1. Chapter 8
Light Reactions

2. Need To Know
How photosystems convert light energy into chemical energy.
How linear electron flow in the light reactions results in the formation of ATP, NADPH, and O2.

3. Calvin Cycle Light Reactions
Figure 8.UN02
H2O
CO2
Light
NADP
ADP
Calvin
Cycle
Light
Reactions
ATP
NADPH O2
[CH2O] (sugar) 3

4. (a) Excitation of isolated chlorophyll molecule Energy of electron
Figure 8.11
Photon
(fluorescence)
Ground
state
(b) Fluorescence
Excited
Chlorophyll
molecule
Heat e−
(a) Excitation of isolated chlorophyll molecule
Energy of electron
Photon
(fluorescence)
Ground
state
Excited
Chlorophyll
molecule
Heat e−
(a) Excitation of isolated chlorophyll molecule
Energy of electron
Ground
state
Excited
Chlorophyll
molecule e−
(a) Excitation of isolated chlorophyll molecule
Energy of electron
Chlorophyll
molecule
Excited
state e−
Photon
Photon
When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable.
When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence.
If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat. 4

5. Mill makes ATP NADPH Photosystem II Photosystem I Photon Photon
Figure 8.14
Mill
makes
ATP
NADPH
Photon
Photon
Photosystem II
Photosystem I 5

6. There are two reaction centers
Linear electron flow
P680
P700
There are two types of photosystems in the thylakoid membrane
Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm.
The reaction-center chlorophyll a of PS II is called P680.
Photosystem I (PS I) is best at absorbing a wavelength of 700 nm
The reaction-center chlorophyll a of PS I is called P700.
Linear electron flow involves the flow of electrons through both photosystems to produce ATP and NADPH using light energy.
P680
P700

7. How a photosystem harvests light!
Figure 8.12a
Photosystem
STROMA
Photon
Light-
harvesting
complexes
Reaction-
center
complex
Primary
electron
acceptor e−
Thylakoid membrane
Pigment
molecules
Transfer
of energy
Special pair of
chlorophyll a
molecules
A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes.
The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center.
A primary electron acceptor in the reaction center accepts excited electrons and is reduced as a result.
Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions.
THYLAKOID SPACE
(INTERIOR OF THYLAKOID)
How a photosystem harvests light! 7

8. 2 H  Photosystem II (PS II) Light  P680 Electron transport chain
Figure
Primary
acceptor
2 H O2 
ATP
Photosystem II
(PS II)
H2O
Light  2 1
P680
Electron
transport
chain
Pigment
molecules
Photosystem I
(PS I)
P700 3 4 5 6 e−
Primary
acceptor
2 H O2 
ATP
NADPH
Photosystem II
(PS II)
H2O e−
Light  2 1
P680
Electron
transport
chain
Pigment
molecules
Photosystem I
(PS I)
P700 H
NADP
reductase 3 4 5 6 7 8
Primary
acceptor
2 H O2 
Photosystem II
(PS II)
H2O
Light  2 1
P680
Pigment
molecules 3 e−
Primary
acceptor
2 H O2 
ATP
Photosystem II
(PS II)
H2O
Light  2 1
P680 Pq
Electron
transport
chain
Cytochrome
complex Pc
Pigment
molecules 3 4 5 e−
Primary
acceptor 2 e−
P680 1
Light
Linear electron flow can be broken down into a series of steps
A photon hits a pigment and its energy is passed among pigment molecules until it excites P680.
An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680).
H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680, thus reducing it to P680; O2 is released as a by-product.
Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I.
Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane; diffusion of H (protons) across the membrane drives ATP synthesis.
In PS I (like PS II), transferred light energy excites P700, causing it to lose an electron to an electron acceptor (we now call it P700)
P700 accepts an electron passed down from PS II via the electron transport chain
Excited electrons “fall” down an electron transport chain from the primary electron acceptor of PS I. The electrons are transferred to NADP, reducing it to NADPH, and become available for the reactions of the Calvin cycle. This process also removes an H from the stroma
Pigment
molecules
Photosystem II
(PS II) 8

9. Cytochrome complex NADP reductase To Calvin Cycle ATP synthase
Figure 8.16
Cytochrome
complex
NADP
reductase
Photosystem II
Photosystem I
Light
4 H 3
Light
NADP  H
NADPH e− e− 2
H2O 1  2 1 O2
THYLAKOID SPACE
(high H concentration)
2 H
4 H To
Calvin
Cycle
The energy from the electron transport chain is used to pump H+ into the thylakoid space. This creates a proton motive force. The protons diffuse out of the thylakoid space through ATP synthase. The energy from the diffusing protons is used to convert ADP to ATP.
ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place.
In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH
Thylakoid
membrane
ATP
synthase
STROMA
(low H concentration)
ADP 
ATP P i H 9

10. Electron transport chain ATP synthase
Figure 8.15
MITOCHONDRION
STRUCTURE
CHLOROPLAST
STRUCTURE
Inter-
membrane
space H
Diffusion
Thylakoid
space
Electron
transport
chain
Inner
membrane
Thylakoid
membrane
Comparison of chemiosmosis in mitochondria and chloroplasts.
Although very similar, chemiosmosis in cellular respiration and photosynthesis are not identical. In addition to some spatial differences, the key conceptual difference is that mitochondria use chemiosmosis to transfer chemical energy from food molecules to ATP, whereas chloroplasts transform light energy into chemical energy in ATP.
ATP
synthase
Matrix
Stroma
Key
ADP 
P i
ATP
Higher [H] H
Lower [H] 10