Chapter 8 Photosynthesis

Trophic Roles Autotrophs  produce organic molecules from CO2 & inorganic raw materials  “producers”  plants Heterotrophs  “consumers”

Chloroplast Structure Found in mesophyll 1 mesophyll cell may have 30 chloroplasts Stomata regulate passage of CO2, O2 and H2O

Chloroplast Structure

Chloroplast Structure (cont.) Pigments  light receptors Absorb light energy Boost e- Chlorophyll Chlorophyll “a”  main pigment  blue-green Chlorophyll “b”  accessory pigment  yellow-green Accessory pigments  absorb different wavelengths of light Carotenoids  yellow-orange

Photosystems Pigment molecules absorb energy  boost electrons  unstable Passes energy to reaction center of antenna complex (chlorophyll “a” molecule)  transfers energy to primary electron acceptor

Photosynthesis Light reactions Dark reactions Calvin Cycle Cyclic photophosphorylation Non-cyclic photophosphorylation Photolysis (breaking water up with light) Dark reactions Calvin Cycle

Photosystems (cont.) Located in thylakoid membrane Photosystem I  P700  absorbs light of 700 nm Photosystem II  P680  absorbs light of 680 nm Both are primarily chlorophyll “a” Electron acceptor  NADP+  NADPH

Non-cyclic photophosphorylation

Cytochrome complex synthase reductase LIGHT REACTOR NADP+ ADP ATP NADPH CALVIN CYCLE [CH2O] (sugar) STROMA (Low H+ concentration) Photosystem II H2O CO2 Cytochrome complex O2 1 1⁄2 2 Photosystem I Light THYLAKOID SPACE (High H+ concentration) Thylakoid membrane synthase Pq Pc Fd reductase + H+ NADP+ + 2H+ To Calvin cycle P 3 H+ 2 H+ +2 H+

Cyclic Photophosphorylation Primitive  used by bacteria  only generates energy--no glucose Electron “boosted” out of PI  ETC  returned to PI Electron drives proton pump  chemiosmosis  ATP

Calvin Cycle aka Dark Reactions Occur in the dark or the light Light independent reactions 3 “steps” Carbon fixation Reduction Regeneration of RuBP

Step 1: Carbon Fixation RuBP (ribulose bisphosphate)  5 C sugar  catalyzed by RuBP carboxylase (Rubisco)  unstable compound (splits)  PGA

Step 2: Reduction Phosphorylated by ATP Reduced by NADPH Produces pyruvate some pyruvate  glucose most pyruvate  regenerate RuBP

Step 3: Regeneration of RuBP Pyruvate  rearranged into RuBP Requires input of 3 ATP Takes 12 turns of cycle  1 glucose

http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

Alternative mechanisms: Photorespiration Competitive reaction between RuBP, CO2 and O2 Rubisco substitutes O2 for CO2  2 C compound (phosphoglycerate) Eventually broken down  releases CO2 Non-productive C3 plants  rice, wheat, soybeans (hot, bright days) Photo (light) respiration( releases CO2/consumes O2)

Alternative mechanisms: C4 Plants Sunny ecosystems Carbon “fixed” outside cells (in bundle sheath cells  very efficient  requires extra ATP Balances out photorespiration Corn is a C4 plant

Alternative mechanisms: CAM Plants Crassulacean acid metabolism  Hot/dry climates Orchids, cacti, etc. Stomates open at night to reduce water loss  evaporation CO2 is fixed, used later