Video Transcript
In this video, we will learn how to
describe the process of glycolysis. We will recall the products and the
reactants of glycolysis and describe the intermediate reactions that occur as these
reactants are converted into the products.
Glycolysis is the term used to
describe the first stage of cellular respiration. Cellular respiration is incredibly
important for organisms. It is the process by which sugars
are broken down to release energy. We can then use this energy to
power important metabolic reactions in our cells and to carry out essential
processes like digestion, breathing, and movement. You may recall the overall chemical
equation for cellular respiration. Glucose plus oxygen react to form
carbon dioxide and water. In this process, energy is released
in the form of ATP. We are going to take a look at this
general equation in a bit more detail and understand all the biochemical reactions
that take place.
Glycolysis takes place in the
cytoplasm of cells. The cytoplasm is the jelly-like
fluid that fills the inside of the cell and contains most of the cell’s
organelles. Glycolysis does not require oxygen,
but the following stages of cellular respiration do. If oxygen is not present,
glycolysis is followed by a process called fermentation, which does not produce as
much ATP as cellular respiration. So let’s get started and have a
look at the reactions of glycolysis in more detail.
Here, we can see the overall
sequence of chemical reactions that occur during glycolysis, but this looks quite
complicated. So let’s go through it step by
step. The primary reactant of glycolysis
is glucose. A glucose molecule has a
ring-shaped structure. But in our simplified diagrams,
let’s just use a chain of carbon atoms. Glucose is a monosaccharide. Saccharide is a word part meaning
sugar, and mono- is a word part meaning one. This refers to the fact that
glucose is made up of a single sugar unit. One molecule of glucose has six
carbon atoms, so we refer to it as a six-carbon sugar. We get the glucose needed for
glycolysis from our diet. Foods that contain carbohydrates,
like pasta, potatoes, and bread, are usually a good source of glucose.
In the first stage of glycolysis,
this molecule of glucose will undergo a process known as phosphorylation. Phosphorylation refers to a
reaction in which phosphate groups are added to a molecule. In glycolysis, glucose is
sequentially phosphorylated by two molecules of ATP. ATP stands for adenosine
triphosphate. You may recall that the word part
tri- means three, so triphosphate means three phosphate groups. When a molecule of ATP donates its
phosphate group to a different molecule with the help of an enzyme, it is converted
from ATP to ADP. ADP stands for adenosine
diphosphate. Di- is a word part that means two,
so we know that ADP only has two phosphate groups.
To form ADP, the bond between the
second and third phosphate group in ATP breaks. When this inorganic phosphate is
released from ATP, it forms a new bond with the glucose molecule. The formation of this new chemical
bond between the phosphate group and the glucose molecule releases energy. The amount of energy released in
making this new bond is greater than what was required to break the bond in the ATP
molecule. With the help of enzymes, glucose
is phosphorylated by two molecules of ATP in turn. This means that glucose gains two
phosphate groups, as shown in the diagram by the Ps.
The compound that is formed is
sometimes known as phosphorylated glucose, but more accurately called
fructose-1,6-biphosphate. Bi- is another word part that means
two. One and six refer to the carbon
atoms where these phosphate groups are attached. Fructose is another
monosaccharide. It is very similar to glucose, as
it also contains six carbon atoms, but has a slightly different structure. Glucose is converted into fructose
by the action of an enzyme. So now we have our new sugar
fructose-1,6-bisphosphate.
Let’s take a look at the next set
of reactions that occur in glycolysis. Fructose-1,6-bisphosphate, the
sugar we have created from phosphorylating and converting glucose, is a six-carbon
sugar. Next, this six-carbon compound is
split into two three-carbon compounds, finally a nice, simple process. These three-carbon compounds have a
few different names. This can depend on what country you
are learning in. We are going to use the name
glyceraldehyde-3-phosphate or G3P. But you may also see them called
phosphoglyceraldehyde or PGAL or triose phosphate or TP. So don’t get confused. They are all referring to the same
molecule.
We are nearly at the end of our
glycolysis reactions. So let’s take a look at the final
steps. Remember that from one molecule of
fructose-1,6-bisphosphate, two molecules of G3P are produced. So, for the next set of reactions,
remember that they will all happen twice. Next, our molecule of G3P is
converted into a molecule of pyruvate. Pyruvate or pyruvic acid is an
important chemical compound, which is known as an intermediate. We’ll see why this is shortly.
For a molecule of G3P to become a
molecule of pyruvate, two things need to happen. It needs to lose a hydrogen and a
phosphate group. Let’s go through this in a bit more
detail. NAD+ is a coenzyme. A coenzyme is a nonprotein compound
or molecule that helps enzymes carry out biochemical reactions. When NAD+ is converted into NADH,
it gains a hydrogen ion and two electrons from G3P. We say that NAD+ is reduced because
it has gained electrons. You may therefore see NADH referred
to as reduced NAD+. This reaction is coupled with the
next one. Using the energy from the reduction
of NAD+, G3P gains another phosphate group.
This new compound doesn’t last long
though. Following this, the molecule loses
both of its phosphate groups. And these phosphate groups are
gained by two molecules of ADP. And there we have it. Through a series of biochemical
reactions, our reactant glucose is converted into two molecules of our product
pyruvate. You might be wondering why. Why have we looked at all of these
complex reactions and not really ended up with much?
Firstly, glycolysis itself produces
two molecules of ATP. Actually, the gross product is four
molecules of ATP. But remember, we used two molecules
of ATP in the first stage. So the net amount of ATP produced
that can be used elsewhere is two molecules. This is important because ATP is an
energy-carrying molecule for all of our cells. It is a small molecule, and it can
be easily broken down by breaking the bond between the end and middle phosphate
groups. When a phosphate group released
from ATP bonds with another molecule, the reaction releases a net amount of
energy. This energy can be used for pretty
much all of our important life processes. This includes moving, breathing,
digesting, and, you guessed it, more cellular respiration. But two molecules of ATP doesn’t
sound like much, right? So why is glycolysis so
important?
Glycolysis is actually the start of
a series of stages in the process of cellular respiration. Glycolysis in humans rarely happens
on its own. It is usually followed by reactions
called the link reaction, the Krebs or citric acid cycle, and oxidative
phosphorylation. Combined, all of these reactions
produce a large amount of ATP for us. You may remember that we refer to
pyruvate as an intermediate compound. The next stage of cellular
respiration, the link reaction, cannot happen without pyruvate. So glycolysis is incredibly
important for kicking off all of these subsequent reactions.
Now that we have learned about
glycolysis, let’s try a practice question.
What is the net yield of ATP for
one glucose molecule undergoing glycolysis?
Glycolysis is the first stage of
cellular respiration. Glycolysis takes place in the
cytoplasm of cells of nearly all living organisms, and it happens whether oxygen is
present or not. In glycolysis, a molecule of
glucose undergoes a series of biochemical reactions to form two molecules of
pyruvate or pyruvic acid. In the first set of reactions in
glycolysis, two molecules of ATP are actually used to convert glucose into the
phosphorylated sugar fructose-1,6-bisphosphate. So, currently, the yield of ATP is
minus two.
Next, the six-carbon
fructose-1,6-bisphosphate is split into two three-carbon compounds. This reaction does not use any ATP,
but it also does not produce any ATP. So our current yield of ATP still
stands at minus two. Finally, the two three-carbon
compounds need to be converted into our final product, pyruvate. In this reaction, the three-carbon
compounds donate a hydrogen ion and two electrons to a coenzyme called NAD+ to form
reduced NAD or NADH. This reaction is coupled to another
reaction.
Using the energy from the reduction
of NAD+, G3P gains another phosphate group. This new compound doesn’t last long
though. Following this, G3P loses both of
its phosphate groups. These phosphate groups are gained
by two molecules of ADP to form ATP. For each molecule of pyruvate
formed, two molecules of ATP are produced. Because there are two molecules of
pyruvate, four molecules of ATP are produced in total. So our yield of ATP before this
stage was minus two, and here we have produced four molecules of ATP.
Now, we’re ready to calculate the
net yield of ATP. Minus two ATP plus four ATP gives
us a net yield of two ATP. So, for each single molecule of
glucose that undergoes glycolysis, the net yield of ATP is two molecules.
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. Glycolysis is the first stage of
cellular respiration and does not use oxygen. The overall basic equation for
glycolysis is glucose plus two ADP plus two inorganic phosphate yields two pyruvate
plus two ATP. The first reactions in glycolysis
use ATP to convert glucose into fructose-1,6-bisphosphate. Fructose-1,6-bisphosphate is
converted into two molecules of G3P, and then G3P is converted into pyruvate. The net yield of ATP from
glycolysis is two molecules.
