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(a) Plants (c) Unicellular protist 10 µm (e) Purple sulfur bacteria (b) Multicellular alga (d) Cyanobacteria 1.5 µm 40 µm 1 Leaf cross section Vein Mesophyll Stomata Chloroplast CO2 O2 Mesophyll cell Outer membrane Thylakoid Stroma Granum Thylakoid space Intermembrane space 5 µm Inner membrane 1 µm 2 Reactants: Products: 6 CO2 C6H12O6 12 H2O 6 H2 O 6 O2 3 CO2 H2O Light NADP+ ADP + P i Light Reactions Calvin Cycle ATP NADPH Chloroplast O2 [CH2O] (sugar) 4 10–5 nm 10–3 nm 103 nm 1 nm Gamma X-rays rays UV 106 nm Infrared 1m (109 nm) Microwaves 103 m Radio waves Visible light 380 450 500 Shorter wavelength Higher energy 550 600 650 700 750 nm Longer wavelength Lower energy 5 Light Reflected light Chloroplast Absorbed light Granum Transmitted light 6 RESULTS Chlorophyll a Chlorophyll b Carotenoids (a) Absorption spectra 400 500 600 700 Wavelength of light (nm) (b) Action spectrum Aerobic bacteria Filament of alga (c) Engelmann’s experiment 400 500 600 700 7 CH3 CHO in chlorophyll a in chlorophyll b Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown 8 Energy of electron e– Excited state Heat Photon (fluorescence) Photon Chlorophyll molecule Ground state (a) Excitation of isolated chlorophyll molecule (b) Fluorescence 9 Photosystem STROMA Light-harvesting Reaction-center complex complexes Primary electron acceptor Thylakoid membrane Photon e– Transfer of energy Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) 10 4 Primary acceptor 2 H+ + 1/ O 2 2 H2O e– 2 Primary acceptor e– Pq Cytochrome complex 7 Fd e– e– 8 NADP+ reductase 3 NADPH Pc e– e– NADP+ + H+ P700 5 P680 Light 1 Light 6 ATP Pigment molecules Photosystem II (PS II) Photosystem I (PS I) 11 e– ATP e– e– NADPH e– e– e– Mill makes ATP e– Photosystem II Photosystem I 12 Primary acceptor Primary acceptor Fd Fd Pq NADP+ reductase Cytochrome complex NADP+ + H+ NADPH Pc Photosystem I Photosystem II ATP 13 Mitochondrion Chloroplast MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE H+ Intermembrane space Inner membrane Diffusion Electron transport chain Thylakoid space Thylakoid membrane ATP synthase Stroma Matrix Key ADP + P i [H+] Higher Lower [H+] H+ ATP 14 STROMA (low H+ concentration) Cytochrome Photosystem I complex Light Photosystem II 4 H+ Light Fd NADP+ reductase NADP+ + H+ NADPH Pq H2O THYLAKOID SPACE (high H+ concentration) e– 1 e– 1/ Pc 2 2 3 O2 +2 H+ 4 H+ To Calvin Cycle Thylakoid membrane STROMA (low H+ concentration) ATP synthase ADP + Pi ATP H+ 15 Input 3 (Entering one at a time) CO2 Phase 1: Carbon fixation Rubisco 3 P Short-lived intermediate P 6 P 3-Phosphoglycerate 3P P Ribulose bisphosphate (RuBP) 6 ATP 6 ADP 3 ADP 3 Calvin Cycle 6 P P 1,3-Bisphosphoglycerate ATP 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 Pi P 5 G3P 6 P Glyceraldehyde-3-phosphate (G3P) 1 Output P G3P (a sugar) Phase 2: Reduction Glucose and other organic compounds 16 The C4 pathway C4 leaf anatomy Mesophyll cell Mesophyll cell CO2 PEP carboxylase Photosynthetic cells of C4 Bundleplant leaf sheath cell Oxaloacetate (4C) Vein (vascular tissue) PEP (3C) ADP Malate (4C) Stoma Bundlesheath cell ATP Pyruvate (3C) CO2 Calvin Cycle Sugar Vascular tissue 17 Sugarcane Pineapple C4 CAM CO2 Mesophyll cell Organic acid Bundlesheath cell CO2 1 CO2 incorporated into four-carbon Organic acid organic acids (carbon fixation) CO2 Calvin Cycle CO2 2 Organic acids release CO2 to Calvin cycle Night Day Calvin Cycle Sugar Sugar (a) Spatial separation of steps (b) Temporal separation of steps 18 H2O CO2 Light NADP+ ADP + P i Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain RuBP ATP NADPH 3-Phosphoglycerate Calvin Cycle G3P Starch (storage) Chloroplast O2 Sucrose (export) 19