Question

Hill used a dye for his famous hill reaction is
(A) Sulphur green
(B) Eosine
(C) Methylene blue
(D) Dichlorophenolindophenol

Answer 1

In the year 1937 and 1939, Robert Hill studied the photochemical phase of the photosynthesis. This phase is light dependent. He proved that oxygen is evolved during light reaction of the photosynthesis and it comes from water during its photolysis.

Complete Answer:
- In his experiment, Robert Hill used Dichlorophenol indophenol (DCPIP) as a redox dye and it is a part of Hill reagent family. It can be used to measure the rate of photosynthesis. The dye DCPIP is blue in oxidized state with a maximal absorption at 600 nm.
- The DCPIP becomes colourless when it is reduced. The loss of colour in the DCPIP is due to reducing agents produced by light-dependent reactions in the extracted chloroplasts.
- DCPIP is often used in measurements of the electron transport chain as it has higher electron affinity as compared to ferredoxin.
- The photosynthetic electron transport chain can reduce DCPIP as a substitute for $NADP^+$ which is generally the final electron carrier in during the photochemical reaction of the photosynthesis. DCPIP accepts electrons between the electron carriers’ plastoquinone and cytochrome. The electrons are ultimately derived from water.
- As DCPIP is reduced and becomes colorless, the resultant increase in light transmittance can be measured using a spectrophotometer. DCPIP is also used as an artificial electron acceptor for enzyme assays.

Therefore, option D is the correct answer.

Note: During light reaction assimilatory power ($NADPH_2$ + ATP) is generated. These assimilatory power helps in the fixation of carbon dioxide during Light independent or dark phase of photosynthesis.