Lipids
Lipids Definition: Lipids are a heterogeneous class of organic compounds that are fatty adds or their derivatives and are insoluble in water but soluble in non-polar solvents.
Lipids comprise a group of naturally occurring molecules that includes fats, wax, sterols, triglycerides, phospholipids, etc. Lipid contains carbon, hydrogen and oxygen but the proportion of oxygen is far less than that in carbohydrate (where the ratio of H:0=2:l).
It is a basic building block of biological membranes.
Lipids Sources: Plants—Various plant sources include seeds of soybean, mustard, sunflower, etc. Animals— Various animal sources include butter, ghee, animal fats, eggs, etc.
Lipids Chemical structure: Lipid has no single common structure. A commonly occurring lipid in our body is triglyceride which is constituted of fatty acid and glycerol.
Three molecules of fatty acid combine with one molecule of glycerol by three ester linkages to form a triglyceride molecule.
Lipids definition, structure, classification, and types notes PDF
Fatty acids: Fatty acids are organic acids with a long hydrocarbon chain ending with a carboxylic group.
Properties of fatty acid:
- It is a type of aliphatic organic acid, which is formed as a result of the hydrolysis of lipids.
- There are about 100 types of fatty acids present in nature.
- The fatty acid chains are usually 14-24 carbon atoms long.
- One end of the fatty acid has a COOH group while the other end has a -CH3 group.
- About 4-30 carbon atoms may be present in the fatty acids.
- Most lipids consist of a polar or hydrophilic head (typically glycerol) and one to three nonpolar or hydrophobic fatty acid tails, and therefore they are amphiphilic.
- Fatty acids consist of unbranched chains of carbon atoms that are connected by single bonds only (saturated fatty acids) or by both single, double or triple bonds (unsaturated fatty acids).
- It is generally not available in free form within the cell.
- Some fatty acids may also exist in the ring structure.
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General formula of fatty acids: Fatty acids are straight-chain monocarboxylic acids with a general molecular formula RCOOH.
‘R’ is the variable group.
Fatty acid chains have a methyl group (—CH3) at one end and a carboxyl (-COOH) group at the other end. The general formula is CH3 – (CH2)n – COOH.
The COOH group is attached to the a -carbon, with the next C-atom being J3 -carbon, the next being <5 -carbon and so on.
Similarly the -CH3 group is attached to the OJ1 -carbon, with the previous carbon atom being u2 -carbon, the one previous to it being w3 -carbon and so on.
Types of fatty acids: Fatty acids are classified on the basis of different characteristics.
Classification based on number of carbon atoms: According to the number of carbon atoms, fatty acids are of the following types—
Short-chain fatty acids: These have less than 10 carbon atoms, Example butyric acid (CH3 – (CH2)2 – COOH) which is found in butter and caproic acid (CH3 – (CH2)4 – COOH) which is found in butter, milk, cream, etc.
Short-chain fatty acids show a higher degree of fluidity than long-chain fatty acids because their intermolecular packing is less compact.
Long-chain fatty acids: They have 14- 24 carbon atoms forming a chain.
These are found in biological systems, though 16 and 18-carbon molecules are most common.
Examples of long-chain fatty acids are, palmitic acid (C16H32O2), stearic acid etc-
Classification based on the number of double bonds in the molecule: According to the number of double bonds present in their molecules, fatty acids are classified as follows—
Saturated fatty acids: Here, all carbon atoms are joined by single bonds in the hydrocarbon chain, Example palmitic acid (C16H3202), stearic acid (C18H36°2) caproic acid (C6H1202), butyric acid (C4H802), etc. Saturated fats are found in animals.
Generally, saturated fats have a high melting point, so they are solids at room temperature.
Unsaturated fatty acids: They have one or more double bonds in the hydrocarbon chain, Example oleic acid (C18H3402), linoleic acid (C18H3202), linolenic acid (C18H30O2), arachidonic acid (C20H32O2) or triple bonds (acetylenic fatty acids) on the basis of number of double bonds the unsaturated fatty acids called monoenoic acid (one double bond), dienoic acid (two double bonds), trienoic acid (three double bond), etc.
Unsaturated fats are liquid at room temperature and found in plants. This explains why butter (made from animal fat) is solid while oils (fats from vegetables) are liquids.
Margarine is made by hydrogenating (adding H) unsaturated vegetable oils, increasing the amount of saturation and thus the melting point will be high (so it will be solid).
Classification of lipids with examples and functions
Classification based on requirement: Based on their requirement in our body, fatty acids are of the following types—
Essential fatty acids: These are not synthesised in the human body and therefore exclusively obtained through diet. Examples are linoleic acid, linolenic acid, and arachidonic acid.
Non-essential fatty acids: These are synthesised in the body and therefore are not essential in the diet. Example palmitic acid, stearic acid, oleic acid, etc.
Sources of some essential fatty acids
Humans and other mammals have a dietary requirement for certain essential fatty acids, such as linoleic acids and alpha-linoleic acid.
They cannot be synthesized from simple precursors in the diet. Most vegetable oils are rich in linoleic acid (sunflower).
Alpha-linoleic acid is found in the green leaves of plants and in some seeds, nuts, and legumes.
Fish liver oils are particularly rich in longer-chain omega-3 fatty acids such as icosapentaenoic acid or EPA (C20H30O2) and docosahexaenoic acid or DHA (C22H3202).
Glycerol: A tricarbon alcohol with three hydroxyl groups (-OH) is known as glycerol.
Classification of lipids: Lipids are classified as simple lipids, compound lipids and derived lipids. These are described under a separate head below.
“lipid classification “
Simple lipids
Simple lipids Definition: Simple lipids are esters of fatty acids with glycerol or other alcohol only without containing any other substituent group.
The two types of simple lipids are mentioned below.
Triacylglycerols or Triglycerides: Triglyceride is an ester obtained from glycerol by the esterification of The two types of simple lipids mentioned below.
Examples are Ghee, groundnut oil, mustard oil, sunflower oil, castor oil, Cod liver oil, and Halibut liver oil.
Triglycerides Characteristics: Triglycerides have the following properties—
- These are non-polar and hydrophobic in nature.
- These are stored in large quantities in plants and animals, as an energy source.
Triglycerides Types: Triglycerides are of two types—
Symmetrical or simple triglycerides: When the three fatty acid molecules of a triglyceride are of the same type, they are called symmetrical or simple triglycerides, Example tristearin.
Asymmetrical or mixed triglycerides: When the three fatty acid molecules of a triglyceride are either of two or three different types, they are called asymmetrical or mixed triglycerides. Example: Oleopalmitosteain.
States: Triglycerides are found in two states—
Fats: The triglycerides that are made of saturated fatty acids and are solid at room temperature are called fats.
Oils: The triglycerides that are made of unsaturated fatty acids and are liquid at room temperature are called oils.
Types of lipids and their biological significance
Waxes: Waxes are esters of higher fatty acids and aliphatic, alicyclic and monohydric alcohol other than glycerol.
Wax has the following properties—
- It is saturated in nature but may break in the presence of alcoholic KOH and high temperature.
- It does not get oxidised in nature.
- It is insoluble in water.
Compound lipid
Compound lipid Definition: Compound lipids are fatty acid esters containing wax that have the following properties—
Compound lipid Phospholipids: Phospholipids or phosphatides are heterogeneous groups of compounds. A phospholipid molecule consists of fatty acids and glycerol in addition to phosphoric acid, nitrogen bases and other substituents like Lecithin (DDPC, DOPC), cephalin, etc.“lipid structure diagram “
Compound lipid Characteristics:
- These are mainly found in the cell membrane.
- The molecules contain a hydrophobic non-polar tail and hydrophilic polar head, hence amphipathic in nature.
Glycolipids or glycosphingosides: The compound lipid, in which one of the fatty acids is replaced by an amino alcohol(sphingosine) and one or more fatty acid is replaced by simple sugars, are called glycolipids or glycosphingosides.
The glycolipids are components of cell membranes, particularly in the membrane and myelin sheath of nerve fibres and membranes of chloroplasts.
Glycolipids or glycosphingosides Structure: The glycolipids are made of sugar, fatty acids and sphingosine.
Glycolipids or glycosphingosides Types: Glycolipids are of the following types—
Glycolipids or glycosphingosides Cerebrosides: The glycolipids that contain glucose or galactose as the sugar units, are called cerebrosides. These are mainly found in the brain.
Sulpholipids or sulphatides: The glycolipids that contain S-containing galactose as the sugar units, are called sulpholipids. These are found mainly in the white matter of the brain.
Gangliosides: These glycolipids are composed of sphingolipids linked by glycosidic bonds to oligosaccharide chains as the sugar units, and are called gangliosides.
These are found in nerve cells, spleen and red blood cells. These molecules have important immunological roles and are used for therapeutic purposes.
Lipoproteins: The compound lipid molecules, conjugated with protein, are called lipoproteins.
These molecules contain neutral fats (triglycerides), cholesterol or phospholipids as the lipid part. These are found in cell membranes, milk, egg yolk, etc.
Derived Lipid
Derived Lipid Definition: The lipids that are derived through hydrolysis of simple or compound lipids are called derived lipids.
Some other chemical constituents are also included in this group. Examples are steroid hormones, fatty acids, glycerol, fat-soluble vitamins—A, D, E, K, hydrocarbons, etc.
Derived lipids are of three types—
- Steroids,
- Terpenes and
- Carotenoids.
Steroids: Steroids are derived lipids obtained from cyclohexane pentano per hydro phenanthrene or sterane compound. They do not contain fatty acids, and hence are non-saponifiable.
Several types of steroids are known. Some of them are given below—
Sterol: Hydroxyl group containing steroid.
Cholesterol: This is present in higher animals in a free state or as a fatty acid ester.
It is an important component of some cell membranes and of plasma lipoproteins and also acts as a precursor of different steroid hormones in animals.
Ergosterol: This is found in plants and fungi, such as yeast, Neurospora, etc. and in some protozoa.
Terpenes: Terpenes are a type of derived lipids, which contain less than 40 carbon atoms, and are found mainly in plants.
They are of different types, such as monoterpenes, diterpenes, etc.
Several isoprene units combine to form isoprenoid units. They are found in leaves, flowers etc. Example menthol, camphor, thymol, etc.
Carotenoids: The carotenoids are unsaturated derived lipids, mainly pigments, present in plants. They are responsible for bright red, orange and yellow colouration in flowers and fruits. They are of several types like lycopene, carotene, xanthophyll, etc..
“biochemistry lipids “
Properties of lipid
Different physical and chemical properties of lipids are discussed below.
Physical properties:
- Lipids are insoluble in water, but soluble in organic solvents (like ether, chloroform, alcohol) etc.
- Pure triglycerides are odourless, tasteless and colourless in nature.
- Unsaturated fatty acids are present in a liquid state (oils), at normal temperature while saturated fatty acids are present in the solid state, at normal temperature.
- They have different melting points, as their melting point depends on the length and saturation level of the fatty acid chains.
- The relative molecular weight of the lipids is less than 1.0, hence they are lighter than water.
- Solid lipids are lighter than liquid lipids.
Chemical properties
Chemical properties Hydrolysis: By boiling with acid or alkali or by increasing temperature, fats can be hydrolysed into fatty acids and glycerol. The enzyme lipase hydrolyses lipids into fatty acids and glycerol, within the digestive system.
Saturated vs unsaturated fatty acids in lipids
Chemical properties Saponification: The process by which fats and oils react with alkali (For example sodium hydroxide, potassium hydroxide, etc.) to form soaps is called saponification.
The number of milligrams of KOH / NaOH required to neutralise the total amount of fatty acids derived from the hydrolysis of 1 gm of fat is called the saponification number.
\(\text { Saponification number }=\frac{1}{\text { M.W. of fatty acid of fat }}\)This number is an index of the average molecular size of fatty acids present in a particular fat.
Chemical properties Halogenation: Unsaturated fatty acids have the ability to add halogens (iodine, fluorine, chlorine and bromine) at double bonds. This principle is used to determine the presence of unsaturated fatty acids in lipids.
Chemical properties Hydrogenation: The addition of hydrogen to fats and oils depends on the presence of unsaturation in the fatty acids.
This process occurs in unsaturated fatty acids, changing them to saturated fatty acids. Hydrogen is usually added at high temperatures in the presence of nickel as a catalyst.
This reaction changes oil to fat. It raises the melting point of oils so that they solidify.
This principle is used to make edible vanaspati or margarine from inedible and cheap vegetable oil like cottonseed oil.
Chemical properties Oxidation: The unsaturated fatty acids, present in the fats and oils, react with atmospheric oxygen when exposed to air forming lipid peroxides, fatty aldehydes, ketones and short-chain fatty acids.
Rancidity or acid number: It is a condition in which fat attains a bad taste along with bad odour due to exposure to air. The product obtained is called rancid lipid.
Rancidity occurs due to the action of lipase enzymes secreted by microorganisms present in the air. Lipids, like tannins, vitamin E, etc., obtained from plants do not get rancid easily as they contain some of the antioxidants.
However, lipids with higher unsaturated fatty acid content become rancid easily. Rancidity is absent in lipids obtained from animal sources.
Biological importance of lipid
Lipids play various important roles in living organisms, especially in human beings.
Energy source: Lipids act as a source of energy. They are superior to carbohydrates and protein. They yield twice the energy produced by the same amount of carbohydrates and protein. The calorific value of lipids is 9.3 kcal/g.
Structural component: Lipids are the major components of cell membranes. The lipid bilayer of the cell membrane controls the movement of materials in and out of the cell.
Reserve energy food: In plants, Lipid serves the lipidsas thearestoragestored in the body soil seeds of groundnut, mustard, castor and coconut to provide nourishment to the developing embryo during germination. In animals, fat is stored in the form of adipocytes or fat cells.
Solvent: They act as solvents for the fat-soluble vitamins (A, D, E, K).
Body temperature regulation: Lipids deposited in the subcutaneous adipose tissue help in insulation and protection from cold.
A thick layer of subcutaneous fat, called ‘blubber’, especially in whales, seals, etc. regulates body temperature in ice-cold water.
Transmission of information: Steroid hormones transmit information and mediate communication between cells through blood.
Phospholipids, steroids, and triglycerides structure and functions
Fatty acid transport: Lipids play an important role in absorption and transportation of fatty acids by blood.
Role In Maintaining Membrane Fluidity: Fatty acids influence membrane fluidity which is important in regulating the diffusion of protein molecules embedded within the membrane.
Hormone synthesis: Steroid hormones like sex hormones and adrenocorticoid hormones are synthesised from cholesterol.
Protection: Lipids form a protective covering over the aerial parts of plants to regulate the excessive loss of water by transpiration.
In animals, the layer of subcutaneous (below the skin) fat provides protection against desiccation. Fat deposition around the delicate visceral organs acts as a cushion and absorbs mechanical shock.
Lipids in cell membranes: role of phospholipids and cholesterol
Electrical insulator: Lipids in the myelin sheath outside the medullary nerves, act as electrical insulators.
Lipid storage diseases or lipidoses
Gaucher disease: It is a hereditary disease caused by to accumulation of cerebrosides which affects the liver, lung, bone marrow and spleen.
Niemann-Pick disease or sphingomyelin lipidosis: This disease is caused by the accumulation of excess sphingomyelin, in the brain. It is a hereditary disorder.
Tay-Sachs disease: It is a hereditary disease caused due to accumulation of excess gangliosides in the nerve cells of the brain causing progressive damage to the cells.