Glycolysis
The universal energy gateway. Glycolysis is the first step in breaking down glucose for energy. This ancient pathway occurs in the cytoplasm (not mitochondria) and produces pyruvate, ATP, and NADH. It's the starting point for both aerobic and anaerobic energy production.
The Pathway
Key Regulatory Steps
Hexokinase
First step. Traps glucose in cell. Inhibited by its product (G6P). Requires Mg²⁺.
PFK-1
Rate-limiting step. Main control point. Activated by AMP, inhibited by ATP and citrate.
Pyruvate Kinase
Final step. Produces ATP. Regulated by fructose-1,6-bisphosphate (feed-forward).
Nutrient Requirements
Magnesium
Required for all kinase reactions. 7 of 10 steps need Mg²⁺. Critical cofactor.
NAD⁺
Accepts electrons at GAPDH step. Makes NADH. Recycled by lactate or mitochondria.
Thiamine (B1)
Not directly in glycolysis but needed for pyruvate's next step (pyruvate dehydrogenase).
Niacin (B3)
NAD⁺ is made from niacin. Essential for the pathway to function.
Phosphate
Inorganic phosphate needed. Required for ATP synthesis steps.
ADP
Substrate for ATP production. Pathway regulated by ADP/ATP ratio.
What Happens to Pyruvate?
Aerobic (With Oxygen)
Pyruvate enters mitochondria → Acetyl-CoA → Krebs cycle → 34+ more ATP. Much more efficient.
Anaerobic (No Oxygen)
Pyruvate → Lactate. Regenerates NAD⁺. Only 2 ATP total. Used during intense exercise.