generally not free because they are acidic and they can’t go around by themselves just

like that, so they are mostly incorporated into other molecules. We already know that

the vast majority of them is in triglycerides, and then some in phospholipids or bound to

cholesterol.

The main distinction we have to make is based on their degree of saturation: we have saturated

fatty acids, monounsaturated fatty acids, and polyunsaturated fatty acids.

This is how a saturated fatty acid looks like. As you can see it is a long chain of atoms

of carbon, each carbon is bound to two other atoms of carbons along the chain, one on its

left, one on its right, and then since carbon needs to make four bonds, the remaining two

bonds are made with hydrogen. We say that this chain of atoms of carbons is saturated

with atoms of hydrogen. This is why we call it a saturated fatty acid, because all the

remaining available bonds are filled with hydrogen. The first carbon atom of the chain

binds to three atoms of hydrogen because it doesn’t have to bind to a carbon before.

At the other hand of the chain we have a carboxyl group, carbon double bond oxygen, and bond

oxygen bond hydrogen, and this is what makes the molecule acidic. So just like sugars,

fatty acids are made of the same three elements: oxygen, hydrogen and carbon. However there’s

much less oxygen in fatty acids and chemically this means they are more reduced, which is

why they contain more energy. If we count the numbers of carbons in this

molecule we will count 18. This number can vary, and the difference between the different

saturated fatty acids depends on the length of their carbon chain.

This is the very same molecule, it’s just another way of representing it: each point

is a carbon, each line is a bond between carbons, and hydrogens are not indicated. We can also

write it like this, C18 for 18 atoms of carbons, and a 0 to indicate it’s saturated. It also

has its own name, this is stearic acid, which is the saturated fatty acid 18 carbons long.

It is actually the most abundant saturated fatty acid in our body.

These are some other abundant molecules: they all have an even number of carbons. Most naturally

occurring fatty acids have an even number of carbons.

Now this is a monounsaturated fatty acid, and specifically oleic acid. Again this has

18 carbons, but, this time you see two bonds between these two atoms of carbons. If you

studied chemistry, you know this means that they share more electrons. But the practical

consequence of this is that now these atoms of carbons don’t have other two bonds available

to make with hydrogen, but only one, so there’s two less hydrogen atoms in this molecule compared

with stearic acid. Still 18 carbons, but less hydrogen. Now the molecule is not anymore

saturated with hydrogen, and because there’s only one of these double bonds it’s called

mono-unsaturated. Mono, for one, that is one unsaturation. If we count from this end which

is called the omega end of the fatty acid, in position nine from the omega end we have

the double bond, so this is why we say that this monounsaturated fatty acid belongs to

the omega-9 family. The first and only double bond is 9 carbons away from the omega end.

We can also represent it like that, or we can write C18 for 18 carbons in the chain,

1 for one unsaturation, and omega-9 to indicate that this double bond occurs in position nine

from the omega end of the fatty acid.

Another thing you notice is that after the double bond, there’s a kink in the chain

of carbons. This is because the two atoms of hydrogen are on the same side of the double

bond and they kind of push the molecule so that it bends. The consequence of this is

you have many of these molecules together, you cannot pile them up as neatly as the saturated

fats whose chains are all straight and so they can be packed tightly together. Unsaturated

fats are less compact. Because of this, fats that are rich in saturated fatty acids will

be solid at room temperature, for example butter, while fats that are rich in unsaturated

fatty acids will be liquid at room temperature, for example olive oil.

We said that normally the two hydrogen at the unsaturation end up on the same side of

the double bond, and this causes the kink in the molecule. This is what occurs naturally

most of the times, and is what we call the cis form of the fatty acid. However it can

also happen that the two hydrogens end up on opposite sides of the double bond, and

that is what we call the trans form of the fatty acid. In this case the chain stays linear,

it doesn’t bend, so it will behave very similarly to the saturated fatty acids, for

example it will be more compact and solid at room temperature. Trans fatty acids are

mostly man made and rarely occur naturally in food, although there are some naturally

occurring short chain trans fatty acids in milk. But most of them come from human processing

of food, and in particular the process of fat hydrogenation which we will see later.

When an unsaturated fatty acid has more than one double bond, two or more, we refer to

it as a polyunsaturated fatty acid. This molecule here is linoleic acid, a very

important polyunsaturated fatty acid, one of the two essential fatty acids in our diet.

It has two double bonds. If we count from the omega end, the first double bond is in

position six. So it belongs to the omega 6 family. It is 18 carbons long, so we can indicate

it as C18:2 for two unsaturations. The other nutritionally relevant family of

polyunsaturated fats is the omega 3 family, so fatty acids in which the first double bond

is located three carbons away from the omega end. This molecule here is alpha-linolenic,

which is the other essential fatty acid in our diet. It is 18 carbons long, with 3 double

bonds.

We have made all of our examples with fatty acids that are 18 carbons long, for a reason.

You already know that the length of the chain can vary, but eighteen is the preferred number.

In our body, most fatty acids are 18 carbons long, especially those incorporated in triglycerides

for energy storage in our adipose tissue. It is not surprising then that the only two

essential fatty acids in our body, linoleic omega-6 and alpha-linolenic omega-3, are both

18 carbons long.

Now this is how a triglyceride is made. You should recognize three fatty acids on the

right, in this case three molecules of stearic acid.

The molecule on the left is a sugar made of three carbons, called glycerol. Glycerol forms

covalent bonds on each of its carbons with the carboxyl end of the three fatty acids.

So with a condensation reaction, and the elimination of three molecules of water, a bond is formed

between the oxygen at the acidic end of the fatty acid and a carbon of the glycerol. So

this is a triglyceride: now the three fatty acids are not free anymore but clumped together,

and they are not acidic anymore because the acidic group is occupied in a bond. This triglyceride

can now be stored in our adipose tissue, and then broken down again when needed for energy

production.