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How energy is derived from
food.
AGRI 6203
Cellular Respiration
• Process of oxidizing food
– energy released
• captured in ATP
• Two phases
– glycolysis
• breakdown of glucose to pyruvic acid
– oxidation of pyruvic acid
Glycolysis
• in eukaryotes, it occurs in the cytosol.
• C6H12O6 + 2NAD+ -> 2C3H4O3 + 2NADH
+ 2H+
• The free energy stored in 2 molecules of
pyruvic acid is somewhat less than that in
the original glucose molecule.
• Some of this difference is captured in 2
molecules of ATP.
Glycolysis
Fates of Pyruvic Acid in Yeast
• Pyruvic acid is decarboxylated and reduced by NADH to
form a molecule of carbon dioxide and one of ethanol.
•
C3H4O3 + NADH + H+ -> CO2 + C2H5OH +
NAD+
• This accounts for the bubbles and alcohol in, for
examples, beer and champagne.
• The process is called alcoholic fermentation.
• The process is energetically wasteful because so much of
the free energy of glucose (some 95%) remains in the
alcohol (a good fuel!).
Fate of pyruvic acid in muscles
• Pyruvic acid is reduced by NADH forming a molecule of
lactic acid.
• C3H4O3 + NADH + H+ -> C3H6O3 + NAD+
• The process is called lactic acid fermentation.
• The process is energetically wasteful because so much free
energy remains in the lactic acid molecule. (It can also be
debilitating because of the drop in pH of overworked
muscles.)
Fate of pyruvic acid in
mitochondria
• Pyruvic acid is oxidized completely to form
carbon dioxide and water.
• The process is called cellular respiration.
• Approximately 40% of the energy in the
original glucose molecule is trapped in
molecules of ATP.
Mitochondrion
Mitochondrion
• Outer membrane
– integral membrane proteins
• form channels
• Inner membrane - 5 complexes
–
–
–
–
–
NADH dehydrogenase
succinate dehydrogenase
cytochrome c reductase
cytochrome c oxidase
ATP synthase
Citric Acid Cycle
• Each of the 3 carbons present in pyruvate
that entered the mitochondrion leaves as a
molecule of CO2
• at 4 steps in the cycle, a pair of (2e-) is
removed and transferred to NAD+ reducing
to NADH + H+
• at one step, a pair of electrons is removed
from succinic acid and reduces FAD to
FADH2
• one GTP (ATP equivalent is produced)
The Respiratory Chain- Function
of the four integral membrane
proteins
• NADH (FADH2) - stepwise transfer of
electrons to ________ atoms to form
_______.
• Harness the energy released by e- transfer to
the pumping of protons (H+) from the
matrix to intermembrane space
• Protons are pumped a 3 complexes
– NADH dehydrogenase complex
– cytochrome c reductase complex
– cytochrome c oxidase complex
• an average of three protons are pumped out
at each complex as each pair of electrons
passes through it
– 9 protons are pumped for each pair of electrons
from NADH; 6 from each pair from FADH2
• The gradient of
protons formed across
the inner membrane
by this process forms a
miniature battery
• protons can flow back
down this gradient,
reentering the matrix,
only through ATP
synthase, another
complex of integral
protiens in the inner
membrane
Chemiosmosis in mitochondria
• Energy from electrons is harnessed by
– NADH dehydrogenase complex
– cytochrome c reductase complex
– cytochrome c oxidase complex
• the complexes pump protons against their gradient from
the matrix into the inner membrane space
• a strong diffusion gradient is set up, the only exit of these
protons is through ATP synthase complex
Sources
Kimball’s Biology Pages
The Biology Project
Principles of Biochemisrty, Lehniger, 1982