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.