How Cells Harvest Energy

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How Cells Harvest Energy

• Chapter 9

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Chemical Energy to Drive Metabolism

• Autotrophs harvest sunlight and convert radiant
  energy into chemical energy.
• Heterotrophs live off the energy produced by
  autotrophs.
• extract energy from food via digestion and
  catabolism (breaking of chemical bonds)

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Cellular Respiration

• Cells harvest energy by breaking bonds and
  shifting electrons from one molecule to another.
• aerobic respiration - final electron acceptor is
  oxygen
• anaerobic respiration - final electron acceptor
  is molecule other than oxygen
• fermentation - final electron acceptor is an
  organic molecule (alcohol or organic acid)

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ATP

• Adenosine Triphosphate (ATP) is the energy
  currency of the cell.
• used to drive movement (10 million molecules /
  second in muscle cell)
• used to drive endergonic reactions

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ATP

• Most of the ATP produced in cells is made by the
  enzyme ATP synthase.
• Enzyme is embedded in the membrane and provides a
  channel through which protons can cross the
  membrane down their concentration gradient.
• ATP synthesis is achieved by a rotary motor
  driven by a gradient of protons.

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Glucose Catabolism

• Cells catabolize organic molecules and produce
  ATP in two ways
• substrate-level phosphorylation
• aerobic respiration
• in most organisms, both are combined in a four
  step process
• glycolysis
• pyruvate oxidation (formation of Acetyl-CoA)
• Krebs cycle
• electron transport chain

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Aerobic Respiration
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Stage One - Glycolysis

• For each molecule of glucose that passes through
  glycolysis, the cell nets two ATP molecules.
• Priming
• glucose priming
• cleavage and rearrangement
• Substrate-level phosphorylation
• ATP generation

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Priming Reactions
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Cleavage Reactions
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Energy-Harvesting Reactions
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Recycling NADH

• As long as food molecules are available to be
  converted into glucose, a cell can produce ATP.
• Continual production creates NADH accumulation
  and NAD depletion.
• NADH must be recycled into NAD.
• aerobic respiration
• fermentation

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Recycling NADH
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Stage Two - Oxidation of Pyruvate

• Within mitochondria, pyruvate is decarboxylated,
  yielding acetyl-CoA, NADH, and CO2.
• Pyruvate (C3) CO2 Acetyl (C2)
• Acetyl Coenzyme A Acetyl CoA

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Stage Three - Krebs Cycle

• Acetyl-CoA is oxidized in a series of nine
  reactions.
• two steps
• priming
• energy extraction

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Harvesting Energy by Extracting Electrons

• Glucose catabolism involves a series of
  oxidation-reduction reactions that release energy
  by repositioning electrons closer to oxygen
  atoms.
• Energy is harvested from glucose molecules in
  gradual steps, using NAD as an electron carrier.

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Electron Transport
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Stage Four The Electron Transport Chain

• NADH molecules carry electrons to the inner
  mitochondrial membrane, where they transfer
  electrons to a series of membrane-associated
  proteins.

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Chemiosmosis
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Theoretical ATP Yield of Aerobic Respiration
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Catabolism of Proteins and Fats

• Proteins are utilized by deaminating their amino
  acids, and then metabolizing the product.
• Fats are utilized by oxidation.

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Cellular Extraction of Chemical Energy
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Fermentation

• Electrons that result from the glycolytic
  breakdown of glucose are donated to an organic
  molecule.
• regenerates NAD from NADH
• ethanol fermentation
• lactic acid fermentation

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