Question

how is 2 atp produced in fermentation

Answer 1

In anaerobic respiration( fermentation/respiration in absence of oxygen), processes like Krebs cycle and oxidative phosphorylation do not occur so the yield of ATP is low. Only glycolysis occurs in anaerobic respiration. In glycolysis, there are only net 2 ATP molecules formed made directly through substrate level phosphorylation without involving oxidative phosphorylation. Although in glycolysis 4 ATP molecules are formed through substrate level phosphorylation, but two are consumed also. So, net yield of ATP molecules in anaerobic resoiration is two ATP molecules.
Here is given list of reactions of glycolysis reaction in which ATP molecules are formed and consumed.

Expenditure of two ATP molecules :
Glucose to glucose 6-phosphate (expenditure of 1 ATP)
Fructose-6-phosphate to Fructose1,6-bisphosphate (expenditure of 1 ATP)
Production of four ATP molecules:
1,3-Bisphosphoglycerate (2 molecules) to 3-Phosphoglycerate (2 molecules) (2 ATP gained)
Phosphoenolpyruvate (2 molecules) to Pyruvate (2 molecules) (2 ATP gained)