Q.Define photosynthesis. What are the main events taking place in light reaction?

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. …

Light reactions occur on thylakoids (grana); dark reactions in stroma → (a).

From the NCERT Class 11 Biology account of photosynthesis in higher plants:

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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 …

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:

1. "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.
2. "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. …

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 …

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 …