Biology 102

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Biology 102

• Photosynthesis 1

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Lecture Outline

1. Back to the dinosaurs
2. What is photosynthesis (and who does it)?
3. The light-dependent reactions

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1. Back to the dinosaurs
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2. What is photosynthesis?

• Photo light
• Light energy is stored in the energy of chemical
  bonds.
• Synthesis build
• Organic molecules are built from inorganic
  molecules
• Carbon-carbon bonds are formed!
• The formulas for photosynthesis and cellular
  respiration
• Write it down!

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The relationship between photosynthesis and
cellular respiration
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Organisms that photosynthesize

• Eukaryotes that photosynthesize
• Plants (all!)
• Some protists
• Marine phytoplankton
• Freshwater and marine algae
• Prokaryotes that photosynthesize
• Example Cyanobacteria

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Leaves (plants only) and chloroplasts (eukaryotes
only)

• Chloroplasts within mesophyll cells
• Epidermis is thin and transparent (why?)

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Photosynthesis involves two sets of reactions

• The light-dependent reactions (photo)
• Light energy is captured from the sun and turned
  into chemical energy
• The energy is temporarily stored within energy
  carrier molecules
• The light-independent reactions (synthesis)
• Temporarily-stored energy is used to build the
  glucose molecule from carbon dioxide and water
• Energy is stored in the carbon-carbon bonds!

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3. The light-dependent reactions

• What is accomplished?
• Light energy is turned into chemical energy
• Overview of what happens
• Light is captured by pigments in the chloroplast
• Electrons of the pigments absorb the energy
• Electrons are transferred between compounds
• ATP and NADPH are both formed

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What is light?

• Light is one type of energy within the
  electromagnetic spectrum
• Different wavelengths of light have different
  amounts of energy

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What wavelengths of light do different plant
pigments absorb?

• Note that the color you see is the color that is
  not absorbed, but rather reflected!

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How is the energy of light actually captured?
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Summary of light-dependent reactions

1. Light energy is absorbed by pigments of the
   light-harvesting complex and passed to a special
   chlorophyll molecule, the reaction center
2. An electron in the reaction center chlorophyll is
   excited and jumps from the chlorophyll molecule
   to
3. An electron acceptor at a higher energy level

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• The electron is passed between molecules of the
  electron transport system
• The energy gained (from moving the electron to
  lower energy compounds) is used to synthesize ATP

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1. The electron ends up in another chlorophyll
   molecule (Photosystem I)
2. An electron in the reaction center chlorophyll is
   excited and jumps from the chlorophyll molecule
   to
3. An electron acceptor at a higher energy level

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1. The electrons are transferred along a second
   electron transport chain (along with an H ion)
   to a molecule of NADPH

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Where did the mysterious H come from? What
about the missing electron?

1. Its all about the water!

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Review of light-dependent reactions
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End result of light-dependent reactions

• Energy from sun was captured and converted to
  chemical energy
• ATP and NADPH
• Water was split to provide e- and H and
  generated oxygen as a bi-product!