photosynthesis conversion of light energy from the sun into stored chemical energy in the form of...
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Photosynthesis • Conversion of
light energy from the sun into stored chemical energy in the form of glucose and other organic molecules
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Site of Photosynthesis• Photosynthesis takes place
in mesophyll tissue• Cells containing chloroplasts
– Specialized to carry out photosynthesis
• CO2 enters leaf through stomata (pore)– Exchange of gases occurs
here– Controlled by guard cells
(opening/closing)
• CO2 diffuses into chloroplasts
• CO2 fixed to C6H12O6 (sugar)
• Energy supplied by light
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Chloroplasts• Site of Photosynthesis• Consists of • Stroma– Aqueous environment– Houses enzymes used
for reactions• Thylakoid membranes– Form stacks of flattened
disks called grana– Contains chlorophyll
and other pigments
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Photosynthesis • 2 stages1. Light-dependant reactions
– Photosystem II and I– Occurs in the thylakoid
membrane of chloroplasts– capture energy from sunlight– make ATP and reduce NADP+
to NADPH
2. Calvin Cycle (light-independent reactions)– Occurs in stroma of
chloroplast– use ATP and NADPH to
synthesize organic molecules from CO2
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Capturing Light Energy • Pigments– Absorb photon (wave
of light)– Excited electron moves
to a high energy state– Electron is transferred
to an electron accepting molecule (primary electron acceptor)
• Chloryphyll a – donates electrons to
PEA
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Accessory Pigments• Chlorophyll b and carotenoids
– Known as antenna complex– Transfers light energy to chlorophyll a– Chlorophyll donates electrons to PEA
• A pigment molecule does not absorb all wavelengths of light
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Pigments • Photosynthesis depends on the absorption of light
by chlorophylls and carotenoids
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Pigments and Photosystems• Chlorophylls and
carotenoids do not float freely within thylakoid
• Bound by proteins • Proteins are
organized into photosystems
• Two types– Photosystem I– Photosystem II
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Photosystem I and II• Composed of
– Large antenna complex
– 250-400 pigment molecules surrounding reaction centre
• Reaction Centre– Small number of
proteins bound to chlorophyll a molecules and PEA
• PI - Contains p700• PII - Contains
p680
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Photosystem II 1. Oxidation of p680– Photon absorbed excites p680– Transfers e⁻ to PEA– e⁻ supplied by splitting of a
water molecule inside lumen
2. Oxidation-reduction of plastiquinone– PEA transfers e⁻ to
plastiquinone • Plastiquinone
– shuttles electrons between PII and cytochrome complex
– responsible for increase proton concentration in thylakoid lumen
3. Electron transfer to PI– Cytochrome complex transfers
e⁻ to plastocyanin• Plastocyanin
– Shuttles electrons from cytochrome complex to PI
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Photosystem I1. Oxidation-reduction of p700– Photon absorbed excites
p700– p700 transfers electron to
PEA– P700⁺ forms ready to
accept another e⁻ from plastocyanin
2. Electron transfer to NADP⁺ by ferredoxin– PEA transfer e⁻ to
ferredoxin• Ferredoxin
– Iron-sulfur protein– Oxidation of ferredoxin
reduces NADP⁺ to NADP
3. Formation of NADPH– Ferredoxin transfers
second e⁻ and H⁺– NADP⁺ reductase
reduces NADP to NADPH
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Linear Electron Transport and ATP Synthesis
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The Role of Light Energy • Z scheme– Two photons of light needed for production of NADPH– p700 molecule too electronegative to give up e⁻– Second photon needed to move e⁻ further away from nucleus of
p700 so it can transfer to NADP⁺
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Oxygen• How many photons of light are needed to
produce a single molecule of oxygen?– 2 H₂O → 4 H⁺ + 4 e⁻ + O₂
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Chemiosmosis and ATP Synthesis• Proton gradient inside lumen increases
– e⁻ transfer by plastoquinone between PII and cytochrome complex– Water molecule splitting inside lumen – Removal of H⁺ from stroma for each NADPH molecule produced
• Proton-motive force created inside thylakoid lumen • ATP synthase uses proton-motive force to synthesize ATP
molecule
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Cyclic Electron Transport• PI can function independently from PII• Ferredoxin does not transfer e⁻ to NADP⁺• Ferredoxin transfers e⁻ back to plastoquinone• Plastoquinone continually moves protons into thylakoid lumen• Splitting of water molecule not needed • Produces additional ATP molecules (photophosphorylation)
– Reduction of CO₂ requires ATP – Occur during drought (no water) or abundance of NADPH
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Light-Independent Reactions• Carbon Fixation
– Series of 11 enzyme-catalyzed reactions
– NADPH reduces CO₂ into sugars
– Overall process is endergonic
– ATP is hydrolyzed to supply energy of reactions
• Divided into three phases– Fixation– Reduction– Regeneration
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Calvin Cycle: Fixation• CO₂ is attached to 5C
RuBP molecule• 6C molecule is produced– 6C splits into 2 3C
molecules (3PG)• RuBisco– RuBP carboxylase– Most abundant protein
on earth– Involvd in first major step
of carbon fixation • CO₂ is now fixed– Becomes part of
carbohydrate
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Calvin Cycle: Reduction• Two 3PG is
phosphorylated– ATP is used
• Molecule is reduced by NADPH
• Two G3P are produced
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Calvin Cycle: Regeneration• RuBP is regenerated for cycle to continue
– Takes 3 cycles – 3 molecules of CO₂– Produces 3 RuBP molecules
• Process (3 turns of cycle)– 3CO₂ combine with 3 molecules of RuBP– 6 molecules of 3PG are formed– 6 3PG converted to 6 G3P– 5 G3P used to regenerate 3 RuBP molecules– 1 G3P left over (This process occurs 2x – 6CO₂ found in reactants)
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Glyceraldehyde-3-phosphate (G3P)• Ultimate goal of photosynthesis• Raw material used to synthesize all other organic plant
compounds (glucose, sucrose, starch, cellulose)• What is required to make 1 molecule of G3P?– 9 ATP– 6 NADPH
• What is required to make 1 molecule of glucose?– 18 ATP– 12 NADPH– 2 G3P
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Alternate Mechanisms of Carbon Fixation • Problems with photosynthesis in C₃
plants • Not enough CO₂ - 0.04% of atmosphere• Rubisco
– can also catalyze O₂– Slows Calvin Cycle, consumes ATP,
releases carbon (photorespiration)• Decrease carbon fixation up to 50%
– Wasteful to cell – Costs 1 ATP and 1 NADPH
• Stomata– Hot dry climates – closes to prevent
water loss – Low levels of CO₂
• Instead of plant producing 2 G3P molecules
• Plant produces 1 G3P molecule and 1 phosphoglycolate (toxic)
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C₄ Cycle• Minimize photorespiration• Calvin Cycle
– Performed by bundle-sheath cells• Separates exposure of Rubisco to O₂
• C₄ Cycle– CO₂ combines with PEP (3 carbon molecule)– Produces oxaloacetate (4 carbon molecule)– Oxaloacetate reduced to malate– Malate diffuses into bundle-sheath cells and enters chloroplast– Malate oxidized to pyruvate releasing CO₂
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Benefits of C4 Plants • Can open
stomata less• Require 1/3 to
1/6 as much rubisco
• Lower nitrogen demand
• Run C3 and C4 cycles simultaneously
• Corn
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CAM Plants• Crassulacean
Acid Metabolism– Run Calvin
Cycle and C4 at different time of the day
– C4 - night– Calvin Cycle
– day • Cactus