Photosynthesis

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Photosynthesis

• Process in which light energy is used to drive
  the conversion of CO2 to carbohydrate.

light

• 6H2O 6CO2 C6H12O6 6O2

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Importance of Photosynthesis

• Source of atmospheric oxygen
• Earths atm 78 N2, 21 O2, 0.035 CO2
• (or 350 ppm rising)
• Source of energy for food chain
• photoautotrophs vs. heterotrophs
• Direct/indirect involvement in all products
• Maintain stable ecosystem

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Photosynthesis and TranspirationWhole Plant
Perspective

• CO2 enters leaf through open stomates water is
  lost
• CO2 is converted to carbohydrate (photosynthate)
  in the leaf and transported as sucrose in phloem
• Water is taken up by roots and transported via
  xylem
• Loss of water from the leaf via stomates is
  evapotranspiration

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Leaf Cross-Section
Cuticle Epidermis
guard cells stomate
Palisade parenchyma Spongy parenchyma
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Chloroplasts

• Site of photosynthesis several per cell
• Bounded by 2 smooth membranes
• Inner thylakoid membrane system
• grana stacks of thylakoids
• contain PS pigments
• site of light reactions
• Stroma
• liquid matrix
• site of dark reactions (CO2 conversion)

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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

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3 Processes in Photosynthesis
(1) light O2 CO2
sugar starch
NADPH
H2O
ATP
(2) Light Reactions (3) Dark Reactions
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Light Striking Leaf

• Reflected back to atmosphere
• Passes through leaf back to atmosphere
• Absorbed by the leaf and used for PS

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WAVELENGTH
ENERGY
GAMMA RAYS X-RAYS UV V
INFRARED RADIO WAVES
Visible Light
380 nm 430 nm 650 nm
750 nm
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PAR

• PAR Photosynthetically Active Radiation
• 400-700 nm wavelengths
• PPF photosynthetic photon flux
• PPF at full sun 2000 µmol/m2/sec

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Action Spectrum
Absorption Spectrum
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Photosynthetic Pigments

• Chlorophylls
• Chl a
• Chl b
• Carotenoids
• carotenes
• xanthophylls

• Reaction Center Pigments
• PSI P700 and PSII P680
• Chl a
• Accessory Pigments
• Chl a
• Chl b
• carotenes
• xanthophylls

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Chlorophyll

• Chl a in all higher plants
• Chl b also in most higher plants
• Chl c or d replaces chl b in some algae
• Chl porphyrin-like ring (loose e-s) and phytol
  tail to anchor it in thylakoid membrane
• Center atom of Mg surrounded by 4 N
• Fe necessary for the synthesis of chl

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CHO
Chlorophyll a
polar head
Reaction Center Pigment PSI and PSII
phytol tail
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beta-carotene
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carotenoids
chlorophyll
thylakoid membrane