Energy Metabolism – Part 1: Body’s Sources of Energy

Do you remember Dieter? Well, his physician told him that he should
exercise more. So, that’s why Dieter’s going for a run
today. The source of energy his muscles initially
use is ATP. ATP, or adenosine triphosphate, is a molecule
containing high energy phosphate-oxygen bonds. When a phosphate group cleaves off from the
ATP molecule, ADP, or adenosine diphosphate, is formed. Losing a second phosphate group yields AMP,
or adenosine monophosphate. Each cleavage of a phosphate-oxygen bond releases
the energy stored within. Only water is required to enable the use of
energy from ATP in the cells of the body. Therefore, ATP is considered the “universal
energy currency’ of the body. Since ATP easily releases phosphate, it’s
unstable and not available in large quantities. The energy supply from ATP is only sufficient
to fuel a few seconds of activity. So although ATP is a good energy carrier,
it doesn’t represent an energy store. After a few seconds of jogging, Dieter’s
muscles switch over to another energy source, creatine phosphate. Like ATP, creatine phosphate has a high-energy
bond. By the way, it cleaves off its phosphoryl
group more easily than ATP. So, it’s not surprising that the supply
of ATP in Dieter’s muscles is only enough to last for around one minute. Afterward, Dieter’s metabolism needs to
gain energy from his body’s energy stores. You’ll learn how this works in detail in
the various episodes of this Chalk Talk course on energy metabolism. First, Dieter’s muscles turn to sugar as
a source of energy: Free sugar, especially glucose, is usually available to the body
in such large quantities that they last for around an hour of physical activity. You can find out how glucose is broken down
in episodes 2 and 3 on glycolysis. Gradually, Dieter’s muscles begin to use
fat reserves as an energy source. The breakdown of fatty acids takes longer;
however, the stored fat in the body usually lasts for several days of physical activity. If you’d like to find out more about the
breakdown of fatty acids, stay tuned for episodes 4 and 5 on beta-oxidation. Through the metabolism of glucose and fatty
acids in Dieter’s body, acetyl coenzyme A, in short acetyl-CoA, is formed. Degradation of this two-carbon unit yields
carbon dioxide in the citric acid cycle. Dieter exhales the carbon dioxide during his
jog. If you’d like to know more about how the
citric acid cycle functions, then keep a lookout for episode 6 of our Chalk Talk course. In all these metabolic processes, Dieter’s
cells not only generate energy in the form of ATP but also reducing equivalents, namely
NADH and H+ and FADH2. These reduced coenzymes temporarily store
two protons and two electrons, which can be utilized again in different reactions. How these alleged reducing equivalents also
yield ATP for energy is explained in episode 7 on the electron transport chain. After jogging for a while, Dieter’s tired,
so he hurries home. Dieter even sprints the last few meters. Since he’s not very fit, the blood supply
in Dieter’s muscles isn’t sufficient to deliver all cells with oxygen at this speed. Also, there isn’t a large number of mitochondria
in Dieter’s muscle cells. To be able to sprint, his muscles need to
utilize energy without oxygen. Differences between anaerobic and aerobic
metabolism will be discussed in episode 8 of our Chalk Talk course on energy metabolism. Now, the anaerobic reactions occurring in
Dieter’s cells produce lactate. As excess lactate can lead to acidosis, Dieter’s
body transforms it. How this occurs will be explained in episodes
9 to 11, which focus on the Cori cycle and gluconeogenesis. So, what if Dieter had started his morning
on an empty stomach and depleted the glucose stores during his run? Well, his body could turn to gluconeogenesis
to produce glucose. Some of the body’s cells, such as those
in the brain, can’t switch to fatty acid metabolism as an energy source. Aside from glucose, the brain can also use
ketone bodies as an energy source. Their metabolism will be discussed in episode
12 of our Chalk Talk course. Back home, Dieter’s hungry after his exhaustive
training. He remembers once reading that pasta is a
great source of energy for runners. So, he eats a large serving of spaghetti. Because Dieter’s very hungry, he consumes
more calories than his body needs. With these additional calories, his body forms
new stores of fat. After all, you never know if Dieter might
come up with the crazy idea of going for a run again tomorrow…..In episode 13 on fatty
acid synthesis, we’ll show you how Dieter’s body creates these new reserves. Now, while Dieter’s taking a well-deserved
rest, he wishes you good luck and hopes you enjoy our biochemistry course.

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