Energy Generation in Mitochondria and Chloroplasts

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Chapter 14

• Energy Generation in Mitochondria and Chloroplasts

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Protons In H2O

• H can move along the H-bonds in H2O
• Dissociating from one molecule to associate with
  the next one

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Transfer of H and e-

• The transfer of an e- sets up a negative charge
  which is rapidly neutralized by adding a H, the
  molecule is reduced
• Reverse is true when things are oxidized

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e- Transport Chain
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Redox Potential

• Redox oxidation-reduction reactions
• Depends on the affinity for electrons of the
  molecules involved in each reaction
• Redox pairs two molecules such as NADH and NAD
  - NADH is a strong electron donor (reducing
  agent) while NAD is a weak electron acceptor
  (oxidizing agent)
• Redox Potential a measure of the tendency of a
  given system to donate or accept electrons

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Versatile Electron Carriers

• The respiratory complexes are made up of a metal
  ion bound to a protein molecule
• The metal ion is responsible for the movement of
  the e-, skipping from one ion to another
• Ubiquinone, a hydrophobic molecule, that can move
  electrons without being bound to a protein

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Quinone Electron Carriers

• Can carry either 1 or 2 e- and picks up 1 H for
  each e-

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Other e- Carriers

• Dehydrogenation complex
• Flavin group
• Iron-sulfur centers carry 1 e- at a time
• Cytochrome b-c1 and cytochrome oxidase complexes
• Proteins that contain a heme group that can
  accept an e-
• Cytochromes are colored due to the Fe

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Chloroplasts and Photosynthesis

• Photosynthesis is process using the energy in
  sunlight and CO2 to create the organic materials
  required of present day cells
• The chloroplast is the special organelle in
  plants responsible for photosynthesis

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Chloroplasts

• Similar to mitochondria
• Uses a proton pump to create ATP
• Stroma instead of matrix
• Has own RNA, DNA and ribosomes
• Difference is that the e- transport chain is in
  the thylakoid membrane 3rd membrane that makes
  up the thylakoids, a sac-like structure, so have
  a thylakoid space
• Granum stack of thylakoids

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Chloroplasts vs Mitochondria
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Light and Dark Reactions

• Light or photosynthetic e- transfer reactions
• Sunlight energizes e- in chlorophyll which then
  moves down the e- transport chain in the
  thylakoid membrane
• e- gotten from H2O to make O2
• Electrochemical gradient is made in the stroma
  across the thylakoid membrane making ATP
• Generate NADPH from NADP
• Dark or carbon-fixation reactions
• ATP and NADPH produced in light reaction used as
  energy and reducing power to take CO2 and convert
  it to a carbohydrate glucose

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Photosynthesis Reactions
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Chlorophyll

• Sunlight is composed of many different
  wavelengths ranging from violet to red
• Chlorophyll is green because it absorbs all the
  wavelengths but green
• The e- in chlorophyll gain a higher energy level
  when a wavelength is absorbed and then bounce
  around the ring porphyrin (blue)

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Photosystem

• Chlorophylls are in a multiprotein complex called
  a photosystem
• Antenna is many molecules of chlorophyll that
  capture the sunlights energy that ultimately
  goes to the reaction center

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Reaction Center

• Set of proteins in the thylakoid membrane
• Special chlorophyll molecule that is an
  irreversible trap for an excited e-
• Transfers the e- to a more stable environment

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ATP and NADPH

• The light reaction makes the ATP and NADPH to
  synthesize the sugar
• ATP made with the first photon of light absorbed
  and NADPH is made from the second photon of light

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Summary of Light Reactions

• Electron from chlorophyll in photosystem II is
  donated to NADPH
• The replacement electron comes from the splitting
  of water
• When 4 electrons are removed (4 photons hit
  chlorophyll) O2 is released

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Carbon FixationorDarkReaction
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Carbon-Fixation or Calvin Cycle

• CO2 joins with a ribulose 1,5-bisphosphate (5 C)
  by a carboxylase called rubisco
• Rubisco is slow compared to other enzymes so
  therefore there is a large amount in the cell to
  compensate for this
• 1 molecule of glyceraldehyde 3-phosphate (net
  product ) is generated and goes to make the sugar
• A large amount of energy goes to regenerating the
  ribulose 1,5-bisphosphate
• 3 ATP and 2 NADPH required for each CO2 molecule
  converted to carbohydrate

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Glyceraldehyde 3-phosphate

• Converted into glucose
• Can be shuttled into the glycolytic pathway in
  the mitochondria of plants to become pyruvate and
  eventually ATP
• Excess is converted into starch in the stroma
  which can be used at night as an energy source