Photosynthesis

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Chapter 10

• Photosynthesis

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Photosynthesis

• conversion of light energy to chemical energy
  that is stored in glucose or other organic
  compounds
• occurs in plants, algae, certain prokaryotes

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Nutritional modes

• Two criteria
• energy source
• light or chemicals
• carbon source
• carbon dioxide or organic compounds

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Nutritional modes, cont.

• energy source
• light photo
• chemicals chemo
• carbon source
• carbon dioxide auto
• organic compounds hetero

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Chemoautotroph

• Energy source
• chemicals, inorganic
• Carbon source
• carbon dioxide
• ex. sulfur bacteria

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Chemoheterotroph

• Energy source
• chemicals, organic
• Carbon source
• organic compounds
• ex. Animals, most protists, most fungi, most
  bacteria

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Photoheterotroph

• Energy source
• light
• Carbon source
• organic compounds
• ex. Rare bacteria

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Photoautotroph

• Energy source
• light
• Carbon source
• carbon dioxide
• ex. Plants, algae, some bacteria (cyanobacteria)

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Biophysics of light

• photon - a quantum or discrete amount, of light
• electromagnetic spectrum
• 10 - 5 nm to over 10 3 m
• sun radiates the full spectrum

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Biophysics of light, cont.

• energy of photons is inversely proportional to
  wavelength of light

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Biophysics of light, cont.

• the shorter the ? the greater the energy of each
  photon of that light
• violet light (380nm) vs. red light (750nm)
• ex. photon of violet light (380 nm) has twice the
  energy as photon of red light (750 nm)

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Electromagnetic Spectrum

• 10 3 nm- 10 6 nm
• infrared
• 10 6 nm- 1 m
• microwaves
• 1 m - 10 3 m
• radio waves

• 10 - 5nm - 10 - 3nm
• gamma rays
• 10 - 3 nm - 1 nm
• x-rays
• 1 nm- 10 3 nm
• UV
• 380 nm- 750 nm
• visible light

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Pigments

• light may be absorbed, reflected, or transmitted
• pigments
• are molecules that absorb particular light
• ex. melanin, retinal

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Photopigments

• two categories
• carotenoids, accessory pigments
• chlorophylls

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Carotenoids

• xanthophylls
• yellow
• carotenes
• orange

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Chlorophylls

• chlorophyll a
• chlorophyll b

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Chlorophyll a

• blue-green
• methyl group (-CH3) in porphyrin ring
• directly participates in light reactions

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Chlorophyll b

• yellow-green
• aldehyde group (-CHO) in porphyrin ring

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Chloroplast

• site of photosynthesis
• thylakoid membranes -
• light-dependent reactions
• stroma - light-independent reactions

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Chloroplasts in Leaves

• leaf c.s.
• mesophyll tissue with spongy and palisade
  cells filled with chloroplasts

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Photosystem

• cluster of few hundred pigment molecules
  (carotenoids and chlorophylls)
• location thylakoid membranes
• a reaction-center (chl. a) in the photosystem

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Photosystem Types

• Photosystem I
• Photosystem II
• named in the order of discovery

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Photosystem I

• chlorophyll a known as P700
• absorbs best at 700 nm

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Photosystem II

• chlorophyll a known as P680
• absorbs best at 680 nm

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P700 and P680

• are identical chl. a molecules
• associated with different proteins on the
  thylakoid memb. which affects their absorption of
  light

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P700 and P680, cont.

• different from other chl. a molecules because of
  the bound proteins
• are closer to their primary electron acceptor

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Electron Flow

• noncyclic electron flow
• cyclic electron flow

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Noncyclic Electron Flow

• electrons leave reaction center and do not return
• involves photosystem I, II
• generates NADPH, oxygen, ATP (photophosphorylation
  )
• most bacteria, plants, algae

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Cyclic Electron Flow

• electrons leave reaction center and return
• involves photosystem I
• generates ATP (photophosphorylation)
• few bacteria

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Photosynthesis equation

• The most important chemical process on earth!!!
• 6 CO2 12 H2O
• (ox agent) (red agent)
• C6H12O6 6 O2 6 H2O
• (red mlc) (ox mlc)

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Tracking the atoms

• 6 CO2 12 H2O
• C6H12O6 6 H2O 6 O2

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Stages of Photosynthesis

• light-independent reactions
• 6CO2 12H2O C6H12O6 6O2 6H2O
• light-dependent reactions

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Light-dependent reactions

• also known as light reactions, photochemical
  reactions
• location thylakoid membranes

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Light-dependent reactions, cont.

• 12 H2O
• 24 ADP 24 Pi 24 ATP
• 6 NADP 6 NADPH
• 6 O2 12 H2

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Light-independent reactions

• also known as dark reactions, biochemical
  reactions, Calvin cycle, Calvin-Benson cycle,
  carbon fixation reactions
• location stroma

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Light-independent reactions, cont.

• 6 CO2 12 H2
• 18 ATP 18 ADP 18 Pi
• 12 NADPH 12 NADP
• C6H12O6 6 H2O

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• Light-dependent
• releases
• 24 ATP
• 6 NADPH
• 42 ATP

• Light-independent
• requires
• 18 ATP
• 12 NADPH
• 54 ATP

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Alternative mechanisms of carbon fixation

• Plants conquered land 425 million years ago
• threat desiccation
• atmosphere initially had low O2
  levels, high CO2 levels

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Photorespiration

• photo - occurs in the light
• respiration - consumes oxygen and produces CO2
  H2O
• evolutionary relic

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Photorespiration, cont.

• On hot, dry days stomata close
• (to conserve water)
• Oxygen from light rx builds up
• (more O2 than CO2)
• enzyme rubisco combines with O2 instead of CO2
  in Calvin cycle
• generates no ATP, makes no food

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C3 plants

• Should
• CO2 RuBP C3 Carbohydrate
• rubisco
• Instead, in hot, arid conditions
• O2 RuBP C2 2 CO2 H2O
• rubisco
• rice, wheat, soybeans

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Evolutionary solutions to photorespiration

• C4 plants
• corn, sugarcane
• CAM plants
• cacti, pineapple

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C4 plantstropical origin plants

• adapted to hot, arid conditions
• spatially separated
• 2 types of photosynthetic cells involved,
• mesophyll bundle sheath cells
• corn, sugarcane

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C4 plants, cont.

• CO2 C4 CO2
• gas enters ...stored in org. acids ...released
• mesophyll bundle sheath for Calvin
  cycle

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CAM plantsdesert plants

• Crassulacean acid metabolism - CAM
• adapted to hot, arid conditions
• temporally separated
• 2 steps separated by time in one cell
• cacti, pineapple

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CAM plants, cont.

• during the night during the day
• CO2 C4 CO2
• gas enters ...stored in org. acids ...released
• mesophyll vacuole of for Calvin
  mesophyll cycle

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Chapter 10

• The End