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PRESENTED TO; MAM RABIA
PRESENTED BY; AZKA JAVED
TOPICPHOTOSYSTEM I & II ELECTRON TRANSPOT CHAIN
Chloroplasts absorb light energy and convert it to chemical energy
LightReflected
light
Absorbedlight
Transmittedlight
Chloroplast
THE COLOR OF LIGHT SEEN IS THE COLOR NOT ABSORBED
Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water
AN OVERVIEW OF PHOTOSYNTHESIS
Carbondioxide
Water Glucose Oxygengas
PHOTOSYNTHESIS
Photosystem:Reaction center surrounded by several light-harvesting complexes
Light-harvesting complex:Pigment molecules bound to proteins (act as antenna for reaction center)
PHOTOSYSTEMS
Reaction center : Protein complex that includes 2 special chlorophyll a molecules + primary e- acceptor molecule
First step of light reactions: special chlorophyll a molecule transfers its excited e- to the primary e- acceptor
A PHOTOSYSTEM: A REACTION CENTER ASSOCIATED WITH LIGHT-HARVESTING COMPLEXES
A photosystemIs composed of a reaction center surrounded by a number of light-harvesting complexes
Primary electionacceptor
Photon
Thylakoid
Light-harvestingcomplexes
Reactioncenter
Photosystem
STROMA
Th
yla
koid
mem
bra
ne
Transferof energy
Specialchlorophyll amolecules
Pigmentmolecules
THYLAKOID SPACE(INTERIOR OF THYLAKOID)
e–
LIGHT-HARVESTING COMPLEXES AND REACTION CENTERS
The light-harvesting complexes consist of pigment molecules bound to particular protein
They funnel the energy from photons of light to the reaction center
When a reaction-center chlorophyll a molecule absorbs energy, one of its electrons gets bumped up to a primary electron acceptor
Two types of photosystems embedded in the thylakoid membranes of land plants (fig 10.13)
1. Photosystem I (PS I)Rxn center chlorophyll a = P700 Cyclic and noncyclic e- flow
2. Photosystem II (PS II)Rxn center chlorophyll a = P680Noncyclic e- flow
Noncyclic e- flow (fig 10.13) Uses PS II & PS IExcited e- from PS II goes through ETC produces ATP Excited e- from PS I ETC used to reduce NADP+Electrons ultimately supplied from splitting water releases O2 and H+
Cyclic e- flow (fig 10.15)Uses only PS IOnly generates ATPExcited e- from PS I cycle back from 1st ETCNo O2 release & no NADPH made
PHOTOSYSTEMS
TWO PHOTOSYSTEMS
The thylakoid membrane Is populated by two types of photosystem I and II.
NONCYCLIC ELECTRON FLOW – INVOLVES BOTH PHOTOSYSTEMS
Produces NADPH, ATP, and oxygen, and is the primary pathway of energy transformation in the light rxns.
Figure 10.13Photosystem II
(PS II)
Photosystem-I(PS I)
ATP
NADPH
NADP+
ADP
CALVINCYCLE
CO2H2O
O2 [CH2O] (sugar)
LIGHTREACTIONS
Light
Primaryacceptor
Pq
Cytochromecomplex
PC
e
P680
e–
e–
O2
+
H2O2 H+
Light
ATP
Primaryacceptor
Fd
ee–
NADP+
reductase
ElectronTransportchain
Electron transport chain
P700
Light
NADPH
NADP+
+ 2 H+
+ H+
1
5
7
2
3
4
6
8
CYCLIC ELECTRON FLOW;
Under certain conditions Photoexcited electrons take an alternative path Uses Photosystem I only
IN CYCLIC ELECTRON FLOW In cyclic electron flow
Electrons cycle back to the first ETC Only ATP is produced
Primaryacceptor
Pq
Fd
Cytochromecomplex
Pc
Primaryacceptor
Fd
NADP+
reductaseNADPH
ATPFigure 10.15
Photosystem II Photosystem I
NADP+
Primaryelectron acceptor
Primaryelectron acceptor
Electron transport
chain
Electron
transport
Photons
PHOTOSYSTEM I
PHOTOSYSTEM II
Energy forsynthesis of
by chemiosmosis
Noncyclic Photophosphorylation
Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product
PLANTS PRODUCE O2 GAS BY SPLITTING H2O
The O2 liberated by photosynthesis is made from the oxygen in water (H+ and e-)
2 H + 1/2
Water-splittingphotosystem
Reaction-center
chlorophyll
Light
Primaryelectronacceptor
Energyto make
Electron transport chain
Primaryelectronacceptor
Primaryelectronacceptor
NADPH-producingphotosystem
Light
NADP
1
23
HOW THE LIGHT REACTIONS GENERATE ATP AND NADPH
SUMMARY—LIGHT DEPENDENT REACTIONS
a. Overall inputlight energy, H2O.
b. Overall output ATP, NADPH, O2.
