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1. Antenna Complex
LIGHT-HARVESTING COMPLEX

2. Introduction to Photosynthesis:
 It is an energy driven bio-chemical process.
 It involves the reduction of energy-poor inorganic compounds (C&H) to energy-rich
carbohydrates

3.  There are mainly two process in which the photosynthesis is completed:
Light-Dependent Reactions:
 Non-cyclic Z-scheme
 Photolysis of Water
Light-Independent Reaction:
 Calvin-Malvin Cycle
 Mechanisms of carbon concentration

4. Light-Harvesting Antenna Complex
 The antenna complex also known as the light harvesting complex.
 It is an association of protein and chlorophyll molecules
 It transfers the light energy to chlorophyll-a molecules present in the reaction center of the
associated photosystem.
 It is the part of Light-Dependent Reactions.
 It acts as an assistance to the reaction center in gathering and absorbing more of the incoming
light.

5. Function & Pigments:
Function:
 Absorption of light
 Broaden the absorption spectrum
 Transfer of energy to reaction center
 Protection of plant from high intensity
sunlight by absorbing it
Pigments:
 Chlorophyll-a ; a blue-green pigment.
 Chlorophyll-b ; a yellow-green pigment.
 Carotene; an orange pigment.
 Carotenoid; a yellow-orange pigment.
 Xanthophyll; a yellow pigment.
 Phaeophytin-a; a gray-brown pigment.
 Phaeophytin-b; a yellow-brown pigment

6. Composition:
Inner Part
 The inner part of antenna complex
consists of the core complex
 It mainly includes chlorophyll-a
molecules
 It transfers light energy from peripheral
part to reaction center.
Outer Part
 The outer part of antenna complex is
also called its peripheral region
 It composed of the necessary and
accessory pigments
 It is called Light Harvesting Complex
(LHC).

7. Resonance
Energy Transfer
When the photosynthetic pigments
are lying very close to each other the
irradiated energy photon is transferred
from one photosynthetic pigment to
the other lying nearby.
Non-radioactive process.
One quantum of energy obtained
from the sun is referred as photon.
One quantum of excitation energy
which is transferred from one
photosynthetic pigment molecule to
the other is called exciton.
It is also called forster transfer

8. Light Harvesting Photosystem
RESONANCE ENERYGY TRANSFER

9. In Plants:
Photosystem-I
 Contains100-120 molecules of
chlorophyll-a.
 Approximately 15-20 molecules of
beta-carotene are present.
 Linked with the reaction center of
P700.
Photosystem-II
 Approximately 20-25 molecules of
chlorophyll-a
 Several molecules of beta-carotene
molecules
 Linked with the reaction center of
P680.

10. Importance of Antenna complex in the
Photosynthesis:
1.Role in Light-harvesting:
The absorption done by the photosynthetic pigments present in the reaction center is very less
than the capacity of electron transport chain of photosynthesis and this would not provide enough
energy to drive the process of photosynthesis in the plants.
The accessory pigments pigments present in antenna complex absorb those wavelengths of light
that are weakly absorbed by the chlorophyll-a molecules, thus the range of absorption spectrum
increases and as a result the process of light absorption becomes maximum.

11. 2. Defense against Active Oxygen
Species:
 As a result of light absorption by the chlorophyll pigments they become readily excited
 The chlorophyll molecules become quite unstable and reach their highest energy state
 Reaction of excited chl-a with Oxygen:
3Chlorophyll-a* + O2 ――― → Chlorophyll-a + [ O]

12. Quenching Process:
 Quenching by Carotenoids with chl-a:
3Chlorophyll-a + Carotenoid ―――→ Chlorophyll-a + 3Carotenoids *
Chlorophyll-a + 3Carotenoids *―――→ Chlorophyll-a + Carotenoid +
heat
 Quenching by Carotenoids with [O]:
[O] + Carotenoids ―――→ O2 + 3Carotenoids*
O2 + 3Carotenoids ―――→ Carotenoids + Heat

13. 3. Management of efficient utilization of
light:
 As the light intensity increases the rate of photosynthetic process increases and gets
highly saturated and ultimately becomes independent of the intensity of falling light due
to incapacity of dark reactions because of to the slow regeneration of NADP+, ADP and
Inorganic Phosphate.
 The presence of limited amount of NADP+ allows the molecular O2 to vie with ferredoxin
process leads to the production of superoxide along with other types of free radicals
which cause damage to the plant
 This damage is reduced by the transfer of excited state energy within the antenna
complex pigments

14. 4. Regulation of Distribution of energy
between the PS-I and PS-II:
 When the availability of NADP+ becomes limited either due to the high intensity of light or
due to the limitation of the dark reactions, the rate at which PQH2 is oxidized by the PS-I
becomes slow.
 Reduction in the level of PQ pool activates a protein called kinase that performs the
phosphorylation of a pool of LHC-II.
 The phosphor-LHC II becomes able to interact and deliver the available excitation energy
to PS-I at the expense of PS-II.
 When the PQ pool gets oxidized the kinase protein turns into inactive form.

15. Energy transfer cycles between PS-I &
PS-II:

16. In Purple
photosynthetic
Bacteria:
Main photosynthetic pigment that is the
major part of the antenna complex is
bacterio-chlorophyll
Accompanied by the carotenoids
They arrange themselves in cylindrical
form forming a ring-like structure.

17. In Green photosynthetic
Bacteria:
There are present some
specialized structures, having
ellipsoidal shape, to capture light for
photosynthesis called “chlorosome”.
The type of bacterio-chlorophyll
present in them is green in color.

18. In Cyano-bacteria:
Specialized light gathering
structures called
phycobillisomes.
Contains phycocyanobillin and
phycoerythrobillin ,also known as
the chromophores
The chromophore and
apoprotein bind together and
form a unique structures called
phycocyanin, phycoerythrine and
allophycocyanin depending on
the nature of associated
chromophore.