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Vital Harvest
Deriving Energy from Food
Energy Currency and Energy Carrier Molecules
• Energy harvesting—Everest’s deadly problem: having only 30
percent of the oxygen that would be available in the
atmosphere at sea level. How many people know why we need
to breathe oxygen? To extract energy from food (covered in
this lecture). Does all energy harvesting require oxygen?
• Relevant questions to be answered.
– How do cells convert food into energy? Do they convert fats
differently from carbohydrates? Do you get fat by eating fat
or calories?
– Why do we need to breathe? How does exercise affect
metabolism?
– What about energy supplements? What does it mean to have
a fast or slow metabolism?
Energy Currency and Energy Carrier Molecules
• ATP is the most important energy storage molecule.
– Potential energy from food breakdown is used to
drive the endergonic synthesis of ATP (like
recharging a battery).
– The charged ATP has energy that can be released
at any time (by breaking off the third phosphate to
do a variety of actions).
Energy Currency and Energy Carrier Molecules
• Electrons from food carry energy to make ATP.
– Electrons from glucose run downhill. Transferred by
carriers, the electron drop powers uphill synthesis
of ATP.
– Electron transfers to molecules—redox reactions
occur side by side.
• One molecule is oxidized—loses electrons.
• Another molecule is reduced—gains those
electrons (reduces charge).
Energy Currency and Energy Carrier Molecules
• Intermediate electron carriers serve to shuttle electrons
through these reactions, transferring energy as they go.
– NAD+ (empty city cab) in redox reaction is an oxidizing agent
(removes electrons causing a substance to be oxidized); it
accepts a hydrogen atom and one electron becomes NADH
(full cab).
– NADH can carry electrons (proceed down energy hill) on to
another acceptor, thus being regenerated (empty cab).
– NAD+ is made by cells from the vitamin niacin
• Enzymes coordinate all these transfers by bringing together
the glucose derivatives with energy carrier molecules.
Cellular Respiration
• Overview of the three stages.
– General reaction.
• C6H12O6 +6O2 + ADP → 6CO2 + 6H2O + ATP
– Energy coupling—downhill breakdown of glucose
releases electrons, carried along and used to
transfer energy to drive uphill synthesis of ATP.
Cellular Respiration
• Overview of the three stages.
– Glycolysis—for eukaryotes, this is the first stage. It begins
breakdown of glucose, yielding little energy, but it does
transfer electrons to NAD+. On the plus side, it doesn’t
require oxygen and occurs in the cytoplasm, and some
prokaryotes and single-celled eukaryotes have long used it
as the sole source of energy.
– Krebs cycle and electron transport chain—evolved later, but
generate larger quantities of energy; only problem is they
occur only in mitochondria (only eukaryotes) and the
electron transport chain requires oxygen.
Glycolysis
• Steps in the process.
– Sugar in bloodstream enters cytoplasm, where this
breakdown begins. Enzymes catalyze each reaction
in metabolic pathway (first is hexokinase, which
adds a phosphate from ATP called
phosphorylation).
– Although breaking sugar apart generates energy, it
requires some activation energy (another ATP is
used to attach another phosphate: –2 ATP total).
Glycolysis
• Steps in the process.
– Rearrangement eventually leads to splitting the
molecule in half from one 6-carbon sugar into two
3-carbon sugars (pyruvic acid is end product).
– Oxidation by 2 NAD+ transfers electrons, and leads
to the attachment of a high-energy phosphate to
each sugar. Enough energy is generated by the
eventual release of these four total phosphates in
the next two steps to attach them to ADP to make
ATP (+ 4 ATP).
Glycolysis
• Ledger.
– Plus side—Very fast reactions cut glucose in half,
generating small amount of energy (net 2 ATP), and
electrons (2 NADH), but no oxygen was required.
– Minus side—What is the next redox reaction where
electrons can be transferred to empty the cab
(NADH) for more passengers (NAD+)? Not much ATP
made for all the work.
Essay: When Energy Harvesting Ends at
Glycolysis
• Bacteria and certain eukaryotes can use only glycolysis.
Problem—How can they recycle the NAD+?
• Solution—alcoholic fermentation, yeast in absence of oxygen
(bread and wine) must regenerate NAD+, so they dump
electrons from NADH onto the acetaldehyde (converted from
pyruvic acid and spewing off CO2), reducing it to ethanol, but
regenerating the NAD+.
• Solution—lactate fermentation, in animals in absence of
oxygen (muscle fatigue), pyruvate accepts electrons from
NADH and regenerates NAD+, but is converted into lactic acid
(muscle burn).
Essay: Energy and Exercise
• Huge quantities of ATP are required to contract
skeletal muscle, but ATP is generated in different
ways depending on circumstances.
• First burst of activity (6 seconds): cells have stockpile
of ATP and phosphocreatine.
• What is greatest source of energy at 30 seconds?
Aerobic or anaerobic? What about at 10 minutes?
Krebs Cycle
• Sugar derivatives are oxidized to yield electrons in
interior of inner membrane of mitochondria.
The Krebs Cycle
The Krebs Cycle
• Steps in the process.
– Acetyl CoA combines with oxaloacetic acid to
make citric acid, continues around through a series
of reactions that finally yield oxaloacetic acid again
(cycle).
– During the cycle of reactions, as the acetyl CoA is
transformed, it is being oxidized by electron carrier
molecules NAD+ and FAD.
– Also, ATP and CO2 are produced.
The Krebs Cycle
• Ledger—6 NADH, 2 FADH2, and 2 ATP; acetyl CoA
completely broken down into CO2.
• Majority of electrons for next stage (electron
transport chain).
Electron Transport Chain (ETC)
• Series of molecules in the mitochondrial inner membrane that
are the destination of the electrons carried by NADH and
FAHD2.
• Steps in the process.
– NADH arrives, and it bumps the ETC’s first carrier, which
accepts the electrons, then passes them on along the chain
(like a hot potato).
– Movement of electrons at each transfer releases enough
energy to power the movement of H+ ions from the inner
compartment into the outer compartment (like heat of a hot
potato dissipating as it is passed). The ions are being
pumped against their concentration gradient (uphill).
The Electron Transport Chain (ETC)
• Steps in the process.
– Hydrogen ions are allowed to flow downhill through an
enzyme in the membrane called ATP synthase, like a water
wheel spinning; as the ions pass, energy is used to transfer
phosphate onto ADP to make ATP.
• Greatest amount of ATP is made in this stage (32 ATP per
glucose).
• At the end of the ETC , which carrier accepts the electron?
– 1/2 O2 + 2 electrons + 2 H+ = H2O
Other Foods, Other Respiratory Pathways
• Fats, proteins, and other sugars can also enter
pathway to be converted to energy, but not in exactly
the same way.
• Food eaten in excess of caloric demands can also be
converted from amino acids, fatty acids, and sugars
into proteins, fats, and carbohydrates for structure or
storage (98 percent of energy reserves of animals are
fats).
Other Foods, Other Respiratory Pathways
• Example: Fats.
– Triglyceride is converted to fatty acids and
glycerol.
– Glycerol is converted to glyceraldehyde phosphate
(glycolysis intermediary) downstream.
– Fatty acids can be broken apart and used to make
acetyl CoA.