Photosynthesis in Higher pLants 1 Flashcards
features of photosynthesis
1.Endothermic (Endergonic) Process – Requires sunlight as source of
energy.
2.Uphill Process
3.Redox Process – Light reaction (splitting of water) is an oxidation & dark
reaction (CO2 fixation) is a reduction step.
4.Physico-chemical process – Mechanism of photosynthesis convert light
energy (physical form) into glucose or starch (chemical form).
5.Anabolic Process - Building up reaction
expt 1
Look for starch formation in two leaves :-
1.A variegated leaf or a leaf partially covered with black paper.
2.One that was exposed to light.
On testing these leaves for starch it was clear that photosynthesis
occurred only in the green parts of the leaves in the presence of light.
expt 2:Moll’s half-leaf experiment
1.Part of leaf is enclosed in a test tube (wide mouth bottle) containing KOH soaked
cotton (which absorbs CO2) while other half is exposed to air.
2.The set up is placed in light for some time.
3.On testing for the presence of starch later in the two parts of the leaf, you must have
found that the exposed part of the leaf tested positive for starch while the portion that
was in the tube, tested negative.
4.This showed that CO2 was required for photosynthesis.
Stephen Hales
Discovered photosynthesis – Green plants synthesize food with the help
of leaves.
*Father of Plant Physiology.
joseph priestley expt
*Revealed the essential role of air in the growth of green plants.
*Discovered oxygen.
Priestley’s Observation :-
*A candle burning in a closed space – a bell jar, soon gets extinguished.
*Similarly, a mouse would soon suffocate in a closed space.
Priestley’s Conclusion :-
*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 the candle continued to burn.
Priestley’s Hypothesis :-
*Plants restore to the air whatever breathing animals and burning candles
remove.
jan ingenhousz expt
Using a similar setup as the one used by Priestley, but by placing it
once in the dark and once in the sunlight, Jan Ingenhousz (1730-1799)
showed that sunlight is essential to the plant process that somehow
purifies the air fouled by burning candles or breathing animals.
Ingenhousz in an 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. Later he identified these bubbles to be of
oxygen. Hence he showed that it is only the green part of the plants that
could release oxygen.
Julius von Sachs
It was not until about 1854 that Julius von Sachs provided evidence
for production of glucose when plants grow. Glucose is usually stored as
starch. His later studies showed that the green substance in plants
(chlorophyll as we know it now) is located in special bodies (later called
chloroplasts) within plant cells. He found that the green parts in plants is
where glucose is made, and that the glucose is usually stored as starch.
T.W Engelmann
Using a prism he split light into its spectral components
and then illuminated a green alga, Cladophora, placed in a suspension
of aerobic bacteria. The bacteria were used to detect the sites of O2
evolution. He observed that the bacteria accumulated mainly in the region
of blue and red light of the split spectrum. A first action spectrum of
photosynthesis was thus described. It resembles roughly the absorption
spectra of chlorophyll a and b
Cornelius van Nie
a microbiologist, Cornelius van Niel (1897-1985), who,
based on 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 dioxide
to carbohydrates. This can be expressed by:
H2A+CO2—>A +CH2O+H2O
In green plants H2O is the hydrogen donor and is oxidised to O2
. Some
organisms do not release O2 during photosynthesis. When H2
S, instead
is the hydrogen donor for purple and green sulphur bacteria, the
‘oxidation’ product is sulphur or sulphate depending on the organism
and not O2
. Hence, he inferred that the O2 evolved by the green plant
comes from H2O, not from carbon dioxide. This was later proved by using
radioisotopic techniques.
(A) Chlorophylls
*Green colour pigments that occur inside chloroplast.
*Lipid in nature, insoluble in water.
Types of chlorophyll :-
*Chlorophylls are of following types :
i.Chl-a (C55H72O5N4Mg) - is universal pigment, which is found in all O2
liberating photosynthetic organisms. Its color is blue green.
ii.Chl-b (C55H70O6N4Mg) - is accessory photosynthetic pigment found
in euglenoids, green algae and higher plants. Its color is yellowish
green.
iii.Chl-c - Lacks phytol tail.
iv.Chl-d
v.Chl-e
(B) Carotenoids
*Yellow to yellow orange
*Occurs alone inside chromoplast and along with chlorophyll inside chloroplast.
*Universal in occurrence (except eubacteria)
*Chemically they are terpenes and are most stable pigments.
*Types of carotenoids-
1.Carotene → yellow orange in color
2.Xanthophyll → yellow colored (Diatoxanthin → Dinoflagellates & Fucoxanthin
→ Brown algae).
*Functions of carotenoids-
3.Accessory pigments
4.Protect Chl a from photo oxidation and also protect photosynthetic machinery by
converting lethal nascent oxygen into molecular oxygen (shield pigments).
5.Beta carotene acts as precursor of Vitamin A.
Phycobilins-
*They are water soluble pigments .
*They lack Mg and phytol tail.
*Types-
1.Phycocyanin - Blue
2.Phycoerythrin - Red
3.Allophycocyanin - Light Blue
*These occur exclusively in BGA and Red Algae as an accessory pigment
Redox potential
Redox potential is a
measure of the ease with
which a molecule will
accept electron.
describe flow of e-s in non cylic photophosphorylation
PSII - phaeophytin- Plastoquinone- Cytochrome b6f- Plastocyanin- PSI-Iron sulphur protein(Fe-S)- Ferrodoxin(Fd)- Ferrodoxin NADP reducatse(FNR)
describe flow of e-s in cylic photophosphorylation
PSI- iron sulphur protein- ferrodoxin- plastoquinone- cy. b6f-plastocyanin to PSI
