Introduction and Function of Lipids

Introduction of lipid:

Lipids may be regarded as the organic substances relatively insoluble in water, soluble in organic solvents (alcohol,chloroform,benzene, acetone) actually or potentially related to fatty acids and utilized by living cells. They are esters of different alcohol and utilizable by living cells,Thus the lipids include fats, oils, steroids and related compounds which are abundantly found in plants,animals, and microbes.

Lipids are a group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins ( such as vitamins A, D ,E, and K), monoglycerides, diglycerides,triglycerides, phospholipids, and other . The main biological functions of lipids include storing energy, signaling, and acting as structural components of cell membranes. Lipids have applications in the cosmetic and food industries as well as in nanotechnology.

Lipids may be broadly defined as hydrophobic or amphiphilic small molecules; the amphiphilic nature of some lipids allows them to form structures such as vesicles, multilamellar/ unilamellar liposomes, or membranes in an aqueous environment. Biological lipids originate entirely or in part from two distinct types of biochemical subunits or " building- blocks". Ketoacyl and isoprene groups. Using this approach, lipids may be divided into eight categories: fatty acids, glycerolipids, glycerophospholipids,sphingolipids, saccharolipids, and polyketides ( derived from condensation of ketoacyl subunits); and sterol lipids and phenol lipids ( derived from condensation of isoprene subunits.)

Although the term lipids are sometimes used as a synonym of fats, fats are a subgroup of lipids called triglycerides. Lipids also encompass molecules such as fatty acids and their derivatives ( including tri-, di-, monoglycerides, and phospholipids), as well as other sterol-containing metabolites such as cholesterol. Although humans and other mammals use various biosynthetic pathways both to break down and to synthesize lipids, some essential lipids cannot be made this way and must be obtained from the diet.

Function of Lipids:

The different function of lipids are described below:

1) source of energy:

lipids are important dietary constituents containing the high degree of calorie value. They provide double energy with compare to carbohydrate and proteins, vitamins which are necessary for human body. 1 gm lipids contain 4.2kcal energy.

2) Structural component of cell :

Eukaryotic cells feature compartmentalized membrane- bound organelles that carry out different biological functions. The glycerophospholipids are the main structural component of biological membranes, such as the cellular plasma membrane and the intracellular membranes of organelles' in animal cells the plasma membrane physically separates the intracellular components from the extracellular environment . The glycerophospholipids are amphipathic molecules ( containing both hydrophobic and hydrophobic regions ) that contain a glycerol core linked to two fatty acids-derived " tails" by ester linkages and to one " head " group by a phosphate ester linkage. while glycerophospholipids are the major component of biological membranes, other non- glyceride lipid components such as sphingomyelin and sterols are also found in biological membranes. In plants and algae, the galactosyldiacylglycerols, which lack a phosphate group, are important components of membranes of chloroplasts and related organelles and are the most abundant lipids in photosynthetic tissues, including those of higher plants, algae, and certain bacteria. The cytoplasmic membrane is composed of phospholipids bilayer which acts as the barrier for the passes of polar molecules and ions.

3) Mechanical protection:

Lipids components are involved in the protection of different organs from mechanical injury which is due to the entry of harmful substances as well as microorganisms. In addition to this lipid gives proper shape to our body.

4) Energy storage:

triglycerides, stored in adipose tissue, are a major form of energy storage both in animals and plants. The adipocytes, or fat cell, is designed for continuous synthesis and breakdown of triglycerides in animals, with breakdown controlled mainly by the activation of hormone-sensitive enzyme lipase. The complete oxidation of fatty acids provides high caloric content. Migratory birds that must fly long distances without eating use stored energy of triglycerides to fuel their flight.

5) Insulation of heat:

Lipid serves as the thermal insulator in the subcutaneous tissues and around certain organs. Sub-cutaneous deposits of fat are higher in aquatic animals. They act as heat insulator as well as maintain buoyancy ( upthrust in water )

6) Dissolve vitamins:

Vitamins A,D,E, K are dissolved by lipid and make absorbance in GI tract.

7) Enzyme activation :

Some enzymes required lipid molecules for their maximal catalytic activity eg; mitochondrialβ-hydroxy butyric acid dehydrogenase.

8) As a precaution for synthesis of regulatory compounds;

Lipids serve as regulatory biomolecules which are responsible for the maintenance of homeostasis . Many hormones are lipid soluble ie; glucocorticoids, Thyroid hormones and sex hormones which regulate the metabolic pathways in the body. These hormones are derived from lipids. Vitamins D3 is synthesized from lipid derivative.

9) Signaling:

In recent years, evidence has emerged showing that lipid signaling is a vital part of the cell signaling . Lipid signaling may occur via activation of G protein -coupled or nuclear receptors and members of several different lipid categories have been identified as signaling molecules and cellular messengers. These include sphingosine -1- phosphate, a sphingolipid derived from ceramide that is a potent messenger molecule involved in regulating calcium mobilization, cell growth, and apoptosis,Diacylglycerol (DAG) and the phosphatidylinositol phosphates involved in calcium -mediated activation of protein kinaseC, the prostaglandins, which are one type of fatty-acid involved in inflammation and immunity, the steroid hormones such as estrogen,testosterone and cortisol, which modulate a host of functions such as reproduction ; metabolism and blood pressure. Phosphatidylserine lipids are known to be involved in signaling for the phagocytosis of apoptotic cells and pieces of cells.

Fatty acid

Fatty acids are the building block component of most of the lipid. They are long chain organic acids, having 4-24 carbon atoms. In chemistry ,particularly in biochemistry, a fatty acid is a carboxylic acid with a long aliphatic chain,which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of the even number of carbon. Fatty acids are usually derived from triglycerides or phospholipids. The fatty acid is important sources of fuel because ,when metabolized, they yield large quantities of ATP. Many cell types can use either glucose or fatty acids for this purpose. Long-chain fatty acids cannot bind blood -brain barrier (BBB) and so cannot be used as fuel by the cells of the central nervous system, however, free short-chain fatty acids and medium-chain fatty acids can cross the BBB, in addition to glucose and ketone bodies.

The fatty acid has a single carboxyl group called as the head region and long hydrocarbon chain called as the tail region which makes the lipid-water soluble and oily and greasy in nature. They do not occur in free form in cells or tissues but are lipids. Nearly all the fatty acids present in nature have an even number of carbon atoms and those with c₁₆- c₁₈ are the most abundant. The long hydrocarbon tail may fully saturate or may contain a double bond.

Classification of fatty acids:

Fatty acid is classified into following groups;

1) Saturated fatty acid:

The fatty acid which contains single bond in their hydrophobic tail region is included in this group. Palmitic acid and stearic acid are the most abundant saturated fatty acids in animal lipids .The MP and BP increase with the increase in the hydrocarbon chain.

2) Unsaturated fatty acids;

The fatty acids which contain the double bond in their hydrocarbon chain are called unsaturated fatty acid. The double bond generally occurs in Cis configuration. Most of the unsaturated fatty acid contain C₁₈-C_{20 .} They are twice more abundant than saturated fatty acids in both plants and animal lipids. They are mainly of two types.

a) Mono- saturated fatty acids;

Fatty acids having only one double bond eg; oleic acid.

b) Polyunsaturated fatty acids;

fatty acids having more than one double bond, eg, Linoleic acid.

3) Branched chain fatty acid;

Some fatty acids contain branched chain hydrocarbon structure and they are mainly found in different microorganism but not in plants and animals. Tuberculostearic acid (C 19) inMIcobacterium tuberculosis and Lactobacillic acid (c20) in Lactobacillus spare the examples of branched chain fatty acids.