Chapter 4 - Photosynthesis

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Chapter 4 - Photosynthesis

• 6CO2 6H2O light ? C6H12O6 6O2

The capture and conversion of solar energy to
chemical bond energy

• Photosynthesis occurs in
• land plants,
• algae,
• cyanobacteria

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How is photosynthesis important?

• Universal energy source
• 100-200 billion metric tons/yr.
• Humans use 40 entire output

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Photosynthesis produces oxygen
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Photosynthesis the carbon cycle
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Summary of photosynthesis
See Figure 4-12
Light-dependent reactions
Light-independent reactions
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Chlorophyll absorbs some wavelengths of light
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Accessory pigments
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What happens when light is absorbed by a molecule?
Free electrons are a source of kinetic energy
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Summary of photosynthesis
See Figure 4-12
Light-dependent reactions
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Light-dependent reactions

• Photosystems embedded in the thylakoid membranes
  consist of chlorophyll molecules (100s) and
  electron transport systems
• Light energy captured and stored in energy
  carriers (ATP NADPH) via chemiosmosis

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Light absorption by chlorophyll
Photolysis replaces electrons removed from
chlorophyll
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Fig. 4-10
H20
?2H 2e- O
e-
Moving electrons provide the energy to move H
ions inside the thylakoid membrane
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Summary of photosynthesis
Light-dependent reactions
Light-independent reactions
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Light-independent reactions (Calvin Cycle)

• Occurs in the stroma
• Several enzyme-catalyzed steps
• Requires energy
• Produces sugars

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1940s-50s Melvin Calvin
Nobel prize in chemistry - 1961
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Calvins experiment (see Fig. 4-6)
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Calvins results
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Photosynthesis animation
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Rate of photosynthesis varies with environmental
conditions

• What is meant by the rate of photosynthesis?
• What environmental conditions affect the rate of
  photosynthesis?

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Factors influencing the rate of photosynthesis

• Carbon dioxide concentration
• Water availability
• Light intensity
• Temperature

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Photorespiration and rubisco

• Rubisco Ribulose 1,5-bisphosphate
  carboxylase/oxygenase
• Most abundant protein on earth (25-40 of total
  plant protein)
• Activity depends on carbon dioxide concentration
  in the leaf

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Rubisco as a carboxylase
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Rubisco as a carboxylase
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Rubisco activity and CO2 concentration

• If CO2 gt 50ppm, carboxylase activity
• If CO2 lt 50ppm, oxygenase activity

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When would photorespiration occur and why?

• When are CO2 concentrations low?

• When stomata close
• Hot days
• Moisture stress

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Variations of photosynthesis

• 85 of plants use C3 photosynthesis
• C3 cycle Calvin cycle
• C4 photosynthesis
• C4 plants include many tropical monocots, other
  grasses (0.4 of plants)
• Crassulacean Acid Metabolism (CAM)
• CAM plants cacti, orchids, bromeliads,
  succulents (10 of plants)

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C4 photosynthesis is more efficient than C3 under
certain conditions
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C4 photosynthesis
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C4 plants reduce photorespiration by
concentrating CO2 in the bundle sheath cells
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Crassulacean Acid Metabolism

• Evolved in areas of high light intensity, little
  precipitation
• Stomata closed during the day, open at night

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Photosynthesis research

• Why study photosynthesis?

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Photosynthesis Research

• Photosynthesis Center at Arizona State
• Oak Ridge National Laboratory
• Photosynthesis Research Unit

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Photosynthesis Research

• Photosynthesis and temperature tolerance
• Photosynthesis at high CO2 levels
• CO2 Science Magazine
• Engineering C4 genes in C3 plants
• Production of pharmaceuticals in chloroplasts

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Cellular respiration

• Plants utilize aerobic respiration as do animals