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
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 collecting
  complexes called Photosynthetic Units (PSI,
  PSII).
• 2 parts to each Photosystem
• Antenna molecules (many)
• Reaction center molecules (2 chlorophyll a
  molecules)

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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 Part I
Photosystem II

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

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Overview of Light Reactions Part 2 Photosystem
I

• Light strikes photosystem pigments
• 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
  no oxygen is released and no water is needed

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Bottom Line of Light Reactions

• 6CO2 12H2O light energy --gt C6H12O6 6O2
  6H2O
• Energy from sun is captured and converted to
  chemical form (ATP, NADPH)
• Oxygen is released

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Review Questions

• Why is water needed?

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Review Questions

• Why is water needed?
• Supplies the 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?

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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?

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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?

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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?

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Review Questions

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

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Is this all?

• NO!
• The light independent reactions follow the light
  dependent reactions.
• The energy molecules formed in the light
  dependent reactions are used, along with the
  carbon from carbon dioxide, to make glucose.

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Why do plants grow better under certain lighting
conditions than others?
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What factors can affect photosynthesis?
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Hydroponic Farming
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Pre-Lab Tasks

• Read the information on photosynthesis and leaf
  anatomy from your textbook
• Complete online tutorial on photosynthesis
• Complete the Planning Form for Why Do Plants
  Grow Better Under Certain Lighting Conditions
  than Others and turn in by the deadline

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Experimental Set Up
Flask filled with H2O and covered with cellophane
(blue, red, or clear)
Cuvette with buffered spinach solution with DPIP
(blue indicator dye) added
Desk Lamp with 60w or 100w bulb
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Other Materials
Vernier Lab Pro Colorimeter Measures the amount
of light transmitted through a sample
Cellophane film Allows only certain wavelengths
of light to be transmitted through the film
Light sensor (Vernier Lab Pro) Measures light
intensity for visible light in lux
Blue filter 413nm 107 lux Red filter 647
nm 161 lux
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More About DPIP (2,6-dichloropheno-indophonl)

• Replaces NAD
• Blue (oxidized) i.e. NAD
• Colorless (reduced) i.e. NADPH
• What will the DPIP allow us to measure?

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Other Questions

• What does the desk lamp supply to the experiment?
• What information can the light sensor provide?
• What is the purpose of covering the flask with
  cellophane?
• Why is the flask filled with water?
• What information will the colorimeter provide?