Glycolysis: The First Step in Energy Extraction
Imagine you have a glucose molecule — a six-carbon sugar packed with energy. Your cells need that energy, but they can't burn glucose directly like fuel in a furnace. They need to break it down step by step, capturing the released energy in usable forms. Glycolysis is that first, universal breakdown pathway.
It happens in the cytoplasm of every living cell — bacteria, yeast, plant, or human. No oxygen required. That's why it's ancient: it worked before Earth's atmosphere had oxygen.
The Big Picture
Glycolysis takes one molecule of glucose (6 carbons) and splits it into two molecules of pyruvate (3 carbons each). Along the way, it produces a small net gain of ATP (the cell's energy currency) and NADH (an electron carrier that stores energy for later use).
The full name is the Embden-Meyerhof-Parnas (EMP) pathway, after the scientists who worked it out.
Glucose (6C)+2 NAD++2 ADP+2 Pi→2 Pyruvate (3C)+2 NADH+2 H++2 ATP+2 H2O
The Two Phases
Glycolysis has ten enzyme-catalysed steps, but they fall into two clear phases.
Phase 1: Energy Investment (Steps 1–5)
You spend 2 ATP molecules to phosphorylate glucose, making it reactive. The 6-carbon sugar is then split into two 3-carbon molecules called glyceraldehyde-3-phosphate (G3P). No energy has been earned yet — you're in the red by 2 ATP.
Phase 2: Energy Payoff (Steps 6–10)
Each G3P is oxidised and converted to pyruvate. For each G3P, you gain 2 ATP and 1 NADH. Since you started with two G3P molecules, the total gain is 4 ATP and 2 NADH.
Net result: 4 ATP earned − 2 ATP spent = 2 ATP net gain, plus 2 NADH.
Remember the net ATP as 2 — one from each of the two 3-carbon fragments after the investment phase.
Key Details for Exams
The substrate-level phosphorylation happens in two steps: one produces ATP from 1,3-bisphosphoglycerate (via phosphoglycerate kinase), the other from phosphoenolpyruvate (via pyruvate kinase). Both are direct transfers of a phosphate group to ADP — no electron transport chain involved.
The oxidation step is step 6: G3P dehydrogenase removes two hydrogens (as a hydride ion and a proton) and passes them to NAD⁺, forming NADH. This is the only oxidation step in glycolysis. …