Lipids: Definition, Properties, Structure, Classification and Examples

Lipids are organic compounds with hydrogen, carbon, and oxygen atoms that provide living cells with their structural and functional structure. These organic compounds are non-polar molecules that are only soluble in non-polar solvents and insoluble in water because water is a polar molecule. However, Lipids are soluble in many non-polar organic solvents like ether, benzene, chloroform, acetone, etc. Oil, butter, whole milk, cheese, fried foods, and various red meats are examples of lipids, some of which can be synthesised in the liver. Let’s look at Lipid’s definition, properties, structure, classification and examples in more detail.

Definition of Lipids

Lipids are a complex category of chemical molecules that include fats, oils, hormones, and certain membrane components that are grouped together because they have less interaction with water. Because the hydrophobic nature of the hydrocarbon chain of most biological fatty acids outweighs the hydrophilic nature of the carboxylic acid group, these molecules have extremely low water solubility.

Fig : Lipids

Properties of Lipids

Lipids are an organic chemical family made up of fats and oils. These molecules have a lot of energy and are involved in a variety of actions in the human body. Lipids are made up of a glycerol molecule attached to three fatty acid molecules. Such a lipid is called triglyceride. Lipids have a number of significant features, which are listed below.

1. Lipids are non-polar oily or greasy molecules that are stored in the body’s adipose tissue.
2. Lipids are a diverse collection of chemicals that are primarily made up of hydrocarbon chains.
3. Lipids form a colloidal complex and get dispersed uniformly in water in the form of minute droplets. The complex is called emulsion.
4. Lipids are organic compounds that are high in energy and supply energy for a variety of living functions.
5. Lipids are a family of chemicals distinguished by their insolubility in water and solubility in non-polar solvents.
6. In either nonenzymatic or enzymatically catalysed oxidation reactions, carbon-carbon double bonds can react with oxygen. This process produces a number of by-products, some of which contribute to the rancid odour of decaying meat and vegetables.
7. Lipids are important in biological systems because they constitute the cell membrane, which is a mechanical barrier that separates a cell from its surroundings.

Structure of Lipids

Pure fatty acids crystallise as stacked layers of molecules, each layer being the thickness of two extended molecules. The hydrophobic (water-fearing) hydrocarbon chains constitute the layer’s core, while the hydrophilic (water-loving) carboxylic acid groups form the layer’s two faces. The following figure depicts the lipid structure:

Fig: Lipid Structure – Saturated and Unsaturated Fatty Acids

Types of Lipids

Fig: Types of Lipids

Numerous particular forms of lipids, including fatty acids, triglycerides, glycerophospholipids, sphingolipids, and steroids, exist within these two primary classes of lipids. Simple lipids and complex lipids are two types of lipids.

Simple Lipids

Simple lipids are formed from fatty acids and alcohol. They do not have any additional groups e.g., fats, suberin, cutin, wax.

Complex Lipids

1. Esters of fatty acids are fatty acid esters that contain groups other than alcohol and fatty acid.
2. Phospholipids are lipids that contain a phosphoric acid residue in addition to fatty acids and alcohol. They usually contain nitrogen-containing bases and other substituents, such as glycerol in glycerophospholipids and sphingosine in sphingophospholipids.
3. Glycolipids (glycosphingolipids) are lipids that contain a fatty acid, sphingosine, and carbohydrate molecule.
4. Other lipids with a complex structure: Sulfolipids and amino lipids are examples of lipids. Lipoproteins fall into this category as well.

Derived Lipids

Derived Lipids are either lipid-like chemicals (e.g., sterols) or derivatives of lipids, e.g., terpenes, prostaglandins. Fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, and ketone bodies, as well as hydrocarbons, lipid-soluble vitamins, and hormones, are among these substances. Neutral lipids include acylglycerols (glycerides), cholesterol, and cholesteryl esters because they are uncharged. The hydrolysis of simple and complex lipids produces these chemicals.

Some of the numerous forms of lipids are detailed here in-depth.

Neutral or True Fats

They’re triglycerides made by esterifying three fatty acid molecules with one molecule of glycerol, a trihydric alcohol (glycerine or trihydroxy propane). Three water molecules are removed from the equation. The number of three fatty acid molecules esterified to a molecule of glycerol is referred to as a triglyceride. The ester is called diglyceride if there are two fatty acids connected to a glycerol molecule, or monoglyceride if there is just one fatty acid coupled to a glycerol molecule.

The three fatty acids are extremely rarely identical in fats (e.g., tripalmitin, tristearin, triolein). Pure fats are known as fatty acids. Typically, two of the three fatty acids are dissimilar, or two of the three are similar. Mixed fats, such as butter, and fats termed after fatty acid names are referred to as mixed fats. Dipalmito-stearin, palmito-oleo-stearin, and stereo-oleio-palmitin are other examples.

Fig: Structure of True Fats

Fatty Acids

Fatty acids are unsaturated or saturated carboxylic acids (or organic acids) having long aliphatic tails (long chains). Organic acids with hydrocarbon chains that terminate in a carboxylic group are known as fatty acids (-COOH). Hydrocarbon chains can be either straight or ring-shaped. Hydroxyl groups can be found in several fatty acids. The length ranges from 4 to 24 carbons.

Fatty acids with 16 or 18 carbons are the most frequent. Plants possess the biosynthetic machinery required to produce all forms of fatty acids. Animals have this machinery as well, although many don’t have the ability to synthesise the three fatty acids linoleic, linolenic, and arachidonic acids. Essential fatty acids are what they’re termed. They have to be in the animal’s diet. Most edible oils, such as those from sunflower, groundnut, cottonseed, and coconut, contain essential fatty acids. Phrynoderma or follicular hyperkeratosis are symptoms of their absence.

Fatty acids are of two main types – Saturated and unsaturated.

Saturated fatty acids 

1. The fatty acid is saturated if there are no carbon-carbon double bonds. 
2. Because of their tendency to pack their molecules together, saturated fatty acids have greater melting temperatures than unsaturated acids of the same size. 
3. This results in a straight rod-like structure.

Unsaturated fatty acids

1. When a fatty acid has more than one double bond, it is called an unsaturated fatty acid. Oleic acids, linoleic acids, linolenic acids, and arachidonic acids are among examples.
2. “Often, naturally occurring fatty acids comprise an even number of carbon atoms and are unbranched.”
3. Unsaturated fatty acids, on the other hand, have a cis-double bond(s) that form a structural kink that prevents them from grouping their molecules in a straight rod-like shape.
4. The melting point of unsaturated fatty acids is lower than that of saturated fatty acids. Unsaturated fatty acids make up the majority of plant lipids, whereas saturated fatty acids make up the majority of animal lipids.

Differences between Saturated and Unsaturated Fats

Function of Lipids

Fats play a number of important roles in our bodies. The following are some of the major functions of fats:

1. Plants and animals both use fats as food reserves. Prior to the advent of winter, hibernating mammals accumulate extra fat. Migrating birds perform the same thing before they migrate.
2. They operate as concentrated meals because they produce more than twice as much energy per unit weight as carbs (9.3 kcal/gm vs. 4.5 kcal/gm).
3. Fats in the right amounts are required for our bodies to function properly.
4. To be properly absorbed by the body, several fat-soluble vitamins must be linked with lipids.
5. Fat is required for the absorption of fat-soluble elements. These vitamins are best absorbed when eaten alongside fat-rich foods.
6. Omega-3 and omega-6 essential fatty acids help reduce inflammation in the joints, tissues, and bloodstream by regulating cholesterol and blood clotting.
7. Fats help in the formation of nerve cell membranes, the insulation of neurons, and the transmission of electrical impulses throughout the brain.
8. Animal and human body shapes are rounded out by subcutaneous fat. Fats form a shock-absorbing cushion around eyeballs, gonads, kidneys, and other critical organs in animals.
9. Fats are found as droplets inside adipocyte cells in animals. Cold-blooded or poikilothermic animals’ adipocytes contain more unsaturated fatty acids than warm-blooded or homoeothermic species’ adipocytes.
10. Plant oils are used as cholesterol-lowering fats. They’re also used to make vegetable ghee, which is hydrogenated.

Trans Fats

1. Oils are artificially hydrogenated in the food business to make them semi-solid and of a consistency suitable for many processed foods.
2. Also, hydrogen gas is pumped into oils to solidify them. Double bonds in the cis– conformation of the hydrocarbon chain may be changed to double bonds in the trans– conformation during this hydrogenation process.
3. Artificially hydrogenated trans fats can be found in margarine, some varieties of peanut butter, and shortening.
4. Recent research has found that increasing trans fats in the human diet can lead to higher levels of low-density lipoproteins (LDL), or “bad” cholesterol, which can lead to plaque buildup in the arteries and cause heart disease.
5. Trans fats have lately been prohibited in many fast-food restaurants, and product labels must now list the number of trans fats present.

Omega Fatty Acids

1. Essential fatty acids are fatty acids that the human body needs but cannot produce. As a result, they must be supplemented by dietary consumption.
2. Omega-3 fatty acids are polyunsaturated fatty acids with a double bond connecting the third carbon from the end of the hydrocarbon chain to its surrounding carbon.
3. Alpha-linoleic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) are polyunsaturated omega-3 fatty acids. 
4. Omega-3 fatty acids can be found in salmon, trout, and tuna. Omega-3 fatty acids have been shown in studies to reduce the risk of sudden mortality from heart attacks, lower blood pressure, and prevent thrombosis by slowing blood coagulation. 
5. They also help to lower inflammation and the risk of some malignancies in animals and human beings. 

Waxes

1. Wax is a substance that may be found practically anywhere. Many plants have waxy coverings on their fruits and foliage that protect them from tiny predators and dryness.
2. Long-alcohols and long-chain carboxylic acids combine to make waxes, which are “esters” (an organic compound generated by replacing the hydrogen with acid by an alkyl or another chemical group).
3. The fur of a few animals and the feathers of birds have the same water-repellent coats.
4. Carnauba wax is recognised for its hardness and resistance to water (significant for car wax).
5. Petroleum is used to make paraffin wax. Paraffin wax and stearic acid are used to make candles. Cosmetics and polishes contain wax. Wrapping is done with waxed paper.

Phospholipids

Fig: Phospholipid

1. Membranes are primarily composed of phospholipids that are Phosphoacylglycerols.
2. Triacylglycerols and phosphoacylglycerols are the same, but, the terminal OH group of the phosphoacylglycerol is esterified with phosphoric acid in place of fatty acid which results in the formation of phosphatidic acid.
3. The name phospholipid is derived from the fact that phosphoacylglycerols are lipids containing a phosphate group.

Steroids

Fig: Structure of Steroids

1. Steroids are hormones that have a tetracyclic structure, which is made up of three fused six-membered rings and one five-membered ring. 
2. The four rings are labelled A, B, C, and D, with the numerals indicating the carbons.
3. Steroids, unlike the phospholipids and fats mentioned above, have a fused ring structure. Despite the fact that they don’t look like the other lipids, they’re grouped alongside them since they’re both hydrophobic and water-insoluble. 
4. All steroids have four connected carbon rings, and some of them have a short tail, such as cholesterol. Many steroids also have the –OH functional group, which classifies them as alcohol (sterols).

Cholesterol

Fig: Structure of Cholesterol

1. Cholesterol is a waxy chemical that can only be found in animal-based foods. Different forms of cholesterol found in blood cells include triglycerides, LDL, HDL, and VLDL. Cholesterol is the most common steroid.
2. Cholesterol is primarily generated in the liver and is a precursor to numerous steroid hormones secreted by the gonads and endocrine glands, such as testosterone and estradiol. It also serves as a precursor to Vitamin D. 
3. Cholesterol is also a precursor to bile salts, which facilitate the emulsification of fats and subsequent cell absorption. Although cholesterol is frequently criticised by the wider population, it is essential for the body’s normal functioning. 
4. It’s a phospholipid bilayer component present within the plasma membrane of animal cells. 
5. The plasma membrane is the outermost structure in animal cells, and it is responsible for material transport, cellular recognition, and cell-to-cell communication.
6. The steroid ring shape of cholesterol in the cell membrane produces a rigid hydrophobic structure that helps in cell membrane rigidity. The cell membrane would be overly fluid if cholesterol wasn’t there.
7. It’s a crucial component of cell membranes and the starting point for the production of other steroid hormones like estradiol and testosterone, as well as other steroids like cortisone and vitamin D.

Summary

Lipids are a type of macromolecule that is both non-polar and hydrophobic. Fats and oils, waxes, phospholipids, and steroids are the most common forms. Fats, also known as triacylglycerols or triglycerides, are a type of energy that is stored in the body. Fatty acids are combined with glycerol or sphingosine to form fats. The presence or absence of double bonds in the hydrocarbon chain determines whether fatty acids are unsaturated or saturated. Saturated fatty acids are those that have only single bonds.

The hydrocarbon chain of unsaturated fatty acids may have one or more double bonds. The membrane matrix is made up of phospholipids. Two fatty acid chains and a phosphate-containing group are connected to a glycerol or sphingosine backbone. Steroids are different types of lipids. Four fused carbon rings make up their basic structure. Cholesterol is a form of steroid that is a key component of the plasma membrane, where it helps in maintaining the membrane’s fluidity. It also serves as a precursor to steroid hormones like testosterone.

Frequently Asked Questions (FAQs) on Lipids

Q.1. What are lipids?
Ans: Lipids are chemical substances made up of fatty acids or fatty acid derivatives that are water-insoluble but soluble in organic solvents. For example, Natural oils, steroids, and waxes.

Q.2. How are lipids important to our body?
Ans: Lipids play a critical role in our bodies. They are a component of the cell membrane’s structure. They help in the production of hormones and provide energy to our bodies. They help in appropriate meal digestion and absorption. If we eat them in the right amounts, they constitute a nutritious element of our diet. They play a vital function in signalling as well.

Q.3. How are lipids digested?
Ans: The enzyme lipase, which is facilitated by bile in the liver, breaks down fats into fatty acids and glycerol.

Q.4. What is lipid emulsion?
Ans: It’s a lipid emulsion for intravenous usage in humans. Intralipids are an emulsion of soybean oil, glycerin, and egg phospholipids that is also known as intralipids. It comes in 10 per cent, 20 per cent, and 30 per cent concentrations.

Q.5. What are the main types of lipids?
Ans: Simple lipids and complex lipids are the two main forms of lipids. Simple lipids are fatty acid esters with different alcohols. Fats and waxes, for example. Complex lipids, on the other hand, are fatty acid esters having groups other than alcohol and fatty acids. Phospholipids and sphingolipids, for example.

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