Photosynthesis In Higher Plants
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Stephen Hales : He is credited for discovery of photosynthesis and known as father of plant physiology.
Joseph Priestley (1733-1804) :
In 1770 he performed a series of experiments that revealed the essentail role of air in the growth of green
plants. He also discovered oxygen in 1774.
Priestley’s observation :
(i) A candle burning in a closed space i.e. a bell jar, soon
gets extinguished.
(ii) Similarly, a mouse would soon suffocate in a closed space.
Priestley’s conclusion :
He concluded that a burning candle or an animal that breathe
the air, both somehow, damage the air. But when he placed
a mint plant in the same bell jar, he found that the mouse
stayed alive and candle continued to burn.
Priestley’s hypothesis :
Plants restore to the air whatever breathing animals
and burning candles remove.
Jan Ingenhousz (1730-1799) :
Using a similar setup as the one used by Priestley but by placing it once in the dark and once in the light, he
showed that sunlight is essential to the plant process (photosynthesis) that somehow purifies the air fouled by
burning candles or breathing animals.
In an another elegant experiment with an aquatic plant showed that in bright sunlight small bubbles were
formed around the green parts, while in the dark they did not.
Conclusion :
It is only the green parts of the plants that could release oxygen in the presence of sunlight.
Julius Von Sachs (1854) :
He provided evidences for production of glucose when plants grow. Glucose is usually stored as starch.
Glucose – Chemically reactive and soluble
Sucrose – Chemically inactive and soluble
Starch – Chemically inactive and insoluble
His later studies showed that the green substance in plants is located in special bodies within plant cells. (Today
the green colour substance is known as chlorophyll and special bodies are chloroplasts)
By the middle of nineteenth century the empirical equation representing the total process of
photosynthesis for oxygen evolving organisms was then understood as :
Cornelius Van Niel (1891-1985)
A milestone contribution to the understanding of photosynthesis was made by him. He was a microbiologist.
He based upon his studies of purple and green bacteria, demonstrated that
“Photosynthesis is essentially a light dependent reaction in which hydrogen from a suitable oxidisable
compound reduces carbon di-oxide to carbohydrates”
In green plants and BGA, H2O is the hydrogen donor and is oxidised to O2.
· In purple and green sulphur bacteria H2S is the hydrogen donor and is oxidised to sulphur or sulphate
depending on the organism.
“He inferred that the O2 evolved by the green plant comes from H2O, not from CO2
Types of chlorophyll :
(1) Chlorophyll-a
· Its a bluish green or bright green pigment with molecular formula C55H72O5N4 Mg.
· Chlorophyll-a is a universal photosynthetic pigment
Reason : Present in every photosyntehtic organism (except eubacteria)
· Chlorophyll-a is a primary photosynthetic pigment.
Reason : Primary reaction of photosynthesis which involve conversion of light energy into chemical energy
(ATP and NADPH) is mediated by chl-a molecule (Reaction center)
· Chlorophyll-a is most abundant photosynthetic pigment.
(2) Chlorophyll-b
It is yellow green pigment, molecular formula is C55H70O6N4 Mg.
It is structurally similar to chl-a, except it has –CHO group in place of –CH3 at III position of II pyrrole ring.
(3) Chlorophyll-c
Chlorophyll-c lacks phytol tail.
(4) Chlorophyll-d
(5) Chlorophyll-e
(B) Carotenoids :
Carotenoids are yellow to yellow orange colour pigments occur alone inside chromoplast and occur alongwith
chlorophylls inside chloroplast. These pigments are universal in occurence (except eubacteria) and insoluble
in water.
Chemically they are terpenes and considered as most stable pigments. Light is not necessary for their synthesis.
They are hydro carbons with conjugated double bonds (–CH=CH–CH=CH–)
Functions of carotenoids :
(1) They are accessory pigments and make photosynthesis more efficient by absorbing different wavelengths of light.
(2) They protect chl-a from photo oxidation and they also protect photosynthetic machinery by converting lethal
nascent oxygen into unharmful molecular oxygen, thus called shield pigments
(3) b-carotene is acts as a precursor of vitamin-A
(4) They help in entomophilly and zoochory
(C) Phycobillins :
· They are hot water soluble, open tetrapyrrole pigments which are associated with proteins.
· They lack Mg and phytol tail.
Types :
(i) Phycocyanin – Blue (ii) Phycoerythrin – Red (iii) Allophycocyanin – Light blue
They occur exclusively in BGA and Red algae as an accessory pigments.
Special points :
· Etiolation : Yellowing of leaves due to deficiency of light.
· Phytochrome : It is a proteinaceous (chromoprotein) pigment in angiosperms and responsible for seed
germination and flowering
Absorption spectrum :
It is a graphic representation of absorption of different wavelength of light by various pigment molecules. (chl-
a, chl-b and carotenoids)
T.W. Engelmann (1843-1909) :
He described the first action spectrum of photosynthesis. By using a prism he split sunlight into its
spectral components (VIBGYOR) and then illuminated a green algae, Cladophora placed in a suspension of
aerobic bacteria.“The bacteria were used to detect the sites of O2 evolution”
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