PHOTOSYNTHESIS

LIGHT REACTIONLIGHT REACTION DARK REACTIONDARK REACTION

Photons to energy

Z-scheme

Photophosphorylation

CO2 fixation

Calvin Cycle

**

What is Photosynthesis?

Ans:

A process in which electrons from excited chlorophyll molecules are passed to acceptorsthat convert electronic energy to chemical energy: ATP and NADPH

Chloroplasts

Anatomy is Essentialto Photosynthesis

Absorption of Light

300 800700600500400 900

Chlorophyll a

Chlorophyll b

PhycoerythrinAbsorptionIntensity

Wavelength, nm

Allowable Transitions

Excited state

Photon

E = hv

h = Planck’s constant

Ground State Electron

= 6.626 x 10-34 J sec

ABSORPTION OF LIGHT ENERGY (Photons)

Depends on frequency of wave length of light

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

• 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

GroundState

First ExcitedState

Second ExcitedState

Dissipation of Excitation Energy of Chlorophyll

Photooxidation(transfer to asecond molecule)

Fluorescence(rare)

Energy

Internal

Chlorophyll canabsorb energy in2 energy states, Blue or shortwave and Red or first energy state

BlueBlue RedRed

Options to Absorption

1. Fluorescence:

2. Resonance Transfer: Excite neighbor (trampoline)

3. Electron Transfer: Pass excited electron

Absorb low , emit high (visible)

2 and 3 are used in the light reaction ofPHOTOSYNTHESIS

Capture Light Energy

LIGHT HARVESTING COMPLEXES

Multi-subunit Protein Complexes in the thylakoidmembranes

Two Components:

Antenna molecules that absorb and direct energy by resonance transfer,

Reaction centers that split water moleculesand conduct electron transfer

The Hill ReactionLeaf extracts rich in chloroplasts, when radiated,evolve oxygen and acquire the capacity to reduce hydrogen acceptors.

Robert Hill, 1937

Leaf extracts rich in chloroplasts, when radiated,evolve oxygen and acquire the capacity to reduce hydrogen acceptors.

Robert Hill, 1937

2H2O + 2A 2AH2 + O2

light

Hill used 2,6-dichlorophenolindophenol

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.

light

3H+

3H+

Thylacoid lumen

Stroma

Thylacoid lumen

Stroma

3H+

3H+Inner Membrane

Matrix

NADPH

Z Scheme of the Light Reaction

PS-II

PS-I

NADP+

ATP

(P680)

(P700)

What is a Photosynthetic Reaction Center?

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

• 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)

HM L

Rps. viridis

Cytoplasm