Q.What are the important events and end products of Light Reaction ? OR Describe Chemiosmotic Production of ATP.

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:

…

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 …