Welcome, back everyone. Today we're going to talk about aerobic respiration. We're
going to go over an overview, and we're going to talk about something called
glycolysis, and also a part called the prep steps. The first thing that we
should know about respiration is that it's actually pretty similar to
photosynthesis, only run in reverse. In photosynthesis, we're converting water,
carbon dioxide, and ATP into a form of sugar called glucose, and we're also
producing oxygen as a by-product. Respiration is almost exactly the
opposite. Instead of converting ATP into glucose, we're starting with glucose and
converting it into ATP. We're also using oxygen and converting it into carbon
dioxide, and along the way we're going to release some heat and some water vapor
as well. So, you're going to want to keep these two equations in mind as we go
through respiration. Respiration is also different from photosynthesis in that it
occurs mostly in the mitochondria of the cell rather than in the chloroplasts.
Both plant and animal cells have mitochondria, which is the major site of
respiration. So, here we can see that we put in glucose and oxygen, and we export
carbon dioxide, ATP, water, and some heat as a by-product. There are two main
categories of cellular respiration, depending on whether or not oxygen is
present. We have aerobic respiration which is more efficient, makes more ATP,
and is also sustainable for a long period of time, and we also have in
aerobic respiration, where there is no oxygen present. This process is much less
efficient, makes much less ATP, and can only be sustained for short periods of
time.
Here is a slightly more detailed view of the two kinds of respiration. On
the right, we have anaerobic respiration which occurs when there is no oxygen
present. On the left, we have aerobic respiration which occurs when there is
oxygen present. Both of these two processes begin with something called
glycolysis, which we're going to go into detail about in a couple of minutes.
Glycolysis produces two molecules of ATP, and then we go into either aerobic
respiration or anaerobic respiration depending on whether or not there is
oxygen present. If there is no oxygen, you will go into anaerobic respiration.
Depending on what kind of organism is respirating, it might produce lactic acid
or it might produce alcohol, but in either case it won't produce any more
ATP. In anaerobic respiration, you only produce two ATP molecules per molecule
of glucose; all of the production happens during glycolysis. Neither making lactic
acid nor making alcohol will generate any more ATP molecules. So, you can see
that anaerobic respiration is not very good at generating a lot of ATP. Aerobic
respiration on the other hand is very good at generating large amounts of ATP,
although it does have more steps. Just like anaerobic respiration, it begins
with glycolysis, where it generates two ATP molecules. Then it goes into what we
call the prep steps, and then the Krebs cycle, where it generates two more ATP.
Finally, it goes into something called the electron transport chain where it
generates a whopping 34 ATP per molecule of glucose. So the total amount
of molecules of ATP for aerobic respiration is something like 38
molecules of ATP per molecule of glucose, whereas anaerobic respiration only makes
two. So, you can see that aerobic respiration is significantly better at
generating ATP than anaerobic respiration is. In this screencast, we're
going to talk about the first two portions of aerobic respiration. We're
going to talk about glycolysis, and then we're going to discuss the prep steps.
We're not going to talk about anaerobic respiration any more today. Something to
be on the lookout for as we discuss respiration is going to be the various
kinds of stored energy that are present. You should already be familiar with ATP;
adenosine triphosphate which holds energy stored in its bonds. In this
class, we choose to liken ATP to a battery because it can be recharged and
reused over and over and over again. There are other kinds of molecules that
hold energy in their bonds and form storable energy sources. These include
NADH and FADH2. These are all rechargeable storable forms of energy
that can be cashed in later on, just like ATP. I've drawn these two look like
batteries but different sizes. When NADH and FADH2 react with other molecules
later on during the respiration process, they will generate ATP molecules which
we will learn about later on. For now, just remember that anything shaped like a
battery is a stored energy source that will be used later on. Just to make sure
we don't get confused, don't forget that NADH is NOT the same as NADPH from
photosynthesis. A helpful way to remember this might be to remember that NADPH has
the letter P for photosynthesis. So, remember, these are two totally different things.
Okay, so now let's get started with glycolysis. We're going to start off by
breaking down one molecule of glucose, and remember that glucose has six
carbons in it. So, one two three four five six. This six carbon molecule can be broken
down into other molecules that will total up to six carbons. It does take some
energy to break apart glucose molecules, though, which is why we're going to spend
two molecules of ATP to break apart this green molecule. When we've broken apart
the glucose molecule, instead of one large molecule with six carbons, we now
have two smaller molecules each that have three carbons. This three carbon
molecule is called Pyruvate. This reaction also has enough energy left to
recharge two molecules of NAD+. We recharged NAD+
and we get molecules of NADH, which is the energy-rich form. In addition to
producing two pyruvate molecules and two molecules of NADH, this set of reactions
called glycolysis will also produce four molecules of ATP. However, because we
already spent two of those molecules to get the reaction going, the total number
that we've produced after glycolysis is only 2 ATP. You see a similar concept all
the time in business. In order to figure out your profit, you subtract your
expenses from your income. You might spend two dollars on things like wages
or on filling your stock, however if your customers buy lots of your product you
still will make a profit of two dollars. This is exactly what we do during the
set of reactions that we call glycolysis. We spend two ATP in expenses, but we
make back four ATP in income. This gives us a profit of two ATP for every
molecule of glucose. Okay so, so far we have taken a molecule of glucose and we
have gone through the process of glycolysis and so now we're going to
take the aerobic pathway and we're going to move on to the next part which is
called the prep steps. Now, remember whenever you
do the prep steps, this means that you have plenty of oxygen. You're going to
undergo aerobic respiration, you don't do the prep steps in anaerobic respiration.
We're going to take some of the products of glycolysis and rearrange them in
order to be ready for the next step, which is the kreb's cycle. One of the most
important things we made during the glycolysis process was two molecules of
pyruvate, which each have three carbons. We're now going to add that pyruvate to
something called Co-enzyme A, and also to two more molecules of energy poor NAD+.
A co-enzyme also stores energy and its bonds it can help enzymes to work
more effectively. When we mix all of these things together we're going to get
new molecules, but look out for how we keep the same number
of carbons. Once all of these molecules have been rearranged, we come up with two
molecules of something called Acetyl Co-enzyme A or "Acetyl Co A", which each
have two carbons. We also generate two molecules of carbon dioxide, which each
have one carbon, and we generate two molecules of NADH which again is packed
with potential energy. So, we haven't lost any carbons during this process. In the
first place with pyruvate, we had two sets of three carbons and then when we
rearrange them, we had two sets of two and two sets of one, again for a total of
six. We've also now discovered where some of the carbon dioxide comes from during
aerobic respiration. We'll be making more later on but here is where part of it
comes from. The last thing that we're going to do before we end the screencast
is we're going to tally up the score of how many of each kind of molecule we've
created. At this point in respiration, so far we made a total of two molecules of
ATP. (We actually made for but remember we spent two of them to get Glycolysis going).
We made four molecules of NADH, and we haven't made any FADH2 yet. So, that's
where we're at. Next time, we'll talk about the Kreb's cycle and about the
electron transport chain which is where we really make a lot of ATP.