Photosynthesis is the process by which plants convert carbon dioxide and water into glucose and oxygen using light energy, occurring in chloroplasts. It involves two main reactions: light-dependent reactions (cyclic and non-cyclic photophosphorylation) occurring in the thylakoid membranes, and light-independent reactions (dark reactions) happening in the stroma. Key components include photosystems I and II, which facilitate light absorption and energy transfer.

1.  Photosynthesis is the formation of sugars(g
lucose) with the help of CO2 and H2O in p
resence of light
Photosynthesis
Light and Dark reaction
 Photosynthesis is the formation of sugars(
glucose) with the help of CO2 and H2O in
presence of light
 Why plants are in green? Plants absorbs bl
ue and red light from sun and reflects gre
en light.
 Photosynthesis is the formation of sugars(
glucose) with the help of CO2 and H2O in p
resence of light
6CO2 + 6H2O C6H12O6 + 6O2
 It takes place in chloroplast.

3.  Photosynthesis
Light reaction
Dark reaction
LIGHT REACTION DARK REACTION
Light dependent reaction
.
Light independent reacti
on. Carbon assimilation r
eaction
Occur in grana. Occur in stroma.
Includes cyclic & non cyc
lic photophosphorylation.
Include C3 cycle.
C4, C2, CAM pathway
Light is absorbed & conv
erted to ATP & NADPH.
ATP & NADPH are utiliz
ed and triose phosphate,
starch& sucrose are form
ed
LIGHT REACTION DARK REACTION

4. PHOTOSYSTEMS (PIGMENT SYSTEMS)
 Photosynthesis is driven by 2 photochemical p
ocesses (cyclic & non cyclic)
 These are associated with 2 pigment systems o
r photosystems
 Photosystems are the light harvesting complex
 Made up of 250-400 pigment molecules, access
ory pigments and associated proteins.
 Photosystems are found on the thylakoid mem
brane of the chloroplast in plants and algae an
d in the cell membrane of photosynthetic bact
eria
 Function:
 Absorption of light energy and
 Transfer of energy & electrons
 Each photosystem consists of 2 closely linked c
omponents
 Reaction center (chl a) and
 antenna molecules (accessory pigmen
ts.)

5.  Photosystems are two types Photosystem I (
PS I / P700) and Photosystem II (PS II / P68
0)
Photosystem I
(PS I / P700)
Photosystem II
(PS II / P680)
Maximum absorption at 700n
m or far red light region or ph
otocenter is p700
Maximum absorption at 680nm o
r red light region or photocenter i
s p680
Pigments absorbs long wavele
ngth of light
Pigments absorbs shorter wavele
ngth of light
It has FeS type reaction center It has quinone type reaction cente
r
Rich in chl a Rich in chl b
Located on exposed part of gr
ana thylakoid membrane (inne
r part)
Located in the stroma exposed me
mbrane (outer part)
Participate in both cyclic and
non cyclic photophosphorylati
on
Participate only in non cyclic pho
tophosphorylation
Not associated with photolysis
of water
Associated with photolysis of wate
r
Involved in production of ATP
in cyclic and NADPH in non c
yclic photophosphoration
Involved in ATP production
PS I in cyclic photophosphory
lation
PS I in non-cyclic phot
ophosphorylation
PS II in non-cyclic phot
ophosphorylation

6. LIGHT REACTION
 Dependent on light
 Also called Hill’s reaction
 Takes palce in thylakoid membrane of me
sophyll cells
 Includes cyclic & non- cyclic photophosph
orylation
 Involves 2 pigment system, PSI &PSII
 ATP, NADPH & molecular oxygen (OEC) ar
e produced
CYCLIC PHOTOPHOSPHORYLATION
 Light absorbed by reaction centre of PSI
 Electron passed to primary e acceptor FR
S (ferridoxin reducing substance)
 Electrons are passed throgh e carriers an
d returned back to PSI
 ATP are produced.
1.
PHOTOPHOSPHORYLATION: Process of fo
rmation of ATP from ADP in presence of li
ght in chloroplast.

7. NON CYCLIC PHOTOPHOSPHORYLATION
 Also called Z SHEME (ZIG ZAG)
 Photons of light is absorbed by reaction c
enter of PSII and the electrons are excited
.
 The electrons are replaced by electrons g
enerated from photolysis of water and pr
oduces molecular oxygen
 Electrons are then passed through carriers
to reach reaction centre of PSI
 This generates a proton gradient and it dr
ives the ATP synthase complex and produ
ces ATP.
 The PSI absorbs photons of light and elec
trons are excited.
 The high energy electron is then absorbe
d by ferridoxin
 The electron transport reduces NADP to
NADPH by activating NADP reductase enz
yme.
2.
The electron removed is replaced by electr
on from PSII
NADP + H+ NADPH
Electron carriers of PSII:
Pheophytin ->plastoquinone -> cytochrome b6-f ->plastocyanin
Electron carriers of PSI:
Membrane bound iron sulphur protein -> ferridoxin

8. Thylakoid
e
e
e
e
H
+
H+
H+
e
e
NADP + H+ ->
NADPH
STROMA
ATMOSPHERE

9. CYCLIC PHOTOPHOSPHORYLATION NON -CYCLIC PHOTOPHOSPHORYLATION
Only PSI is involved Both PSI and PSII are involved
The electron excited from reaction center ret
urn back to the reaction center of PSI
The electron excited from reaction center d
oesnot return back to the reaction center
Only ATP is produced ATP, NADPH and O are produced and transf
erred to stroma for dark reaction
No requirement of water Requires water for photolysis

10. ELECTRON CARRIERS
Ferridoxin : Small, water soluble, iron sulph
ur protein
Plastocyanin: Cu containing protein, blue co
lored, 8 stranded antiparallel beta barrel co
ntainng 1 Cu center
Plastoquinone: Isoprenoid quinone molecule
, PQ A, PQB, PQ C and PQ D, structurally si
milar to ubiquinone
Cytb6f: Found in thylakoid membrane
Pheophytin: Chlorophyll molecule lacking ce
ntral Mg2+ ion. Dark bluish waxy pigment
INHIBITORS OF LIGHT REACTION
COMMERCIAL HERBICIDES
DCMU: 3-(3,4- DICHLOROPHENYL) 1,1- DI
METHYL UREA
CMU: 3,4- CHLOROPHENYL 1,1- DIMETHYL
UREA