Fats and Oils

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The structures of glycerol, a fatty acid, and a triacylglycerol are shown. Glycerol is a chain of three carbons, with a hydroxyl (upper O upper H) group attached to each carbon. A fatty acid has an acetyl (upper C upper O upper O upper H) group attached to a long carbon chain. In triacylglycerol, a fatty acid is attached to each of glycerols three hydroxyl groups via the carboxyl group. A water molecule is lost in the reaction so the structure of the linkage is C dash O dash C, with an oxygen double bonded to the second carbon.
Joining three fatty acids to a glycerol backbone in a dehydration reaction forms triacylglycerol. Three water molecules release in the process.

Source: OpenStax Biology 2e

Fats and Oils (OpenStax Biology 2e)

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. Fatty acids have a long chain of hydrocarbons to which a carboxyl group is attached, hence the name “fatty acid.” The number of carbons in the fatty acid may range from 4 to 36. The most common are those containing 12–18 carbons. In a fat molecule, the fatty acids attach to each of the glycerol molecule’s three carbons with an ester bond through an oxygen atom.

– What is an organic compound (alcohol) with three carbons, five hydrogens, and three hydroxyl (OH) groups?

During this ester bond formation, three water molecules are released. The three fatty acids in the triacylglycerol may be similar or dissimilar. We also call fats  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, is derived from the palm tree. Arachidic acid is derived from Arachis hypogea, the scientific name for groundnuts or peanuts.

– What is a chemical compound derived from an acid (organic or inorganic) in which at least one –OH (hydroxyl) group is replaced by an –O–alkyl (alkoxy) group?

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 hydrogen atoms attached to the carbon skeleton is maximized. Stearic acid is an example of a saturated fatty acid.

Due to having antimicrobial and antiviral properties, glycerol is widely used in FDA approved wound and burn treatments. It can also be used as an effective marker to measure liver disease. It is also widely used as a sweetener in the food industry and as a humectant in pharmaceutical formulations

The structure of stearic acid is shown. This fatty acid has a hydrocarbon chain seventeen residues long attached to an acetyl group. All bonds between the carbons are single bonds.
When the hydrocarbon chain contains a double bond, the fatty acid is unsaturated. Oleic acid is an example of an unsaturated fatty acid.

Source: OpenStax Biology 2e
The structure of oleic acid is shown. This fatty acid has a hydrocarbon chain seventeen residues long attached to an acetyl group. The bond between carbon eight and carbon nine is a double bond.
Oleic acid is a common unsaturated fatty acid.

Source: OpenStax Biology 2e

Most unsaturated fats are liquid at room temperature. We call these oils. If there is one double bond in the molecule, then it is a monounsaturated fat (e.g., olive oil), and if there is more than one double bond, then it is a polyunsaturated fat (e.g., canola oil).

When a fatty acid has no double bonds, it is a saturated fatty acid because it is not possible to add more hydrogen to the chain’s carbon atoms. A fat may contain similar or different fatty acids attached to glycerol. Long straight fatty acids with single bonds generally pack tightly and are solid at room temperature. 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, or adipocytes, where fat globules occupy most of the cell’s volume. Plants store fat or oil in many seeds and use them 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. If hydrogens are present in the same plane, it is a cis fat. If the hydrogen atoms are on two different planes, it is a trans fat. The cis double bond causes a bend or a “kink” that prevents the fatty acids from packing tightly, keeping them liquid at room temperature. 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.

– What are the cells that primarily compose adipose tissue and are specialized in storing energy as fat?

Brown adipocytes are fascinating cells and they can only be found in mammals. Their main function is to generate endogenous heat in a process called thermogenesis. This is made possible by their unique expression of a mitochondrial membrane protein called uncoupling protein 1. The energy expending properties of brown fat, and the recent realization that adult humans have brown fat, has made them a target for therapies aimed at fighting over-nutrition. Active brown adipocytes also have one of the most intriguing metabolic programs. They take up and consume large amounts of diverse nutrients simultaneously (glucose, lipids, amino acids) and can simultaneously engage both anabolic and catabolic metabolism.

A comparison of saturated and unsaturated fatty acids is shown. Stearic acid, a saturated fatty acid, has a hydrocarbon chain seventeen residues long attached to an acetyl group. Oleic acid also has a seventeen-residue hydrocarbon chain, but a double bond exists between the eighth and ninth carbon in the chain. In cis oleic acid, the hydrogens are on the same side of the double bond.  In the cis oleic acid, the 2 hydrogens on the same side cuase the chain to bend. In trans oleic acid, they are on opposite sides.
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.

Source: OpenStax Biology 2e

Source:

Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e

https://en.wikipedia.org/wiki/Glycerol

https://en.wikipedia.org/wiki/Adipocyte

https://www.umassmed.edu/guertinlab/research/adipocytes/

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