Meaning of Lipids
What are Lipids:
Lipids are hydrophobic and insoluble in water molecules, composed mainly of carbon, oxygen and hydrogen and generally linked to chains of carbohydrates called fatty acids.
In this way, most lipids at the biological level are classified as saponifiable lipids, that is, they are formed by fatty acids.
Lipids are called simple lipids when only carbon, oxygen and hydrogen molecules are found in their composition, such as fats, oils and waxes.
On the other hand, complex lipids are called those that in their structure contain other elements in addition to those that make up simple lipids, such as phospholipids of the plasma membrane, which also contain a modified phosphate group.
Function of lipids
The different types of lipids that exist in the body, in general, have the main function of storing energy. In this sense, each gram of lipid contains twice the energy of a carbohydrate, for example.
In the animal kingdom, lipids also have the function of providing thermal insulation and is a fundamental unit for the formation of:
- vitamins and their absorption, such as vitamins A, D, K and E,
- hormones such as testosterone and estradiol
- bile acids that aid digestion,
- plasma membranes, made up of specialized lipids called phospholipids.
In addition, in humans, some lipids, such as essential fatty acids, regulate inflammation and mood, reduce the risk of sudden death from heart attacks, lower blood triglycerides, lower blood pressure, and It prevents formation of blood clots.
On the other hand, lipids in the form of wax help in the waterproofing function of the leaves in plants and the feathers of birds.
Types of lipids
At the biological level, the most important simple lipids are divided into fats, oils and waxes and within the complex lipids we can find phospholipids and steroids.
Fats are one of the best known types of lipids. They are considered simple lipids because they are composed of carbon, oxygen and hydrogen and are called saponifiable lipids because they are made up of fatty acids.
Fats are made up of a glycerol backbone and at least one fatty acid linked by an ester bond (C = O). Depending on the amount of fatty acid tails, they are classified into monoacylglycerides (1 fatty acid), diacylglycerides (2 fatty acids) or triacylglycerides (3 fatty acids).
Fats are characterized by their single-bonded saturated fatty acids that give them solidity, such as specialized fat cells called adipocytes that make up fat tissue and butter.
Oils are simple and saponifiable lipids. They are characterized by being liquid due to their unsaturated fatty acid tails with double bonds of cis configuration. Examples of them can be found essential fatty acids, also known as omega fatty acids.
Waxes are simple and saponifiable lipids whose structure is generally made up of long chains of fatty acids linked to alcohols (glycerin) by ester bonds (C = O). Waxes can be found on plant leaves and bird feathers that give it hydrophobic properties.
Phospholipids are complex lipids, since, in addition to its glycerin backbone and its 2 fatty acid tails, it has a modified phosphate group. Phospholipids are specialized lipids and are major components of the plasma or cell membrane.
They form the phospholipid bilayer of the cell membrane, where the fatty acid tails form the hydrophobic part of the layer located between the hydrophilic heads of the phosphate groups.
Steroids are complex lipid molecules, since their structure is made up of 4 fused carbon rings. Steroids share the hydrophobic characteristics of lipids, such as their insolubility in water. Examples of steroids are cholesterol, mainly synthesized by the liver, and the raw materials of sex hormones such as testosterone.
Chemical structure of lipids
Most lipids, whether they are fats, oils, waxes or phospholipids, are made up of a glycerol skeleton (C3H8O3) or also known as glycerin, an alcohol composed of 3 hydroxyl groups (OH).
Generally, the hydroxyl groups of glycerol are attached to fatty acids via ester bonds (C = O) in a reaction called dehydration synthesis. The lipids formed by fatty acids are called saponifiable lipids.
Depending on the amount of fatty acids that bind to the glycerol molecule, the following types of lipids will be obtained:
- Monoacylglycerides: 1 fatty acid tail attached to 1 glycerin molecule,
- Diacylglycerides: 2 fatty acid tails attached to 1 glycerin molecule,
- Triacylglycerides: 3 fatty acid tails attached to 1 glycerin molecule
Fatty acid structure
Fatty acids form the tails of saponifiable lipids, which make up the majority of lipids. Fatty acids are long chains of carbohydrates (between 4 to 36 carbons) attached to a carboxyl group.
Fatty acids are classified as saturated and unsaturated:
Saturated fatty acids
Saturated fatty acids are made up of single bonds between neighboring (C) carbons. It is called saturated because it is saturated with hydrogen molecules (H), that is, the carbons are linked to the largest amount of hydrogens possible.
Simple bonds produce straight, compact tails characteristic of solid fats with high melting points, such as butter.
Unsaturated fatty acids
The structure of unsaturated fatty acids are formed by double bonds which means that they have less hydrogens (H). Unsaturated fatty acids that contain 1 double bond are called monounsaturated and those that have several double bonds are polyunsaturated.
Depending on the type of configuration of the double bonds of fatty acids, there are cis double bonds and trans double bonds.
Cis double bonds that are characterized by joining 2 hydrogens on the same side. These types of bonds are typical of liquid lipids or oils, as they have a low melting point, such as olive oil.
Another example is essential fatty acids, so called because they are necessary in the diet of the human body, since it does not synthesize them naturally. Essential fatty acids are unsaturated and contain at least 2 cis bonds. Among them we can find those derived from alpha-linolenic acid (ALA), known as omega-3, and from linoleic acid (LA), called omega-6.
See also Food Pyramid.
The trans double bonds, on the other hand, are characterized by joining their 2 hydrogens but they are located on opposite sides. These types of fatty acids are obtained from industrial processes called partial hydrogenation, which convert double bonds into single bonds in order to give the oils solid properties like saturated fats, such as vegetable shortenings.