Plant Biology Fall 2006BISC 367 - Plant Physiology

LabSpring 2009

Notices:•

• General reading• Taiz and Zeiger: Chapter 7, Photosynthesis: the Light Reactions

• Photosynthesis (P/S) uses light energy to build carbon compounds

CO2 + H2O (CH2O)n + O2

Inputs: CO2

H2O

Light

Outputs: Carbohydrate (CHO)

O2

Photosynthesis - light

Light energy

• P/S is divided into 2 stages

Photosynthesis - light

Light NRG oxidizes H2O

Reductant & NRG are used to reduce CO2

P/S takes place in chloroplasts (cp)

Photosynthesis - light

Thylakoid

Grana lamellaeStacked membranes

Stroma lamellaeUnstacked membrane

Stroma

Light rxns occur here

Carbon reduction rxns occur here

Pigments (chlorophylls and carotenoids) exist in antenna complexes– Harvest and transfer light NRG to chlorophylls at the reaction centre

• NRG is transferred from pigment to pigment by resonance transfer

– At the reaction centre NRG is converted to chemical (redox) NRG

Photosynthesis - light

Antennal pigment/ reaction centre complexes are also referred to as photosystems

There are 2 photosystems• PSI• PSII

• Light NRG moves down an energy gradient towards the reaction centre

Photosynthesis - light

• PSI and PSII operate in series– PSI absorbs far-red light (>680 nm) = P700

• Produces a very strong reductant

– PSII absorbs red light (680 nm) = P680• Produces a strong very strong oxidant

– Can oxidize water

Photosynthesis - light

Z scheme of Photosynthesis

Ultimate source of e-

• 4 protein complexes embedded in the thylakoid membrane are involved in the light reactions

• PSII

• Cytochrome b6f complex

• PSI

• ATP synthase

Photosynthesis - light

Photosynthesis - light

Oxidation of water:• Performed by the oxygen evolving centre (OEC) of PSII

2H2O O2 + 4e- + 4H+

Water is extremely stable - oxidation is not easy!

4H+ are released into thylakoid lumen• OEC is located on lumenal side of membrane

Photosynthesis in an intact plant

Photosynthesis in isolated cps

O2 is evolved as e- are passed to Ferricyanide

Some herbicides block e- flow

Photosynthesis – blocking e- transport

DCMU

Electron flow is blocked, O2 evolution is inhibited

Photosynthesis – flavin mononucleotide (FMN)

DCMU

O2 is consumed as FMN passes e- to O2 to generated H2O2

O2 can accept e- from PSI

• Mehler reaction (light dependent consumption of O2):

O2 + Fdred O2- + Fdox

• 5 - 10% of e- generated by PSI react with O2

• Plants have built in systems to “scavange” ROS• SOD (superoxide dismutase):

O2- + O2

- + 2H+ H2O2 + O2

• H2O2 must be reduced to prevent reaction with O2-

to form the highly toxic hydroxyl radical (OH.)

O2 can accept e- from PSI

• Asada-Halliwell pathway reduces H2O2:

H2O2

H2O

Ascorbate

Monodehydro-ascorbate

GSSH

GSH

NADPH

NADP+

PSI