Lecture 3 Outline (Ch. 8) Photosynthesis overview A. Purpose B. Location The light vs. the “dark” reaction Chloroplasts pigments A. Light absorption B. Types Light reactions A. Photosystems B. Photophosphorylation V. The light independent reaction (“dark” reaction) A. Carbon “fixation” B. Reduction C. Regeneration VI. Alternative plants

Photosynthesis - overview

Photosynthesis - overview Overall purpose: • photosynthesis – light chemical energy • complements respiration Energy for all life on earth ultimately comes from photosynthesis

Cellular Respiration vs. Photosynthesis (Exergonic) Photosynthesis: (Endergonic)

Photosynthesis – chloroplast recap Outer membrane Inner membrane Thylakoid membrane Stroma Thylakoid space Intermembrane space

Photosynthesis - overview 1. light rxn: store energy & split water – “photo” NADPH & ATP 2. dark rxn: “fix” CO2 & make sugars – “synthesis” Calvin cycle

Redox Reactions 6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O Equation for photosynthesis

Photosynthesis - overview • light reactions: • dark reactions: – thylakoid membrane – stroma Light H2O Chloroplast Reactions NADP+ P ADP + ATP NADPH O2 Calvin Cycle CO2 [CH2O] (sugar) – thylakoid space

Photosynthesis – light absorption • visible light ~380 to 750 nm • chloroplast pigments – abs blue-violet & red - transmit and reflect green

Photosynthesis – light absorption • pigments: • chlorophyll a -energy-absorbing ring -hydrocarbon tail • accessory pigments - chlorophyll b - carotenoids - photoprotective

Photosynthesis – light absorption • chlorophyll a – abs blue-violet, red 400-450, 650-700 nm • chlorophyll b & carotenoids – abs broadly blue-violet mid-400s • more wavelengths used for photosynthesis = more light energy absorbed

Photosynthesis – light absorption Pigments have two states: ground & excited • chlorophyll abs light • e- excited • more energy • energy transferred

Photosynthesis – light absorption Pigments are held by proteins in the thylakoid membranes light harvesting complex • energy absorbed from light - to pigments • to reaction center - two special chlorophyll a - proteins - 1° electron acceptor • light harvesting complex & reaction center = photosystem (PS)

Photosynthesis – energy transfer Light Thylakoid membrane THYLAKOID SPACE STROMA Photosystem II Photosystem I • Photosystem I (PS I) & PS II • Difference – light wavelength, proteins,  where e- from

Photosynthesis – energy transfer • PSII: absorbs 680 nm, splits water, powerful ETC, ATP made • PS I: absorbs 700 nm, (less energy) e- from PSII, short ETC, NADPH made

Photosynthesis – energy transfer • e- in PS II, from split H20 • e- from PS II electron transport chain (ETC) PS I • e- from PS I 2nd ETC e- carrier: NADP+  NADPH

Photosynthesis – chemiosmosis • How is ATP produced? Chemiosmosis • e- down ETC, H+ to thylakoid space • H+ conc. gradient • H+ down gradient, ATP synthase photophosphorylation

Light reaction - summary • inputs: light energy, H2O • PS II, ETC, PS I, ETC • outputs: ATP NADPH O2 (waste)

Self-Check Know figures of chloroplast reactions/locations!  Step of Photosynthesis Location IN chloroplast Inputs Outputs ATP produced? (don’t need #) e- carriers loaded? Light reaction overall PSII PSI “Dark” reaction overall Know figures of chloroplast reactions/locations! 

Photosynthesis – energy transfer

“Dark” reaction (Light-independent Reaction) 6CO2 + 12H2O + light energy C6H12O6 + 6O2 + 6H2O • “Dark” reaction: Calvin cycle CO2 NADP+ ADP P i + RuBP 3-Phosphoglycerate Calvin Cycle G3P ATP NADPH Starch (storage) Sucrose (export) Chloroplast Light H2O O2 • regenerative • anabolic • CO2 in, sugar out • during daylight

Carbon fixation • 3 stages of Calvin-cycle: • #1 – carbon fixation • CO2 link to 5-C • 5-C: ribulose bisphosphate (RuBP) - enzyme: Rubisco abundant • 6-C unstable – split  2(3-C)

Reduction • 3 stages of Calvin-cycle: • #2 – reduction • 3-C reduced • e- from NADPH • reduced 3-C: G3P

Regeneration of C-acceptor • 3 stages of Calvin-cycle: • #3 – regenerate C-acceptor • still 5 G3P  3 RuBP • multiple steps • uses ATP • every 3 cycles: 1 G3P made 3 RuBP regenerated • C3 plants – CO2 fixed into 3-C

Step of Photosynthesis Location IN chloroplast Self-Check Step of Photosynthesis Location IN chloroplast Inputs Outputs ATP produced (don’t need #) e- carriers loaded Light reaction overall PSII PSI “Dark” reaction overall

Alternate methods of C fixation

Alternate methods of C fixation CO2 NADP+ ADP P i + RuBP 3-Phosphoglycerate Calvin Cycle G3P ATP NADPH Starch (storage) Sucrose (export) Chloroplast Light H2O O2 • CO2 in  stomata • open, lose water • hot, dry – open stomata less; lowers water loss, lowers CO2 • O2 fixed – photorespiration – inefficient • fix CO2 into 4-C molecules

Photosynthesis – summary • light reaction: Light energy + H2O O2, NADPH, ATP Thylakoids • light-independent: CO2, NADPH, ATP G3P (sugar), RuBP Stroma

Photosynthesis – summary Where do photosynthetic products go?

Photosynthesis – in context of big picture

Lecture 3 Summary 1. Photosynthesis Overview (Ch. 8) Purpose Redox reactions Electron carriers & sugars 2. Light (Ch. 8) Absorption pigments Light spectra/wavelengths 3. Locations of steps, inputs/outputs, purpose, description (Ch. 8) PSI vs. PS II Whole light reaction [includes chemiosmosis] “dark” reaction/Calvin cycle [3 steps] 4. Alternate modes of photosynthesis (Ch. 8) 5. Photosynthesis context (Ch. 8) - Uses for products - Relationship of cell respiration and photosynthesis