5.2.3 Glycolysis

What is Glycolysis?

• Glycolysis is the first stage of both aerobic and anaerobic respiration.
• It occurs in the cytoplasm of the cell and does not require oxygen.

Steps of Glycolysis

1. Phosphorylation of Glucose:

• Glucose (6C) is phosphorylated using 2 ATP molecules, forming glucose-6-phosphate and eventually fructose-1,6-bisphosphate.
• This makes glucose more reactive and traps it within the cell.

2. Splitting of Fructose-1,6-bisphosphate:

• The 6-carbon sugar splits into two molecules of triose phosphate (3C).

3. Oxidation of Triose Phosphate:

• Each triose phosphate molecule is oxidised, reducing NAD to NADH (reduced NAD).
• During this step, inorganic phosphate (Pi) is added to the triose phosphate, forming intermediates.

4. ATP Production:

• The intermediates are converted into pyruvate (3C).
• 4 ATP molecules are produced by substrate-level phosphorylation, giving a net gain of 2 ATP (as 2 ATP were used in the phosphorylation step).

Products of Glycolysis

From one molecule of glucose:

• 2 Pyruvate: Used in the next stages (aerobic or anaerobic respiration).
• 2 ATP (net): A small, immediate energy supply for the cell.
• 2 NADH: Used in oxidative phosphorylation to generate more ATP during aerobic respiration.

Fate of Pyruvate

1. Aerobic Respiration:

• Pyruvate enters the mitochondrial matrix for the link reaction and subsequent stages of respiration.

2. Anaerobic Respiration:

• Pyruvate is converted into lactate (in animals) or ethanol and CO₂ (in plants and yeast) using NADH.
• This regenerates NAD, allowing glycolysis to continue when oxygen is not available.

Summary Table

Stage	Reactants	Products	Energy Yield
Phosphorylation	Glucose, 2 ATP	Fructose-1,6-bisphosphate	-2 ATP (used)
Splitting	Fructose-1,6-bisphosphate	2 Triose Phosphate	No energy produced
Oxidation	Triose Phosphate, NAD	2 NADH	+2 NADH
ATP Production	Intermediates	4 ATP, 2 Pyruvate	+4 ATP
Net Yield			+2 ATP, 2 NADH