Video Transcript
In this video, we will learn about
the structure and the function of different carbohydrates and about the roles they
play in the cells of organisms. We’ll also learn how simple sugars
bond together to form polysaccharides and how the structure and function of
carbohydrates are related.
A carbohydrate is a molecule which
contains carbon, hydrogen, and oxygen atoms in a particular ratio. Carbo- is a word part which means
carbon and hydrate refers to water, which we know contains hydrogen and oxygen. The ratio that most carbohydrates
follow is C𝑚H2O𝑛, meaning that there are 𝑚-number of carbon atoms and 𝑛-number
of water molecules arranged in a particular carbohydrate. Certain carbohydrates called
complex carbohydrates are macromolecules, which are also referred to as
polymers. A polymer is a large molecule made
of several smaller repeating units called monomers. The monomers that make up complex
carbohydrates are called simple carbohydrates.
Different types of carbohydrates
play different roles in living things. Carbohydrates have many
functions. In animal cells, carbohydrates are
used for energy transfer and energy storage. Also, carbohydrates are attached to
proteins embedded in the cell membrane. These are called glycoproteins and
they are an important part of how cells communicate with each other. In addition to these roles, in
plant cells, carbohydrates are also responsible for providing rigidity and structure
to the cell wall. In biology, we know that structure
is always directly related to function. So different types of carbohydrates
that are made of different parts in different arrangements have different
properties.
The simplest carbohydrates are
simple sugars. These are also called
monosaccharides. Mono- is a word part that means
one, and saccharide is another word for sugar. Monosaccharides usually follow the
ratio CH2O𝑛, where 𝑛 is a number between three and seven. Monosaccharides can also be
classified based on the number of carbon atoms that they possess. Examples of monosaccharides include
glucose, galactose, and fructose. These are all hexose sugars, since
they have six carbons each. All three of these simple sugars
are also isomers of each other. They have the same chemical
formula, which means that they possess the same atoms just in slightly different
arrangements. The ribose and the deoxyribose
sugars that you find in nucleic acids are also monosaccharides. And these are pentose sugars, since
they each have five carbon atoms.
At this point, you’ve probably
noticed that many of these words seem to rhyme. The suffix -ose is a word part that
refers to sugar. Glucose is particularly important
to cells, since it’s the sugar that’s a reactant in cellular respiration. During cellular respiration, the
energy in the bonds between the atoms in the glucose molecule is released and
transferred to ATP, which powers our life processes. Monosaccharides, like glucose, are
also soluble in water, which means that their concentration within the cell must be
carefully monitored and controlled.
Next, let’s take a look at the
polymerization of carbohydrates. Monosaccharides combine together
during a chemical process known as a condensation reaction. A condensation reaction occurs when
the hydroxyl groups on two monosaccharides rearrange to share just one oxygen atom
in what’s called a glycosidic bond. To form this glycosidic bond, two
hydrogen atoms and one oxygen atom must be removed. They bond together to form one
water molecule, which is released. The release of this water molecule
is what gives the condensation reaction its name. You can remember by thinking about
how water is released from the air when it condenses into droplets. When just two monosaccharides bond
together, they form a disaccharide, which is another type of simple sugar. Di- is a prefix that means two.
When glucose bonds with glucose,
the disaccharide that is formed is known as maltose. When galactose bonds with glucose,
the disaccharide that is formed is known as lactose, which you may recall is a sugar
found in milk. And when glucose bonds with
fructose, the disaccharide that’s formed is known as sucrose, which you may be
familiar with as common table sugar. Disaccharides usually follow the
chemical formula C𝑚H2O𝑛, where 𝑛 is equal to 𝑚 minus one, since one water
molecule had to be released during the condensation reaction. All three of these disaccharides
have the chemical formula C12H22O11.
More than two monosaccharides can
also bond together into large macromolecules called polysaccharides. Poly- is a word part that means
many. Polysaccharides are insoluble in
water, so they’re ideal for functions like structure and support and long-term
energy storage. Polysaccharides can exist in higher
concentrations, since they won’t affect the water balance of the cell. When a cell needs to release some
of this energy from storage, the polysaccharide must first be broken apart into
monosaccharides using a chemical process known as a hydrolysis reaction.
Hydro- is a word part that refers
to water and lysis means to break apart. A hydrolysis reaction consumes a
water molecule and breaks the glycosidic bond apart. It’s basically the reverse of the
condensation reaction that we learned about a little bit earlier. Once they’re broken down again, the
monosaccharides can be used in processes that transfer energy such as cellular
respiration. Examples of polysaccharides include
starch, glycogen, and cellulose, and these three macromolecules differ in both
structure and in function. The monomers in starch and glycogen
are a different isomer of glucose than the monomers in cellulose.
Recall that an isomer is a molecule
with the same chemical formula but a different arrangement of the atoms. These two isomers are called
alpha-glucose and beta-glucose. And the difference is here in the
atoms attached to the first carbon. In alpha-glucose, the hydroxyl
group attached to the first carbon is on the opposite side of the molecule from this
CH2OH group that you see here. And in beta-glucose, that hydroxyl
is on the same side as the CH2OH group. So I’ve added a little bit more
detail to the diagrams of each of these monomers to help us to tell them apart.
Starch and glycogen are made of
alpha-glucose monomers. They both function as energy
storage molecules. But starches are only made by plant
cells, and glycogen is made by animal cells. Starch can occur in long branched
or unbranched chains, while glycogen is always a highly branched molecule. Cellulose is made of beta-glucose
monomers. Because of their structure,
beta-glucose molecules bond together in an alternating orientation, forming long
rigid unbranched chains. And these chains easily form
hydrogen bonds between them, allowing them to stack together into larger
structures. For this reason, cellulose is used
for the structure of plant cell walls and not for energy storage.
Now that we’ve learned about
monosaccharides, disaccharides, and polysaccharides and we familiarized ourselves
with their structure and their function, let’s try a practice question.
The structure of monosaccharides
and disaccharides can be determined using general formulas. What is the general molecular
formula of a monosaccharide? A monosaccharide has the formula
C6H12O6. Two of these monosaccharides join
by a condensation reaction. What is the molecular formula of
the resulting disaccharide?
This question is asking us to
recall what we know about the chemical formulas of monosaccharides and
disaccharides, which are both types of carbohydrates. Let’s just focus on the first part
of this question for now. Here, we’re interested in the
general molecular formula of a monosaccharide. And since it’s the general
molecular formula, it should be able to apply to more than one type of
monosaccharide. Let’s also take a moment to recall
that mono- is a word part that means one and saccharide means sugar. So we’re looking for the general
molecular formula of one sugar.
We know that sugars are a type of
carbohydrate, and a great clue to their chemical formula is present right here in
the name. Carbo- is a word part that refers
to carbon and hydrate means water. So we know that these molecules
consist of some carbon and some H2O. A monosaccharide that you’re likely
familiar with already is glucose. Glucose is the simple sugar used by
ourselves in cellular respiration. One molecule of glucose looks
something like this and has the chemical formula C6H12O6 because glucose possesses
six atoms of carbon, 12 atoms of hydrogen, and six atoms of oxygen.
So, like our carbohydrate ratio,
there are twice as many atoms of hydrogen as there are carbon and oxygen. But glucose has six times the atoms
of our base ratio. So in order to complete our general
formula, we must multiply the entire ratio by a variable, which we’ll call 𝑛. Continuing our example, when 𝑛 is
equal to six, that means we have six times the carbon, hydrogen, and oxygen atoms in
our molecule, which comes out to C6H12O6. And this is the correct chemical
formula for our example monosaccharide, glucose. So the general molecular formula
for a monosaccharide is CH2O𝑛.
Now let’s take a look at the second
part of this question. It’s asking us about the resulting
disaccharide molecule that forms when two monosaccharides join. Di- is a prefix that means two. So let’s take a look at what
happens when two monosaccharides bond together to form a disaccharide. We’ll use glucose as the
monosaccharides in our example, since they have the chemical formula C6H12O6 and
we’re already familiar with their structure.
When glucose bonds with glucose,
the disaccharide that’s formed is called maltose. During the condensation reaction,
two of the hydroxyl groups are rearranged so that both molecules are now sharing one
oxygen atom. This process releases two hydrogen
and one oxygen atoms, and they bond to form one molecule of water. The fact that a molecule of water
is released is how the condensation reaction gets its name. So to figure out the chemical
formula, we could count up the number of atoms in our maltose molecule. Or we can recall that that maltose
molecule was made by joining two molecules of glucose together and releasing a water
molecule in the process. Either way, you’ll come up with the
same solution. The molecular formula for the
resulting disaccharide is C12H22O11.
Let’s wrap up our lesson by taking
a moment to review what we’ve learned. In this video, we learned about the
structure of carbohydrates and their importance in living organisms. We learned how bonds form between
monosaccharides through condensation reactions, forming disaccharides and
polysaccharides. We learned about three examples of
polysaccharides — glycogen, starch, and cellulose — and how their structure is
related to their function.
