Biology · Photosynthesis in Higher Plants · NEET
Because 12 water molecules are used as reactants but only the net change matters. NCERT writes the full balanced form: 6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2. Six water molecules are re-formed during the reaction, so 6 appear again as products. The net water consumed is 12 minus 6 = 6, but the equation must show 12 on the left because 12 waters are actually split.
Because the O2 released comes only from water, not from CO2. To release 6 molecules of O2 you must split 12 molecules of water (each O2 needs 2 water molecules). This was proved using radioisotope (heavy oxygen) tracing. If only 6 water were used, the oxygen count would not balance.
From water. Cornelius van Niel showed photosynthesis is a light reaction where hydrogen from a compound reduces CO2. In green plants that hydrogen donor is H2O, which is oxidised to O2. So the O2 evolved comes from the splitting of water (photolysis), not from carbon dioxide. This is why 12 H2O appear in the correct equation.
It is the older simplified (empirical) form and is not the fully correct one for NEET. NCERT calls the empirical equation 6CO2 + 6H2O → C6H12O6 + 6O2, but after van Niel's work the correct equation became 6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2. For NEET, remember the 12-water version as the correct balanced equation.
The arrow carries 'light' and 'chlorophyll' because photosynthesis needs light energy captured by chlorophyll pigments. Without light or chlorophyll the reaction does not proceed, which is why they are written as conditions, not as reactants or products.
Oxygen is not produced during photosynthesis by
Try the real previous-year questions from this chapter — each with the answer and a full solution.
6CO2 + 12H2O → C6H12O6 + 6H2O + 6O2, taking place in the presence of light and chlorophyll.
From water. Water is split (photolysis) and oxidised to O2. The oxygen does not come from carbon dioxide.
Because 12 water molecules must be split to release 6 molecules of O2, since the oxygen comes only from water.
Cornelius van Niel proposed water is the hydrogen donor; it was later confirmed using radioisotope (heavy oxygen) techniques.
The empirical equation uses 6 water (6CO2 + 6H2O → C6H12O6 + 6O2). The correct equation uses 12 water and shows 6 water re-formed as product.
