Oxygen in cellular
respiration
Most all physiology can be
explained by the following equation, which describes the process of cellular
respiration:
Glucose + 6 Oxygen (O2) --> 6 Carbon dioxide (CO2) + 6 Water (H2O) + 34 ATP
Glucose is a simple
carbohydrate (sugar) that we use as a direct energy source. Glucose, which is 6 carbons long, gets broken
down step by step in a series of chemical reactions. At each step, a little bit of energy is
released by the reaction and that is stored in carrier molecules. These carriers then donate the energy in the
form of electrons, which is then harnessed to make another molecule called
ATP. ATP is cellular energy. The chemical bonds in ATP store high energy
and can be used to drive other cellular reactions, like pumping ions, or the
process that causes muscle contraction.
Without ATP we die.
But what does oxygen have to
do with this? Well, the electrons that are
donated by the carrier molecules must hop from protein to protein in what is
known as the “electron transport chain”.
The final electron acceptor is oxygen (O2), which forms water with that
extra electron. That’s it. That’s why we breathe, that’s why our heart
pumps blood— our tissues need energy (ATP) to perform cellular tasks and in
order to get energy from glucose, we need oxygen to accept the final electron. Carbon dioxide is produced as a byproduct and
is removed from the body during exhalation.
 |
| Glycolysis is anaerobic respiration and does not use oxygen. Note that oxygen is used as the last step of the electron transport chain to make the majority of the ATP. (www.phschool.com/science/biology_place/biocoach/cellresp) |
I should also mention here
that the more our tissues are active and working, the more ATP they need and
the more O2 needs to get to the cells.
When we exercise our muscles are very active, so that’s why the heart rate
and breathing rate increase; our body needs to intake more oxygen and
distribute it faster to our muscles.
You can see that oxygen
plays a critical role in our cells, so the mice at high altitudes are going to
have a harder time getting their cellular energy. How do they manage to run around when there
is so little oxygen?
Energy sources
As I mentioned above, we can
make ATP directly from glucose (a carbohydrate). We can also make ATP by using fats as an
energy source. There are two differences
between these two energy sources:
1) When we use fats as an
energy source, it always requires oxygen.
Glucose, on the other hand, can make a limited amount of ATP without
oxygen, which is called anaerobic respiration.
This is useful during short vigorous activity, but we cannot make enough
ATP by anaerobic respiration for sustained exercise.
2) For a given amount of
oxygen, more ATP is produced from carbohydrates, like glucose, than from
fats. However, the amount of ATP created
from a single fat molecule is greater than from a glucose molecule. In other words, if you have plenty of oxygen,
you should be burning fats. But once
oxygen becomes limiting, either because you’re working so hard, or because
you’re at a high altitude, then carbohydrates should be used.
Given this information, the
authors hypothesized that the mice at high altitudes will burn more
carbohydrates than mice at sea level.
They have a limited amount of oxygen in the air, so they need to use it
in the most efficient way to produce the energy they need.
High altitude mice burn more
carbohydrates, but fatigue sooner
The authors did all their
tests in the same experimental conditions, with the same amount of oxygen in
the air for both sets of mice. Under
normal oxygen conditions and when there was low oxygen content, the high
altitude mice burned more carbohydrates than the other mice during moderate
exercise. At rest, they also burned more
carbohydrates under low oxygen conditions.
The authors found that the activity of enzymes associated with breaking
down carbohydrates were greater in the high altitude mice, specifically in the
heart muscles. The heart has to work
harder at high altitude to get enough oxygen to the tissues, so it makes sense
that these muscles, in particular, would be burning carbohydrates
preferentially.
The high altitude mice, therefore,
have adapted to the low oxygen environment by having more active enzymes to
break down carbohydrates rather than fats.
One problem with that, though, is that the carbohydrate storage is less
extensive than fat storage. The
researchers found that high altitude mice fatigued more quickly than the sea
level mice, which burned more fats. They
suggest that the fast fatigue is a result of using up all the carbohydrate
stores. These mice, though, don’t travel
long distances and just need short bursts of speed to escape predators.
I really like how so many
physiological processes can be explained through understanding cellular
respiration. It’s so logical that animals
at high altitude need to use oxygen more efficiently, so they use carbohydrates
more for energy. It’s simple.
