Announcements, March 12

• Please remove homework from folders• One homework and 2 quizzes (Q14, Q18) without names

still unclaimed – please see me to claim.• Reading for today: pp. 288-302 on photosynthesis; pp.

302-307 will not be covered due to snow day.• Review Session from 4-5 PM, March 13 in BR 203• Exam 2 on Wednesday, March 14 over the same

material as before.• Reading for Friday, March 16: pp. 318-358 on

intracellular trafficking

Chloroplast Ultrastructure

Photosynthesis Overview

• Light + 3 CO2 + 6 H2O → C3H6O3 + 3 O2 + 3 H2O

• Function: ATP synthesis and synthetic rxns• Energy source: sunlight• Energy conversion:

– solar H+ gradient ATP

– H2O electron transport NADPH

• Carbon source: CO2

• Electron source: H2O

• Intermediate electron acceptor: NADP+

• Byproduct: O2

Reduced C:3C sugar

Energy Transduction reactions:Photoreduction (NADPH synthesis)

Photophosphorylation (ATP synthesis)

NADP+ and NADPH

Carry e- in syntheticreactions, whereasNAD+/NADP usedin degradative rxns.

Absorption spectra of plant pigments

Chlorophyll

Chlorophyll is special because it can accept a low energy electron and donate

a high energy electron.

Light Harvesting Complex

1. Heat,light2.

3.

Fate of light energy when e* eo

1. Energy is lost as heat and lower energy light.

2. Resonance energy transfer to an electron in an adjacent pigment molecule.

3. Photochemical reduction, where e* is itself transferred to a carrier (only in reaction center chlorophyll).

Resonance Energy Transfer to Adjacent Pigments

e*

eo

e*

eo

e*

eo

e*

eo

Antenna chlorophyllsReaction center

chlorophyll P680PSII

Groundstate

Excitedstate

photon

Photochemical Reduction:e* is transferred to NADP+

e*

eo

Reaction centerchlorophyll P680

PSII

e

H2O 2H+ + O2

e*

e*

eo

ChlorophyllP700PSI

e*e*

Electron transportSystem (ETS)

photonETS

NADP+

NADPH

OEC

Energy conversion in thylakoid MB

AnalogousTo RCIII

The Z Scheme: Steps

1. Water splitting: 2 H2O → 4 H+, O2 to lumen

2. P680o P680* (PSII)

3. Electron transfer to P700o (PSI)

4. 4 H+ pumped to lumen through cytochrome b6/f

5. P700o P700* (PSI)

6. Electron transfer to ferredoxin

7. Ferredoxin-NADP+ reductase catalyzes: 2 NADP+ + 2 H+

stroma + 2 e- 2 NADPH

Understanding how the water-splitting enzyme works is a major unsolved problem,

with practical applications

4

Z scheme

Photophosphorylation(Light-driven ATP synthesis)

Current estimates (know this):• pHlumen = 6• pHstroma = 8 pH = -2 (in-out)• Em = +30 mV (flow of Mg2+ and

Cl- balances much of flow of H+)• pmf = 30 - 60 mV(-2) = +150 mV• 4 H+ flow/ATP through CF0CF1

ATP synthase• 4 e- Z scheme 2 ATP, 2

NADPH• But need 3 ATP/2 NADPH in

Calvin cycle, so cyclic electron flow occurs:

H+ Cl-

H+ H+ Mg2+

lumen

stroma

lumen

stroma

Cyclic e- Flow• Regulates NADPH/ATP

formed (need 2:3).• 2 electrons flow back

through cyt b6/f to P700, pumping 4 H+ into lumen

• 2 photons re-excite the 2 electrons.

• No water oxidized and no O2 released, since PSII not involved.

NADP+

PSI

Photosynthesis Reactions(Don’t memorize)

1. Energy transduction reactions (Equation 11-14)26 photons + 9 ADP + 9 Pi + 6 NADP+ + 6 H2O 3 O2 + 9 ATP + 6 NADPH + 3 H2O

2. Calvin cycle reactions (Equation 11-13)3 CO2 + 9 ATP + 6 NADPH + 6 H2O C3H6O6 + 6 NADP+ + 9 ADP + 9 Pi

3. Overall (Equation 11-16):26 photons + 3 CO2 + 6 H2O C3H6O6 + 3 O2 + 3 H2O

What % of mass of C3H6O6 comes from CO2 ?MW of CO2 is 44, of H2O is 18, of C3H6O6 is 186.

Carbon fixation

occurs in the

stroma