Photosynthesis Autotrophs/ producers

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

Who? Bacteria Cyanobacteria Plants – Aquatic Photo-zone – Terrestrial Temperate Desert

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

Where Else? Cyanobacteria use electrons from water & solar energy to convert atmospheric CO 2 into organic compounds. nH 2 O + nCO 2  (CH 2 O) n + nO 2 No chloroplasts are needed.

When? Light-dependent reactions – Daylight hours – Daylight hours with suspended processes C 4 & CAM Light-independent reactions – Day or night – Calvin cycle – Carbon-fixation reactions

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

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

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

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 H 2 O  2 H 2 O  4 H e - + O 2 (via water-splitting enzyme nearby on thylakoid membrane)

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

Calvin Cycle {“Carbon fixation”} Occurs within the stroma of chloroplast ATP & NADPH’s energy used to make 3-C sugar Atmospheric CO 2 is source of carbons Cycle of enzymes accept C from CO 2 (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.

Alternative Pathways Hot, dry climates – Would lose water through stomata which is port of entry for CO 2 – High O 2 & low CO 2 levels inhibit photosynthes C 4 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

Factors affecting photosynthesis Light intensity – Directly correlated until it reaches a plateau CO 2 levels – Directly correlated until it plateaus. Temperature – Has a peak optimal range Enzyme-specific Water & carbon dioxide loss with closing stomata