Chlorophyll pigments are
present in a cell organelles
Four 5-membered rings, called pyrolle
• central core oF magnesium.
• a side chain, called the phytol chain,
extends From one oF the pyrolle rings
in the chlorophyll molecule.
• chl a and chl b are similar in structure
except that the-cho group in
chlorophyll a is replaced with –ch3
pigment is a substance that
absorbs light oF diFFerent
chlorophyll absorbs light in the
violet and blue wavelengths an
also in the red region oF the visible
spectrum oF light.
this portion oF the spectrum
between 400nm and 700nm is also
known as photo synthetically
• They absorb light of different
wavelengths and funneled to
• I t is a major pigment the reaction centre (accessory
converts the light pigments or antennae
energy into electrical molecules)
• They also protect the
chlorophyll from oxidation.
photo system photo system ii
• it includes chl a • it includes chl a
with maximum with maximum
absorption at absorption
accessory er accessory
• they are present • they are present
in stroma in the oppressed
thylakoids & non regions oF grana
There are two steps of
• Light reaction
• Dark reaction
• Chlorophyll absorbs light energy in the
form of photons, get excited and release
• Electron passes through electron
• NADP reduces to NADPH+ H+
• Energy release in the form of ATP
• ADP+Pi ATP
Formation oF atp in
the presence oF
it occurs in two
• it includes two photo systems-psi and psii.
• when psii(p680)absorbs light , excited and its
electrons are transFerred to an electron
• now psii becomes a strong oxidizing agent and
splits a molecule oF water(photolysis oF
water).psii restores the electrons From the water
• the primary e- acceptor (phepophytin in plants) is
• the reduced acceptor is strong a reducing agent
now denotes the e- to an electron transport chain,
consisting oF plastoquinone, cytochrome complex
and plastocyanin to psi(p700).
• p700 is excited by absorbing light and emits
electron which is accepted by primary electron
• the primary electron acceptor is reduced. it
• splitting oF water in presence
h 2o 2h + + 1 / 2 o 2 + 2e -
• the electrons generated in
this process are used to
replace the electrons lost by
• the protons are used For
reducing nadp to nadph.
• oxygen is liberated as a
When chloroplast is illuminated with light greater than
680nm,only PSI is activated, as it has a maximum absorption
at 700nm,and PSII absorbs at 680nm.remains inactivated.
In cyclic only PSI is involved. When it is activated by light,
it emits electrons.
These electrons are accepted by the primary acceptor, then
transfer to ELECTRON TRANSPORT CHAIN i.e.
Ferridoxin,Plastoquinone, cytochromeb6&bf and then
finally come back to PSI(P700).
This downhill movement of electrons from an electron
acceptor to P700 results in the formation of ATP from ADP,
and is termed as cyclic photophosphorylation.
In cyclic photophosphorylation oxygen is not released (as
there is no photolysis of water ) and NADPH2 is also not
• carbon dioxide is reduced to
carbohydrates (carbon Fixation).
• carbon Fixation occurs in the
stroma oF chloroplast.
• the process makes use oF the atp
and nadph produced in the light
it consists oF three phases.
• regeneration oF rubp
the First acceptor oF co2 is ribulose 1,5
biphosphate.6 molecules oF co2 reacts with
molecules oF 6 rubp (ribulose bi phosphate) to
Form a short lived 6c compound.
the reaction is catalyzed by rubp carboxylase
6c compound breaks into 2 molecules oF 3c-
compound i.e.phosphoglyceric acid.
pga is the First stable compound in this pathway.
12 molecules of PGA is converted into 12 molecules of 1,3-di-
phosphoglycerate and then reduced to phosphoglyceraldehyde
(PGAL) using ATP and NADPH.
Two molecules of PGAL are used for the synthesis of glucose
molecule than starch.
10 molecules of PGAL by a series of complex reactions are
converted into 5Ccompound,RUBP.
The formation of 6 molecules of RuBP needs 6 ATP molecules.
Total 18 ATP and 12NADPH is required in the synthesis of one
molecule of Glucose.
–Hatch and Slack Pathway:
This pathway of carbon fixation occurs in plants
like maize, sugarcane, pearl millet and amaranth.
These plants have two types of photosynthetic cells
i.e. mesophyll cells and bundle sheath cells (Kranz
The chloroplast are dimorphic i.e. those in the
mesophyll cells are granal and those in the bundle
sheath cells agranal.
Phospho-enol-pyruvate(PEP) is the CO2 acceptor ,
present in the mesophyll cells and the reaction is
catalyzed by the enzyme PEP case .
The first stable product is oxalo acetic acid (OAA) which
is a 4C-compound
OAA is converted into malic acid and transported to
bundle sheath cells where it is decarboxylated into pyruvic
The CO2 librated is used for Calvin cycle in the bundle
Pyruvic acid is transported back to mesophyll cells
where it converts into PEP. During this conversion 2 ATP
C4 plants are special they tolerate higher temperature, they show a response to highlight
intensity , they lack the process called photorespiration and have greater productivity of biomass.
Characteristics C3plants C4plants Choose from
the Calvin cycle Cell type in Mesophyll/bundle
which takes place sheath/both
Cell type in which the initial Mesophyll/bundle
carboxylation reaction sheath/both
How many cell types does the leaf
have that fixCO2 2. .Mesophyll
Which is the primary CO2 RuBP/PEWP/PGA
Number of carbons in the 5/4/3
primary CO2 acceptor
Which is the first stable PGA/OAA/RuBP/PEP
Does the plant have PEP Yes/no/not always
• At high temperatures and limited CO2 supply Rubisco enzyme
binds with O2 instead of CO2 in C3 plants and enters in the cycle
•It involves interaction pf three organelles-chloroplast,
peroxisome and mitochondria.
• RUBP+O2 Phosphoglycote+PGA
•Phosphoglycolate passes through series of reactions and finally
•75% of the carbon lost by the oxygenation of RUBP is recovered
and 25% is lost as release of one molecule of CO2.
•No doubt it is a wasteful process but it has a useful
role in protecting the plants from oxidative damage
light-the chlorophylls absorb
mostly the blue and red regions oF
the spectrum. the rate oF
photosynthesis increases at the
lower intensities oF light and
decreases at higher intensities.
reason: i-other Factors required
For photosynthesis become limiting.
2. destruction oF chlorophyll
•the rate oF photosynthesis increases
with increase in temperature up to a
maximum oF 30oc. however the rate
starts decreasing iF the temperature
rises beyond 3o0c.
•reason: photosynthesis involves number
oF enzymes. the Function oF enzyme is
maximum at optimum temperature. low
temperature inhibits the rate oF
photosynthesis as it lowers the activity
oF enymes.high temperature inhibits the
rate oF photosynthesis as as it denatures
• The rate of photosynthesis increases with an increase in
carbon dioxide concentration up to a certain level. Beyond
that, carbon dioxide concentration has no effect on the rate of
• In C3 plants rate of photosynthesis increase up to 500µl.l-1
but in C4 plants up to300µl.l-1due to two factors:
• 1.high availability of substrate for the carboxylation reaction.
• 2. reduced photorespiration due to more available CO2to
• The rate of photosynthesis is slow in water
deficient conditions because of two
• 1. Under water deficient conditions
stomata remain closed to reduce the rate
of transpiration. This reduce or stops the
entry of CO2 into the leaves.
• 2.Reduced water potential.
To make a temporary stained mount of lily and petunia leaf
and count the number of stomata on upper and lower
surface of epidermis