Photosynthesis
The Light Reactions
Chapter 3.3
Photosynthesis Review
light
CO2 + H2O
C6H12O6 + O2
photosynthesis – creating sugar using light
Only chloroplast organelles and special bacteria
have the proteins necessary to carry out
photosynthesis.
Photosynthesis: Two Major
Processes
1.
2.
The Light
Reactions
Calvin cycle

harvests light energy to
split water, creating O2
, ATP and NADPH

process of producing
C6H12O6 , using ATP
for energy and
producing NADP+
Photosynthesis: Two Major
Processes
Chloroplast Structure: A
Review
(thylakoid space)
The Light Reactions
1.
Photoexcitation
 Absorption
of light
photons
2.
Electron transport
3.
Photophosphorylati
on (chemiosmosis)
 Similar
to ETC in
mitochondria
 ATP
synthesis due to
electrochemical
gradient
Photoexcitation
 e-
gain energy when
atoms absorb energy.
 e-
fall back to lowest
energy level (ground
state) if it isn’t
transferred to another
molecule
Isolated Chlorphyll
 If
an isolated solution of
chlorophyll is
illuminated

It will fluoresce, giving
off heat and light
Photosystems
•
Made up of a
variety of proteins
–
•
•
reaction center
surrounded by a
number of lightharvesting
complexes
Contain chlorophyll
and other light
absorbing pigments
Located in the
thylakoid
membrane
Reaction Centre
 Contains
a primary
electron acceptor
 contains chlorophyll a
molecule which the
light energy is focused
in a photosystem
Two Types of Photosystems
Photosystem I (PS I)
 Has
Phosystem II (PS II)
P700 chlorophyll a
within reaction centre
 Has
Best at absorbing
700 nm wavelength
(far red part of
spectrum)


P680 chlorophyll a
within reaction centre
Best at absorbing
680 nm wavelength
(red)
Purposes of Photosystems
Two purposes:
1.
to collect as much light energy as possible
2.
excite chlorophyll a and transfer its electrons
to an electron acceptor and through a series
of proteins (electron transport)
Electron Transport
Electron transport occurs in the thylakoid
membrane.
Two mechanisms of electron transport:
1.
Non-cyclic electron flow (the primary pathway)
2.
Cyclic electron flow
Non-Cyclic Electron Flow: An Overview
H2O
CO2
LIGHT
NADP+
ADP
LIGHT
REACTOR
CALVIN
CYCLE
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Photosystem I
NADP+
reductase
Light
2 H+
3
Fd
NADPH
Pq
NADP+ + 2H+
+ H+
Pc
2
H2 O
THYLAKOID SPACE
(High H+ concentration)
1⁄
1
2
O2
+2 H+
2 H+
To
Calvin
cycle
STROMA
(Low H+ concentration)
Thylakoid
membrane
ATP
synthase
ADP
ATP
P
H+
Step 1: PS II

A photon of light strikes a pigment
molecule in a light harvesting complex
and is relayed to other pigment
molecules until it reaches one of the
P680 chlorophyll a pigments within
the reaction centre.

This excites one of the P680
electrons and is captured by the
primary electron acceptor.

PSII splits a water molecule into 2
electrons, 2 hydrogen ions (2 H+) and
1 oxygen atom.

These electrons replace (one by one)
the electrons lost to the primary
electron acceptor.
PS II: The Details
STROMA
(Low H+ Concentration)
THYLAKOID
MEMBRANE
THYLAKOID SPACE
(LUMEN)
(High H+ Concentration)
Step 2: Pq, Cytochrome
Complex, Pc
 Each photoexcited electron
passes from the primary
electron acceptor of PSII to PSI
via an ETC (similar to the ETC
in cellular respiration)
 The ETC between PSII and PSI
is made up of:
 Pq (plastiquinone) - mobile
 Cytochrome Complex
 Pc (Plastocyanin) - mobile
Pq, CC, Pc: The Details
STROMA

(Low H+ Concentration)
As Plastoquinone (Pq)
transfers electrons to the
Cytochrome Complex, protons
are pumped across the
membrane into the thylakoid
space (lumen)

THYLAKOID
MEMBRANE
THYLAKOID SPACE

(LUMEN)
(High H+ Concentration)
This exergonic “fall” of
electrons to a lower energy
level provides energy for the
active transport of H+ ions
against its concentration
gradient.
Electrons are then transferred
to Plastocyanin (Pc), also a
moveable component on
thylakoid surface in lumen
Step 3: PS I
 A photon of light strikes a pigment
molecule in a light harvesting
complex and is relayed to other
pigment molecules until it reaches
one of the P700 chlorophyll a
pigments within the reaction
centre.
 This excites one of the P700
electrons and is captured by the
primary electron acceptor, creating
an electron “hole” in the P700.
 This hole is filled by an electron
that reaches the bottom of the
ETC from PS II.
PS I: The Details
Step 4: Fd and NADP+
Reductase
•
Electrons are transferred to ferrodoxin (Fd) – moveable
component on thylakoid surface in stroma
•
Electrons are transferred to NADP+ reductase
•
final electron acceptor is NADP+ that is reduced to NADPH
NADH vs. NADPH: A Review
Step 5: ATP Synthase

protons pumped into the lumen (from Step 1 and Step 2)pass
through ATP synthase by facilitated diffusion

ATP produced in stroma

photophosphorylation – light-dependent formation of ATP by
chemiosmosis
 The

spatial organization of chemiosmosis
Differs in chloroplasts and mitochondria
Key
Higher [H+]
Lower [H+]
Chloroplast
Mitochondrion
CHLOROPLAST
STRUCTURE
MITOCHONDRION
STRUCTURE
Intermembrance
space
Membrance
Matrix
H+ Diffusion
Electron
transport
chain
ATP
Synthase
ADP+
Thylakoid
space
Stroma
P
H+
ATP
Non-Cyclic Electron Flow
H2O
CO2
LIGHT
NADP+
ADP
LIGHT
REACTOR
CALVIN
CYCLE
ATP
NADPH
STROMA
(Low H+ concentration)
O2
[CH2O] (sugar)
Cytochrome
complex
Photosystem II
Photosystem I
NADP+
reductase
Light
2 H+
3
Fd
NADPH
Pq
NADP+ + 2H+
+ H+
Pc
2
H2 O
THYLAKOID SPACE
(High H+ concentration)
1⁄
1
2
O2
+2 H+
2 H+
To
Calvin
cycle
STROMA
(Low H+ concentration)
Thylakoid
membrane
ATP
synthase
ADP
ATP
P
H+
H2 O
CO2
Energy Diagram
(Z scheme)
Light
NADP+
ADP
CALVIN
CYCLE
LIGHT
REACTIONS
ATP
NADPH
O2
[CH2O] (sugar)
Primary
acceptor
Primary
acceptor
Fd
Energy of Electrons
Pq
H2 O
e
e
Cytochrome
complex
2 H+
+
O2
NADP+
reductase
NADP+
+ 2 H+
NADPH
PC
e–
Light
e–
+ H+
P700
e–
P680
Light
ATP
Photosystem II
(PS II)
Photosystem-I
(PS I)
Non-Cyclic Electron Flow
Summary
1.
H2O is split to produce O2 (released from cell) and
H+ ions (released into lumen)
2.
enzyme complexes pump protons from stroma to
lumen
3.
NADP+ is final electron acceptor and produces
NADPH
4.
chemiosmosis to synthesize ATP
Light Reaction Animation
http://www.youtube.com/
watch?v=hj_WKgnL6MI
Cyclic Electron Flow
•
•
Non-cyclic electron
flow produces roughly
equal amounts of ATP
and NADPH
However, Calvin Cycle
uses more ATP than
NADPH
–
Cyclic electron flow
makes up the
difference in ATP
(without producing
more NADPH).
H2O
CO2
Light
LIGHT
REACTIO
NS
NAD
P
ADP
+ P
CALVIN
CYCLE
ATP
NADPH
Chloroplast
O2
[CH
2O]
(sug
ar)
Cyclic Electron Flow
Primary
acceptor
Primary
acceptor
Fd
Fd
Pq
NADP+
reductase
Cytochrome
complex
Pc
Photosystem II
ATP
Photosystem I
NADP+
NADPH
Cyclic Electron Flow Summary
1.
only involves photosystem I (P700)
2.
ferrodoxin returns electrons back to cytochrome
complex
3.
Only ATP produced, no NADPH
To Do:
 Section

# 1-3, 11
 Section

3.2 Questions (pg. 154-155)
3.3 Questions (pg. 166-167)
#1-4, 6, 8a(i-iii), 8b