Four Important Events
Occur in the Glycolytic Pathway
1.
2.
3.
4.
Substrate level phosphorylation: the transfer of
phosphate groups from substrates to ATP
Breaking of a six-carbon molecule (glucose) into two
three-carbon molecules of pyruvate
The reduction of two coenzymes of NAD to NADH
The capture of energy in ATP
Fermentation
• Pyruvic Acid
– metabolized in the absence of oxygen
– NAD must be recycled
– NAD passes electrons to other
molecules
• Types of fermentation
– Lactic acid
• Pyruvic acid converted to lactic acid
• uses electrons from NAD
– Alcoholic
• carbon dioxide released from pyruvic acid
• Acetaldehyde formed
• reduced to ethanol
Lactic Acid Fermentation: Pyruvic acid
reduced to lactic acid by NAD from glycolysis
Alcoholic Fermentation:
Carbon dioxide removed from pyruvic acid to acetaldehyde;
Acetaldehyde reduced to ethyl alcohol by NAD
Results of Glucose Fermentation
•
Natural waste products useful to
humans
–
–
–
–
•
Fermented beverages
Bread
Cheese
Yogurt
Infectious microbes may cause
disease
Fermentation
Pathways
Anaerobic
Respiration
•
•
Glycolysis
yields 2 ATP’s
net
Partial
oxidation of
carbon atoms
Aerobic
Respiration
• Starts with glycolysis
• Yields 36-38 ATP’s
• Complete oxidation
of substrate molecules
to C02 in the Krebs
Cycle
Transition to the Krebs cycle: Pyruvic acid loses CO2 & gets
oxidized by NAD; two-carbon acetyl group attaches to
coenzyme A, forming acetyl-CoA.
The Reactions of
the Krebs Cycle
What has been accomplished in the
Krebs Cycle?
• 8 reactions instead of just 1
• Each reaction has a separate reactant-specific enzyme
• The initial reactants are two 2-carbon molecules of acetyl
Co-A and the final products are four 1-carbon molecules
of CO2 .
• Transfer of electrons and/or H+ to coenzymes – 3 pairs
to NAD and 1 to FAD for each turn of the cycle
• One ATP molecule produced for each turn of the cycle
• The cycle turns two times for each molecule of glucose
because glucose has been converted to two molecules
of acetyl Co-A
• The cycle regenerates the starting molecule,
oxaloacetate
Why is Krebs a Cycle?
• Oxaloacetic acid combines with acetylCoA in the first step
• Oxaloacetic acid is regenerated at the end
What happens to the Coenzymes?
• They must be reoxidized!
– Their chemical bonds have stored chemical energy.
– Coenzymes are in short supply.
– Must be available to continue their oxidation work in
the Krebs Cycle
http://www.youtube.com/watch?v=26EE3jG5thM&feature=related
Waterfall model
of the electron transport chain
• As electrons pass from
carrier to carrier, they
decrease in energy.
• Some of the energy they
lose is harnessed to make
ATP.
The Electron Transport Chain
•
The electron transport chain performs two
functions:
– Accepts electrons from NADH and FADH2 &
transfers them to electron acceptors
– Uses the energy released during the electron
transfers to pump H+ across the inner
mitochondrial membrane to the
intermembrane space creating a concentration
gradient
Enzyme Complexes Involved
in Electron Transport
Oxidation/reduction enzymes:
1. NADH dehydrogenase
2. Flavoproteins (FAD)
3. Iron-sulfur proteins
4. Cytochromes
5. Quinones (lipid-soluble)
http://www.youtube.com/watch?v=xbJ0nbzt5Kw&feature=related
Electron Transport Chain
• Transfers electrons from one substrate to
another and finally to oxygen
• Pumps H+ across the inner mitochondrial
membrane into the inter-membrane space
• Oxygen is the final electron and hydrogen
acceptor!
Chemiosmosis
• The electron transport chain creates a H+ gradient
potential across the inner mitochondrial membrane
• ATP Synthase allows H+ back across the membrane
• ATP is produced by a proton motive force (pmf)
when H+ pass through ATP Synthase and cause ADP
+ P to form ATP.
http://lecturer.ukdw.ac.id/dhira/Metabolism/ETLP.html
http://www.youtube.com/watch?v=3y1dO4nNaKY
Oxidative Phosphorylation
ATP formation involving molecular
oxygen and chemiosmosis
http://www.youtube.com/watch?v=9Z2A6qJyURY&NR=1
http://www.youtube.com/watch?v=26EE3jG5thM&feature=related
• What part of the electron transport chain is
represented by the marbles being pushed
upward?
– The hydrogens being pumped into the intermembrane space
How does electronegativity play a
part in the electron transport chain?
– Each electron acceptor in the chain is more
electronegative than the previous one
– the electrons move from one electron transport
chain molecule to the next, falling closer and
closer to the nucleus of the last electron
acceptor.
Where do the electrons for the electron
transport chain come from?
• NADH and FADH2 which got their
electrons from glucose originally, in the
previous two phases of cell respiration.
Why does FADH2 drop its electron onto
a different initial acceptor than NADH?
• It has a different electronegativity.
• FADH2 is more electronegative and
therefore the initial acceptor for FADH2 must
be stronger in order to pull the electrons
away.
What molecule is the final
electron acceptor?
• Water is made from the splitting of an oxygen
gas molecule. Each oxygen atom grabs
– two electrons from the electron transport
chain and
– two hydrogen ions from the inter-membrane
space.
• What is consumed during this process?
• Oxygen gas
What is gained by this process?
• A chemiosmotic gradient of H+ ions inside
the inter-membrane space.
The electron transport chain does not generate
ATP directly, so what good does it do?
• It generates a chemiosmotic gradient that
will eventually generate ATP.
How does this gradient generate ATP?
• A special protein, ATP Synthase,
embedded in the inner membrane can use
the flow of hydrogen ions, H+, to
phosphorylate ADP molecules.
How does the specialized
membrane protein work?
• It turns mechanically like a rotary motor
• As the hydrogen ions, H+, rush through the
protein channel, it causes chemical energy
to be converted into mechanical energy
• This energy drives a phosphorylation
reaction.
• This is called oxidative phosphorylation.
Is cell respiration
endergonic or exergonic?
• Exergonic – energy is released.
Is it a catabolic or
anabolic reaction?
• Catabolic – a molecule is being broken
down into smaller molecules
If one molecule of ATP holds 7.3 kcal of
potential energy, how much potential
energy does one molecule of glucose
produce in cell respiration?
• At its maximum output:
38 X 7.3 kcal = 277.4 kcal
One molecule of glucose actually
contains 686 kcal of potential energy.
Where does the remaining energy go
when glucose is oxidized?
• It is lost as heat which is why we’re warm.
What is the net efficiency of cell
respiration if glucose contains 686 kcal
and only 277.4 kcal are produced?
• 277.4 / 686 x 100 = 40%
How does this rate of efficiency
compare to energy capture processes
that you see in everyday life?
• Incandescent light bulb = 5% efficient
• Electricity from coal
= 21% efficient
• Car engines
= 23% efficient
Define cellular respiration.
• Cellular respiration is the release of
energy from food by oxidation.
Compare anaerobic and
aerobic respiration.
Anaerobic
Respiration
•
•
Glycolysis
yields 2 ATP’s
net
Partial
oxidation of
carbon atoms
Aerobic
Respiration
• Starts with glycolysis
• Yields 36-38 ATP’s
• Complete oxidation
of substrate molecules
to C02 in the Krebs
Cycle
Why doesn’t the reaction stop once the
ATP’s are produced from glycolysis
leaving the oxidized glucose in the form
of pyruvate?
• Because the reactions that produce CO2 +
alcohol or lactic acid are needed to
reoxidize NADH. Without this the lack of
NAD+ would stop glycolysis.
Given what you know about the
process of fermentation, what
are some of the requirements
for making wine or beer?
• A culture of yeast or other anaerobic
organisms
• An oxygen-free environment so the
organisms are forced to perform only
glycolysis
• A source of glucose or fructose
Total Energy Released
“The oxidation of glucose to carbon dioxide releases
approximately 277.4 kcal of energy. If all of this energy is
released at one time, then most of it would be lost as
heat. Burning the energy all at once would be akin to
igniting your gas tank in order to run your car, rather than
burning small amounts of gasoline slowly in the engine.
If the energy of glucose is released slowly, in several
small steps, then the potential energy available could be
captured at each small step, just as the gasoline energy
is captured (to move a car) in a slow release rather than
in a single, explosive release.”