Question

The idea of two pigment systems in light reaction of photosynthesis was given by
a. Arnon
b. Hill
c. Blackman
d. None of the above

Answer 1

Hint: In the light reaction of photosynthesis, two important products are formed, NADPH and ATP. It also involves evolution of oxygen gas as a byproduct. It has been proven through experimental evidence.

Complete answer:
- Hill conducted an experiment with isolated chloroplasts from Stellaria media and used some oxidants like ferrocyanides and benzoquinone which is also called the Hill reagent. By placing them in sunlight, he noticed the evolution of oxygen from water. Hill therefore concluded that if there was a hydrogen acceptor in the medium, it would have been reduced.
- Arnon also repeated the experiment with isolated chloroplasts and concluded that:
- NADP (Nicotinamide Adenine Dinucleotide Phosphate), which was earlier called as TPN (TriphosphoPyridine Nucleotide), and also called as Co II is the hydrogen acceptor present in plants. It is reduced to NADPH.
- An ADP molecule is converted to ATP in light by utilizing inorganic phosphate. This phenomenon is called photophosphorylation.
- Arnon observed that isolated chloroplasts are capable of fixing carbon dioxide resulting in the formation of carbohydrates. He suggested that the light reaction occurs in the granum of the chloroplast and the dark reaction in the stroma.
- Blackmann gave the law of limiting factors that states that rate of photosynthesis is affected by the pace of the slowest factor or limiting factor, e.g. light or carbon dioxide.
- Emerson noticed that giving shorter and longer wavelengths simultaneously increases the rate of photosynthesis. This is called Emerson enhancement or Emerson effect. This showed the presence of two photosynthetic processes which were associated with two different photo pigments.
- So, the idea of two pigment systems in light reaction of photosynthesis was given by Robert Emerson.

Hence, the correct answer is option (D).

Note: Each photosystem has all the photosynthetic pigments except one molecule of chlorophyll a forming a light harvesting system also called antennae and one molecule of chlorophyll forms the reaction centre. Two forms of chlorophyll are Chl a 683 $(P6_80)$ and Chl a 703 $(P_700)$.