Products of the Light Reaction: ATP, NADPH and Oxygen
Biology · Photosynthesis in Higher Plants · NEET
The light reaction of photosynthesis makes three products: ATP, NADPH and oxygen (O2). ATP and NADPH move into the stroma to power the dark reaction (Calvin cycle), so together they are called "assimilatory power". Oxygen is a by-product that comes only from the splitting of water. Memory hook: light reaction gives "A-N-O" — ATP, NADPH, Oxygen.
The light reaction in the thylakoid uses light and water to make three products: ATP and NADPH (the assimilatory power that feeds the Calvin cycle) plus oxygen, which comes only from the splitting of water.
Your doubts, answered
Is glucose or sugar a product of the light reaction?
No. The light reaction only makes ATP, NADPH and oxygen. Glucose (sugar) is made later in the stroma during the dark reaction (Calvin cycle), using the ATP and NADPH supplied by the light reaction. NEET often puts glucose as a wrong option here, so never pick it as a light-reaction product.
Where does the oxygen released in photosynthesis come from?
The oxygen comes only from water, not from CO2. During the light reaction, water is split (photolysis) at Photosystem II into 2H+, [O] and electrons. The [O] atoms join to form O2. This was proved by Van Niel and by experiments using heavy oxygen (isotope) labelling of water.
What is 'assimilatory power'?
ATP and NADPH made in the light reaction are together called assimilatory power. This name is used because these two molecules carry the energy (ATP) and reducing power/hydrogen (NADPH) needed to 'assimilate' or fix CO2 into sugar in the Calvin cycle. Oxygen is NOT part of assimilatory power.
Is it NADH or NADPH that is formed in the light reaction?
It is NADPH (NADP is reduced to NADPH). NADH belongs to respiration, not the light reaction of photosynthesis. NEET 2018 asked exactly this — the odd one out was NADH. Watch the extra 'P': NADPH for photosynthesis light reaction.
Are ATP and NADPH used up in the light reaction or the dark reaction?
They are MADE in the light reaction (in the grana/thylakoids) and USED in the dark reaction (in the stroma). So the same two molecules are products of one stage and reactants of the next. The Calvin cycle needs them to reduce CO2 to sugar.
⚠️ The NEET trap ✗ Oxygen released in photosynthesis comes from CO2. ✓ Oxygen comes only from the splitting of water (photolysis) at Photosystem II, not from CO2. 🧠 Follow the water: label water with heavy oxygen and the O2 released is heavy — so O2 traces back to H2O, never to CO2.
Real NEET questions
NEET 2018
Which of the following is not a product of light reaction of photosynthesis?
A · NADPH
B · NADH ✓
C · ATP
D · Oxygen
Solution: The light reaction produces ATP, NADPH and oxygen. NADH is a product of respiration, not the photosynthetic light reaction — note the missing 'P'. So NADH is the odd one out.
NEET 2018
Oxygen is not produced during photosynthesis by
A · Cycas
B · Nostoc
C · Green sulphur bacteria ✓
D · Chara
Solution: Oxygen is released only when water is used as the hydrogen (electron) donor and is split. Green sulphur bacteria use H2S instead of H2O, so they release sulphur, not oxygen. Cycas, Nostoc (cyanobacterium) and Chara all split water and evolve O2.
NEET 2023 Phase 1
Which micronutrient is required for splitting of water molecule during photosynthesis?
A · manganese ✓
B · molybdenum
C · magnesium
D · copper
Solution: Water splitting (photolysis), the source of the oxygen product, is carried out by the water-splitting complex of PS II, which uses manganese (with Cl- and Ca2+). So manganese is the micronutrient linked to O2 evolution.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
