Botany 130, Lecture 12

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ATP synthesis Botany 130 Lecture 12
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Converting reducing power to ATP

• Electrons are passed down a chain of carriers
• In the case of the cytochromes, iron is the
  carrier. Other carriers are iron sulfur centers
  and quinones.
• At three different steps, the electron passage
  causes H to move from the matrix to the
  intermembrane space.
• The return of the protons to the matrix is
  coupled to ATP synthesis

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Converting reducing power to ATP
NADH
H
H
H
H
NAD
H
H
H
H
½ O2 2 H 2 e-
H
H2O
This is where all of the oxygen is used in
respiration.
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Converting reducing power to ATP
NADH
Complex I
Coenzyme Q, also called ubiquinone, swims through
the membrane to -
NAD
Complex III (also called the cytochrome
b/c complex)
½ O2 2 H 2 e-
Cytochrome c, a small protein, swims through the
intermembrane space to -
Complex IV
H2O
(Complex II feeds electrons into the chain from
other sources)
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Converting reducing power to ATP
NADH
Coupling factor
H
H
H
H
NAD
H
H
H
H
H
½ O2 2 H 2 e-
H
H
H
H
H
H2O
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Converting reducing power to ATP
NADH
Coupling factor
H
H
H
Pumping protons into this area creates a Proton
Motive Force (PMF)
The PMF has both concentration (pH) and
electrical components
H
NAD
H
H
H
H
H
½ O2 2 H 2 e-
H
H
H
H
H
H2O
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Nature 427, 407 - 408 (29 January 2004)
The binding change mechanism of ATP synthesis
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Converting reducing power to ATP
NADH
It was known for some time that electron
transport caused H to move across the membrane.
But could this electrochemical gradient make ATP?
Peter Mitchell said yes and was scorned. He was
then proved right and given the Nobel Prize.
NAD
½ O2 2 H 2 e-
H2O
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Cyanide and plant respiration
NADH
NADH
NAD
NAD
½ O2 2 H 2 e-
½ O2 2 H 2 e-
H2O
H2O
Cyanide blocks the terminal oxidase
Plants have an alternative, cyanide-insenstive,
oxidase
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Cyanide and plant respiration

• The alternative oxidase of plants allows some ATP
  synthesis in the presence of cyanide
• Plant mitochondria may have ways of avoiding even
  that little bit of ATP synthesis
• Alternative oxidase respiration or
  cyanide-insensitive respiration may be a method
  for plants to heat up

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Some fly-pollinated plants use sulfur gases to
attract the flies. By heating up, they can make
the sulfur compounds evaporate. The heating is
accomplished by engaging cyanide-insensitive
respiration so that glucose is consumed but no
energy is saved as ATP. All energy is
converted to heat. One of the signals for this
is salicylic acid.
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Reducing power and ATP for photosynthesis comes
from the Light Reactions, or photosynthetic
electron transport

• Photosynthetic electron transport occurs on the
  thylakoid membranes
• Compare oxidative electron transport on the
  christae of the mitochondrion
• Many of the essentials of electron transport are
  the same in respiration and photosynthesis

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Electron transport
Complex I Photosystem II
Ubiquinone Plastoquinone swims through the
membrane to
Cytochrome b/c complex Cyt. b6/f complex
Cytochrome c Plastocyanin swims through the
intermembrane space to
Complex IV Photosystem I
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Photosynthetic Electron Transport
H2O
Photosystem II
Plastoquinone swims through the membrane to -
½ O2 2 H
H
Cyt. b6/f complex
H
Plastocyanin swims through the intermembrane
space to -
Photosystem I
NADP
NADPH
Stroma Thylakoid lumen
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Photosynthetic Electron Transport
NADP
Plastoquinone
? Redox potential
Cyt. b6/f complex
NADPH
This way of looking at photosynthetic electron
transport is called the Z scheme
H2O
Plastocyanin
½ O2 2 H
Photosystem I
Photosystem II
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Important herbicides affect photosynthetic
electron transport
Atrazine
NADP
Plastoquinone
? Redox potential
Cyt. b6/f complex
NADPH
Paraquat
H2O
O2 2 H
½ O2 2 H
H2O2
Photosystem I
Photosystem II
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ATP can be made while NADPH is made
NADP
Plastoquinone
? Redox potential
Cyt. b6/f complex
NADPH
H2O
½ O2 2 H
H
Photosystem I
Photosystem II
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ATP can be made without NADPH production
NADP
? Redox potential
Cyt. b6/f complex
NADPH
Cyclic phosphorylation ATP made but no NADPH and
no O2
H2O
½ O2 2 H
H
Photosystem I
Photosystem II
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ATP synthesis in mitochondria and chloroplasts
are similar and it is thought they have a common
ancestry.
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Nature 427, 407 - 408 (29 January 2004)
The binding change mechanism of ATP synthesis
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ATP synthesis

• We have now covered three major types of ATP
  synthesis
• Substrate level phosphorylation (for example
  glycolysis)
• Oxidative phosphorylation
• Photophosphorylation

These two depend on proton gradients