Glycolysis

Glycolysis is the first stage of cellular respiration and the most universal energy-extraction pathway in biology. Every living cell on Earth runs glycolysis. The process splits one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each) through 10 enzymatic steps, all of which happen in the cytoplasm without requiring oxygen. The net yield is 2 ATP and 2 NADH per glucose. It’s a small payoff compared to the full aerobic respiration that follows, but glycolysis is what makes anaerobic life possible and what bridges glucose into the larger respiratory pathway.

The Net Reaction

Summarized in one line:

$$ \text{Glucose} + 2\,NAD^+ + 2\,ADP + 2\,P_i \;\longrightarrow\; 2\,\text{Pyruvate} + 2\,NADH + 2\,ATP + 2\,H^+ + 2\,H_2O $$

Read the equation backwards from the right side: the cell starts with glucose and a bit of metabolic raw material (ADP, inorganic phosphate, NAD+). It ends with two pyruvate molecules ready for the Krebs cycle, plus 2 ATP and 2 NADH worth of harvested energy. All in the cytoplasm. No oxygen needed.

The Two Phases

Phase 1: Energy Investment (Steps 1-5)

The cell spends 2 ATP up front to phosphorylate and prepare glucose for splitting. This may seem counter-productive, but the investment makes the molecule unstable enough to fall apart in phase 2 with a big energy payoff.

1. Step 1. Hexokinase phosphorylates glucose using 1 ATP, producing glucose-6-phosphate.
2. Step 2. Phosphoglucose isomerase rearranges glucose-6-phosphate into fructose-6-phosphate.
3. Step 3. Phosphofructokinase adds a second phosphate using another ATP, producing fructose-1,6-bisphosphate. This is the rate-limiting step of glycolysis and the main regulatory checkpoint.
4. Step 4. Aldolase splits fructose-1,6-bisphosphate into two 3-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).
5. Step 5. Triose phosphate isomerase rapidly interconverts DHAP and G3P. Only G3P continues; DHAP gets converted to G3P. So we now have 2 G3P molecules per starting glucose.

Phase 2: Energy Payoff (Steps 6-10)

All five steps below happen twice — once per G3P. The numbers below are per glucose (so 2x per G3P).

1. Step 6. Glyceraldehyde-3-phosphate dehydrogenase oxidizes G3P to 1,3-bisphosphoglycerate, producing 2 NADH per glucose. This is where the NADH comes from.
2. Step 7. Phosphoglycerate kinase transfers a phosphate from 1,3-bisphosphoglycerate to ADP, producing 2 ATP per glucose. The first substrate-level phosphorylation.
3. Step 8. Phosphoglycerate mutase rearranges 3-phosphoglycerate into 2-phosphoglycerate.
4. Step 9. Enolase dehydrates 2-phosphoglycerate into phosphoenolpyruvate (PEP).
5. Step 10. Pyruvate kinase transfers a phosphate from PEP to ADP, producing 2 more ATP per glucose and 2 pyruvate. Second substrate-level phosphorylation.

The Stoichiometry

Per glucose	Phase 1 (Investment)	Phase 2 (Payoff)	Net
ATP	-2 (consumed)	+4 (produced)	+2
NADH	0	+2 (produced)	+2
NAD+	0	-2 (consumed)	-2
Carbon (output)	—	2 pyruvate (3C each)	2 pyruvate

What Happens to the Pyruvate

Pyruvate has two main fates, depending on whether oxygen is available.

• With oxygen (aerobic): Pyruvate enters the mitochondrion and gets converted to acetyl-CoA, releasing 1 CO2 and producing 1 NADH per pyruvate. Acetyl-CoA then enters the Krebs cycle for the full aerobic-respiration payoff (~30-32 ATP per glucose total).
• Without oxygen (anaerobic): The cell ferments pyruvate to regenerate NAD+ so glycolysis can keep running. In animal muscle, pyruvate becomes lactate (lactic acid fermentation). In yeast, it becomes ethanol and CO2 (alcoholic fermentation). Either way, the cell stays stuck at 2 ATP per glucose — much less efficient than aerobic respiration but enough to keep going.

Why Glycolysis Is Universal

Glycolysis is present in essentially every living cell — bacteria, archaea, plants, animals, fungi. The pathway is highly conserved across all three domains of life. Several reasons it shows up everywhere:

• No oxygen required. Earth’s atmosphere had little free oxygen for the first 2 billion years of life. Glycolysis evolved before oxygen was available, and it still works without it.
• No specialized organelles required. All enzymes are cytoplasmic. Prokaryotes (which lack mitochondria entirely) can still run glycolysis.
• Fast. Substrate-level phosphorylation is much faster than oxidative phosphorylation. Cells that need a quick ATP burst (sprinting muscle, fermenting yeast) rely on glycolysis.
• Backbone for biosynthesis. Glycolytic intermediates feed multiple biosynthetic pathways: glucose-6-phosphate goes to the pentose phosphate pathway; pyruvate goes to fatty acid synthesis; DHAP goes to lipid synthesis.

The Pasteur Effect

Louis Pasteur noticed in the 1860s that yeast cells consume sugar much faster anaerobically than aerobically. This is the Pasteur effect: when oxygen is unavailable, glycolysis speeds up dramatically because the cell needs to compensate for the lower ATP yield per glucose. The biochemical mechanism: AMP and ADP accumulate, activating phosphofructokinase (the rate-limiting enzyme of glycolysis). When oxygen returns, ATP levels recover, phosphofructokinase is inhibited, and glycolysis slows back down.

Related study notes: Cellular Respiration, Krebs Cycle, Mitochondria, Enzyme.

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