Transcript
PHOTOSYNTHESIS
Light Energy to Chemical Energy
Importance of Photosynthesis
• Source of atmospheric oxygen– Earth’s atmosphere
• 78% N2, 21% O2, 0.035% CO2
• Source of energy for food chain– Photoautotrophs vs. Heterotrophs
• Direct/indirect involvement in all products• Maintain stable ecosystem
Photosynthesis and TranspirationWhole Plant Perspective
• CO2 enters leaf through open stomata; water is lost
• CO2 is converted to carbohydrate in the leaf & transported as sucrose in phloem
• Water is taken up by roots & transported via xylem
• Loss of water from the leaf via stomata is evapotranspiration
Photosynthesis
6 H20 + 6 CO2 + Light Energy C6H12O6 + 6 O2
Chloroplast
GranumThylakoid
Stroma
Outer Membrane
Inner Membrane
Thylakoid
Thylakoid Membrane
Thylakoid SpaceGranum
Grana – Site of ‘Light Reactions’. Pigments Embedded in Thylakoid Membranes.
Stroma – Site of ‘Light-Independent Reactions’. CO2 Fixed into Sugar (Glucose). Membrane – Bounded by
2 smooth membranes
Visible light
Chlorophyll pigments harvest energy (photons) by absorbing certain wavelengths– Blue (420 nm)– Red (660 nm)
Plants are green because the green wavelength is reflected, not absorbed.
Photosynthetic Pigments
• Main photosynthetic pigment– Chlorophyll a
• Accessory Pigments– Absorb light & transfer energy to Chl a– Chlorophyll b– Carotenoids (Aids in photoprotection)
• Carotenes• Xanthophylls
Photosynthetic Unit
• Chlorophyll bound to proteins in PS
• Light harvesting molecules pass their energy at a reaction center
• Two photosystems– PS I (P700)– PS II (P680)
Processes in Photosynthesis
(1) light O2 CO2
sugar
starchH2O
(2) Light Reactions (3) Dark Reactions
NADPH
ATP
3 Processes in Photosynthesis
• Absorption of light energy– Pigments in thylakoids
• Light Reactions– Pigments & proteins in thylakoids– Generates ATP and NADPH
• “Dark” Reactions– Enzymes in stroma
– Reduce CO2 to carbohydrates using ATP & NADPH
The light-dependent reactions of photosynthesis produce chemical energy in the form of ATP and NADPH.
Lab 4B
• Expose samples to red wavelength– Why?
• Role of DPIP– Electron acceptor
• Works in place of NADP+– When DPIP is reduced, resultant increases
light transmittance
Light Reaction
• Light Dependent Reaction• Captures Light Energy
– Electrons exited to higher energy state • Produces energy from solar power (photons) in the
form of ATP and NADPH
• Splits H2O, Uses H+ & Electrons, Spits out O2 as waste
• Two possible routes for electron flow:– Noncyclic Electron Flow– Cyclic Electron Flow
Light ReactionNoncyclic Electron Flow
• Uses Photosystem II and Photosystem I• Uses Electron Transport Chain (ETC)
– Pq (Plastoquinone)– Cytochrome complex– Pc (Plastocyanin – Cu-containing proteins)
• Other proteins involved (P700)– Fd (Ferredoxin – Fe-containing protein)– NADP+ reductase
• Generates O2, ATP, and NADPH
Light ReactionCyclic Electron Flow
• Uses Photosystem I only• Uses Electron Transport Chain (ETC)
– Fd (Ferredoxin)– Cytochrome complex
• Generates ATP only– No production of NADPH
– No release of O2
Cyclic Electron Flow
Why Cyclic Electron Flow?
• Calvin cycle consumes more ATP than NADPH
• Makes up the difference needed for the Calvin cycle to function properly
Light Reactions
Inputs Outputs
Light
H2O
ADP
NADP+
O2 (Waste)
ATP
NADPH + H+
Energy Carrier Molecules Take Converted ‘Light’
Energy to Stroma
Chemiosmosis
• Powers ATP synthesis• Located in the thylakoid membranes• Uses ETC and ATP synthase (enzyme) to
make ATP• Photophosphorylation
– Addition of phosphate to ADP to make ATP
Dark Reaction
• Light Independent Reaction, Calvin Cycle, Carbon Fixation
• Occurs in the stroma • Uses energy (ATP and NADPH) from light reaction
to make sugar (glucose) from CO2
• Uses CO2 • Regenerates ‘Empty’ Carrier Molecules (NADP+ &
ADP) • To produce glucose
– It takes 6 turns and uses 18 ATP and 12 NADPH
3 Phases – Calvin Cycle
• Carbon fixation
• Reduction• G3P routes
PhotosynthesisPhotosynthesis
HH22OO COCO22
OO22 CC66HH1212OO66
Light Light ReactionReaction
Dark ReactionDark Reaction
Light is AdsorbedLight is AdsorbedBy By
ChlorophyllChlorophyll
Which splitsWhich splitswaterwater
ChloroplastChloroplast
ATP andATP andNADPHNADPH22
ADPADPNADPNADP
Calvin CycleCalvin Cycle
EnergyEnergy
Used Energy and is Used Energy and is recycled.recycled.
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Types of plantsProcesses of Photosynthesis
• C3 plants• C4 plants• CAM plants
C3 photosynthesis
• Called ‘C3’ - 3-carbon acid (PGA)• Catalyzed by enzyme Rubisco
– Most abundant enzyme on Earth! • C3 pathway used by 85% of plants• Most trees and many crops use C3
photosynthesis
Relatively high water loss
Photorespiration
• Occurs on hot, dry, bright days• Stomata close• Fixation of O2 instead of CO2
• Produces 2-C molecules instead of 3-C sugar molecules
• Produces no sugar molecules or no ATP• Plants have special adaptations to limit the
effect of photorespiration.– C4 plants & CAM plants
C4 Plants
• Hot, moist environments• 15% of plants (grasses, corn, sugarcane)• Divides photosynthesis spatially• Light rxn - mesophyll cells• Calvin cycle - bundle sheath cells
C4 photosynthesis
• Four carbon molecule as 1st intermediate• Catalyzed by phosphoenolpyruvate
carboxylase (PEP carboxylase)• PEP carboxylase reacts effectively with CO2
at low concentrations• C4 plants can achieve high photosynthesis
with small stomatal opening, thereby saving water
• Carbon Fixation & Calvin cycle occur in two types of cells
CAM Plants
• Crassulacean Acid Metabolism • Hot, dry environments• 5% of plants (cactus and ice plants)• Stomates closed during day• Stomates open during the night
– Lower temperatures at night reduce water loss• Light rxn - occurs during the day• Calvin Cycle - occurs when CO2 is present• Carbon Fixation and Calvin cycle occur in the
same cell but at different times
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