What is Hill's reaction and its purpose?

• A. Demonstrates that isolated chloroplasts can release oxygen in the presence of an artificial electron acceptor, indicating light-driven electron flow.
• B. Measures the pigment content of leaves.
• C. Describes how mitochondria produce ATP in dark conditions.
• D. Describes how stomata open in response to light.

Answer: (A)

Hill's reaction demonstrates that light energy drives electron flow in the chloroplasts by showing that isolated chloroplasts can evolve oxygen when an artificial electron acceptor is present. In this setup, light-energized photosystem II transfers electrons to a high-potential electron acceptor (such as ferricyanide) instead of NADP+. The chloroplasts continue to split water to supply these electrons, producing oxygen as a by-product. This experiment proves that the light-dependent reactions occur within the chloroplasts and that light provides the energy to push electrons through the photosynthetic chain, with oxygen release linked to water splitting. The other options describe different plant processes (pigment measurement, mitochondrial ATP production in the dark, stomatal movement) that are unrelated to the light-driven electron transport demonstrated by Hill’s reaction.