Photosynthesis

1. Photosynthesis Autotrophs/ producers

2. Why? • Energy = the ability to do work • Energy cannot be created nor destroyed, only transformed • Electromagnetic energy (sun)  chemical bond energy + heat energy • Increase in order within the cell is coupled with a decrease in order outside the cell

3. Who? • Bacteria • Cyanobacteria • Plants • Aquatic • Photo-zone • Terrestrial • Temperate • Desert

4. Where ? • Plant cells: Organelle = Chloroplast • Chloroplast contains 3 distinct membranes • Outer membrane • Inner membrane • Thylakoid membrane *** energy site *** • Interconnected • Form stacks called grana • Surrounded by the stroma • Chlorophyll located within thylakoids

5. Where Else? • Cyanobacteria use electrons from water &solar energy to convert atmospheric CO2 into organic compounds. nH2O + nCO2 (CH2O)n+ nO2 • No chloroplasts are needed.

6. When? • Light-dependent reactions • Daylight hours • Daylight hours with suspended processes • C4 & CAM • Light-independent reactions • Day or night • Calvin cycle • Carbon-fixation reactions

7. How? • Sunlight hits chlorophyll & carotenoid pigments • Excites pigments’ electrons • Electrons move down thylakoid membrane • Electron-transport proteins pump protons (H+) across thylakoid membrane • H+-pump drives ATP synthesis in the stroma • Electron transport also drives NADP+NADPH

8. Absorption Ranges • Chlorophyll a – indigo/purple (~425nm) • Chlorophyll a - orange/red (~ 665 nm) • Chlorophyll b – indigo/ blue (~450 nm) • Carotenoids – green (~480 nm) • Not as plentiful as chlorophyll pigments • Responsible for Fall leaves, blossom & fruit colors • Only chlorophyll a is directly involved in photosynthesis; the others are accessory pigments

9. Light Reaction Details(within thylakoid membranes) • Photosystem II: light energy excites electrons • Electrons(e-) are passed to primary e- acceptor • Primary electron acceptor passes electrons to electron transport chain • Photosystem I: light excites chlorophyll a’s e- • e- are passed to different primary e- acceptor • This passes e- to a different transport chain • Energy e- lose being passed is used to move H+ in

10. Replenishing electrons • Reduction = gaining electrons • Oxidation = losing electrons • Reduction reactions couple to oxidation rxns • Photosystem II gives e- to photosystem I • NADP+ accepts e-; is reduced to NADPH • Replacement e- for photosystem II is from H2O • 2 H2O  4 H+ + 4 e- + O2(via water-splitting enzyme nearby on thylakoid membrane)

11. Making ATP • Chemiosmosis = ATP-making process • Relies on H+ concentration gradient across the thylakoid membranes • ATP synthase in the thylakoids harnesses the potential energy of the H+ gradient into chemical energy of ATP • The movement of e- drives these reactions

12. Calvin Cycle {“Carbon fixation”} • Occurs within the stroma of chloroplast • ATP & NADPH’s energy used to make 3-C sugar • Atmospheric CO2 is source of carbons • Cycle of enzymes accept C from CO2 (x 3) • 5-C ribulose bisphosphate (RuBP) accepts 1 C • RuBP+C  into two 3-phosphoglycerates (3-PGA) • ATP/NADPH drives formation of glyceraldehyde 3-phosphate (G3P) & reformation of RuBP.

13. Alternative Pathways • Hot, dry climates • Would lose water through stomata which is port of entry for CO2 • High O2 & low CO2 levels inhibit photosynthes • C4 plants (corn, sugar cane, crab grass) • Tropical climates • Make a 4-C compound & transport to other cells • CAM (cactus, pineapple, et al.) • Open stomata at night & close during day

14. Factors affecting photosynthesis • Light intensity • Directly correlated until it reaches a plateau • CO2 levels • Directly correlated until it plateaus. • Temperature • Has a peak optimal range • Enzyme-specific • Water & carbon dioxide loss with closing stomata