1. The document discusses photosynthesis and the light dependent reactions that take place in chloroplasts. 2. It describes the two photosystems, photosystem I and photosystem II, their reaction centers and electron transport chains, and how they work together to produce ATP, NADPH, and oxygen through noncyclic electron flow. 3. It also explains cyclic electron flow which uses only photosystem I and generates ATP without producing NADPH or oxygen.

1. PRESENTED TO;
MAM RABIA
PRESENTED BY;
AZKA JAVED

2. TOPIC
PHOTOSYSTEM I & II
ELECTRON
TRANSPOT CHAIN

3. Chloroplasts
absorb light
energy and
convert it to
chemical energy
Light
Reflected
light
Absorbed
light
Transmitted
light
Chloroplast
THE COLOR OF LIGHT SEEN IS THE
COLOR NOT ABSORBED

4.  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
Carbon
dioxide
Water Glucose Oxygen
gas
PHOTOSYNTHESIS

5. Photosystem:
Reaction center surrounded by several
light-harvesting complexes
Light-harvesting complex:
Pigment molecules bound to proteins
(act as antenna for reaction center)
PHOTOSYSTEMS

6. 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

7. A PHOTOSYSTEM: A REACTION CENTER
ASSOCIATED WITH LIGHT-HARVESTING
COMPLEXES
A photosystem
 Is composed of a
reaction center
surrounded by a
number of light-
harvesting
complexes
Primary election
acceptor
Photon
Thylakoid
Light-harvesting
complexes
Reaction
center
Photosystem
STROMA
Thylakoidmembrane Transfer
of energy
Special
chlorophyll a
molecules
Pigment
molecules
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
e–

8. 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

9. 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 = P680
Noncyclic e- flow
Noncyclic e- flow (fig 10.13)
Uses PS II & PS I
Excited 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 I
Only generates ATP
Excited e- from PS I cycle back from 1st ETC
No O2 release & no NADPH made
PHOTOSYSTEMS

10. TWO PHOTOSYSTEMS
 The thylakoid membrane
 Is populated by two types of photosystem I and II.

11. NONCYCLIC ELECTRON FLOW – INVOLVES BOTH PHOTOSYSTEMS
 Produces NADPH, ATP, and oxygen, and is the primary pathway of energy
transformation in the light rxns.
Figure 10.13
Photosystem II
(PS II)
Photosystem-I
(PS I)
ATP
NADPH
NADP+
ADP
CALVIN
CYCLE
CO2H2O
O2 [CH2O] (sugar)
LIGHT
REACTIONS
Light
Primary
acceptor
Pq
Cytochrome
complex
PC
e
P680
e–
e–
O2
+
H2O
2 H+
Light
ATP
Primary
acceptor
Fd
e
e–
NADP+
reductase
P700
Light
NADPH
NADP+
+ 2 H+
+ H+
1
5
7
2
3
4
6
8

12. CYCLIC ELECTRON FLOW;
 Under certain conditions
 Photoexcited electrons take an alternative path
 Uses Photosystem I only

13. IN CYCLIC ELECTRON FLOW
 In cyclic electron flow
 Electrons cycle back to the first ETC
 Only ATP is produced
Primary
acceptor
Pq
Fd
Cytochrome
complex
Pc
Primary
acceptor
Fd
NADP+
reductase
NADPH
ATP
Figure 10.15
Photosystem II
Photosystem I
NADP+

14. Primary
electron acceptor
Primary
electron acceptor
Photons
PHOTOSYSTEM I
PHOTOSYSTEM II
Energy for
synthesis of
by chemiosmosis
Noncyclic Photophosphorylation
 Photosystem II regains electrons by splitting
water, leaving O2 gas as a by-product

15. PLANTS PRODUCE O2 GAS BY SPLITTING H2O
 The O2 liberated by photosynthesis is made from the
oxygen in water (H+ and e-)

16. 2 H + 1/2
Water-splitting
photosystem
Reaction-
center
chlorophyll
Light
Primary
electron
acceptor
Energy
to make
Primary
electron
acceptor
Primary
electron
acceptor
NADPH-producing
photosystem
Light
NADP
1
2
3
HOW THE LIGHT REACTIONS GENERATE ATP AND NADPH

17. SUMMARY—LIGHT DEPENDENT REACTIONS
a. Overall input
light energy, H2O.
b. Overall output
ATP, NADPH, O2.