Glycolysis (cytoplasm)
Glucose split into 2 pyruvate; net 2 ATP + 2 NADH per glucose.
Glycolysis ('sugar splitting') is the first stage of respiration. It takes place in the cytoplasm of all living cells and does not require oxygen — every organism on Earth uses glycolysis.
The pathway has three phases:
1. Phosphorylation (activation). Glucose (6C) is first phosphorylated by 2 ATP to form fructose-1,6-bisphosphate. The 2 ATP are an investment — they 'destabilise' the glucose so it can be split. The enzyme hexokinase catalyses the first phosphorylation.
2. Lysis (splitting). Fructose-1,6-bisphosphate is split into two molecules of triose phosphate (TP, 3-carbon each).
3. Oxidation and phosphorylation. Each TP is oxidised — hydrogen atoms are removed and accepted by NAD (NAD → NADH) — and is phosphorylated by an inorganic phosphate from the cytoplasm. The phosphate groups (one was already on the TP, and one added now) are transferred to ADP in two substrate-level phosphorylation steps, producing 4 ATP and 2 pyruvate.
Net yield per glucose:
- 2 ATP (4 made − 2 invested)
- 2 NADH (each TP donates H to NAD)
- 2 pyruvate (3-carbon end product)
Pyruvate now has two possible fates:
- Aerobic (O₂ present): Pyruvate is transported into the mitochondrion and enters the link reaction.
- Anaerobic (no O₂): Pyruvate undergoes fermentation (see later) in the cytoplasm.
- Cytoplasm; anaerobic.
- Phosphorylation: glucose → fructose-1,6-bisphosphate (uses 2 ATP).
- Lysis: split into 2 TP (3C each).
- Oxidation: NAD → NADH; substrate-level phosphorylation → 4 ATP.
- Net per glucose: 2 ATP, 2 NADH, 2 pyruvate.