Q. 5: Write a note on Hill’s reaction.
Give an account
of photolysis of water.
‘‘Hill’s
experiment does not prove that the source of oxygen evolved during photo
synthesis is water.” This statement is
true or false? Explain.
Give the
significance of photolysis of water.
Ans.: 1) Splitting of water molecules into its components i.e.
H+ and OH– ions in the presence of light and chlorophyll
is called as photolysis of water.
2) It is one of the
important reactions in photosynthesis and its experimental proof was given by
Robert Hill in 1937.
3) He stated that
during photosynthesis water molecule splits up into H+ and OH– ions.
4) He used isolated
chloroplast from unicellular algae chlorella in CO2 free water, ferric salt (Fe3+) as
hydrogen acceptor and haemoglobin as oxygen carrier.
5) He carried out
photosynthesis and observed the splitting of water molecules. Ferric salt (Fe3+) reduced to
ferrous salt. (Fe2+)
and haemoglobin to oxyhaemoglobin.
i) 4H2O 4H + 4OH–
ii) 4H+ + 4OH– 4H+ 4e + 2H2O + O2
He used suitable hydrogen accepter for his experiment
“A”, where “A” is Hill’s oxidant.
6) Hill’s reaction
can be represented as follows;
2H2O + 2A2AH2 + O2
Where `A’ is Hill’s oxidant.
7) Significance of
photolysis of water: -
i) Splitting of
water molecules is observed.
ii) Oxygen is
released as by-product.
iii) Water acts
as hydrogen donor.;
iv) Assimilatory
power of photosynthesis i.e. NADPH2 is
produced.
v) Loss of electrons
of PS-II or P680 in non – cyclic photophosphorylation is
compensated by photolysis of water.
D) Arnon’s concept:
(1) Arnon, discovered the nature of Hill’s oxidant as
NADP i.e. Nicotinamide Adenine Dinucleotide phosphate:
(2) Arnon coined the term “Assimilatory power’ for
photosynthesis as ATP and NADPH2.
2H2O + 2NADP----------2NADPH2 + O2
E) Ruben, Kamen and
Randall reaction:
(1) These three-scientist confirmed water as a source of
oxygen during photosynthesis.
(2) They used isotope tracer technique in their
experiment. They used two isotopes of oxygen as O16 and O18
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