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Autotrophs
- Photosynthesis is performed by autotrophs
- --make their own food by CO2, H20, light
- Specifically photoautotrophs (vs/ chemoautotrophs)
- Autotrophs are the primary producers
- --store solar energy in sugars
- --first source of organic compounds
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Leaf Structure
- "Middle" leaf tissue captures light
- Gases enter/exit leaf via stomata (pores
- Water absorbed by roots, transported to leaves
- CO2+H2O+light are the requirements for photosynthesis
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Chloroplasts structure
- Site of photosynthesis in plants (organelles)
- All green parts of the plant have chloroplasts
- green pigments
- absorbs light energy
- Two outer membranes
- -small intermembrane space
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Photosynthesis equation
- 6CO2+6H20+light->C6H12O6+6O2
- equation is revers of respiration, but not the reverse of respiration
- Water is split and O2 is released
- Carbon in CO2 fixed in sugar (glucose)
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Stroma
- Internal fluid of chloroplast
- DNA, ribosomes, enzymes
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Thylakoids
- Internal membrane system of chloroplast
- flattened sacs
- Thylakoids can be in stacks
- --Granum (grana plasma)
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Sunlight energy and visible spectrum
- Light is electromagnetic energy
- variable by wavelength
- Visible light is absorbed by chloroplasts (includes colors that we see (ROYGBIV)
- Pigments are light receptors
- Ex. Chlorophyll absorbs red/blue and reflects green
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Photorespiration
- Wasteful process--different than cellular respiration
- Rubisco picks up the O2 and not CO2
- -Only CO2 reacion will fix carbon but O2 preferentially taken in certain conditions/ (no net carbon uptake, sugar can't be made)
- Occurs with high temperature and water stress
- Alternative mechanisms decrease photorespiration (C4, CAM photosynthesis)
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CAM Photosynthesis
- Crassulacean acid metabolism (specimen in hot dry climate-like cacti)
- Open stomata (pores) at night and closed during the day saves water, less drought, less photorespiration.
- -Avoid evaporation during the heat of the day
- -Store CO2 cells until daylight
- CO2 extracted, then normal Calvin cycle occurs. ATP and NADPH (light rxns) are available
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C4 Photosynthesis
- Plants in hot, dry climate (sugarcane)
- Calvin cycle preceded by different fixation
- -initially by rubisco
- CO2 is fixed, transported, and concentrated
- -Fixed into a 4c molecule first (mesophyll)
- -4C molecule moves to bundle sheath cell (unique leave anatomy)
- -CO2 extracted and normal Calvin cycle occurs
- Calvin cycle proceeds in high CO2 environment (Rubisco picks up CO2, photorespiration avoided)
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Role of light energy
- light energy excites chlorophyll
- chlorophyll loses electron
- NADP+ accepts the electron then reduces to NADPH
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Role of water
- water molecule is split
- replaces chlorphyll electron, O2 released
- ATP produced via photophosphorylation
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Linear pathways
- Photosystems help synthesize NADPH and ATP
- -light is the energy source (light reactions)
- 1) Light absorbed by PS II, e- released
- 2) excite electron by 1 degree receptor
- 3) Water is split to replace PS II electron (oxygen created as biproduct)
- 4)Electron moves to PS I, via electron transport chain
- 5) ATP synthesis via e- transport chain
- 6) PS I releases excited electron
- 7) Second electron transport chain
- 8) Ends in NADP+ + H+ +e- -> NADPH
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Chlorophyll A/B
- A: Plant pigments directly involved with light reactions
- B: very similar in structure to A.
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Carotenoids
- Orange/Yellow pigments
- Broaden spectrum of light used for photosynthesis
- protects against excessive light energy
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Photosystem I
absorbs 700 nm light best
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Photosystem II
- absorbs 680 nm light best
- functions first and then supplies PSI with e-
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Reaction Centers
- Reaction center complex
- -Has a pair of central Chlorophyll A molecules
- -Recieves most of surrounded absorbed energy
- Reaction center Chlorophyll A loses electron
- -primary electron acceptor recieves excited e-
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Chemiosis
- •Produces ATP in light reactions
- –As a result of electron transport chains
- –Very similar to mitochondrial process
- •Creates high [ H+ ] across membrane
- –PS: sunlight drives electron transport
- •Gradient across thylakoid membrane
- –Resp: food oxidation drives electron transport
- •Gradient across inner mitochondrial membrane
- •Allows ATP synthase to produce ATP
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Products of Light RxN
ATP produced via photophosphorylation
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Calvin Cycle
- Dont require light energy
- generally occur in daytime
- Carbon Fixation- incorporates CO2 from air and "Fixes" carbon into sugar molecules
- Requires NADPH and ATP from light rxns and supply energy and electrons to reduce fixed carbon.
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calvin cycle step 1
- 1). Carbon Fixation
- –CO2 attaches to 5C sugar RuBP
- •RuBP = ribulose bisphosphate
- •Reaction facilitated by RuBP carboxylase
- –“Rubisco”: most abundant enzyme on earth
- –Creates unstable 6C intermediate
- –6C splits and forms two 3C molecules
- •3-phosphoglycerate (3PGA)
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Calvin Cycle step 2
- Reduction
- –3C molecule is phosphorylated & reduced (to
- G3P)
- •Phosphorylated by ATP
- •Receives electrons (reduced) from NADPH
- –Some glyceraldehyde-3-P (G3P) leaves cycle
- •Used to make glucose
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Calvin Cycle Step 3
- Regeneration of RuBPG3P remaining in cycle forms RuBP
- -ATP required to transform G3P
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