Biology 107 Cellular Respiration

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Biology 107Cellular Respiration

• September 29, 2004

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Metabolism Review
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Cellular Respiration I

• Student Objectives As a result of this lecture
  and the assigned reading, you should understand
  the following
• 1. Cell release chemical energy by means of an
  exergonic process called cellular respiration,
  the aerobic harvesting of energy from food
  molecules by cells.
• 2. Cellular respiration is the energy-releasing
  chemical breakdown of molecules and the storage
  of energy from that breakdown in a form (e.g.,
  ATP) the cell can use to perform work.

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Cellular Respiration II

• 3. Normally there is an oxidation of the organic
  molecule causing the hydrogen atoms (electrons
  and their accompanying protons) to be removed
  from the carbon atoms and eventually combined
  with oxygen (which is thereby reduced).
• In cellular respiration, the electrons go from
  higher energy levels to lower energy levels, and
  energy is released. This energy is released over
  many steps as electrons move to successively
  lower energy levels. Some of that energy is lost
  as heat a portion of that energy (40) is
  captured in the terminal phosphate bonds of ATP.
• 5. The efficiency in living systems is due to the
  fact that energy release occurs over the course
  of a series of controlled reaction steps.

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Cellular Respiration I

• The harvesting of energy involves the
  rearrangement of electrons in chemical bonds.
  The common theme is that a cell transfers energy
  from one molecule to another by coupling an
  exergonic reaction (energy-releasing) to an
  endergonic reaction (energy-storing). The energy
  released was stored in the specific arrangement
  of a molecule's covalent bonds, and the energy
  stored is in the new covalent bonds formed.
• In summary, cellular respiration rearranges
  electrons in chemical bonds. These are redox
  reactions. Because an electron transfer requires
  both a donor and an acceptor, an electron leaves
  one molecule only when it contacts another
  molecule that attracts it more strongly.
• 8. In respiration, there are two main coenzymes
  derived from B complex vitamins. First is NAD
  ,(nicotinamide adenine dinucleotide) which in
  part is derived from B3, niacin. The second
  coenzyme is FAD (flavin adenine dinucleotide),
  which in part is derived from B2, riboflavin.

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Cellular Respiration I

• Glucose supplies energy to form ATP by two
  related processes 1) glycolysis and 2)
  cellular respiration. The products of glycolysis
  are reactants used in respiration.
• In glycolysis ("splitting of sugar"), the
  6-carbon glucose molecule is split into two
  3-carbon molecules, pyruvate. The net energy
  harvested from the glycolysis reactions is in the
  form of ATP and NADH.
• a. This production of ATP in glycolysis is by
  the direct, enzyme- mediated transfer of a
  phosphate group from a substrate to ADP by the
  mechanism called substrate phosphorylation. This
  is different from electron transport (oxidative)
  phosphorylation, which requires oxygen and a
  transport system.
• b. Glycolysis occurs in the cytosol and does not
  require oxygen (i.e., it is an anaerobic
  process).

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Cellular Respiration I

• In the presence of oxygen, the pyruvates are fed
  into the second stage of energy capturing,
  cellular respiration.
• In the absence of oxygen, the pyruvate is
  converted to either lactic acid or ethanol. This
  conversion process is known as fermentation, and
  it produces no ATP. Fermentation is a mechanism
  for cells to replenish the supply of NAD that
  the cell is using in glycolysis.
• The reactions of glycolysis follow essentially
  the same routes in prokaryotes and eukaryotes,
  except the products of fermentation are more
  varied under anaerobic conditions.

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Energy Flow in Ecosystems
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ATP Supplies Energy for Cellular Work
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Modes of ATP Synthesis
Substrate Phosphorylation
Oxidative Phosphorylation
Reactions result in the transfer of electrons to
O2. This transfer of energy is used to
phosphorylate ADP with free Pi.
Enzyme transfers a phosphate group from a
substrate to ADP
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Cellular Respiration Involves Oxidation-reduction
Reactions

• C6H12O6 6O2 6CO2 6H2O

oxidation
reduction
Electrons lose potential energy along the way
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Exergonic Reactions Advantage of Multistep
Process in Transfer of Energy to ATP
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Summary of Multistep Reactions Used to Generate
ATP in Eukaryotic Cells
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Glycolysis Energy Scorecard
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Structure of NAD/NADH
e-
e-
H
H
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Early Steps in Glycolysis
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Later Steps in Glycolysis
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Substrate Phosphorylation
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Summary of Net Products of Gycolysis
Two ATPs Two water molecules Two NADHs (2H) Two
pyruvates The electrons in the NADHs can yield
ATPs through the electron transport system, and
the pyruvate can be metabolized in the Krebs
cycle.
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In the Presence of Oxygen, Pyruvates Enter the
Mitochondrion and are Oxidized
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In the Absence of Oxygen, Pyruvates are Fermented
to Liberate NAD
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Lactic Acid Fermentation Compared to Alcohol
Fermentation
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Summary