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 CO2 into organic compounds.

nH2O + nCO2 (CH2O)n + nO2

• No chloroplasts are needed.

When?

• Light-dependent reactions– Daylight hours– Daylight hours with suspended processes• C4 & CAM

• Light-independent reactions– Day or night– Calvin cycle– Carbon-fixation reactions

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

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

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 H2O

2 H2O 4 H+ + 4 e- + O2 (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 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.

Alternative Pathways• Hot, dry climates– Would lose water through stomata which is port

of entry for CO2

– High O2 & low CO2 levels inhibit photosynthesis

• 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

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