Energy Processes from Catabolizing Glucose
The body’s primary energy source is ATP. Although there are a few different ways
in which the body resynthesizes ATP, the most used comes from glucose. Glucose is a
six-carbon molecule that the body uses to produce ATP by using three main energy
processes. These processes/cycles are known as glycolysis, Krebs cycle, and oxidative
phosphorylation.
Glycolysis is the breakdown of carbohydrate – either glycogen stored in the
muscle or glucose delivered in the blood – to resynthesize ATP (Haff & Triplett).
Glycolysis occurs in the sarcoplasm of the muscle cell and is a process involves a “series
of enzymatically catalyzed, coupled reactions” (Powers 2018). Powers separated this
process into two separate phases: the first few steps are the “energy investment phase,”
because stored ATP must be used to “prime” to add phosphate groups (called
phosphorylation) to the glucose by ATP to produce glucose-6-phosphate, which then,
isomerizes into fructose-6-phosphate. A second phosphorylation occurs from ATP and
forms fructose-1,6-bisphosphate that is now split into two pathways in what Powers calls
the “energy generation phase.”
The energy generation phase starts with two, three-carbon molecules, instead of
six-carbon as in the investment phase, with glyceraldehyde-3-phosphate. Oxidation,
followed by phosphorylation, produces 2 NADH molecules and two high-energy
bisphosphoglycerate molecules. The removal of two energized phosphate groups by two
ADP molecules produce two ATP molecules and two 3-phosphoglycerate molecules.
Oxidation then takes places by the removal of water and produces two
phosphoenolpyruvates, which then two ADP break off their phosphate groups to attach to
other ADP and form two ATP molecules, a process called myokinase reaction (Haff &
Triplett). This final step of glycolysis forms the end product pyruvate, which will either
be converted into lactate in the sarcoplasm, or pyruvate will be shuttled into the
mitochondria to undergo the Krebs cycle.
Once pyruvate is shuttled into the mitochondria, the three-carbon molecule breaks
down into a two-carbon molecule called Acetyl COA and begins the Krebs cycle. The
remaining carbon molecule is given off as carbon dioxide. The primary purpose of the
Krebs cycle is to remove hydrogens and the energy associated with those hydrogens from
various substrates involved in the cycle (Powers). AcetylCOA drops the coenzyme A by
the enzyme citrate synthase to form citrate. Citrate isomerizes to become isocitrate. At
this time, NAD is converted to NADH, the first of three NADH’s that are made from one
turn in the Kreb’s cycle. Also, a carbon molecule is released as carbon dioxide, which
makes alpha-ketoglutarate. The second NAD becomes NADH and alpha-ketoglutarate
decarboxylates into succinyl COA, which is a four-carbon compound. Succinyl CoA
forms succinate, which the enzyme succinate dehydrogenase comes in and makes
succinate into fumarate. FAD turns into FADH and is the one and only FADH that one
turn of the Kreb’s cycle produces. One water molecule added converts fumarate into
malate, which then forms the end product of the Kreb’s cycle oxaloacetate. The third and
final NAD is turned into NADH.
NADH’s and FADH’s that were made in the Kreb’s cycle are going to be shuttled
to the mitochondrial membrane to drop off their electrons in a process called the electron
transport chain, or oxidative phosphorylation. The way this process works is sort of a
“hot potato” game as one complex will pass on from one to another. NADH from the
Kreb’s cycle will drop their electrons off to complex one, as simultaneously, the FADH
will drop theirs off to complex two. Complex one and two pass their electrons to
coenzyme Q moves over to complex three to pass off the electrons to cytochrome-c from
one and two. Cytochrome-c will the pass electrons over complex four, which then,
complex four sends electrons to oxygen and hydrogen to make water. During this passing
of the electrons, hydrogen protons are accumulating as complex’s one, two, and three are
pumping out protons into the mitochondrial membrane, charging up ATP synthase, which
is anchored to the mitochondrial membrane and absorbs potential energy. This potential
energy is used by the ATP synthase to have ADP and inorganic phosphate merge together
and react to form ATP. This theory is called the Chemiosmotic Hypothesis.
The three main processes discussed involved carbohydrates in the form of glucose
to make ATP. The human body also has the ability to break down fats and proteins to
convert into ATP. However, the body does not like to break down protein, unless it
becomes necessary for survival or long-term exercise. The reason for this is because our
bodies “don't maintain official reserves of protein for use as fuel. Rather, protein is used
to build, maintain, and repair body tissues, as well as to synthesize important enzymes
and hormones” (Eberle 2014). It is believed that protein breaks down into energy from
branched-chain amino acids. The problem is that its dependability on the availability of
the amino acids. Also, the “nitrogenous waste products of amino acid degradation are
eliminated through the formation...of small amounts of ammonia” (Haff & Triplett).
Ammonia is toxic and can be harmful to the body.
Fats, on the other hand, can be a huge advantage for the body to burn as a primary
source of energy. One advantage is that fats can make a lot more ATP than can
carbohydrates. Free fatty acids enter the mitochondria and undergo beta oxidation, where
“the breakdown of a single triglyceride molecule containing three 16-carbon chain free
fatty acids can be metabolized to yield over 300 ATP molecules” (Haff & Triplett).
Because fatty acid chains have more carbon and hydrogen relative to oxygen, they
provide more energy per gram, which is 9 kcal/gram versus carbohydrates providing 4
kcal/gram.
REFERENCES
Haff, G. Gregory, Triplett, N. Travis, & National Strength & Conditioning
Association (U.S.). (2016). Essentials of strength training and conditioning (Fourth ed.).
Human Kinetics.
Powers, S., Howley, E. T., & Quindry, J. (2021). Exercise physiology: theory and
application to fitness and performance. McGraw-Hill.
Eberle, Suzanne Girard. (2014). Endurance Sports Nutrition (Third Edition).
Human Kinetics.
WILLIAM LAMBO
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