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Are you a morning person?
One of us is and one of us is definitely not.
Mainly because, when I wake up in the morning, it just takes a while for me to get my energy
back.
It takes a lot of time- and coffee -for that to happen for me.
Cells really don’t have that luxury.
They are busy performing cell processes all the time: active transport of many substances
needed for their survival, for example.
And the energy currency they need, specifically, is ATP.
ATP stands for adenosine triphosphate.
It’s a type of nucleic acid actually, and it is action packed with three phosphates.
We have a video all about ATP and how it works as an energy currency.
So where am I going with this?
Well, cells have to make this ATP.
It doesn’t really matter what kind of cell you are - prokaryote or eukaryote - you have
to make ATP.
The process for making that ATP can be different, however, depending on that type of cell.
One way is called aerobic cellular respiration.
Lots of organisms can do aerobic cellular respiration, but this video is specifically
going to go into aerobic cellular respiration in eukaryotic cells.
That means, this video is talking about the process within cells that have membrane-bound
organelles such as a nucleus and mitochondria.
Eukaryotic cells include the cells you’d find in protists, fungi, animals, and plants.
The mitochondria- which can be found in most eukaryotic cells - are going to be kind of
a big deal in this aerobic cellular respiration, because some of the process occurs in the
mitochondria.
So let’s get started.
Remember the major goal for any organism performing this: it’s to make ATP.
Ok, here’s the overall look at the equation for aerobic cellular respiration.
Remember that reactants (inputs) are on the left side of the arrow.
And products (outputs) are on the right side of the arrow.
This equation, by the way, looks remarkably similar to photosynthesis.
Look at how the reactants and products are on different sides.
While that doesn’t mean they’re simply opposites, it does show that they have substances
in common.
In photosynthesis, organisms make glucose.
Notice how glucose is a product.
But in cellular respiration, organisms break the glucose down to make ATP.
Fun fact: You know how when a bean seed first germinates in the ground, it isn’t able
to do photosynthesis yet?
Yeah, the germinating bean is relying on glucose that it has stored and it’s doing cellular
respiration to break it down to make ATP so it can grow.
Of course, once it starts to mature and develop leaves, it can do photosynthesis.
It will be able to do both photosynthesis and cellular respiration.
Plants make glucose in photosynthesis and they break it down in cellular respiration:
they can do both.
But if you aren’t photosynthetic, such as a human or an amoeba, you have to find a food
source to get your glucose.
You need glucose to get this process started, and we’re going to assume that we’re starting
with one glucose molecule.
Step #1 Glycolysis.
This step takes place in the cytoplasm, and this step does not require oxygen.
It’s considered anaerobic.
In glycolysis, glucose, the sugar from the equation, is converted into a more usable
form called pyruvate.
Glycolysis usually takes a little ATP itself to start up.
The net yield from this step is approximately 2 pyruvate and 2 ATP molecules.
And 2 NADH.
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.
Now, an intermediate step occurs.
The 2 pyruvate are transported by active transport into the mitochondria, specifically the mitochondrial
matrix.
In the mitochondria, pyruvate is oxidized.
The 2 pyruvate are converted to 2 acetyl CoA, which will be used by the next step.
Carbon dioxide is released, and 2 NADH are produced.
Step #2 The Krebs Cycle - also called the Citric Acid Cycle.
Still in the mitochondrial matrix.
The Citric Acid Cycle is considered an aerobic process.
While the cycle doesn’t directly consume oxygen, some of the events in the cycle need
oxygen to continue.
To learn more about this intricate cycle where the 2 acetyl CoA will enter, check out the
further reading suggestions in the video details.
Carbon dioxide is released.
We also produce 2 ATP, 6 NADH, and 2 FADH2.
FADH2 is also a coenzyme, like NADH, and it will also assist in transferring electrons
to make even more ATP.
Step #3 The electron transport chain and chemiosmosis.
This is, just, a beautiful thing, really.
In eukaryotic cells, this is still in the mitochondria, and to be more specific, it’s
involving the inner mitochondrial membrane.
We do require oxygen for this aerobic step.
This is a very complex process, and we are greatly simplifying it by saying that electrons
are transferred from the NADH and FADH2 to protein complexes and electron carriers.
The electrons are used to generate a proton gradient as protons are pumped across to the
intermembrane space.
All these protons being pumped out into this intermembrane space generates an electrical
and chemical gradient.
The thing is, if you remember from our cell transport video, ions (like the H+) don’t
easily travel across membranes directly without something to travel through.
The protons can travel through an amazing enzyme called ATP synthase.
If I could be any enzyme, I’d be ATP synthase, because it has the ability to make ATP by
adding a phosphate to ADP.
ADP is a precursor to ATP.
ADP has two phosphates, but if it obtains a third phosphate, it becomes ATP.
So, in chemiosmosis, the protons travel down their electrochemical gradient through a portion
of the ATP synthase, powering it to make ATP.
The ultimate goal.
Oxygen is the final acceptor of the electrons.
When oxygen combines with two hydrogens, you get H20 - water.
If you remember from our equation, water is a listed product.
Now, the electron transport chain and chemiosmosis step makes a lot more ATP compared to the
other two previous steps.
How much?
So, I’ve noticed in my years of teaching and the assortment of textbooks I’ve collected
over the years, there are some different numbers in cellular respiration charts.
In fact, you can even see them change a bit in different editions of the same book.
And it’s made me want to emphasize that it’s important to not just memorize a number,
because it really is more of a range.
I want to focus less on that number of ATP made per glucose molecule because it depends
on a lot of variables.
One variable is the gradient we were talking about: how many protons were pumped across
that mitochondrial membrane.
You can see more variables discussed in the factual references, and those references have
estimates ranging from 26-34 molecules of ATP per glucose molecule in the electron transport
chain and chemiosmosis step alone.
Then if you add the other two steps: Krebs – otherwise known as the Citric Acid Cycle
- and glycolysis, you could estimate a range of anywhere between 30-38 net ATP molecules
total per molecule of glucose.
Again, to emphasize, a range.
Now, this was just one way of creating ATP.
But like we had said at the beginning, all cells have to make ATP, but the way that they
do it can differ.
If there's no oxygen available, some cells have the ability to perform a process known
as fermentation.
It’s not as efficient, but it can still make ATP when there isn’t oxygen.
We have a video about that!
We can’t emphasize enough how important the process of making ATP is for cells.
For example, cyanide - which is found in some rat poisons - can block a step in the electron
transport chain, which would block ATP production.
A poison that prevents ATP from being produced can be deadly.
With the important role that mitochondria have in ATP production, there is also a demand
for increased research on mitochondrial diseases.
We are confident that the understanding of how to treat these diseases 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.