Download - Plant Biology Fall 2006
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