Introduction to the Light Reactions of Photosynthesis

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Introduction to the Light Reactions of
Photosynthesis

• "Life is woven out of air by light"

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Introduction

• Life on Earth is solar powered.

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PHOTOSYNTHESIS

• Plants and other autotrophs are the producers of
  the biosphere.
• On a global scale, photosynthesis is the most
  important process to the welfare of life on
  Earth.

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Where does photosynthesis occur?

• Green parts of plants
• Leaves are the specialized plant organs where
  most photosynthesis occurs

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• A typical leaf parenchyma cell has 30-40
  chloroplasts, each about 2-4 microns by 4-7
  microns long.

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Parts of the Chloroplast

• Thylakoids
• Grana
• Chlorophyll
• a - 2-3x, 430nm (violet-blue), 662nm (orange-red)
• b - 453nm (blue), 642nm (orange)
• Accessory pigments (i.e.carotenoids,
  xanthophylls)
• Stroma

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Photosynthesis

• Two steps
• Light reactions (light dependent reactions)
• Light independent reactions ( Calvin Cycle,
  Calvin-Benson Cycle, Dark Reactions)

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PHOTOSYNTHESIS Light Reactions
6CO2 12H2O light energy --gt C6H12O6 6O2
6H2O
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Major Components of Light Reactions

• 6CO2 12H2O light energy --gt C6H12O6 6O2
  6H2O
• Light
• Where does it come from?
• How does it get into the leaf?
• How does it get into the mesophyll cell?
• Why is it needed?

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• When light meets matter, it may be reflected,
  transmitted, or absorbed.

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• The light reactions work with those wavelengths
  of light that are absorbed.
• In the thylakoids are several pigments that
  differ in their absorption spectrum.

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• Pigments are grouped into two light harvesting
  complexes called photosynthetic units (I, II).
• Antenna molecules
• 200 chlorophyll a
• Chlorophyll b
• Accessory pigments
• Reaction center
• 2 chlorophyll a molecules

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• There are two types of photosystems.
• Photosystem I has a reaction center chlorophyll,
  the P700 center, that has an absorption peak at
  700nm.
• Photosystem II has a reaction center with a peak
  at 680nm.
• These two photosystems work together to use light
  energy to generate energy carrier molecules ATP
  and NADPH.

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Major Components of Light Reactions

• 6CO2 12H2O light energy --gt C6H12O6 6O2
  6H2O
• Water
• Where does it come from?
• How does it get into the leaf?
• How does it get into the mesophyll cell?
• Why is it needed?

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Overview of Light Reactions Photosystem II

• Light strikes chlorophyll
• Reaction center molecules excited to a higher
  energy level
• Excited electrons captured by electron acceptor
• The electrons in hydrogen from water replace the
  hole left by excited electrons of reactive
  chlorophyll oxygen released
• Electron acceptor passes excited electrons to
  another acceptordown an electron transport
  chain, ATP formed by chemiosmosis

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Overview of Light Reactions Photosystem I

• Light strikes chlorophyll
• Reaction molecules excited to a higher energy
  level
• Excited electrons captured by electron acceptor
  NADP nicotinamide adenine dinucleotide
  phosphate--gt NADPH formed
• Excited electrons from reaction center replaced
  by electrons from PII electron transport chain

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Bottom Line of Light Reactions

• Overall Photosynthetic Reaction
• 6CO2 12H2O light energy --gt C6H12O6 6O2
  6H2O
• Light Reactions
• H2O NADP ADP Pi light energy---chlorophyl
  l---gt ATP NADPHH O2
• Energy from sun is captured and converted to
  chemical form (ATP, NADPH)
• Oxygen is released
• NADPHH and ATP are used to power the Calvin
  Cycle (Light Independent Rxns).
• Occurs in the thylakoid membrane of chloroplasts

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Light Independent Reactions Calvin Cycle

• Overall Photosynthetic Reactions
• 6CO2 12H2O light energy --gt C6H12O6 6O2
  6H2O
• Light Independent Reactions (Calvin Cycle)
• CO2ATPNADPHH---gt C6H12O6 ADP Pi NADP
• NADPHH and ATP are used to power the Calvin
  Cycle (Light Independent Rxns) in the stroma of
  the chloroplast.
• The stored energy in ATP NADPH is used to
  convert CO2 (from atmosphere) into glucose
• Glucose is then used to make cell structures like
  cellulose or is converted in the mitochondria to
  make ATP via cellular respiration just like in
  you!

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Review Questions

• Why is water needed?
• Supplies the electrons (from hydrogens) to
  replace the electrons lost from the chlorophyll

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Review Questions

• What energy molecules are formed by the end of
  the light reactions?
• ATP and NADPH

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Review Questions

• What is the source of energy for ATP and NADPH?
• Sunlight

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Review Questions

• What wavelengths of light are most important for
  the light reactions?
• Violet-blue (400-500nm) and orange-red (600-700nm)

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Review Questions

• Is oxygen needed for the light reactions?
• No, it is a waste product