Video Transcript
In this video, we will learn about
the link reaction and the Krebs cycle as stages of cellular respiration. We will recall the reactants, the
products, and the intermediate compounds in these reactions. Finally, we will be able to explain
why the products of these stages are so important for the entire process of cellular
respiration.
Cellular respiration is the
biological process which, in short, breaks down glucose in our cells to release
energy. Glycolysis, the first stage of
cellular respiration, converts one molecule of glucose into two molecules of
pyruvate, also known as pyruvic acid. Glycolysis takes place in the
cytoplasm of most living cells. We are going to be looking at the
next stages of cellular respiration. And for this, we’re going to move
out of the cytoplasm and into the mitochondria of cells.
The link reaction takes place on
the inner membrane of the mitochondria, and the Krebs, or citric acid, cycle takes
place in the mitochondrial matrix. Both the link reaction and the
Krebs cycle are considered aerobic processes. This means that oxygen must be
present for the reactions to take place. So let’s start by having a detailed
look at the link reaction.
The link reaction is so called
because it links the stages of glycolysis and the Krebs cycle. Calling it a reaction is a little
misleading. It is actually a process made up of
multiple reactions. The primary reactant of the link
reaction is pyruvate, also known as pyruvic acid. You may remember that this was the
final product of glycolysis. In the link reaction, a molecule of
pyruvate is modified by a couple of biochemical reactions. Firstly, pyruvate loses a carbon
atom. Alongside this, two oxygen atoms
are also lost. These atoms are released in the
form of carbon dioxide. This process converts the
three-carbon pyruvate into a two-carbon molecule. Next, this two carbon compound is
oxidized.
But what does this mean? In chemistry, oxidation refers to
the loss of electrons from a molecule. Here, pyruvate is oxidized because
it loses electrons. These electrons, as well as a
hydrogen ion, are gained by the coenzyme NAD plus. Because NAD plus has gained
electrons and a hydrogen ion, we refer to it as reduced. Reduction is the opposite of
oxidation. So typically in a chemical
reaction, when one molecule is oxidized, another is reduced. Reduced NAD is also called NADH for
short, which we will use here. So we have a two-carbon compound
which has lost electrons and become oxidized. The final step of the link reaction
is when coenzyme A bonds to this two-carbon compound. This forms the primary product of
the link reaction, acetyl coenzyme A or acetyl CoA for short.
Let’s have a quick think back to
our general equation for cellular respiration. Glucose plus oxygen yield carbon
dioxide plus water plus energy. We know that glucose has already
been broken down in glycolysis to give us the pyruvate needed for the link
reaction. We can now see that the link
reaction is responsible for producing some of the carbon dioxide given out by
cellular respiration. Importantly, we can also see that
the link reaction does not produce any molecules of ATP. However, some energy is stored in
the product of the link reaction, acetyl CoA. So now, we’ve got our product of
the link reaction acetyl coenzyme A.
Let’s take a look at the next
stage, the Krebs cycle, to see why this little compound is so important. Here, we have a diagram to outline
the overall process of the Krebs cycle. We call it the Krebs cycle after
the scientist who discovered it Hans Krebs. However, you may also see it
referred to as the citric acid cycle. It looks a little complicated at
the moment, so let’s go through it step by step first. Acetyl coenzyme A provides two
carbon atoms to a four-carbon compound called oxaloacetic acid or oxaloacetate. By adding two carbon atoms to a
four-carbon compound, we make a six-carbon compound. This newly formed six-carbon
compound is called citric acid or citrate. This is where the name the citric
acid cycle comes from.
Next, this six-carbon compound is
converted into a new five-carbon compound by two key reactions. Citric acid loses a carbon atom and
this is given off in the form of carbon dioxide. It also loses electrons and a
hydrogen ion. These are gained by NAD plus, the
coenzyme we saw in the link reaction. This converts NAD plus into reduced
NAD, or NADH. Remember, when it comes to
electrons, oxidation is loss and reduction is gain. So we say that citric acid is
oxidized and NAD plus is reduced. Now, our five-carbon compound is
going to go through a series of reactions to form a four-carbon compound. Again, the compound loses electrons
and a hydrogen ion. And exactly as before, these are
gained by NAD plus. So we form another molecule of
reduced NAD, or NADH. And again, an atom of carbon is
lost from the compound in the form of carbon dioxide.
Next there is quite an important
reaction that occurs. During the conversion of the
five-carbon compound to a four-carbon one, ADP is phosphorylated to form ATP. This means that adenosine
diphosphate gains a phosphate group to form adenosine triphosphate. You might remember that ATP is the
energy-carrying molecule of cells and provides an immediate supply of energy. So this is a very important product
of cellular respiration.
Finally, we just need to convert
our intermediate four-carbon compound into oxaloacetic acid. To do this, two coenzymes gain
electrons and hydrogen ions, FAD and NAD plus. They form one molecule of reduced
FAD or FADH2 and one molecule of reduced NAD or NADH. And there we go. We now have a molecule of
oxaloacetic acid ready to join with our next incoming molecules of acetyl coenzyme
A. When it does this, the whole Krebs
cycle will start over again.
But now that we have come full
circle, you might be wondering, what is the benefit of the Krebs cycle? Firstly, as we saw, the Krebs cycle
produces one molecule of ATP. We know that ATP is essential for
acting as an energy-carrying molecule for our cells. But one molecule isn’t really that
much. Secondly, the Krebs cycle produces
multiple molecules of reduced coenzymes. In total, we gain one molecule of
FADH2 and three molecules of NADH from each turn of the Krebs cycle. These molecules are incredibly
important for the next stage of cellular respiration, oxidative phosphorylation. It’s in this stage where many more
molecules of ATP are produced.
Now, let’s have a quick recap of
the link reaction, the Krebs cycle, and the key products we gain from them. Firstly, the link reaction produces
one molecule of carbon dioxide and one molecule of reduced NAD. Its final product, acetyl coenzyme
A, becomes a primary reactant of the Krebs cycle. The Krebs cycle produces one
molecule of FADH2 and three molecules of NADH. It also produces two molecules of
carbon dioxide. And let’s not forget that one
molecule of ATP is also produced.
So here are the products of the
link reaction and the Krebs cycle combined. But for each molecule of glucose
that enters glycolysis, the link reaction and Krebs cycle happen twice. This is because glycolysis produces
two molecules of pyruvate for every one molecule of glucose. So to get the total number of
products for the link reaction and Krebs cycle per one molecule of glucose, we need
to multiply everything by two. Now that we’ve learned about the
link reaction and the Krebs cycle, let’s have a go at a practice question.
The diagram provided shows a basic
outline of the Krebs cycle. What are the products of the Krebs
cycle? (A) ATP, reduced FAD, reduced NAD,
and carbon dioxide. (B) ADP, FAD plus, and NAD
plus. (C) Oxaloacetic acid and acetyl
coenzyme A. (D) Carbon dioxide and acetyl
coenzyme A.
The Krebs cycle is the third major
stage of cellular respiration. Let’s briefly recap the other
stages to see where this cycle fits in. Firstly, glycolysis takes a
molecule of glucose and, through a series of biochemical reactions, converts it into
two molecules of pyruvate. Then, the link reaction converts a
molecule of pyruvate into a compound called acetyl coenzyme A. Acetyl coenzyme A then becomes the
primary reactant of the Krebs cycle, also known as the citric acid cycle. After the Krebs cycle, the final
stage of cellular respiration is oxidative phosphorylation. But we won’t worry about this too
much for now.
Let’s take a look at the diagram to
determine what the products of the Krebs cycle are. Firstly, the two-carbon acetyl
coenzyme A joins with a four-carbon compound called oxaloacetic acid. You may also see this called
oxaloacetate. This forms a six-carbon compound
called citric acid or citrate. So far, no products have been given
out by the Krebs cycle. Next, citric acid is converted into
a five-carbon compound. We can see two molecules are
produced in this process, one molecule of carbon dioxide and one molecule of reduced
NAD. Let’s use this table to continue to
record the products of the Krebs cycle.
Now, this five-carbon compound is
converted into a four-carbon compound. As we can see from the diagram,
this conversion results in another molecule of reduced NAD, another molecule of
carbon dioxide, and one molecule of ATP. Our new four-carbon compound is now
converted into oxaloacetic acid. As this happens, we see that
another molecule of reduced NAD is produced and one molecule of reduced FAD. Finally, we’ve come full circle
around the Krebs cycle. So let’s see, using our tally of
products, what our correct answer is. The only option to correctly show
all of the products of the Krebs cycle is option (A): ATP, reduced FAD, reduced NAD,
and carbon dioxide.
Let’s summarize what we’ve learned
with some key points. Cellular respiration is the process
by which living organisms break down glucose and other substrates to release
energy. The link reaction occurs after
glycolysis and is responsible for converting pyruvate into acetyl coenzyme A. In the Krebs cycle, acetyl coenzyme
A donates two carbon atoms to oxaloacetic acid, which forms citric acid. The products of the link reaction
and Krebs cycle include carbon dioxide, reduced NAD, reduced FAD, and ATP.
