BISC 367 Plant Physiology Lab Spring 2009 Plant
BISC 367 - Plant Physiology Lab Spring 2009 Plant Biology Fall 2006 Notices: • • General reading • Taiz and Zeiger: Chapter 7, Photosynthesis: the Light Reactions
Photosynthesis - light • Photosynthesis (P/S) uses light energy to build carbon compounds Light energy CO 2 + H 2 O Inputs: CO 2 H 2 O Light Outputs: Carbohydrate (CHO) O 2 (CH 2 O)n + O 2
Photosynthesis - light • P/S is divided into 2 stages Light NRG oxidizes H 2 O Reductant & NRG are used to reduce CO 2
Photosynthesis - light P/S takes place in chloroplasts (cp) Stroma lamellae Unstacked membrane Thylakoid Light rxns occur here Grana lamellae Stacked membranes Carbon reduction rxns occur here Stroma
Photosynthesis - light 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 Antennal pigment/ reaction centre complexes are also referred to as photosystems There are 2 photosystems • PSII
Photosynthesis - light • 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) = P 700 • – Produces a very strong reductant PSII absorbs red light (680 nm) = P 680 • Produces a strong very strong oxidant – Can oxidize water Ultimate source of e- Z scheme of Photosynthesis
Photosynthesis - light • 4 protein complexes embedded in the thylakoid membrane are involved in the light reactions • • PSII Cytochrome b 6 f complex PSI ATP synthase
Photosynthesis - light Oxidation of water: • Performed by the oxygen evolving centre (OEC) of PSII 2 H 2 O O 2 + 4 e- + 4 H+ Water is extremely stable - oxidation is not easy! 4 H+ are released into thylakoid lumen • OEC is located on lumenal side of membrane
Photosynthesis in an intact plant
Photosynthesis in isolated cps [Fe(CN)6]3[Fe(CN)6]4 O 2 is evolved as e- are passed to Ferricyanide
Some herbicides block e- flow
Photosynthesis – blocking e- transport + + DCMU - + + + + [Fe(CN)6]3+ + + Oxidized - - Reduced Electron flow is blocked, O 2 evolution is inhibited
Photosynthesis – flavin mononucleotide (FMN) DCMU 2 FMN + 2 H 2 O 2 FMNH 2 + 2 O 2 2 FMNH 2 + O 2 2 FMN + 2 H 2 O 2 is consumed as FMN passes e- to O 2 to generated H 2 O 2
O 2 can accept e- from PSI • Mehler reaction (light dependent consumption of O 2): O 2 + Fdred O 2 - + Fdox • 5 - 10% of e- generated by PSI react with O 2 • Plants have built in systems to “scavange” ROS • SOD (superoxide dismutase): O 2 - + 2 H+ H 2 O 2 + O 2 • H 2 O 2 must be reduced to prevent reaction with O 2. to form the highly toxic hydroxyl radical (OH )
O 2 can accept e- from PSI • Asada-Halliwell pathway reduces H 2 O 2: H 2 O 2 H 2 O Ascorbate GSSH NADPH Monodehydroascorbate GSH NADP+ PSI
- Slides: 16