PinkMonkey Online Study Guide-Biology
Photo-excitation of chlorophyll-a
When a molecule of chlorophyll-a (acting as the reaction center)
receives light energy in the form of photons (quanta), it becomes energy-rich
and activated. This is called the excited state of chlorophyll.
In this excited state, the chlorophyll-a expels one electron. With the
loss of an electron, the chlorophyll-a develops a positive charge. This
is called ionized chlorophyll-a. The expelled electron contains
an extra amount of energy received from light. In other words, the energy-rich
expelled electron represents light energy. This electron energy is used
for the formation of ATP during photosynthesis.
Photolysis of Water
In the photosynthesis of green plants, water is used as a source
of hydrogen required for the reduction of carbon dioxide to form carbohydrates.
In this process, water is oxidized in presence of light and chlorophyll. Hydrogen is removed from water, and oxygen is released as follows :
This is described as photolysis of water, or photochemical
oxidation of water. This was first indicated by Van Niel (1931). It was
demonstrated with the help of an experiment by R. Hill (1937). Hill suggested
that during the photolysis of water, hydrogen combines with some hydrogen
acceptor in the plant, while oxygen is released.
(where A = unknown H-acceptor in plants).
Later on, Arnon (1951) discovered this hydrogen acceptor to
be a coenzyme called NADP (nicotinamide adenine dinucleotide phosphate).
Substance A = NADP
Similarly, Ruben, Kamen, et al. (1941) confirmed that
the oxygen evolved during photosynthesis comes from the splitting of water
only. They used heavy isotope of oxygen (O18) to demonstrate the fact
as follows :
Table of Contents
4. 2 Overall Equation of Photosynthesis
4.3 Primary Process of Photosynthesis
4.4 Secondary Processes of Photosynthesis (Biochemical Phase, Dark Reaction)
4.5 Diversity in Photosynthesis Pathway
4.6 Significance of Photosynthesis