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  • Fatty acids are the main building blocks of lipids in food and in our body but they are

  • 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.

Fatty acids are the main building blocks of lipids in food and in our body but they are

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B1 中級 美國腔

營養學步驟 5.5 - 脂肪酸 (Nutrition Steps 5.5 - Fatty Acids)

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    陳秋汝 發佈於 2021 年 01 月 14 日
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