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Photosynthesis : Photosynthesis is an enzyme regulated anabolic process
of manufacture of organic compounds inside the chlorophyll containing cells
from carbon dioxide and water with the help of sunlight as a source of energy.
Historical Perspective
Joseph Priestley (1770) : Showed that plants have the ability to take up
CO
2
from atmosphere and release O
.
Jan Ingenhousz (1779) : Release of O
2
by plants was possible only in sunlight
and only by the green parts of plants.
Theodore de Saussure (1804) : Water is an essential requirement for photosynthesis
to occur.
Julius Von Sachs (1854) : Green parts in plant produce glucose which is
stored as starch.
T. W. Engelmann (1888) : The effect of different wavelength of light on
photosynthesis and plotted the first action spectrum of photosynthesis.
C. B.
Van Niel (1931) : Photosynthesis is essentially a light dependent reaction
in which hydrogen from an oxidisable compound reduces CO
to form
sugar. He gave a simplified chemical equation of photosynthesis.
Hill (1937) : Evolution of oxygen occurs in light reaction.
Calvin (1954-55) : Traced the pathway of carbon fixation.
Hatch and Slack (1965) : Discovered C
4
pathway of CO
fixation.
Site for photosynthesis : Photosynthesis takes place only in green parts of
2
the plant, mostly in leaves. Within a leaf, photosynthesis occurs in mesophyll
cells which contain the chloroplasts. Chloroplasts are the actual sites for photosynthesis.
The thylakoids in chloroplast contain most of pigments required for
capturing
solar energy
to initiate photosynthesis. The membrane system (grana)
2 is responsible for trapping the light energy and for the synthesis of ATP and
NADPH. Biosynthetic phase (dark reaction) is carried in stroma.
Chlorophyll a : (Bright or blue green in chromatograph). Major pigment, act as reaction centre, involved in trapping and converting light into chemical energy.
Chlorophyll b : (Yellow green)
Xanthophyll : (Yellow)
Carotenoids : (Yellow to yellow-orange)
In the blue and red regions of spectrum shows higher rate of photosynthesis.
Light Harvesting Complexes (LHC) : The light harvesting complexes are
made up of hundreds of pigment molecules bound to protein within the
photosystem I (PSI) and photosystem II (PSII). Each photosystem has all the
pigments except one molecule of chlorophyll ‘a’ forming a light harvesting system
(antennae). The reaction centre (chlorophyll a) is different
in both the photosystems.
Photosystem I (PSI) : Chlorophyll ‘a’ has an absorption peak at 700 nm
(P700).
Photosystem II (PSII) : Chlorophyll ‘a’ has absorption peak at 680 nm
(P680).
Process of photosynthesis : It includes two phases - Photochemical phase
and biosynthetic phase.
(i) Photochemical phase (Light reaction) : This phase includes - light
absorption, splitting of water, oxygen release and formation of ATP and
NADPH.
(ii) Biosynthetic phase (Dark reaction) : It is light independent phase,
synthesis of food material (sugars).
Photophosphorylation : The process of formation of high-energy
chemicals (ATP and NADPH).
Cyclic photophosphorylation : Two photosystems work in series − First
PSII and then PSI. These two photosystems are connected through an electron
transport chain (Z. Scheme). Both ATP and NADPH + H
are synthesised by this
process. PSI and PSII are found in lamellae of grana, hence this process is
carried here.
Non-cyclic photophosphorylation : Only PSI works, the electron
circulates within the photosystem. It happens in the stroma lamellae (possible
location) because in this region PSII and NADP rectase enzyme are absent. Hence
only ATP molecules are synthesised.
The electron transport (Z-Scheme) : In PS II, reaction centre (chlo. a)
absorbs 680 nm wavelength of red light which make the electrons to become
excited. These electrons are taken up by the electron acceptor that passes them
to an electron transport system (ETS) consisting of cytochromes. The
movement of electron is down hill. Then, the electron pass to PSI and move
down hill further.
The splitting of water : It is linked to PS II. Water splits into H
, O and
electrons.
Chemiosmotic Hypothesis : Chemiosmotic hypothesis explain the
mechanism of ATP synthesis in chloroplast. In photosynthesis, ATP synthesis is
linked to development of a proton gradient across a membrane. The electrons
are accumulated inside of membrane of thylakoids (in lumen). ATPase has a
channel that allows diffusion of protons back across the membrane. This
releases energy to activate ATPase enzyme that catalyses the formation of ATP.
Plants : ATP and NADH, the products of light reaction are used in synthesis of
Biosynthetic phase in C
food. The first CO
2
3
fixation product in C
plant is 3-phosphoglyceric acid or
PGA. The CO
2
3
acceptor molecule is RuBP (ribulose bisphosphate). The cyclic
path of sugar formation is called Calvin cycle on the name of Melvin Calvin, the
discoverer of this pathway.
Calvin cycle proceeds in three stages :
(1) Carboxylation : CO
combines with ribulose 1, 5 bisphosphate to
form 3 PGA in the presence of RuBisCo enzyme.
2
(2) Reduction : Carbohydrate is formed at the expense of ATP and
NADPH.
(3) Regeneration : The CO
acceptor ribulose 1, 5-bisphosphate is formed
again .
2
6 turns of Calvin cycles and 18 ATP molecules are required to synthesize
one molecule of glucose.
The C
4
pathway : C
plants have special type of leaf anatomy, they tolerate
higher temperatures. In this pathway, oxaloacetic acid (OAA) is the first stable
product formed. It is 4 carbon atoms compound, hence called C
4
pathway (Hatch
and Slack Cycle).
The leaf has two types of cells : mesophyll cells and Bundle
sheath cells (Kranz anatomy). Initially CO
2
4
is taken up by phosphoenol pyruvate
(PEP) in mesophyll cells and changed to oxaloacetic acid (OAA) in the presence
of PEP carboxylase. Oxaloacetate is reduced to maltate/asparate that reach into
bundle sheath cells.
and
formation of pyruvate (3C). In high CO
The oxidation of maltate/asparate occurs with the release of O
concentration RuBisCo functions as
carboxylase and not as oxygenase, the photosynthetic losses are prevented. RuBP
operates now under Calvin cycle and pyruvate transported back to mesophyll
cells and changed into phosphoenol pyruvate to keep the cycle continue.
2 Photosrespiration : The light induced respiration in green plants is called
photorespiration. In C
3
plants some O
2
binds with RuBisCo and hence CO
fixation is decreased. In this process RuBP instead of being converted to 2
molecules of PGA binds with O
to form one molecule of PGA and
phosphoglycolate.
2
Law of Limiting Factors : If a chemical process is affected by more than
one factor, then its rate will be determined by the factor which is nearest to its
minimal value. It is the factor which directly affects the process if its quantity is
changed.
Factors affecting photosynthesis :
1. Light
2. Carbondioxide
3. Temperature
4. Water
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