(Narrator) This is Armando.
This video was created as part of the Hungry Microbiome Project,
which I made at CSIRO.
Carbohydrates are the most abundant biomolecules on earth.
They are found everywhere.
Carbohydrates are a major source of energy for all living organisms,
such as animals and plants.
But they are not only important for energy.
Carbohydrates also serve as important structural components.
For example, DNA contain the carbohydrate ribose,
and the plant cell wall are made up of the carbohydrate cellulose.
Carbohydrates mainly contain carbon,
hydrogen and oxygen atoms in a molar ratio of 1:2:1.
One carbon, two hydrogen’s, and one oxygen.
Carbohydrates can be divided into four types.
These are monosaccharides, disaccharides,
oligosaccharides and polysaccharides.
The word saccharide is derived from the Greek word for sugar.
Now let us look at each of these types of carbohydrates
and learn a bit more about their structure and how they are formed.
This is a biochemistry lesson.
Let’s begin with monosaccharides.
Monosaccharides are also referred to as simple sugars,
and they are the smallest units that make up any carbohydrate.
They are the building blocks.
The three main monosaccharides in the human diet include glucose,
galactose and fructose.
Now these structures may look intimidating,
but all you need to know is that they contain carbons,
hydrogens and oxygens.
Let’s look at glucose first, which you all probably have heard of.
Glucose is the main source of energy for humans.
Here you can see glucose in its cyclic chemical form.
What’s important to know about glucose is that it contains six carbon atoms,
so let’s have a closer look.
Here C represents a carbon atom,
and we can label these carbon atoms of glucose with numbers,
one, two, three, four, five, six, in this specific order.
So glucose has six carbon atoms.
This particular type of glucose is actually an alpha glucose
because it has an alpha configuration.
Alpha carbohydrates is where the hydroxyl group,
the OH group of carbon number one is pointing in the opposite direction
to the carbon number six, so these are opposite each other.
And there is another type of glucose known as the beta glucose.
This is essentially where the hydroxyl group of carbon number one
and carbon number six are pointing in the same direction.
So beta carbohydrates is where the hydroxyl group of carbon number one
is pointing in the same direction as carbon number six.
And again these alpha and beta carbohydrates,
they also apply to other types of carbohydrates,
such as galactose, as well as fructose.
So for example this galactose molecule is actually a beta galactose,
because the hydroxyl group here and the
carbon number six is pointing in the same direction.
Similarly this fructose here is actually in a beta configuration,
so it’s a beta fructose,
because the hydroxyl group here is
pointing in the same direction as carbon number six.
So I hope you understood the structure of the
three major monosaccharides in the human diet.
Now let’s look at disaccharides.
Disaccharides are made up of two monosaccharides.
So for example a glucose molecule,
and another glucose molecule, can form a bond with each other.
This new disaccharide is called maltose.
Maltose is essentially two glucose molecules linked together.
It is linked together by an alpha 1-4 glycosidic bond.
It’s called alpha 1-4 glycosidic bond
because carbon number one of this glucose,
and carbon number four of this glucose,
are involved in the linking process.
And it’s called alpha because
both these glucose molecules are in an alpha configuration.
Now the process of linking monosaccharides with one another
is called condensation, and here water is released.
Therefore the reverse reaction is hydrolysis,
and this is where we add water.
Adding water to an alpha 1-4 glycosidic bond will break the bond.
So maltose is only one example of a disaccharide.
Let’s look at some other common examples.
Now this galactose molecule can link with a glucose molecule,
so this particular glucose molecule is actually in a
beta configuration because remember
the hydroxyl group here is
pointing the same direction as carbon number six.
So this galactose molecule and the glucose molecule can form a link,
and through the condensation process it will form lactose.
Lactose is made up of galactose and glucose.
The galactose and glucose is linked together
by a beta 1-4 glycosidic bond.
Now it’s called a beta 1-4 glycosidic bond
because the galactose and glucose are in a beta configuration,
and also carbon number one and carbon number four are involved.
The reverse reaction, to break lactose,
requires hydrolysis, by adding water.
Now lactose, as you all probably know, is found naturally in milk.
The third type of disaccharide I want to talk about
is where we form a bond between one glucose molecule and one fructose.
This glucose molecule is an alpha glucose because the...
as you can see the hydroxyl and carbon number six
are pointing in the opposite direction.
So glucose and fructose can form a link,
and through the condensation reaction, removal of water,
it can form a disaccharide called sucrose.
Sucrose is made up of one glucose and one fructose.
The bond between the glucose and fructose is a little more complicated,
as it is a glucose alpha one and fructose beta two bond.
This sort of linking occurs.
So the glucose is in an alpha configuration,
and the fructose is in a beta configuration.
And its carbon number one of glucose
and carbon number two of fructose that are involved in the linking process.
So essentially fructose flips over.
Sucrose, as you all know, is table sugar
and is formed by plants, and not formed by animals.
So humans cannot form sucrose.
Sucrose is broken down through a hydrolysis reaction.
So the disaccharides, maltose, lactose and sucrose,
are all good examples that we encounter in our normal diet.
Now let’s look at oligosaccharides.
Oligosaccharides basically consist of short chains of monosaccharides,
typically less than 20 monosaccharides linked together.
Actually a disaccharide can be referred to as an oligosaccharide.
Now let’s look at an example of an oligosaccharide.
So if we were to take this maltose and add another glucose molecule to it,
through a condensation reaction again
we can form additional alpha 1-4 glycosidic bond.
This oligosaccharide is called Maltotirose, tri as in three.
And this maltotriose is made up of glucose and they are linked together,
as I mentioned, by alpha 1-4 glycosidic bonds.
To break down these bonds requires hydrolysis, the addition of water.
And this structure can keep growing
with the addition of more glucose molecules.
But when the oligosaccharide eventually exceeds
20 monosaccharides with 20 bonds,
the carbohydrate is then referred to as a polysaccharide.
So from a oligosaccharide it becomes a polysaccharide.
Most carbohydrates found in nature occur as polysaccharides.
Polysaccharides are also known as glycans.
To simplify things polysaccharides can be a homopolysaccharide,
or they can be a heteropolysaccharide.
A homopolysaccharide means
the polysaccharide only contains a single type of monosaccharide.
For example it only contains glucose molecules linked together.
A heteropolysaccharide means that the polysaccharide contains
two or more different monosaccharides,
so for example a long chain of fructose and glucose molecules.
To make things a little bit more interesting
a polysaccharide can also be unbranched, like what you see here.
Or it can be branched.
This goes for both homopolysaccharides as well as heteropolysaccharides.
Heteropolysaccharides can also be unbranched or branched.
What you have to understand is that polysaccharides
I am currently drawing are very small.
In reality the polysaccharide contains thousands,
are made up of thousands of monosaccharides linked together.
In this section of the video we won’t focus on heteropolysaccharides,
but we will look at homopolysaccharides,
because homopolysaccharides serve as storage,
forms of monosaccharides in both humans and plants,
and even bacteria, so they’re very, very important.
So let’s look at some examples of homopolysaccharides.
Starch is a storage form of monosaccharides in plants.
Starch is the main carbohydrate in a human diet
and are found in our bread, cereal and rice.
Starch is only made up of glucose, because it is a homopolysaccharide.
Let us zoom into this area here and learn a bit about the bonds.
So here we have our regular
alpha 1-4 glycosidic bond between two glucose molecules,
and this is because carbon number one and carbon number four of
these glucose molecules are involved in the linking process.
However the branching points here is actually an alpha 1-6 glycosidic bond
between two glucose molecules.
And that is because carbon number one of this
glucose and carbon number six here are involved in the linking process.
So what to take out of this is that starch has two forms,
it can be branched, like what I just explained,
or starch can be unbranched.
If starch is unbranched, so it is only a chain of glucose
linked together by alpha 1-4 glycosidic bonds,
it is referred to as amylose.
If starch is branched it contains both alpha 1-4
and alpha 1-6 glycosidic bonds between glucose,
and therefore it is referred to as amylopectin.
Amylose and amylopectin are two forms of glucose polymers.
The other good example of a polysaccharide is glycogen.
Now glycogen is a homopolysaccharide because it is made up of glucose.
Glycogen can also be branched or unbranched.
Glycogen is a storage form of glucose in animals, such as humans.
Starch and glycogen are actually very similar in structure.
They both are made up of glucose,
and they can either be branched or unbranched,
so both starch and glycogen contain amylose and amylopectin.
The only difference is that glycogen has
these branch points occurring every eight to 12 glucose residues.
In starch these branch points occur every 24-30 glucose residues.
So the branch points occur more frequently in glycogen than starch,
and this of course will influence the structure in some way.
Another type of polysaccharide is dextrans.
Dextrans are structural components in bacteria and yeast.
These polysaccharides are made up of
alpha 1-3 and alpha 1-6 glycosidic bonds.
So here we have glucose units with alpha 1-3 bonds and alpha 1-6 bonds.
However the dextrans
can also contain alpha 1-2 and alpha 1-4 glycosidic bonds.
Finally the other polysaccharide worth mentioning is cellulose.
Cellulose are structural components in plants.
They make up the plant cell wall.
They are unbranched homopolysaccharide
consisting of thousands of glucose molecules.
So here you can see unbranched cellulose on top of each other.
In cellulose the glucose molecules have a beta configuration,
and therefore the bonds between these glucose molecules
are beta bonds.
The glucose molecules are linked together by beta 1-4 glycosidic bonds.
Humans do not have enzymes that break down,
that hydrolyse beta 1-4 glycosidic bonds of cellulose,
and so humans cannot digest cellulose.
Now even though cellulose are only chains of beta glucose,
so unbranched beta glucose,
these chains can form hydrogen bonds with each other,
forming a very strong structure.
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