Photosynthesis
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Transcript of Photosynthesis
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Photosynthesis
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Photosynthesis
• Plants capture light energy from the sun • Energy is converted to chemical energy (sugars
& organic molecule)
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Autotrophs• Photosynthesizers are autotrophs – organisms that
produce organic molecules from CO2 & inorganics from environment.
• Photoautotrophs - plants, algae, some other protists, and some prokaryotes
• Chemoautotrophs – oxidize inorganics (S, NH3).
Unique to bacteria.
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Heterotrophs
• Live on products of other organisms • Consumers• Decomposers• Completely dependent on autotrophs
for byproducts of photosynthesis
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Location of Photosynthesis
• Chloroplasts – any green part of plant, primarily leaves
• ½ million chloroplasts/mm2 of leaf surface• Green color derived from pigment
chlorophyll• Chlorophyll important in light absorption
(more on that later)
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Chloroplasts in Elodea
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Location of Chloroplasts
• Found mainly in mesophyll cells – interior of leaf
• O2 exits and CO2 enters leaf through stomata
• Stomata in close proximity to chloroplasts – WHY?
• Veins deliver H20 from roots and carry off sugar to other areas where needed
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• Typical mesophyll cell has 30-40 chloroplasts
• Chloroplast structure – remember? • Thylakoids/
grana, stroma
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Leaf cross section
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Photosynthesis – Redox Rxn Recall -
RESPIRATION • C6H12O6 + 6 O2 + 6 H2O 6 CO2 + 12 H2O +
Energy OR
• C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy
• Glucose is oxidized to form CO2
• Oxygen is reduced, forming water• Reaction is EXERGONIC
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Photosynthesis Equation• Photosynthesis reverses aerobic respiration• Net process of photosynthesis is:
6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
• Water split and e- transferred to CO2, reducing it to sugar
• Byproduct: 6O2
• Reaction is ENDERGONIC
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Free Oxygen
• Plants give off O2, split from H2O not CO2
• C.B. van Neil, studies with H2S in bacteria
• Later scientists used radioactive tracer 18O
to confirm van Neil’s H2O hypothesis
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Photosynthesis Equation
• Where does the energy to power the reaction come from?
• In reality, photosynthesis adds one CO2 at a time (carbon fixation)CO2 + H2O + light energy -> [CH2O]* + O2
*CH2O represents the general formula for a sugar.
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Photosynthesis: A closer look…
Two major components• LIGHT REACTIONS
(PHOTOPHOSPHORYLATION) - conversion of light energy to chemical energy
• CALVIN CYCLE (DARK REACTIONS) – transforms atmospheric CO2 to organic molecule; uses energy from light rxn to reduce to sugar.
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Light Reactions: Overview
• Light energy absorbed by chlorophyll in
thylakoids
• Drives the transfer of e- to NADP+
(nicotinamide adenine dinucleotide
phosphate), forming NADPH
• Generates ATP by photophosphorylation
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What is LIGHT?
• Electromagnetic energy – travels in waves• Distance between waves is the
wavelength• ↓ wavelength = ↑ energy• ↑ wavelength = ↓ energy • Measured via electromagnetic spectrum• Visible light = 380 – 750 nm
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Photons
• Direct particles of energy
• Intensity inversely related to wavelength
• Purple/blue light carries much more
energy than orange/red range of spectrum
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• When light meets matter, it is either reflected, transmitted or absorbed
• Different pigments absorb photons of different wavelengths
• WHY ARE LEAVES GREEN?
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Spectrophotometer• Measures
pigment’s ability to absorb wavelengths
• Uses transmittance
• Absorption spectrum
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• Thylakoids - 3 major pigments• Chlorophyll a – dominant pigment.
Red & blue absorption• Chlorophyll b and carotenoids
• Slightly different absorption• Funnel energy to chloro a• PHOTOPROTECTION (carotenoids)
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Action Spectrum • All the pigments together determine “action
spectrum” for photosynthesis
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• Action spectrum ≠ absorption spectrum of ONE pigment
• Engelmann 1883 – aerobic bacteria indic. O2
& absorption
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Capturing Light Energy • Molecule absorbs photon• Causes e- to elevate to orbital with more
potential energy• “Ground” state to “excited” state• Molecules absorb photons that match the
energy difference between ground and excited state of e-
• Corresponds to specific wavelengths, absorption spectrum
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• Photons are absorbed by clusters of pigment molecules in thylakoid membranes
• Energy of photon converted to potential energy of e- raised from ground state to excited state
• In chlorophyll a and b, an electron from Mg in the porphyrin ring is excited
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Chlorophyll “head”
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• Excited e- unstable• Drop to ground state in billionth of a
second, releasing heat energy• Chlorophyll & other pigments release
photon of light (fluorescence) without an e- acceptor
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Photosystems• In thylakoid membrane, chlorophyll
organized photosystems• Acts like a light-gathering “antenna” • Hundreds of chloro a, b, and carotenoids• Some proteins, other small organic molec.
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• Photon absorbed by any antenna molecule
• Transmitted from molecule to molecule until reaches reaction center
• At reaction center is a primary electron acceptor
• Removes an excited e- from chloro a in reaction center • This starts the light reactions
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Photosystem I & II
• Photosystem I has an absorption peak at 700nm - its rxn center is called the P700 center
• Photosystem II - rxn center at 680nm.• Differences between reaction centers
due to the associated proteins• Photosystems work together to
generate ATP and NADPH.
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Cyclic & Noncyclic Electron Flow• During light rxn, e- can flow 1 of 2 ways:
• Noncyclic electron flow, the predominant route, produces both ATP and NADPH
• Under certain conditions, photoexcited electrons from photosystem I, but not photosystem II, can take an alternative pathway, cyclic electron flow
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Noncyclic Pathway• Similar to oxidative phosphorylation
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1. Photosystem II absorbs light, captures an excited electron (rxn ctr oxidized)
2. Enzyme extracts e- from H2O and donates to oxidized reaction center
• P680 is the strongest oxidizing agent known – it must be filled with e-
3. Photoexcited e- pass along ETC from PSII to PSI
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• Electron carriers: • Pq (plastoquinone), a cytochrome complex • Pc (plastocyanin), a protein
4. Exergonic fall of e- provides energy for ATP synthesis
Meanwhile - in Photosystem I…
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5. PS I rxn center excited, releasing photoexcited electron
• e- captured by acceptor creating an e- hole in P700 center
• Hole filled by e- from the PS II ETC
6. 2nd ETC in PS I. Electron carrier is Fd (ferredoxin), a protein
7. NADP reductase transfers e- from Fd to NADP, reducing to NADPH
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Cyclic Pathway • Under certain conditions, photoexcited e-
from PS I (not PS II), take an alternative pathway, cyclic electron flow
• “Short circuit” – no NADPH or O2 produced• Excited e- cycle from rxn center to primary
acceptor, along ETC, and return to oxidized P700 chlorophyll
• As e- flow along ETC, they generate ATP by cyclic photophosphorylation.
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Benefits of Cyclic Pathway• Noncyclic e- flow produces ATP and
NADPH in roughly equal quantities• Calvin cycle consumes more ATP than
NADPH• Cyclic electron flow allows chloroplast to
generate extra ATP to satisfy the Calvin cycle
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Chemiosmosis• Chloroplasts and mitochondria generate
ATP by the same mechanism• ETC pumps protons across membrane as e-
are passed along a series electronegative carriers.
• Builds proton-motive force in the form of H+ gradient
• ATP synthase harness this force to generate ATP as H+ diffuses back across membrane.
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• The proton gradient, or pH gradient, across thylakoid membrane is substantial• When illuminated, the pH in thylakoid space
drops to about 5 and the pH in stroma increases to about 8, a thousandfold difference in H+ concentration
• Produces ATP and NADPH on the stroma side of the thylakoid
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Overall products
• Noncyclic flow pushes e- from H2O to NADPH, where they have high potential energy• This process also produces ATP• Oxygen is a byproduct
• Cyclic flow converts light energy to chemical energy in the form of ATP
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