Glycolysis
Steps, Regulation, and Energy Yield
Master the ten-step pathway that converts glucose into pyruvate, generating ATP and NADH in every living cell.
Biochemistry
What is Glycolysis?
Glycolysis (Greek: glyco = sweet, lysis = splitting) is the metabolic pathway that breaks one molecule of glucose (\(\text{C}_6\text{H}_{12}\text{O}_6\)) into two molecules of pyruvate (\(\text{C}_3\text{H}_4\text{O}_3\)).
Location
Cytoplasm of virtually all cells: prokaryotic and eukaryotic.
Oxygen?
Does not require \(\text{O}_2\): occurs under both aerobic and anaerobic conditions.
Ancient
Evolved before atmospheric \(\text{O}_2\) existed: one of the oldest metabolic pathways.
Key Molecules You Need to Know
ATP
Energy currency. Hydrolysis of one phosphate releases ~\(30.5\;\text{kJ/mol}\).
NADH
Electron carrier. Formed when \(\text{NAD}^+\) accepts \(2\text{e}^-\) and \(\text{H}^+\).
ADP
Adenosine diphosphate: the "spent" form of ATP that gets recycled.
Pi
Inorganic phosphate (\(\text{P}_i\)): combined with ADP to regenerate ATP via substrate-level phosphorylation.
The Big Picture: Two Phases
Energy Investment Phase: Steps 1-5
1
Hexokinase phosphorylates glucose to glucose-6-phosphate (G6P). Costs 1 ATP. Irreversible.
2
Phosphoglucose isomerase converts G6P to fructose-6-phosphate (F6P). Reversible isomerization.
3
PFK-1 (phosphofructokinase-1) phosphorylates F6P to fructose-1,6-bisphosphate. Costs 1 ATP. Rate-limiting step.
4
Aldolase cleaves the 6-carbon sugar into DHAP and G3P (3 carbons each).
5
Triosephosphate isomerase converts DHAP to G3P. Now we have 2 G3P.
Energy Payoff Phase: Steps 6-10
6
G3P dehydrogenase oxidizes G3P, reducing \(\text{NAD}^+\) to NADH. Attaches \(\text{P}_i\) to form 1,3-BPG.
7
Phosphoglycerate kinase transfers a phosphate from 1,3-BPG to ADP, producing 1 ATP per G3P (2 total).
8
Phosphoglycerate mutase rearranges 3-PG to 2-phosphoglycerate (2-PG).
9
Enolase dehydrates 2-PG to phosphoenolpyruvate (PEP), a high-energy intermediate.
10
Pyruvate kinase transfers PEP's phosphate to ADP, producing 1 ATP per G3P (2 total). Irreversible.
The Three Irreversible Steps
Glycolysis has three irreversible reactions that serve as key regulatory checkpoints. Each is catalyzed by a different enzyme with a large negative \(\Delta G\).
Step 1
Hexokinase
\(\Delta G = -33.4\;\text{kJ/mol}\)
Traps glucose inside the cell.
\(\Delta G = -33.4\;\text{kJ/mol}\)
Traps glucose inside the cell.
Step 3
PFK-1
\(\Delta G = -22.2\;\text{kJ/mol}\)
The committed, rate-limiting step.
\(\Delta G = -22.2\;\text{kJ/mol}\)
The committed, rate-limiting step.
Step 10
Pyruvate kinase
\(\Delta G = -16.7\;\text{kJ/mol}\)
Final ATP-generating step.
\(\Delta G = -16.7\;\text{kJ/mol}\)
Final ATP-generating step.
Regulation: PFK-1 is the Master Switch
Substrate-Level vs Oxidative Phosphorylation
| Substrate-Level | Oxidative | |
|---|---|---|
| Where | Cytoplasm (glycolysis) | Inner mitochondrial membrane |
| Mechanism | Phosphate transferred directly from substrate to ADP | Chemiosmosis via \(\text{H}^+\) gradient |
| \(\text{O}_2\) Required? | No | Yes |
| ATP per glucose | 2 net (glycolysis) | ~26-28 (from ETC) |
| Speed | Fast | Slower but more efficient |
The Net Equation of Glycolysis
Input
1 Glucose (6C)
2 \(\text{NAD}^+\)
2 ADP + 2 \(\text{P}_i\)
2 \(\text{NAD}^+\)
2 ADP + 2 \(\text{P}_i\)
Output
2 Pyruvate (3C each)
2 NADH
2 ATP (net)
2 NADH
2 ATP (net)
Fate of Pyruvate
How to Analyze Glycolysis Problems
1
Identify the starting molecule and count carbons. Glucose = 6C, each G3P = 3C.
2
Track ATP: Investment phase uses 2 ATP. Payoff phase produces 4 ATP. Net = 2 ATP.
3
Track electron carriers: 2 \(\text{NAD}^+\) are reduced to 2 NADH in step 6.
4
Remember the "doubling": After step 5, everything happens twice (one per G3P).
5
Identify regulation: Is the question about PFK-1 activators/inhibitors? Think energy charge.
Worked Example: Energy Accounting
Problem: A cell metabolizes 3 molecules of glucose through glycolysis. How many net ATP and NADH molecules are produced?
1
Per glucose: net gain = 2 ATP + 2 NADH.
2
For 3 glucose: \(3 \times 2 = \) 6 ATP and \(3 \times 2 = \) 6 NADH.
3
Also produced: \(3 \times 2 = 6\) pyruvate molecules.
Worked Example: Regulation Logic
Problem: A cell has high levels of ATP and citrate. What happens to the rate of glycolysis, and why?
1
High ATP signals the cell has abundant energy: inhibits PFK-1.
2
Citrate (from Krebs cycle) also inhibits PFK-1, signaling downstream pathways are saturated.
3
Since PFK-1 is the rate-limiting enzyme, glycolysis slows down.
Worked Example: Fermentation Logic
Problem: A yeast cell is placed in an anaerobic environment. It consumes 10 glucose molecules. How many ethanol and \(\text{CO}_2\) molecules are produced?
1
Each glucose produces 2 pyruvate via glycolysis.
2
In alcoholic fermentation, each pyruvate yields 1 ethanol + 1 \(\text{CO}_2\).
3
Per glucose: 2 ethanol + 2 \(\text{CO}_2\).
4
For 10 glucose: \(10 \times 2 = \) 20 ethanol and 20 \(\text{CO}_2\).
Common Mistakes to Avoid
✕ Forgetting the investment
Saying glycolysis produces 4 ATP instead of 2 net ATP. Always subtract the 2 ATP spent in steps 1 and 3.
✕ Wrong location
Placing glycolysis in the mitochondria. It occurs in the cytoplasm: no membranes required.
✕ Confusing NADH and FADH2
Glycolysis produces NADH, not \(\text{FADH}_2\). \(\text{FADH}_2\) is made in the Krebs cycle (step 6: succinate dehydrogenase).
✕ Ignoring the doubling
Forgetting that after aldolase splits the 6C sugar, every product is doubled: 2 NADH, 4 ATP, 2 pyruvate.
✕ Fermentation = extra ATP
Fermentation does not produce additional ATP. Its sole purpose is regenerating \(\text{NAD}^+\) so glycolysis can continue.
Question 1 of 5
What is the net ATP yield from one molecule of glucose undergoing glycolysis?
Glycolysis produces 4 ATP in the payoff phase but consumes 2 ATP in the investment phase. Net = \(4 - 2 = 2\) ATP. The ~30-32 ATP figure refers to the approximate total yield from complete aerobic respiration.
Question 2 of 5
Which enzyme catalyzes the rate-limiting step of glycolysis?
PFK-1 catalyzes step 3: the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate. This is the committed step and the most heavily regulated point in glycolysis. Hexokinase (step 1) and pyruvate kinase (step 10) are also irreversible but are not the primary rate-limiting enzyme.
Question 3 of 5
High levels of AMP in a cell would have what effect on glycolysis?
High AMP indicates the cell is low on energy (ATP has been broken down to AMP). AMP is an allosteric activator of PFK-1, speeding up glycolysis to generate more ATP. Citrate and high ATP would do the opposite: inhibit PFK-1.
Question 4 of 5
During lactic acid fermentation, the primary purpose is to:
Fermentation does not produce extra ATP. Its sole purpose is to oxidize NADH back to \(\text{NAD}^+\), which is needed as an electron acceptor in step 6 of glycolysis. Without \(\text{NAD}^+\) regeneration, glycolysis would halt.
Question 5 of 5
In glycolysis, which step directly produces \(\text{NADH}\) from \(\text{NAD}^+\)?
Step 6 is the only oxidation step in glycolysis. Glyceraldehyde-3-phosphate dehydrogenase oxidizes G3P while reducing \(\text{NAD}^+\) to NADH. Steps 1 and 3 consume ATP, while steps 7 and 10 produce ATP via substrate-level phosphorylation, but none of them involve \(\text{NAD}^+\).