-1?" PFK-1 (step 3). Hexokinase is regulated too, but its product (glucose-6-phosphate) can still be diverted to other pathways, so step 1 does not commit glucose to glycolysis. PFK-1 makes fructose-1,6-bisphosphate, which is committed to glycolysis, so PFK-1 is the rate-limiting and committed step. Many sources loosely call hexokinase rate-limiting; PFK-1 is the correct answer.

"Why do the payoff numbers double?" Because glucose is split into two three-carbon molecules at step 5. Every reaction from step 6 onward therefore happens twice, once for each molecule, so 1 ATP at step 7 becomes 2, and so on. The doubling is not a separate rule; it is just "two molecules, each going through the same steps."

"What does aldolase actually split?" Fructose-1,6-bisphosphate, not fructose-6-phosphate. Step 3 (PFK-1) adds the second phosphate to make fructose-1,6-bisphosphate; step 4 (aldolase) then cleaves that six-carbon bisphosphate into two three-carbon molecules. A common error is to say aldolase splits F6P.

"Is glycolysis the same as the EMP pathway?" Yes. Embden-Meyerhof-Parnas (EMP) is the full name, used especially in microbiology. It is the standard glycolysis. In microbes there are also alternative glucose-breakdown routes (the pentose phosphate and Entner-Doudoroff pathways), but "EMP pathway" and "glycolysis" mean the same thing.

"Why is fermentation needed if it makes no extra ATP?" Its job is not to make ATP; it is to regenerate NAD⁺. Glycolysis uses up NAD⁺ at step 6, and without oxygen there is no other way to recycle it. Fermentation (making lactate or ethanol) regenerates NAD⁺ so glycolysis can keep running and keep making its 2 ATP.

References

• Nelson DL, Cox MM. Lehninger Principles of Biochemistry. 8th ed. New York: W.H. Freeman; 2021.
• Berg JM, Tymoczko JL, Gatto GJ, Stryer L. Biochemistry. 9th ed. New York: W.H. Freeman; 2019.
• Madigan MT, Bender KS, Buckley DH, Sattley WM, Stahl DA. Brock Biology of Microorganisms. 16th ed. Pearson; 2021. (For the EMP, pentose phosphate, and Entner-Doudoroff pathways.)
• Rodwell VW, Bender DA, Botham KM, Kennelly PJ, Weil PA. Harper's Illustrated Biochemistry. 32nd ed. New York: McGraw Hill; 2022.