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Are you a morning person? One of us is and one if us is definitely not. Mainly because,
when I wake up in the morning, it just takes a while for me to feel like I get my energy
back. It takes a lot of time---and coffee---for that to happen for me.
Cells don’t really have that luxury. They are busy performing cell processes all the
time and many of the processes
that they do require energy. Specifically, ATP energy.
ATP stands for adenosine tri phosphate. It’s a type of nucleic acid actually, and it is
action packed with three phosphates. When the chemical bond that holds the third phosphate
is broken, it releases a great amount of energy. It also is converted into ADP, adenosine di
phosphate. And really, that’s just a fancy way of saying that it has two phosphates after
losing one.
So where am I going with this? Well, cells have to make this ATP energy. It doesn’t
really matter what kind of cell you are---prokaryote or eukaryote---you have to make ATP energy.
The process for making that ATP energy can be different, however, depending on the type
of cell. But you have to make ATP energy.
One way that this can be done efficiently is called aerobic cellular respiration. We
are going to focus on aerobic in eukaryote
cells which have many membrane bound organelles such as mitochondria. The mitochondria is
are going to be kind of a big deal in this.
So let’s get started. Remember we are trying to make ATP energy. Let’s take a look at
this formula. Remember that reactants (inputs) are on the left side of the arrow. And products
(outputs) are on the right side of the arrow.
This formula, by the way, looks remarkably similar to photosynthesis. Look how the reactants
and products just seem to be on different sides.
You know why? See, in photosynthesis, organisms (like plants and protists for example) made
glucose. Notice how glucose is a product. But in cellular respiration, we break the
glucose. Notice how glucose is a reactant. In order to make ATP energy.
So photosynthesis makes glucose---and cellular respiration, it breaks glucose. Kind of cool.
Photosynthetic organisms have the best of both worlds because they not only do photosynthesis
to make their glucose but they do cellular respiration to break it. I say that’s pretty
great, because glucose is the starter molecule in cellular respiration and needed in order
to get this going. If you aren’t photosynthetic, such as a human or an amoeba, you have to
find a food source to get your glucose. Cellular respiration involves three major steps. We
are going to assume that we are starting with one glucose molecule so that you can see what
is produced from one glucose molecule.
#1 Glycolysis- This step takes place in the cytoplasm, and this step does not require
oxygen. Glucose, the sugar from the formula, is converted into a more usable form called
pyruvate. It actually takes a little ATP energy itself to get this process started. The net
yield from this step is approximately 2 ATP molecules. And 2 NADH molecules. What is NADH?
NADH is a coenzyme, and it has the ability to transfer electrons, which will be very
useful in making even more ATP later on. We’ll get to that in a minute.
#2 Krebs Cycle-This is also called the Citric Acid Cycle. We are now involved in the mitochondria,
and this step requires oxygen. The pyruvate that was made is converted and will be oxidized.
CO2 (carbon dioxide) is produced. We produce
2 ATP, 6 NADH, and 2FADH2. FADH is also a coenzyme, like NADH, and it will also assist
in transferring electrons to make even more ATP.
#3 The electron transport chain. This is, just, a beautiful thing. Really. We’re still
in the mitochondria, and we do require oxygen for this step. This is a very complicated
process, and we are greatly simplifying it by saying that electrons are transferred from
the NADH and FADH2 to several electron carriers. They are used to create a proton gradient.
The protons are used to power an amazing enzyme called ATP synthase. Remember that the word synthase
means to “make” so that’s what ATP synthase does. All the time. It makes the ATP by adding
phosphates to ADP. Oxygen is the final acceptor of the electrons. When oxygen combines with
two protons, you get H20---aka
water. The electron transport chain produces a lot of ATP compared to the other two steps.
There isn’t an exact number on this---many textbooks will say 34 ATP. Meaning that the
net amount of ATP made when you add all the steps together is 38 ATP. But you need to
understand that this is a “perfect case” scenario and in general, you can expect a
lot less ATP made.
If we look at our formula again, we can see how the glucose and oxygen on the reactant
side was used to produce carbon dioxide (a waste product), water (a waste product), and
ATP energy. ATP energy was our goal.
Now, this was just one way of creating ATP energy---and a very efficient way at that.
But like we had said at the beginning, all cells have to make ATP energy. But the way
that they do it can differ. If there is no oxygen available, some cells have the ability
to perform a process known as fermentation. It is not nearly as efficient, but it can
still can make ATP when there isn’t oxygen.
We really can’t emphasize enough how important the process of making ATP energy is. If you
doubt how powerful it is, consider cyanide. This toxin is found in some rat poisons and
highly toxic. It works by blocking a step in the electron transport chain. Without being
able to continue the electron transport chain, cells cannot produce their ATP, and this poison
can be deadly in a very short timeframe.
There is also a demand for increased research on various mitochondrial disorders. Many mitochondrial
disorders can be deadly, because the role of the mitochondria in our body cells is so
essential for our ATP production. We are confident that the understanding of how to treat these
disorders will continue to improve as more people, like you, ask questions. Well that’s
it for the amoeba sisters and we remind you to stay curious.