0
Case Study: Cardiovascular System
Dr. Bosak
EXSC 510: Advanced Exercise Physiology
Raekwon J. Parker
EXSC 510 1
To generate adenosine triphosphate (ATP) from the breakdown of a single glucose
molecule, the human body relies on three essential energy-producing processes known as the
phosphagen, glycolytic, and the oxidative. Numerous chemical reactions take place within the
human body every minute of every day. These reactions collectively form is known as
metabolism, which is the sum of all cellular reactions, including the chemical pathways that lead
to the synthesis of various molecules (Powers et al., 2018). This process includes both anabolic
(building up) and catabolic (breaking down) reactions. The energy systems involved must
efficiently convert fats, proteins, and carbohydrates into energy that the body can use for
different actions. The metabolic process known as bioenergetics is crucial for converting
carbohydrates into forms of energy that all cells can utilize. It is vital for the body to have the
ability to extract energy from food nutrients consistently and to sustain that energy source
throughout the duration of physical activity (Powers et al., 2018).
The structure of a cell plays a significant role within bioenergetics. So, understanding the
structure of a cell is crucial when learning bioenergetics and the primary processes for ATP
production. Yes, a cell is the fundamental unit of life, different cells vary but they typically share
a three-part structure. The cell membrane is a selectively permeable barrier that delineates the
cell from its surrounding environment. Within the cell, the nucleus houses genes made of DNA,
which are the blueprint for genetic information. Genes play a vital role in cells as they oversee
the production of proteins. Not every cell contains a single nucleus; for instance, skeletal muscle
fibers possess multiple nuclei distributed throughout the muscle fiber. The cytoplasm, the
cellular fluid, is home to a variety of organelles, with the mitochondria being particularly
EXSC 510 2
significant. The mitochondria main responsible within the cell is for the oxidative conversion of
nutrients into a form of energy that cells can use.
Now having a strong grasp of foundational understanding of metabolism, bioenergetics,
and cellular structure and function now established. It’s important to recognize the different
sources of energy for these processes and cycles. Energy originates from the sun which then
plants harness this solar energy to drive the chemical reactions for carbohydrates, fats, and
proteins. Animals consume plants and other animals to acquire the necessary energy for
sustaining cellular functions. The body needs necessary energy for cellular processes that will be
released from the chemical bonds for molecules. Endergonic reactions is the energy transfer in
the cell which occurs when a series of chemical reactions within the cell.
To perform daily activities such as exercises or daily movements or even rest activities,
the body utilizes the carbohydrates, fats, and protein consumed within our daily consumption.
During exercise, the main two sources of energy are fats and carbohydrates depending on what
type of resistance training you are incorporating. While protein has a minor/lesser role in overall
energy expenditure when it comes to training. Carbohydrates are carbon, hydrogen and oxygen
atoms that in the stored state can provide the body with a rapid form of energy with 1 g of carbs
yielding approximately 4 kcals of energy (Powers et al., 2018). Carbohydrates exist in three
forms: monosaccharides, disaccharides, and polysaccharides. Monosaccharides are basic sugars
such as fructose and glucose. Disaccharides are formed when two monosaccharides bond
together such as fructose & glucose bond to form sucrose. Polysaccharides are complex
carbohydrates which consist of multiple (3 or more) monosaccharides and it is highly beneficial
of all carbohydrates with the prime example being glycogen. Carbohydrates are stored in the
EXSC 510 3
form of Glycogen which is to minimize the osmotic pressure variance between the intracellular
space and the surrounding extracellular fluid.
Transitioning to the next main energy source which is fat, which are insoluble
(hydrophobic) and share the same basic chemical elements as carbohydrates but with a higher
carbon to oxygen ratio. Stored body fat is the best fuel for prolonged exercise because unlike
carbs, fat molecules contain larger quantities of energy per unit (1g=9 kcals) (Powers et al.,
2018). Fats are categorized into 4 types: fatty acids, triglycerides, phospholipids, and steroids.
Muscle cells mainly use fatty acids for energy, which is stored in the body as triglycerides.
Triglycerides are stored in the fat cells but can also be located in the skeletal muscle. Converting
triglycerides into fatty acids and glycerol is known as lipolysis which this process is helped by
lipase enzymes.
Finally, protein is the last energy source to be taken into account. The reason why is
because they have the least impact into the process of converting to ATP (energy) during
physical activity. The reason for this is because it only contributes to 2% to 15% of the fuel
during exercise (Powers et al., 2018). Protein can be involved in bioenergetic pathways by
breaking down protein into their amino acid components. Majority of these amino acids are
converted into either glucose or pyruvate, and a few into acetyl-CoA. It is very minimal for the
contribution of protein to energy and they require more energy for their metabolism than they
have. For this very reason is why the body reserves protein and only uses protein for energy only
as a last resort if the body doesn’t have anything else to pull energy from.
The quickest and initial pathway for ATP generation is termed the PC pathway, which
stands for phosphocreatine breakdown. This process involves the contribution of a transfer
phosphate group along with its bond energy from phosphocreatine to ADP, resulting in ATP.
EXSC 510 4
The enzyme creatine kinase facilitates this reaction. This reaction is catalyzed by the enzyme
creatine
kinase and as rapidly as ATP is broken down to ADP=Pi when exercise begins it is resynthesized
via the PC reaction making the total amount of ATP formed via this reaction limited (Powers et
al., 2024). Yes, this process is short lived but is essential for short burst high intensity exercise
such as 100m sprints, 5-10 sprints, 3RM Power Cleans, etc.
The second metabolic route that swiftly generates ATP anaerobically (without oxygen) is
known as glycolysis. Glycolysis process breaks down glucose or glycogen into either two
pyruvate or lactate molecules. Glucose is initially a six-carbon structure, while pyruvate and
lactate are with three carbons each. Glycolysis is an anaerobic mechanism that extracts energy
from glucose to create ATP by rebonding an inorganic phosphate (Pi) to ADP within the muscle
cell's sarcoplasm. From each glucose molecule, this pathway yields two ATP molecules and
either two pyruvate or two lactate molecules. Glycolysis does not involve the use of oxygen, but
with the presence of it in the mitochondria pyruvate can participate in aerobic production of
ATP, which leads into talking about the aerobic discussion of ATP production (Powers et al.,
2018).
The aerobic synthesis of ATP is known as oxidative phosphorylation, a process where
energy is conveyed through the electron transport chain, culminating with oxygen as the final
electron acceptor. This process of events takes place within the mitochondria and encompasses
two key metabolic processes: the Krebs cycle and the electron transport chain. Krebs cycle
primary function is to fully oxidize acetyl CoA, utilizing NAD+ and FAD as electron
transporters. The energy derived from this process is then utilized to join ADP and inorganic
phosphate to regenerate ATP within the electron transport chain. While oxygen does not play a
EXSC 510 5
role in the Krebs cycle, it is very essential at the conclusion of the electron transport chain as the
final electron acceptor. The Krebs cycle facilitates the oxidation of carbohydrates, fats, or
proteins, to lead to the production of CO2 and the provision of electrons for the electron
transport chain, which in turn powers the generation of ATP within the mitochondria. The end
result of oxidation-reduction of the Krebs cycle shows that two NADH forms when pyruvate is
converted to acetyl-CoA which results in five ATP. A total of six NADH and two FADH
molecules are produced in the Krebs cycle from one glucose molecule. The total yield from
aerobic oxidation reduction of glucose 32 ATP molecules (Powers et al., 2018).
EXSC 510 6
References
Powers, S.K. and Howley, E. T. (2018) Exercise Physiology: Theory and application to
fitness and performance (10th Ed.). McGraw Hill
