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

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

Chloroplasts Anatomy is Essential to Photosynthesis

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

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 = 6.626 x 10-34 J sec v = frequency of light energy in waves per sec

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

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

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

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

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.

3H+ Stroma Thylacoid lumen 3H+

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

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

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

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