Case Study: Cardiorespiratory System
ATP, which stands for adenosine triphosphate, is a quick source of energy and is the most
important molecule that carries energy in the cell in order for muscles to contract. The three main
energy processes that the human body uses to produce ATP from catabolizing one molecule of
glucose includes the following: the Krebs Cycle, the oxidative phosphorylation, and glycolysis
(Powers & Howley, 2020).
To begin, the Krebs Cycles, also referred to as the citric acid cycle or tricarboxylic acid
cycle, has a main function of completing oxidation, which is the electron removal. The process
of electron removal is important due to the fact that these electrons hold the food molecules’
potential energy. The Krebs Cycle is a series of reactions where cells create energy through
aerobic respiration (Powers & Howley, 2020). This process takes place in the mitochondria of
the cell. As a result of this process, the Krebs Cycle takes in oxygen, gets rid of carbon dioxide
and water, and finally turns ADP into ATP (Powers & Howley, 2020). The Krebs Cycle starts
with the raw materials, acetyl-CoA, glucose, and fatty acids, and the end product of the Krebs
Cycle is the carbon dioxide which is the waste from the process (Powers & Howley, 2020).
The next process that produces ATP is oxidative phosphorylation, which is also termed
aerobic metabolism. This process is comprised of two elements that are fairly linked together,
which are the electron transport chain and chemiosmosis (Powers & Howley, 2020). This
process is the last step in cellular respiration and also takes place in the mitochondria of the cell
and includes oxygen. Oxidative phosphorylation starts with glycolysis, which produces the
pyruvic acid molecules. Those molecules enter the mitochondria and forms acetyl-CoA, which
undergoes oxidation. Ultimately, the end products are ATP, heat, carbon dioxide, and water
(Powers & Howley, 2020).
The final process is glycolysis, which occurs in the cytosol and does not utilize oxygen
and refers to the breakdown of glucose or glycogen. There are two phases of the reactions
involving glucose and pyruvate, and these phases are an energy investment phase and an energy
generation phase (Powers & Howley, 2020). In the energy investment phase, the stored ATP will
be utilized in order to form sugar phosphates (Powers & Howley, 2020). Ultimately, glycolysis
results in the release of energy. Glycolysis is primed by ATP, which adds phosphate groups to
both glucose and fructose 6-phosphate (Powers & Howley, 2020). The last phase is the energy
generation phase, where four ATP and two NADH are produced. In summary of the two phases,
glucose goes in and produces two pyruvate or two lactate, two ADP go in and two ATP come
out, and two NAD+ goes in and two NADH come out. The products of glycolysis then produce
the end products including the following: two pyruvate or two lactate, two ATP, and two NADH.
Finally, it is understood that the body accesses carbohydrates as the first source of fuel;
however, free fatty acids may be a better alternative as a raw source of fuel. Carbohydrates are a
good steady source of fuel as digestion breaks the sugars and starches contained in the
carbohydrates into simple sugars that the body uses as an energy source. With this being said,
free fatty acids contain nearly two times as much energy as carbohydrates. As a matter of fact,
each gram of free fatty acids produces two times more energy than carbohydrates. Understanding
this fact showcases why free fatty acids will ultimately be the better source of energy to access.
After considering free fatty acids and carbohydrates, proteins are the next source of fuel
to consider. The body will only resort to protein for energy if required to do so. The reason for
this is that protein is much more complex than free fatty acids and carbohydrates, and, therefore,
protein takes the longest amount of time to produce energy. The body needs protein to maintain
ew
muscle, which is the reason why the body will only access the protein sources if no other energy
source is available.
Amy Montgomery, L31739627
References
Powers, S., Howley, E., & Quindry, J. (2020). Exercise Physiology: Theory and Application to
Fitness and Performance (11th ed.). McGraw-Hill Higher Education (US).
https://mbsdirect.vitalsource.com/books/9781260813562
