Fats and Oils

Fats and Oils

A fat molecule consists of two main components—glycerol and fatty acids. Glycerol is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups. On the other hand, fatty acids have a long chain of hydrocarbons with an attached carboxyl group. Hence the name “fatty acid.”

The number of carbons in the fatty acid may range from 4 to 36. Most common are those containing 12–18 carbons. In a fat molecule, the fatty acids join to each of the three carbons of the glycerol molecule with an ester bond through an oxygen atom (see image below).

triacylglycerol

Triacylglycerol forms by the joining of three fatty acids to a glycerol backbone in a dehydration reaction. There is usually a release of three molecules of water in the process. Image Attribution: OpenStax Biology

During this ester bond formation, there is a release of three water molecules. Generally, the three fatty acids in the triacylglycerol may be similar or dissimilar. We also refer to fats as triacylglycerols or triglycerides because of their chemical structure. Some fatty acids have common names that specify their origin. For example, palmitic acid, a saturated fatty acid, comes from the palm tree. Arachidic acid comes from Arachis hypogea, the scientific name for groundnuts or peanuts.

Saturated and Unsaturated Fatty Acids

Fatty acids may be saturated or unsaturated. In a fatty acid chain, if there are only single bonds between neighboring carbons in the hydrocarbon chain, the fatty acid is saturated. Saturated fatty acids are saturated with hydrogen. In other words, the number of attached hydrogen atoms on the carbon skeleton is maximized. Stearic acid is an example of a saturated fatty acid (see image below).

stearic-acid

Stearic acid is a common saturated fatty acid. Image Attribution: OpenStax Biology

When the hydrocarbon chain contains a double bond, the fatty acid is said to be unsaturated. Oleic acid is an example of an unsaturated fatty acid (see image below).

oleic-acid

Oleic acid is a common unsaturated fatty acid. Image Attribution: OpenStax Biology

Most unsaturated fats are liquid at room temperature and we refer to them as oils. If there is one double bond in the molecule, then we refer to it as a monounsaturated fat. An example is olive oil. Similarly, if there is more than one double bond, then we refer to it as a polyunsaturated fat. An example in this case is canola oil.

When a fatty acid has no double bonds, we refer to it as a saturated fatty acid. This is because no more hydrogen may be added to the carbon atoms of the chain. A fat may contain similar or different fatty acids attached to glycerol. Long straight fatty acids with single bonds tend to pack tightly and are solid at room temperature.

Fats in Animals and Plants

Animal fats with stearic acid and palmitic acid (common in meat) and the fat with butyric acid (common in butter) are examples of saturated fats. Mammals store fats in specialized cells called adipocytes, where globules of fat occupy most of the cell’s volume.

In plants, fat or oil is stored in many seeds and is used as a source of energy during seedling development. Unsaturated fats or oils are usually of plant origin and contain cis unsaturated fatty acids. Cis and trans indicate the configuration of the molecule around the double bond.

fatty-acids

Saturated fatty acids have hydrocarbon chains connected by single bonds only. Unsaturated fatty acids have one or more double bonds. Each double bond may be in a cis or trans configuration. In the cis configuration, both hydrogens are on the same side of the hydrocarbon chain. In the trans configuration, the hydrogens are on opposite sides. A cis double bond causes a kink in the chain. Image Attribution: OpenStax Biology

If hydrogens are in the same plane, we refer to it as a cis fat. However, if the hydrogen atoms are on two different planes, we refer to it as a trans fat. The cis double bond causes a bend or a “kink”. This bend prevents the fatty acids from packing tightly. As a result, they remain liquid at room temperature (see image above). Olive oil, corn oil, canola oil, and cod liver oil are examples of unsaturated fats. Unsaturated fats help to lower blood cholesterol levels whereas saturated fats contribute to plaque formation in the arteries.

Short Video Describing Fat

As the narrative goes, fat is bad. Well, it’s actually more nuanced than that. The type of fat you eat is more impactful on your health than the quantity. In the TED-Ed video below, George Zaidan examines triglycerides, the varied molecules that make up fat, and how to identify which types of fat you are consuming.

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