Biology · Photosynthesis in Higher Plants · NEET
Photosynthesis is the process where green plants make their own food. They use light energy (usually sunlight), carbon dioxide from the air, and water from the soil. Inside the chlorophyll, these are turned into glucose (a sugar the plant uses as food) and oxygen (which is let out into the air). In short: the plant uses light to build sugar and gives out oxygen.
The full balanced equation used in NCERT is: 6CO2 + 12H2O --light, chlorophyll--> C6H12O6 + 6O2 + 6H2O. Here C6H12O6 is glucose. Note that 12 water molecules go in and 6 water molecules come out. NEET can ask this exact equation, so learn both the reactants and the products.
Twelve water molecules are shown because water is split during photosynthesis to release oxygen. All 6 O2 molecules come from water, and this splitting needs 12 H2O. Six new water molecules form again on the product side. So the net water used is 6, but we write 12 to correctly show the oxygen source. This is a common NEET trap.
The oxygen released in photosynthesis comes only from water (H2O), NOT from carbon dioxide. This was proved using radioactive isotope studies. In green plants water is the hydrogen donor and is oxidised to O2. NEET frequently tests this exact point, so never say oxygen comes from CO2.
Photosynthesis is NOT a single reaction. The equation only summarises it. In reality it is a multi-step process with a light reaction (photochemical phase) and a dark reaction (biosynthetic phase). NCERT clearly states the equation is 'not a single reaction but a description of a multistep process.'
Raw materials (reactants): carbon dioxide (CO2), water (H2O), and light energy, with chlorophyll as the pigment. End products: glucose (C6H12O6, stored as starch) and oxygen (O2). Light energy is trapped and stored as chemical energy in the glucose.
Oxygen is not produced during photosynthesis by
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Julius von Sachs (around 1854) provided evidence for the production of glucose when plants grow. Glucose is usually stored as starch. He also showed that the green substance (chlorophyll) is located in chloroplasts.
The early empirical form is CO2 + H2O --light--> [CH2O] + O2, where [CH2O] represents a carbohydrate such as glucose. This was later refined into the full balanced glucose equation.
Photosynthesis is the source of all food and oxygen on Earth and is a high-weightage NEET chapter. The definition, overall equation, and the water-as-oxygen-source point are direct exam favourites and easy marks if learnt correctly.
It tells us 12 water molecules are used and 6 water molecules are formed. The 12 input waters supply all the oxygen released, which is why the equation is written with 12H2O rather than 6H2O.
