Glycolysis

Glycolysis is a metabolic pathway that breaks down glucose (C₆H₁₂O₆) into pyruvate, producing energy (ATP) and reducing equivalents (NADH). It is the first step in cellular respiration and occurs in the cytoplasm of all cells.


Explanation of Glycolysis

Glycolysis is an anaerobic process, meaning it does not require oxygen. It consists of ten enzymatic steps that can be divided into two phases:

  1. Energy Investment Phase (Steps 1-5) – Uses ATP to prepare glucose for breakdown.
  2. Energy Payoff Phase (Steps 6-10) – Produces ATP and NADH by breaking down glucose.

Net Reaction of Glycolysis:

C6H12O6+2NAD++2ADP+2Pi→2C3H4O3(Pyruvate)+2NADH+2ATP+2H2O+2H+C_6H_{12}O_6 + 2NAD^+ + 2ADP + 2P_i \rightarrow 2C_3H_4O_3 (Pyruvate) + 2NADH + 2ATP + 2H_2O + 2H^+C6​H12​O6​+2NAD++2ADP+2Pi​→2C3​H4​O3​(Pyruvate)+2NADH+2ATP+2H2​O+2H+


Steps of Glycolysis

Phase 1: Energy Investment Phase

  1. Glucose Phosphorylation:
    • Enzyme: Hexokinase
    • Reaction: Glucose → Glucose-6-Phosphate (G6P)
    • ATP Used: 1
  2. Isomerization:
    • Enzyme: Phosphoglucose Isomerase
    • Reaction: G6P → Fructose-6-Phosphate (F6P)
  3. Second Phosphorylation:
    • Enzyme: Phosphofructokinase-1 (PFK-1)
    • Reaction: F6P → Fructose-1,6-Bisphosphate
    • ATP Used: 1
  4. Cleavage into Two Molecules:
    • Enzyme: Aldolase
    • Reaction: Fructose-1,6-Bisphosphate → G3P (Glyceraldehyde-3-Phosphate) + DHAP (Dihydroxyacetone Phosphate)
  5. Isomerization of DHAP to G3P:
    • Enzyme: Triose Phosphate Isomerase
    • Reaction: DHAP → G3P

Phase 2: Energy Payoff Phase

  1. Oxidation and NADH Production:
    • Enzyme: Glyceraldehyde-3-Phosphate Dehydrogenase
    • Reaction: G3P → 1,3-Bisphosphoglycerate
    • NADH Produced: 2 (1 per G3P molecule)
  2. ATP Generation (Substrate-Level Phosphorylation):
    • Enzyme: Phosphoglycerate Kinase
    • Reaction: 1,3-Bisphosphoglycerate → 3-Phosphoglycerate
    • ATP Produced: 2
  3. Molecular Rearrangement:
    • Enzyme: Phosphoglycerate Mutase
    • Reaction: 3-Phosphoglycerate → 2-Phosphoglycerate
  4. Dehydration Reaction:
    • Enzyme: Enolase
    • Reaction: 2-Phosphoglycerate → Phosphoenolpyruvate (PEP)
  5. Final ATP Production (Substrate-Level Phosphorylation):
  • Enzyme: Pyruvate Kinase
  • Reaction: PEP → Pyruvate
  • ATP Produced: 2

Net Yield of Glycolysis

MoleculeProduced
ATP (Total)4
ATP (Net)2
NADH2
Pyruvate2

Fate of Pyruvate After Glycolysis

After glycolysis, pyruvate can follow three different pathways depending on oxygen availability:

  1. Aerobic Conditions (Oxygen Present):
    • Pyruvate enters the Krebs Cycle (Citric Acid Cycle) in the mitochondria.
    • Complete oxidation to CO₂ & ATP via Oxidative Phosphorylation.
  2. Anaerobic Conditions (No Oxygen – Fermentation):
    • In animals & humansLactic Acid Fermentation (Lactate is produced).
    • In yeast & bacteriaAlcoholic Fermentation (Ethanol & CO₂ are produced).

Importance of Glycolysis

Universal Metabolic Pathway – Occurs in all cells, from bacteria to humans.
Provides Quick Energy – Can function without oxygen (important for muscle cells during exercise).
Prepares Substrates for Further Metabolism – Pyruvate fuels cellular respiration or fermentation.
Critical for Brain Function – The brain relies on glucose as its primary energy source.