Glycolysis is the metabolic pathway that breaks one glucose molecule into two pyruvate molecules. Students need this cheat sheet because glycolysis includes many steps, enzymes, energy changes, and carbon changes that are easy to mix up. A step-by-step reference helps connect the names of intermediates to the overall purpose of cellular respiration.
It also supports understanding of fermentation and aerobic respiration because both begin with glycolysis.
The pathway has two main phases: an energy investment phase and an energy payoff phase. The overall net reaction is glucose + 2 NAD+ + 2 ADP + 2 Pi -> 2 pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O. ATP is spent in steps 1 and 3, while ATP is produced in steps 7 and 10 by substrate-level phosphorylation.
NADH is produced in step 6 when glyceraldehyde-3-phosphate is oxidized.
Key Facts
- Glycolysis occurs in the cytosol and does not require oxygen directly.
- The net reaction of glycolysis is glucose + 2 NAD+ + 2 ADP + 2 Pi -> 2 pyruvate + 2 NADH + 2 H+ + 2 ATP + 2 H2O.
- The energy investment phase uses 2 ATP to phosphorylate glucose and fructose-6-phosphate.
- The energy payoff phase produces 4 ATP total, giving a net gain of 2 ATP per glucose.
- One glucose molecule with 6 carbons is split into two 3-carbon molecules that become pyruvate.
- NAD+ is reduced to NADH in step 6 when glyceraldehyde-3-phosphate is converted to 1,3-bisphosphoglycerate.
- Substrate-level phosphorylation makes ATP in glycolysis when a phosphate group is transferred directly to ADP.
- The key regulatory enzymes of glycolysis include hexokinase, phosphofructokinase-1, and pyruvate kinase.
Vocabulary
- Glycolysis
- Glycolysis is a ten-step pathway that breaks glucose into two pyruvate molecules while producing a small amount of ATP and NADH.
- Pyruvate
- Pyruvate is a 3-carbon molecule produced at the end of glycolysis.
- ATP
- ATP is the main energy-carrying molecule that stores usable energy in phosphate bonds.
- NAD+
- NAD+ is an electron carrier that accepts electrons and hydrogen during glycolysis to form NADH.
- Substrate-level phosphorylation
- Substrate-level phosphorylation is the direct transfer of a phosphate group from a metabolic intermediate to ADP to make ATP.
- Phosphofructokinase-1
- Phosphofructokinase-1 is a major regulatory enzyme that converts fructose-6-phosphate into fructose-1,6-bisphosphate.
Common Mistakes to Avoid
- Counting the gross ATP instead of the net ATP is wrong because glycolysis produces 4 ATP but spends 2 ATP, so the net gain is 2 ATP.
- Forgetting that each glucose forms two pyruvate is wrong because the 6-carbon glucose is split into two 3-carbon products.
- Saying glycolysis happens in the mitochondria is wrong because glycolysis occurs in the cytosol before pyruvate enters mitochondrial pathways in aerobic respiration.
- Assuming oxygen is required for glycolysis is wrong because glycolysis can run without oxygen as long as NAD+ is available.
- Mixing up NADH and NAD+ is wrong because NAD+ is the oxidized electron acceptor and NADH is the reduced electron carrier produced during glycolysis.
Practice Questions
- 1 One glucose molecule enters glycolysis. How many ATP are used, how many ATP are produced gross, and what is the net ATP gain?
- 2 If 3 glucose molecules go through glycolysis, how many pyruvate molecules and NADH molecules are produced?
- 3 A cell runs glycolysis for 10 glucose molecules. What is the net number of ATP molecules gained?
- 4 Why must cells regenerate NAD+ for glycolysis to continue when oxygen is not available?
Understanding Glycolysis Step-by-Step Reference
The early reactions do more than prepare a sugar for later steps. Adding phosphate groups gives the molecule a negative charge, which helps keep it inside the cell instead of allowing it to pass back out through transport proteins. Phosphates also make the carbon skeleton easier for enzymes to reshape.
The conversion from glucose-6-phosphate to fructose-6-phosphate moves a carbonyl group to a new position. This change seems small, yet it sets up the molecule for its later split.
The phosphorylation controlled by phosphofructokinase-1 is often called the commitment step. After this point, the cell has strongly directed the sugar toward energy release rather than storage or another pathway.
Carbon tracking prevents many common mistakes. Fructose-1,6-bisphosphate has six carbons when aldolase divides it into two three-carbon molecules. These products are not identical at first.
One is glyceraldehyde-3-phosphate, while the other is dihydroxyacetone phosphate. Triose phosphate isomerase rapidly converts the second form into glyceraldehyde-3-phosphate. From that moment onward, every reaction happens twice for each original glucose molecule.
This is why students must double the products made after the split. A useful habit is to mark the split clearly in a pathway diagram, then write a small two beside each later intermediate. It makes the ATP and NADH totals much easier to justify.
The oxidation reaction is important because it captures electrons before the carbon molecules are converted to pyruvate. Glyceraldehyde-3-phosphate loses electrons, and NAD plus receives them to form NADH. At the same time, an inorganic phosphate becomes attached, producing a molecule with enough stored energy to help form ATP in the next reaction.
This shows that ATP production is not caused simply by breaking down glucose. Energy must be transferred through specific intermediate molecules. NADH must eventually give up its electrons so that NAD plus can be regenerated.
In muscle cells working very hard, this can happen through lactate formation. In yeast, a related process supports ethanol production. Without NAD plus recycling, glycolysis stops even if glucose remains available.
Cells regulate this pathway because glucose is valuable and energy needs change. High ATP levels signal that the cell has abundant usable energy, so phosphofructokinase-1 slows down. AMP signals a lower energy state and tends to increase its activity.
Citrate can reduce the enzyme activity when later stages of respiration are well supplied. These controls help prevent wasteful processing of sugar. Pyruvate is another important branch point.
With suitable conditions, it can enter mitochondrial pathways that extract more energy. Under other conditions, it can support fermentation or serve as material for biosynthesis. When studying the sequence, focus on three linked ideas.
Track carbons, track phosphate groups, and track electrons. Enzyme names matter, but these three patterns explain why the steps occur in their particular order.