Triglycerides are lipids that include fats and oils. You may have heard about triglycerides in relation to medicine, as high levels of triglycerides are a common sign of various health issues. However, there is another side to triglycerides: triglycerides as energy powerhouses! Both their structure and function make them such useful energy storage molecules.
Triglycerides are often referred to simply as fats and are the most common lipids found in living organisms. A lot of them come from foods we eat often, like butter and vegetable oils.
The structure of triglycerides
The building blocks of triglycerides are fatty acids and glycerol. The term triglyceride comes from the fact that they have three (tri-) fatty acids attached to glycerol (glyceride).
Glycerol is an alcohol, and an organic compound, with the formula .
Fatty acids are acids belonging to the carboxylic acid group. They consist of a long hydrocarbon chain, with a carboxyl group ⎼COOH at one end and a methyl group at the other. The simple formula of fatty acids is RCOOH, where R is the hydrocarbon chain with the methyl group.
Depending on the bonds between carbon atoms in the chain, fatty acids can be saturated and unsaturated: mono-unsaturated and poly-unsaturated. Saturated fatty acids have only single bonds. Unsaturated fatty acids have one or more double bonds between carbon atoms: mono-unsaturated have one double bond, while poly-unsaturated have two or more. That is why you will hear fats referred to as saturated and unsaturated fats.
Fig. 1 - Simplified structure of a triglyceride with one saturated (palmitic acid), one mono-unsaturated (oleic acid), and one poly-unsaturated fatty acid (alpha-linolenic acid) attached to a glycerol backbone
Due to a large number of carbons and hydrogens comprising the structure of triglycerides, they are entirely insoluble in water (hydrophobic).
How are triglycerides formed?
Triglycerides are formed during the condensation reaction of fatty acids and glycerol.
Glycerol has three –OH groups to which three fatty acids attach during condensation. A covalent bond called the ester bond forms between glycerol and fatty acids.
It is important to remember that fatty acids do not attach to one another, only to glycerol!
The formation of triglycerides is a condensation reaction. The carboxyl group of each fatty acid loses one hydrogen atom, and glycerol loses three –OH groups. This results in the release of not one but three water molecules since three fatty acids attach to glycerol, and therefore three ester bonds form.
Like all biological macromolecules, triglycerides go through hydrolysis when they need to be broken down into their building blocks of fatty acids and glycerol. For instance, the breakdown of fats stored in fat cells during hunger. During hydrolysis, ester bonds between fatty acids and glycerol break using three water molecules. This results in the breaking down of triglycerides and the release of energy.
Fig. 2 - Hydrolysis of triglycerides (left) results in one molecule of glycerol (blue) and three fatty acids (right). The red bonds are three hydrolysed ester bonds
Remember that the other three biological macromolecules - carbohydrates, proteins, and nucleic acids - are polymers composed of small molecules called monomers. Polymers are built of monomers during condensation and broken down during hydrolysis.
Triglycerides are lipids and, therefore, not polymers, and fatty acids and glycerol are not monomers. This is because fatty acids and glycerol do not form repetitive chains like other monomers. However, triglycerides (and all lipids) go through condensation and hydrolysis to be created or broken down!
The function of triglycerides
The primary function of triglycerides is energy storage and providing energy to the body. They are obtained via the food we eat or are released from the liver. They are then transported via blood plasma, providing nutrients to different body parts.
Triglycerides are large and insoluble in water (hydrophobic). This means that triglycerides can be stored in cells without affecting their osmosis. This, too, makes them excellent energy storage molecules.
Triglycerides are stored as oils in plants, specifically in seeds and fruits. In animals, triglycerides are stored as fats in the liver and adipose tissue (the connective tissue that serves as the primary lipid storage in mammals).
Other functions of triglycerides include:
Insulation - Triglycerides stored beneath the body surface insulate mammals from the environment, keeping their bodies warm. In aquatic animals, a thick layer of fat underneath their skin keeps them warm and dry.
Triglycerides can prove to have a negative effect on our health. If you remember, plants store excess glucose in the form of starch, and animals store it as glycogen. The same thing happens with triglycerides. We don’t need triglycerides short-term, so we store them as body fat. However, human bodies often store an excessive amount of triglycerides, mainly around the organs.
Therefore, hypertriglyceridemia (high triglyceride levels) may occur. It is a serious indication that our bodies are not functioning well and may lead to conditions such as heart disease and stroke. It can be an indication of diabetes as well. Read more on this disease in the article Diabetes.
A piece of general advice is to limit the intake of so-called “bad fats”, i.e. the food high in saturated fats, such as starchy food, baked goods, fast food and other high-calorie food, and even alcohol. This advice extends to include the intake of healthier fats, including fish, white chicken meat, whole grains, low-fat dairy, and vegetable oils like olive and rapeseed oil.
Triglycerides - Key takeaways
- Triglycerides are lipids that include fats and oils, the most common types of lipids found in living organisms.
- The building blocks of triglycerides are fatty acids and glycerol.
- Triglycerides are formed during the condensation of fatty acids and glycerol. A covalent bond called the ester bond forms between glycerol and fatty acids. Three molecules of water are released as three ester bonds are formed.
- During the hydrolysis of triglycerides, ester bonds between fatty acids and glycerol break using three water molecules. This results in the breaking down of triglycerides and the release of energy.
- The primary function of triglycerides is to serve as energy storage.
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