The Living World Chapter 5

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Photosynthesis Chapter 10
Professor Pamela L. Pannozzo Principles of
Biology I BSC 1010
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Space Plants!
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Photosynthesis

• 6CO2 12H20 light gt C6H12O6 6O2 6H2O

• Occurs in the
• Plasma membranes of some bacteria
• Cells of algae
• Chloroplasts of plant cells

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Photosynthesis

• Autotrophs organisms that acquire organic
  compounds from inorganic materials from the
  environment
• Primary producers
• Plants, plant-like protists (algae), some
  prokaryotes
• Photoautotrophs
• Chemoautotrophs
• Heterotrophs organisms that acquire organic
  compounds by consuming other organisms
• Consumers
• Animals, fungi (decomposers), fungi- and
  animal-like protists, some prokaryotes

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The Electromagnetic Spectrum
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The Electromagnetic Spectrum

• Light energy electromagnetic energy
  electromagnetic radiation
• Travels both as electromagnetic waves and as
  particles (photons)
• Electromagnetic spectrum entire range of
  radiation
• Wavelength distance between crests of
  electromagnetic waves
• Photosynthesis driven by visible light 380-750 nm
• Pigments absorb certain wavelengths (chlorophyll
  a, chlorophyll b, carotenoids)

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Photosynthesis (cont)

• Two stages
• Light reactions
• Dark reactions (Calvin Cycle)

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Light Reactions

• Convert light energy to chemical energy of ATP
  and NADPH
• Occur in the thylakoid membranes of the
  chloroplasts
• Steps
• Pigment molecules absorb light energy
• Water molecule splits, O2 released
• Electrons from H2O molecule energized
  (Photosystem II)
• Energized electrons and hydrogen transferred
  along electron transport chain
• ADP phosphorylated to form ATP
• Electrons transferred to NADP, forming NADPH
  (Photosystem I)

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Chlorphyll Excitation
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Dark Reactions (Calvin Cycle)

• Incorporate CO2 into organic molecule
  intermediaries, and then into sugar (CH2O)
• Occur in the stroma of the chloroplasts
• Steps
• Atmospheric CO2 incorporated into ribulose
  bisphosphate (RuBP) by enzyme rubisco, forms
  3-phosphoglycerate
• Phosphate group added from ATP, then electrons
  from NADPH, resulting in G3P sugar, converted
  into glucose
• G3P recycled into RuBP, via ATP

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The Problem of Photorespiration Under Hot
Conditions

• Hot dry conditions? plants close their stomata to
  conserve water
• Prevents uptake of CO2
• Photorespiration under scarcity of CO2, rubisco
  adds O2 to RuBP instead of CO2
• Gives off CO2 made from intermediary compunds
• Produces no sugar
• Why does photorespiration occur??
• A wasteful process!!
• Occurs in most common plants (C3 Plants)
• 3-phosphoglycerate is first product of Calvin
  Cycle

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Two Plant Adaptations to Minimize Photorespiration

• C4 Plants--grasses

• CAM Plants--succulents

• First product of carbon fixation is 4-carbon
  malate
• Unique leaf anatomy
• Spatial segregation of Calvin cycle processes
• Mesophyl cells--PEP carboxylase fixes CO2, makes
  organic acid
• Pumps organic acids (CO2) to bundle sheath cells
• ATP used to make PEP
• Bundle sheath cells-- undergo normal Calvin cycle
• Calvin cycle occurs only in bundle sheath cells

• Named for plant type
• Temporal segregation of Calvin cycle processes in
  mesophyll cells
• Evening fixation of CO2 into organic acid when
  stomata open
• Organic acid stored in vacuoles until day
• Daytime normal Calvin cycle when stomata closed
• CO2 fixed from organic acids

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