1. 3604 – Plant Physiology and
Biochemistry
Unit–01- PHOTOSYNTHESIS
By – ONKAR DINESH SHINDE
Class – TYBSc Botany
Roll. No – 213720
2. • PHOTOPHOSPHORYLATION •
The process in which energy rich ATP molecules are synthesized using solar energy is called
photophosphorylation .
ADP + iP → ATP
There are two types of Photophosphorylation : Non cyclic and Cyclic Photophosphorylation.
• NON CYCLIC PHOTOPHOSPHORYLATION •
• This involves photolysis of water and a continuous unidirectional flow of electrons from water to PS
– II to PS – I via cytochromes and finally to NADP.
• Electrons expelled from chlorophyll – a never come back to the same chlorophyll – a.
• The pigments in PS – II absorb energy of different wavelengths of light which is funneled to the
reaction centre chl-a or P680.
• The energy rich electrons are expelled from P680 and taken up by Plastoquinone (PQ) through
cytochrome quinone (CO-Q) leaving the reaction centre ionized or positively charged and PQ gets
reduced.
3. The energy rich electrons roll down the chain of various electron carriers ( cyt – b6 , cyt – f ,
plastocyanin ) then transferred to reaction centre of PS – I i.e. P700.
During transfer of electrons from cyt – b6 to cyt – f the energy released is used in synthesis of ATP.
The energy rich electrons are expelled from excited chl – a molecule and these are taken up by iron
containing red protein called Ferredoxin(Fd) through Ferredoxin Reducing substance.
From reduced Ferredoxin these electrons are taken up by Coenzyme NADP .
NADP retains electron and becomes negatively charged , while due to loss of electrons P700
becomes positively charged or ionized .
PS II replaces the electrons in PS I as described earlier.
• PHOTOLYSIS OF WATER •
• P680 is left ionized after donating electrons to PS-I , acts as strong oxidizing agent and brings
about splitting of water molecules to release oxygen .
• This light dependent splitting of water molecules is called as the photolysis of water.
• Magnesium , calcium and chloride ions play an important role in photolysis of water.
• 4H2O → Light + chlorophyll → 4e- + 4H+ + 2H2O + O2
4.
5. • CYCLIC PHOTOPHOSPHORYLATION •
This involves only PS I and the flow of electrons is in cyclic manner as same expelled electrons come
back to the reaction centre.
Photolysis of water is not associated with this.
The pigments in PS I absorb energy of different wavelengths of light which is funneled to the reaction
centre, specific molecule of chlorophyll – a , i.e. P700.
It gets excited and expels energy rich electrons, which are accepted by an iron containing red protein
called Ferredoxin (Fd) via an unknown electron acceptor called Ferredoxin reducing substance ( FRS )
P700 becomes positively charged or left ionized. These electrons then roll down the energy gradient
through a series of electron carriers such as cyt –b6 , cyt – f and Plastocyanin ( PC ). From
plastocyanin the electrons are transferred back to P700 . Thus flow of electrons is in closed circuit.
During the transfer of electrons from cytochrome b6 to cytochrome-f and Ferredoxin to cyt – b6 energy
from electrons is released and is used in synthesis of ATP.
Cyclic photophosphorylation is a significant process as it produces additional ATP molecules required
for synthesis of glucose.
Synthesis of one glucose molecule requires 18 ATP and 12 NADPH2 molecules.
6.
7. • C3 PATHWAY/ CALVIN CYCLE •
This the second phase of photosynthesis in which CO2 is fixed or reduced to
glucose . It occurs in the Stroma of Chloroplast.
It is independent of light ( does not require direct light ) hence called dark reaction.
The products of light reaction namely ATP and NADPH2 are used here .
CO2 + 2NADPH2 + 2ATP → (CH2O) + H2O+ 2NADP + 2ADP + 2iP
The first stable product is a 3- Carbon compound, hence Dr.Calvin called it C3 cycle
or C3 pathway
The cycle involves following three main steps :
1. Carboxylation 2. Reduction
3A.Synthesis 3B. Regeneration
8.
9. 5/6 th part of PGAL i.e. out of 12 molecules, remaining 10 molecules are
used for regeneration of 6 molecules of Ribulose Monophosphate
(RUMP).
For Regeneration of 6RUBP , 6 ATPs are required. Hence , RUBP is
regenerated and the process keeps going on.
12 NADPH2 and 18ATP are required for synthesis of 1 Glucose
molecule.
• The plants that fix Atmospheric CO2 through Calvin Cycle are called as C3
PLANTS.
• RUBISCO is thermoliable and requires higher concentration of CO2 for its
activity. At high temperatures the stomata closes partially so that the
availability of CO2 falls.
• At high temperature and low CO2 concentration, RUBPcarboxylase
functions as oxygenase and brings about oxidation of RUBP instead of
carboxylation .
10. • PHOTORESPIRATION •
The process of respiration (oxidation) that is initiated in the chloroplast and takes place only during
day is called as photorespiration .
RUBISCO is the most abundant enzyme in the world and its active site can bind to both CO2 and
O2.
At high temperatures oxidation of RUBP by O2 takes place, which results in the formation of one
molecule of 2 Compound, phosphoglycolate and one molecule of PGA .
Phosphoglycolate gets dephosphorylated to form glycolate within the chloroplast. The glycolates
then gets diffused into peroxisomes where it is oxidised to glyoxylate and then gets converted into
an amino acid glycine (2C).
Glycine enters mitochondria and two molecules of glycine gives rise to one molecule of Serine (3C)
and one CO2.
Serine is taken up by peroxisomes and gets converted into glycerate . The glycerate enters the
chloroplast and gets phosphorylated to form PGA and enters the Calvin cycle .
It protects C3 plants from photo oxidative damage and in C4 plants photorespiration is avoided .
11.
12. • C4 PATHWAY •
In certain tropical plants, CO2 is not directly absorbed by RUBISCO .Due to low CO2
concentration they follow another pathway . In this another pathway, the first stable
compound is a 4- C compound Oxaloacetic acid and hence it is called C4 pathway .
Plants like – Maize , sugarcane, jowar and amaranthus .etc have this C4 pathway,
hence all such plants are called as C4 plants.
C4 pathway works entirely on Krantz Anatomy.
The Enzyme PEP carboxylase is present in mesophyll chloroplast and RUDP or
RUBP carboxylase (RUBISCO) in bundle sheath chloroplast.
In C4 plants , CO2 fixation takes place twice , in two different cells during the day .
PEP Carboxylase can pick up CO2 at very low concentration and C4 plants can
photosynthesize in high light intensity, high temperature and less amount of water.
13. A) In Mesophyll Cells -
Atmospheric CO2 is accepted by PEPA a three carbon compound present in mesophyll
chloroplast. In presence of water and enzyme PEP carboxylase , PEPA gets
carboxylated to form Oxalo Acetic Acid (OAA) a four Carbon compound.
PEP carboxylase can even function in low atmospheric CO2 concentration.
OAA is reduced to malic acid in the presence of NADPH2 and enzyme Malate
dehydrogenase.
B) In Bundle Sheath cells -
• Malic acid is transported to chloroplast of bundle sheath cells. In these agranal
chloroplast malic acid undergoes decarboxylation in presence of NADP to form pyruvic
Acid and CO2 is released.
• The CO2 released by second CO2 acceptor RUBP and is fixed by C3 pathway in the
agranal chloroplast of bundle sheath cells .
• Pyruvic Acid produced due to decarboxylation of malic acid is transported back to
mesophyll cells and is phosphorylated by ATP to form PEPA
14.
15. • CAM PATHWAY •
In certain plants like succulents and plants which grows in dry conditions ( xerophytes )
stomata remain closed in these plants during the day to check the loss of water due to
transpiration. Mechanism of photosynthesis in these plants is different and it is called as
Crassulacean Acid Metabolism.
In these plants CO2 is taken up during night when stomata is open by PEPA and
Oxaloacetic acid is formed.
Malate gets accumulated during the night. During the day, stomata are closed, malate
undergoes decarboxylation and pyruvate is formed.
CO2 released enters the Calvin Cycle and sugar is produced, which gets converted into
Starch. Thus starch gets accumulated during the day.
During Night , from starch , PEPA is regenerated
In CAM plants, C4 pathway occurs during night and C3 occurs during day. Acid
concentration increases during the night and decreases during day . This diurnal
fluctuation in acid concentration is characteristic feature of CAM plant and Krantz
anatomy is absent.
16. • BACTERIAL PHOTOSYNTHESIS •
The Process of Photosynthesis that occurs in Bacteria are called as Bacterial photosynthesis.
A) Anoxygenic Photosynthesis –
• No evolution of Oxygen takes place
• Carried out by Purple and green Sulphur Bacteria.
• Only one photosystem is involved.
• Hydrogen Sulphide is the electron donor.
• Equation – 2H2A + CO2 → C(H2O)+H2O +2A
B) Oxygenic Photosynthesis –
• During light energy the electrons are transferred from water ( H2O )to Carbon dioxide ( CO2 ) to form
Carbohydrates .
• CO2 is reduced and H2O is oxidized
• Evolution of Oxygen takes place.
• Equation – 6CO2 + 12 H2O →Light energy →C6H12O6+ 6H2O + 6O2
• Most Common and seen in plants , algae and Cyanobacteria.
