Imagine a solar panel. Sunlight hits it, and out comes electricity. The light reaction in photosynthesis is exactly that — a solar panel built into every green leaf. But instead of electricity, it produces two chemical fuels: ATP (the energy currency of the cell) and NADPH (a reducing agent that carries high-energy electrons). These two molecules are then used in the next phase (the Calvin cycle) to build glucose from carbon dioxide.
The key insight: the light reaction is the only part of photosynthesis that directly uses light. Everything after it runs on the ATP and NADPH it produces.
Where does it happen?
Inside the chloroplast, there are flattened, disc-like sacs called thylakoids. Their membranes are packed with pigment molecules (chlorophyll a, chlorophyll b, carotenoids) and protein complexes. This is the stage. The space inside the thylakoid is the lumen; the fluid outside is the stroma.
What actually happens? (The precise statement)
When a photon of light strikes a chlorophyll molecule, an electron gets excited — it jumps to a higher energy level. That excited electron is then passed through a chain of carriers embedded in the thylakoid membrane, like a bucket brigade. As it moves, its energy is used to pump protons (H+) from the stroma into the thylakoid lumen, building up a concentration gradient. That gradient drives ATP synthesis (via ATP synthase). Meanwhile, the electron eventually ends up reducing NADP+ to NADPH.
But here's the catch: the chlorophyll that lost an electron needs to replace it. That electron comes from water (H2O). Splitting water releases oxygen gas (O2) as a byproduct — that's the oxygen we breathe.
2H2O+2NADP++3ADP+3PilightO2+2NADPH+3ATP
The two photosystems: a closer look
There are two distinct pigment systems, Photosystem II (PSII) and Photosystem I (PSI), named in the order they were discovered, not the order they work. Light hits both.
PSII absorbs light, excites an electron, and passes it to an electron acceptor. The electron hole in PSII is filled by splitting water. This releases O2 and protons into the lumen.
The electron travels down an electron transport chain (cytochrome b6f complex), pumping protons along the way.
The electron reaches PSI, which also absorbs light and re-excites the electron to an even higher energy level.
That high-energy electron is finally used to reduce NADP+ to NADPH.
The proton gradient built up during step 2 drives ATP synthase to make ATP. This entire flow is called non-cyclic photophosphorylation — the most common pathway.
Note
There is also a cyclic pathway where electrons from PSI cycle back to the electron transport chain instead of reducing NADP+. This produces only ATP, no NADPH or oxygen. It happens when the cell needs extra ATP.
Photosynthesis is the making of food (glucose) by green plants from CO2 and water using light energy; the chloroplast performs the light reaction in its grana and the dark reaction in its stroma.
Definition of photosynthesis:
Photosynthesis is the anabolic process by which green plants (and some other autotrophs) manufacture their own food (carbohydrates) from carbon dioxide and water, using light energy absorbed by the pigment chlorophyll, and release oxygen as a by-product.
Overall equation:
6CO2 + 12H2O → (in the presence of light and chlorophyll) → C6H12O6 + 6O2 + 6H2O
Role of the chloroplast in photosynthesis:
The chloroplast is the organelle where photosynthesis takes place. It has two functional regions:
Grana (stacks of thylakoids) — the site of the light reaction (photochemical phase). Here chlorophyll traps light, water is split (photolysis) releasing O2, and energy-rich ATP and NADPH are formed. …
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
CBSE 2026Set ANNUAL1 markMCQ
Q.Hill reaction occurs in which part of the chloroplast ?
(a) Stroma
(b) Inner membrane
(c) Grana
(d) Outer membrane
›Reveal solutionSolution
The Hill reaction occurs in the grana (thylakoids) of the chloroplast, so the answer is (C).
The chloroplast has two main regions:
Grana (stacks of thylakoids) — site of the light reaction, including the Hill reaction (light-driven splitting of water and release of O2), electron transport and ATP/NADPH formation. …
(a) Photoexcitation of chlorophyll are electron emission
(b) Photolysis of water
(c) Release of oxygen
(d) Synthesis of ATP
›Reveal solutionSolution
The photosynthetic light reaction begins with light being absorbed by chlorophyll, exciting an electron out of the reaction-centre chlorophyll -- this photoexcitation and electron emission is the very first step, and everything else (electron transport, water-splitting, ATP/NADPH synthesis) follows from it.
When light strikes chlorophyll in the reaction centre of a photosystem, it excites an electron to a higher energy state, and that excited electron is emitted/passed to an electron acceptor, starting the electron transport chain. The chlorophyll molecule, now electron-deficient, is replenished by an electron pulled from the splitting of water (photo …
Q.Identify true statement regarding light reaction of photosynthesis.
(a) Splitting of water molecule is associate with PS I.
(b) PS I and PS II are involved in the formation of NADPH+H+.
(c) The reaction center of PS I is chlorophyll "a" with absorption peak at 680 nm.
(d) The reaction center of PS II is chlorophyll "a" with absorption peak at 700 nm.
›Reveal solutionSolution
NADPH formation requires the combined, linear action of both photosystems (the Z-scheme): PS II supplies the electrons (from water splitting) that flow via the electron transport chain to PS I, where they are finally used to reduce NADP+ to NADPH.
Check each option against the actual light reaction mechanism:
"Splitting of water molecule is associated with PS I" — FALSE. Photolysis of water (2H2O -> 4H+ + 4e- + O2) occurs at the oxygen-evolving complex associated with Photosystem II (PS II), not PS I. The electrons released here replace those lost from PS II's reaction centre.
"PS I and PS II are involved in the formation of NADPH+H+" — TRUE. In the Z-scheme (non-cyclic/linear electron flow), light-excited electrons leave PS II, pass down an electron transport chain (plastoquinone -> cytochrome b6f -> plastocyanin) to PS I, are re-energised by light at PS I, and are finally passed via ferredoxin to the enzyme ferredoxin-NADP+ reductase, which reduces NADP+ to NADPH + H+. Since the electrons that end up reducing NADP+ originated at PS II and were re-energised at PS I, both photosystems are genuinely involved in NADPH formation.
…
Q.Fill in the blank: In photosynthesis, the light reaction is completed in __________. (choose: Grana / stroma)
›Reveal solutionSolution
The light-dependent reactions occur in the grana (thylakoid membranes); the light-independent (dark) reactions occur in the stroma.
The chloroplast is differentiated into grana (stacks of thylakoid membranes bearing chlorophyll and other pigments organized into Photosystem I and II) and the stroma (the fluid matrix around the grana). Light energy is absorbed by pigments in the thylakoid membranes, driving photolysis of water, electron transport, and ATP + NADPH synthesis — all within the grana. The stro …
Q.Where does the light reaction of photosynthesis take place?
›Reveal solutionSolution
The light reaction of photosynthesis takes place in the thylakoid membranes (grana) of the chloroplast.
The light reaction (also called the light-dependent reaction, or the Hill reaction) of photosynthesis occurs in the thylakoid membrane system of the chloroplast — the flattened, membrane-bound sacs (thylakoids) stacked into structures called grana, which are embedded in the stroma. This is where the photosynthetic pigments (chlorophylls, carotenoids), the two photosystems (PS I and PS II), and the electron transport chain components are located. Here, light …