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
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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
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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”}
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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 3phosphate (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