1. PHOTOSYNTHESIS * Photons to energy CO2 fixation Z-scheme Calvin Cycle
LIGHT REACTION
DARK REACTION
Photons to energy
CO2 fixation
Z-scheme
Calvin Cycle *
Photophosphorylation

2. What is Photosynthesis?
Ans:
A process in which electrons from excited
chlorophyll molecules are passed to acceptors
that convert electronic energy to chemical
energy: ATP and NADPH

3. Chloroplasts
Anatomy is Essential
to Photosynthesis

4. Absorption of Light Chlorophyll b Chlorophyll a Absorption Intensity
Phycoerythrin
400
500
600
700
800
900
300
Wavelength, nm

6. E = hv Photon h = Planck’s constant = 6.626 x 10-34 J sec
ABSORPTION OF LIGHT ENERGY (Photons)
Excited state
Photon
Allowable Transitions
E = hv
Depends on frequency of
wave length of light
Ground State Electron
h = Planck’s constant
= x J sec
v = frequency of light energy
in waves per sec

7. Rules of the Game
Electromagnetic radiation is transmitted in quanta (discrete energy packages)
A molecule can absorb photons of only certain wavelengths
Absorbed energy must match the energy difference between two states
Energy of absorbed photon is transferred to photosynthetic reaction system

8. Dissipation of Excitation Energy of Chlorophyll
Chlorophyll can
absorb energy in
2 energy states,
Blue or short
wave and Red or
first energy state
Second Excited
State
Internal
First Excited
State
Photooxidation
(transfer to a
second molecule)
Ground
State
Fluorescence
(rare)
Blue
Red

9. Options to Absorption 1. Fluorescence:
Absorb low , emit high (visible)
2. Resonance Transfer: Excite neighbor (trampoline)
3. Electron Transfer: Pass excited electron
2 and 3 are used in the light reaction of
PHOTOSYNTHESIS

10. LIGHT HARVESTING COMPLEXES
Capture Light Energy
Multi-subunit Protein Complexes in the thylakoid
membranes
Two Components:
Antenna molecules that absorb and direct
energy by resonance transfer,
Reaction centers that split water molecules
and conduct electron transfer

11. The Hill Reaction Leaf extracts rich in chloroplasts, when radiated,
evolve oxygen and acquire the capacity to reduce
hydrogen acceptors.
Robert Hill, 1937
light
2H2O + 2A
2AH2 + O2
Hill used 2,6-dichlorophenolindophenol
light
A = BLUE
AH2 = COLORLESS
Hill separated the light from the dark reaction.
Hill showed that light induced an electon flow
that drove an unfavorable energy reaction.

13. 3H+
Stroma
Thylacoid lumen
3H+

14. Inner Membrane
Stroma
3H+
Matrix
Thylacoid lumen
3H+

15. Z Scheme of the Light Reaction
ATP
NADP+
NADPH
(P700)
PS-I
Z Scheme of the Light Reaction
(P680)
PS-II

16. What is a Photosynthetic Reaction Center?
Only purple bacterial centers are known
A transmembrane protein complex with 3 protein components: H, L, M
Typically, 2 chlorophyll, 2 pheophytin, one Fe2+, and 2 ubiquinone or 1 ubiquinoine and one menaquinone
Nearly perfect 2-fold symmetry

17. Photosynthetic Reaction Center (Rhodobacter sphaeroides)
Rps. viridis L M H
Cytoplasm