?Previously, you’ve learned about atomic structure
and how atoms come together to form molecules
and all of that’s based on electrons in the valence shell.
Now, we’re going to venture into exploring how these smaller molecules come together
to form the four major classes of macromolecules or biological molecules.
So in this lecture you’ll learn to identify what the monomers and polymers are
of the four classes of macromolecules,
as well as explain the process of dehydration synthesis;
that is how these molecules come together.
And you’ll also be able to identify carbohydrate molecule structure
and discuss why we can’t eat grass.
So let’s begin by taking a look at how macromolecules come together.
All organic macromolecules are composed of hydrocarbon chains with additional functional groups.
By hydrocarbon we mean simply hydrogens and carbons covalently bonded to each other.
In this example, you can see the hydrogen carbons in black and white
and then a functional group on the far end of that molecule.
This one’s myristic acid but that doesn’t really matter at the moment.
The point here is you can see that there are single covalent bonds
between each of the carbon molecules in that chain and they are fully saturated;
they’re covered in hydrogens.
In this case the chain is straight.
On occasion, you’ll see that the chain has kinks in it
and that might be because of double bonds between the carbons,
but either way it’s a carbon hydrogen chain with functional group on the end.
We’ll discuss those functional groups shortly.
Here’s the comparison of those three molecules again.
Each of them the point here being that they have a hydrocarbon chain
with functional groups added to the basic structure of the hydrocarbon chain.
Now, we’re going to be looking at a variety of different functional groups.
Here are some of the main ones that we’ll see in biology.
First of all, we have the hydroxol group that I previously introduced.
It’s simply an OH.
We’ll see that in a lot of carbohydrates, proteins, nucleic acids and lipids,
so in all four classes of macromolecules.
We also will see the carbonyl group which we’ll see on carbohydrates and nucleic acids.
We can also see the carboxyl group in proteins and lipids.
We will see amino group, again, in proteins and we will see sulfhydryl on occasion in proteins
where we create disulfide bridges and protein folding and phosphate groups in nucleic acids.
Methyl groups we’ll see in proteins and in DNA.
Before we begin our discussion of the various macromolecules,
we need to understand two major terms.
The first of which is monomer and the other is polymer.
Mono means single, -mer means units,
so monomers are single units that are strong together to form polymers, multiple units.
So here we have a bunch of monomers; so for example glucose molecules.
Glucose molecules are polymerized
or brought together to a process called dehydration synthesis,
that we’ll investigate shortly,
and they will form a polymer strands, so many units of glucose.
In this case it could be glycogen for example.
So let’s look briefly at each of the four classes of macromolecules
so that I can introduce them to you
and later we’ll explore them in a much more depth.
First of all, we look at carbohydrates.
Carbohydrates are long chain sugars, often they could be single sugars
or they could be disaccharides or polysaccharides.
The monomer is a monosaccharide, in this case glucose.
Multiple glucoses are strong together to form a starch
or many different forms of polysaccharides.
Then let’s look at the basics of polypeptides or proteins.
Polypeptides are strings of amino acids.
The monomer being the amino acid,
the polymer being the whole polypeptide, the string of amino acids.
Now the polypeptide will fold and form eventually its protein,
but we’ll cover that in much more detail in the following lecture.
A brief look at nucleic acids.
We see that the polymer is the DNA strand or the RNA strand
and that polymer is composed of multiple repeating subunits of a monomer,
which in this case is the nucleotide.
The nucleotide is composed of three subunits itself
but we’ll deal with them in much more detail, again, in the following lecture.
Lipids are a little bit different because they are non-polymer macromolecules,
which means they don’t have repeating subunits of exactly the same type of thing.
For example, amino acids or glucose molecules or monosaccharides.
In this case, we have three fatty acid tails tied together by one glycerol molecule
so it’s not a repeating thing so lipids are a class of non-polymer macromolecules.
The other three classes are polymer macromolecules,
we have monosaccharides linking together to form polysaccharides,
amino acids to form polypeptides, nucleic acids are formed by monomers of nucleotides.
So then we’ll move on into looking at how these come together?
Macromolecules are all stuck together with our friend, the covalent bond.
What happens here is we remove a hydrogen from one end of the molecule
and a hydroxide from the other end of the next molecule, the next monomer,
and we extract H2O and those two molecules will bind together.
They now have enough an affinity to pair electrons with each other.
So because we’re losing water this is called dehydration synthesis.
So two monomers will come together through polymerization reaction, polymer-ization,
where we lose water and that creates the affinity
between the two monomers to form a covalent bond.
And again, the covalent bonds are the strongest bonds that we see;
single, double and clearly triple, the most strong bond that we’ll see in biology.
On the other hand, when we metabolize polymer macromolecules,
for example in the metabolism of say pasta, a complex carbohydrate,
we’re going to separate the monomers from each other
in a process called hydrolysis, hydro-lysis,
so we’re breaking apart the water molecule.
We’ll put hydrogen on one end of a monomer
and a hydroxide group on the other end of the monomer
and now they’re free to separate
and that covalent bond is broken between the monomers.
So contrasting reactions that we see are the hydrolysis reaction, breaking things apart,
and dehydration synthesis is bringing things together.
So now that we have an understanding of how things come together,
how are monomers joined together to form polymers
is the same with every class of macromolecules.
Let’s dig a little bit deeper into carbohydrates.
Carbohydrates are molecules that are composed of monomers or monosaccharides.
Monosaccharides are simple sugars.
They come in a form of either 3 carbon sugars.
There’s our carbon backbone that got hydrogen associated with it.
They could come in the form of 5 carbon sugars,
we’ll see this when we look at ribose and deoxyribose in the structure of DNA,
and then we could have 6 carbon sugars.
Ones we’re familiar with are things like glucose and fructose and galactose.
These monomers can all come together to form polymer macromolecules.
We could have disaccharides or trisaccharides,
but mostly we deal with disaccharides which are two sugars coming together
or polysaccharides which are multiple sugar units coming together.
In the case of disaccharides, we could look at something like glucose.
Glucose is a chain of 6 carbon atoms associated with their hydrogen,
so those are hydrocarbons.
Now that molecule can fold in solution.
It does fold into a ring structure where we see the terminal oxygen bind
with the number 5 carbon in order to form that ring.
Now, something that we’ll come to later on is that this could form in two different ways.
It can either fold in this direction and form alpha-glucose
or it could fold in that direction and form beta-glucose.
The result here is that we have either an H group up and OH group down
or an OH group up and an H group down and that is going to impact
how these two molecules come together.
Again, we can see that there are multiple forms of C6H12O6.
Not only could they fold in a different direction to form the ring,
but they could also be a slightly different structural shape.
So we have the basic formula, C6H12O6.
They all have the same number of carbons, hydrogens and oxygens,
however, they could form a different arrangement with those carbons, hydrogens and oxygens.
We can have a structural isomer which is structurally quite different.
You can see here that we have a carbon oxygen double bond
replacing one of the single bonds in which case it’s quite a different molecule.
Or we can have a stereo isomer in which one piece might be just reflected
and it could be at any one of those carbons.
The point here is that this different structure will require a different enzyme
to hydrolyze or break the sugar apart,
so each of these has a very specific fit to that certain enzyme
that’s responsible for breaking them down or even putting them together.
So disaccharides are just two sugars together and they’re generally responsible for storage
and transport both within an organism and between different organisms.
So we could consume a disaccharide, for example, by consuming something like lactose.
So here let’s look at alpha-glucose and dehydration synthesis forming alpha-glucoses together.
This is just one way the chain could form; we look at alpha versus beta-glucose.
And we could have alpha-glucose linked to fructose
and we come up with a disaccharide that’s called sucrose.
Here, dehydration synthesis we lose an O from one end and OH from the other
and they come together to form the disaccharide sucrose.
Lactose is a disaccharide that provides a great example of enzyme specificity.
There are two sugar monomers in there
that we produce an enzyme for during our lactation years
when we’re feeding on milk up until about two years of age and in adulthood,
unless we’re exposed to a diet very high in milk.
Generally, we don’t produce the enzyme lactase that breaks down lactose.
So that’s just an example of how the enzymes can be very specific to the type of sugar
or the isomer of sugar that’s involved in a polymer.
This is the case for many polymers whether we’re looking at proteins
or nucleic acids or lipids for that matter.
So polysaccharides are more than two sugar monomers.
So more than two monomers of sugar strung together could form starch
or glycogen or chitin for that matter.
Here’s an example of glycogen, a storage polysaccharide that we see in our muscle cells.
This is how we store glucose, comes into the blood,
we pack it away and store it as glycogen in the muscles or in the liver.
We could also see amylose or amylopectin.
This is a storage polysaccharide that we see in plants.
Cellulose is also a linear chain of glucose molecules strung together.
So the question is then why is it that we couldn’t eat grass like a cow could?
Again, it comes down to enzyme specificity and this isomerization issue.
This is an isomer molecule where we see not the alpha-glycosidic linkage
or the alpha form of the molecule,
we see beta-glucose where the OH and H group are reversed.
And this is a beta-1, 4 glycosidic linkage and we don’t contain any enzymes ever in our life
to break down this polysaccharide.
So cows actually have bacteria in their gut that help them break down these bonds.
They have the bacteria have the enzymes that break down the beta-glycosidic linkage
so that cows can actually then break down the carbon chains and release energy
and make ATP and live off of that ATP.
An example of a structural polysaccharide would be chitin.
Chitin is found in the shells of lobsters and crabs and shrimp
and it’s made of a chain of glucose molecules that are cross-linked with proteins
to give it much more integrity and strength.
Again, we couldn’t eat the shell of a lobster
or crab or you could try but it wouldn’t be so good.
So the take home message here about the group of polysaccharides
is that the monomer is called monosaccharide,
the polymer is a polysaccharide no matter which storage form or structural form it is
and the linkage between them is by dehydration synthesis
but it’s called a glycosidic linkage because it is in carbohydrates.
So carbohydrates have a monomer, a polymer and a linkage
and for each of the macromolecules we’re going to discuss the monomer,
the polymer and the linkage form between them.
So hopefully from this lecture you’ve had a nice introduction
to each of the macromolecules that we’re going to cover.
What their monomers and what their polymers are called?
And you could explain the process of dehydration synthesis;
how each of the monomers are held together?
As well as identify some carbohydrate structures like glycogen and amylopectin and chitin
and discuss precisely why you can’t eat grass to your friends.
Thank you so much for joining me for this lecture.
I hope to see you in the next one shortly.