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Overview of mechanism of photosynthesis
Process of photosynthesis
C4 and CAM pathway
OVERVIEW OF PHOTOSYNTHESIS
A process by which autotrophic organisms use
light energy to make sugar & oxygen gas from
carbon dioxide & water.
Occurs in chloroplast, an organelle in mesophyll
PHOTOSYNTHESIS Divided into 2 phases :
a) photolysis of water
Dark reaction (carbon fixation or calvin cycle)
Light reaction is light dependent & takes place in
grana of chloroplast & its product is NADPH & ATP.
Dark reaction is light independent & occurs in
stroma of chloroplast.
FOUR PHASES OF
PHOTOSYNTHESIS 1) light absorption & energy delivery by antenna
2) primary electron transfer in reaction centers
3) energy stabilization by secondary processes
4)synthesis & export of stable products
First 3 phases makeup the light reaction & fourth
encompasses the dark reaction.
PIGMENTS Substances that have ability to absorb specific
wavelengths of light & reflect all others.
Pigments are colored.
Easily excited by light energy.
The color we see is the net effect of all the light
reflecting back at us…!
photosynthetic pigments are of 3 types
1)Chlorohylls (chlorophyll a & b)
2)Accessory photosynthetic pigment or carotenoids
(carotene & xanthophyll)
Most important pigment in photosynthesis.
Absorbs blue,red & violet wavelengths in the visible
Formula is C55H72O5N4Mg
Complex ring structure having 2 parts i.e. head & tail
Head(porphyrin ring) : 4 complex pyrole rings of carbon
In the centre of porphyrin ring a single magnesium atom
is attached to the nitrogen of each pyrrole ring.
Maximum absorption by chlorophyll a occurs in blue &
Tail is attached to one of the pyrrole rings.
It is a long hydrocarbon phytol (C20H39) & anchors
chlorophyll molecule in thylakoid membrane.
Its structure is similar to chlorophyll a but CH3 is
replaced by CHO.
So molecular formula is C55H70O6N4Mg
It primarily absorbs blue light .
It is used to complement absorption spectrum of a
by extending the range of light wavelengths a
photosynthetic organism is able to absorb.
They are not directly involved in LDR .
includes carotenes & xanthophylls.
carotenes are hydrocrbons with general molecular
They absorb wavelengths that are not efficiently
absorbed by chloropyhlls .
Carotenoid is yellow to orange in color
Xanthophyll is yellow in color.
Carotenoids have 2 important roles in plants
1)transfer the light energy they capture to
chlorophyll to use in the LDR.
2)Protect chlorophyll a from photo-oxidation
Photosynthesis occur in two phases :
Phase 1 : Light reaction ( granna -thalakoid
Phase 2 : Dark reaction( stroma)
6 CO2 + 6 H2O sunlight C6 H12 O6 + 6 O2
carbon dioxide + water = glucose(sugar ) + oxygen
PHASE 1.LIGHT REACTION
Also called Light Dependent Reaction which contain Photosystem I
and Photosystem II.
Occurance: chloroplast (granna - thylakoid)
Chlorophyll (thylakoid) traps energy from light
Requirement light ,water
NADP + ADP + Pi
Products O2 ATP & NADPH
Energy is Capture from Sunlight.
light energy, trapped by chlorophyll, is used
Step 1 photolysis of water
Water is Split into Hydrogen Ions ,electron and Oxygen (O2).
The O2 Diffuses out of the Chloroplasts (Byproduct).
Formation of reduced NADPH ( chemical energy )
Step 2 phosphorylation
The Light Energy is Converted to Chemical Energy, which is Temporarily Stored
in ATP (ADP + Pi + energy )
PHASE 2. DARK REACTION
Other names :
Light Independent Reaction
Carbon Fixation or C3 Fixation
Does not require light
Location Occurs in stroma of chloroplast
ATP and NADPH as a fuel
CO2 (air )
Product glucose sugar
CO2 fixation in glucose sugar
The Chemical Energy Stored in
ATP and NADPH powers the
formation of Organic Compound
glucose (Sugar) using CO2
In the process of photosynthesis, the phosphorylation
of ADP to ATP using the energy of sunlight is called
In photophosphorylation, light energy is used to create
a high-energy electron donor and a lower energy
electron acceptor. Electrons then move spontaneously
from donor to acceptor through an electron transport
TYPES OF PHOTOPHOSPHORYLATION
There are two types of photophosphorylation
In bacterial photosynthesis, a single photosystem is
When an electron is energized by absorption of light, it is
ejected from the photosystem reaction centre.
The electron then passes down through an electron
transport system, and finally to the reaction centre.
The energy released during this electron transport is used
to produce ATP.
Since the excited electron returns to the reaction centre,
this mechanism for making ATP is called cyclic
No reducing power needed for biosynthesis generated in
Plants and cyanobacteria utilize two photosystems
which work sequentially to produce both energy and
First, a photon of light ejects a high electron from
The electron lost from photosystem II does not return
to photosystem II, but is replaced by an electron
generated from the enzymatic splitting of water and
the release of oxygen.
The electron then travels from the excited reaction
centre of photosystem II down an electron transport
chain and finally to the reaction centre of photosystem
This transport system generates a photon motive force
that is used to produce ATP.
Since the excited electron does not return to
photosystem II, this mechanism for making ATP is
called non-cyclic photophosphorylation.
Photosystems are involved in light reaction
Photosystems are functional and structural units of protein complexes
involved present in thalykoid membrane of GRANUM.
Both photosystems work together.
Together carry out the primary photochemistry of photosynthesis: the
absorption of light and the transfer of energy and electrons.
They are found in the thylakoid membranes of plants, algae and
cyanobacteria (in plants and algae these are located in the chloroplasts),
or in the cytoplasmic membrane of photosynthetic bacteria.
There are two kinds of photosystems: II and I.
The flow of electrons occurs in two ways
---Non-Cyclic Pthway: passes through both photosystems (Z-
---Cyclic Pathway : occurs only in PSI.
Each photosystem consists of
Antenna Complex :
Cluster of Chlorophyll
a,b and carotenoid
molecules which gather
light ad transfer it to
Reaction Centre: Has
chlorophyll "a" molecules
different from other
Absorbs light of 700nm
Water splits, electrons enter PSII reaction centre.
When P680 gains energy, 2 electrons become excited and
leave the molecule.
The electrons are readily captured by primary electron
Primary electron acceptor>> Plastoquinone>>Cyt b>>Cyt f>>
As the electron pass through the electron trasport chain its
energy is released and is used by thylakoid membrane to
PHOTOSYSTEM I (P680)
Absorbs light of 680nm
The electrons from photosystem II finally
reaches the reaction centre of photosystem I.
Then electrons move to the primary electron
From here they pass to Ferrodoxin (Fd).
The elctrons then passes from Ferrodoxin to
NADP,taking H+ and form NADPH2.
OVERALL RESULT OF Z-SCHEME.
The result of the non-cyclic electron flow is that water is oxidzed
yielding H+,e- and O2.
ATP is produced.
NADP+ becomes NADPH2.
The hydrogen and Energy of NADPH2 and ATP produced in the light
reaction are used in dark reaction.
DARK REACTION OF
The Calvin Cycle
the free energy of cleavage of ~P bonds of ATP,
and reducing power of NADPH, are used to fix
and reduce CO2 to form carbohydrate.
Carbon atoms from CO2 are bonded, or fixed,
into organic compounds = carbon fixation.
THIS OCCURS IN THE STROMA
An enzyme (rubisco),
combines CO2 with a 5-
carbon sugar RuBP
The product, 6-C
2, -3C molecules
PGA is converted to
another 3- Carbon
molecule PGAL in a 2
Each PGA receives a
P group from ATP
compound receives a
releases the P,
( ADP & NADP+
return to light rxn., to
make ATP and
Most of the PGAL
is converted back to
Requires a P
Some PGAL leave
and used by plants
BALANCE SHEET FOR PHOTOSYNTHESIS
How much ATP & NADH are required to make 1 molecule
of PGA from carbon dioxide?
Each turn fixes one CO2
PGAL is a 3-C molecule (takes 3 turns to make each
Each turn of the cycle:
3 ATP ( 2 in step 2 & 1 in step 3)
2 NADPH (step 3)
C4 PLANT OR PHOTOSYNTHESIS
What is C4 plant or photosynthesis ?
Present particularly in monocot
Presence of Kranz anatomy
PEP is the initial acceptor
First product is OAA ( 4 carbon compound /
Oxygen does not inhibit this process
Photorespiration is low
Rate of transpiration is low
No wastage of carbon dioxide
Well adapted in xeric condition
Distinct division of labour
Requires more ATP than C3
It is an adaptation of plants in arid condition
CAM plants use both C3 and C4 pathways
CAM plants use only mesophyll cells
Stomata are close during the day
Prevents carbon dioxide entrance as well while
preventing water loss
Stomata opens at night in cooler temprature
CAM PATHWAY (CONT.)
Carbon dioxide diffuses
Carbon dioxide combines with PEP forming OAA
OAA reduce to form Malate
Malate stored in vacuoles
Photosynthesis commences during the day
Transport of malate to cytoplasm
Formation of pyruvate
Entrance of carbon dioxide to chloroplast
Formation of starch
Fleshy succulants (stone crops )