The typical biological molecule is like a weird lumpy thing.
Imagine a dog, and you throw a blanket over it.
That gives you an idea of the general 3D shape of a dog.
But it’s the eyes, the teeth, the tail...the paws - these are the parts that actually are
the business ends of the dog, and they perform the actions you associate with a dog.
Wagging a tail, running around on 4 paws, biting or barking or panting.
Similarly, in biological molecules, the carbon skeleton determines their general 3D shape.
But what’s on the SURFACE of the molecules determines their chemical behavior.
Small chemical species, hanging off the exterior of these molecules,
bump into each other and react.
These are known as FUNCTIONAL GROUPS.
To understand what kinds of chemical reactions biological molecules can do, you have to understand
the chemical behavior of the functional groups on the surface of the molecule.
For instance, if a molecule has a big patch of functional groups that are hydrophobic,
that side of the molecule is going to repel water.
It’s going to behave very differently from a patch of acidic functional groups, or basic
functional groups, or functional groups that bond to each other forming bridges.
Let’s look at the most common functional groups.
-OH is the HYDROXYL group.
This group is hydrophilic, so if a molecule has a patch of -OH groups,
that region will be hydrophilic.
These are highly reactive groups, participating in a variety of chemical reactions.
One place you’ll see the Hydroxyl group is in dehydration reactions
that link polymers together.
Here you can see how hydroxyl groups are involved in these two sugars joining together.
CARBONYL Group =CO This is a carbon double bonded to an oxygen.
This is a polar group that will increase solubility in water.
If this group is at the end of the carbon skeleton, it’s called an ALDEHYDE.
Otherwise, it’s called a KETONE.
These are groups that you’re going to see in sugars.
For example, here’s a carbonyl group that we’d call an aldehyde
at the end of this glucose molecule:
And here’s a ketone group on fructose - you can see how it’s not on the end.
CARBOXYL Group is - COOH.
So you have a carbon double-bonded to an oxygen and also attached to a hydroxyl group.
These groups confer acidic behavior to molecules, because they tend to donate their H+,
becoming COO-.
You’ll see these on one side of amino acids, for instance.
And on the other side of an amino acid, you see the AMINE Group - NH2.
The amine group confers basic behavior to molecules, because it tends to accept a H+,
becoming NH3+.
PHOSPHATE Group - Phosphoric acid loses multiple H+ ions to become PO4^3-.
So again, by increasing the H+ concentration, it’s acting as an acid.
This functional group is attached to the carbon skeleton by its oxygen - then you get a phosphorus
and then 3 other oxygens.
Phosphates are also really important in energetic reactions, for instance,
those involving the molecule ATP.
For a lot of reactions, the controlled addition or removal of a phosphate group is used to
either store or release energy.
Or sometimes the addition or removal of a phosphate group
changes the conformation of a molecule.
So you’ll hear about phosphorylation or dephosphorylation.
SULFHYDRYL Group is -SH, a sulfur atom bonded to a hydrogen.
Sulfhydryl groups tend to bond to other sulfhydryl groups, forming disulfide bridges.
This is important in protein folding, joining together two regions of an amino acid chain
in a stable conformation.
METHYL Group - CH3 - this is the one functional group we’re going to see today that is hydrophobic.
So if you see a patch of methyl groups on the surface of a biological molecule, that
that region is hydrophobic and will repel water.
Think of molecules like phospholipids - this top part with a phosphate group is hydrophilic,
but these tails which are hydrophobic end in methyl groups.
Compared with these other groups, methyl is less reactive - it just sits there,
repelling water.
However, you will see the function of various biological molecules modified by the addition
or subtraction of a methyl group.
For instance, you’ll learn about the importance of DNA methylation - how that
can affect gene expression.
I suggest you create for yourself a chart of functional groups - or even better, flashcards.
You should be able to recognize the name, structure, and chemical behavior
of these functional groups.
Are you subscribed to Socratica?
You should.
It’s good for you.
And it’s good for us, too.
If you can share this video - tweet about it, put it up on Reddit or some other discussion
board - that will help us find our audience.
And then we can keep making more videos for you.
If you want to REALLY make sure we can stick around to help you learn more, you can become
our Patron on Patreon.
Every bit helps.
Thanks for watching.