Lipids are a wide variety of natural products

Although the human body can manufacture most of the lipids it needs, there are others, called essential fatty acids, that must be consumed in food.. Classification system for lipid molec

TABLE OF CONTENTS

LIST OF FIGURES 3

LIST OF TABLES 4

PART 1: LIPID 5

1 INTRODUCTION 5

2 DEFINITION 6

3 CLASSIFICATION 7

3.1 Classification by the chemical component 8

3.1.1 Simple lipid 8

3.1.2 Complex lipid 9

3.2 Classification by biological role 22

3.2.1 Storage lipid 23

3.2.2 Membrane lipid 23

3.3 Classification by source 24

3.3.1 Animal-based lipid 25

3.3.2 Plant-based lipid 26

4 STRUCTURE 28

5 FUNCTION 29

5.1 Energy storage 29

5.2 Lipid signaling 30

5.3 Cell membrane structuration 31

5.4 Others 31

6 DIGESTION AND ABSORPTION 32

7 METABOLISM 35

PART 2: HEALTHY DIET THROUGH ADULTHOOD 37

1 HOW TO BUILD A HEALTHY DIET PLAN 37

1.1 Diversity in diet plan 37

1.2 Achieving caloric balance 37

1.3 Simple and flexible plan 39

2 THE DIFFERENCES IN DIET FOR ADULT 41

2.1 Calories intake 41

2.2 Schedule 41

2.3 Fiber and water 41

2.4 Fat 41

3 DAILY CALORIES INTAKE 43

3.1 Based on gender 43

3.2 Based on workload 43

4 CONCLUSION 45

PART 3: REFERENCES 46

LIST OF FIGURES

Figure 1 Classification system for lipid molecules 7

Figure 2 Chemical reaction to produce triglycerides 8

Figure 3 Glycerol structure 9

Figure 4 Structure of glycerophosphate-based lipids 10

Figure 5 Glycosylacylglycerols of plant and algal photosynthetic membranes 13

Figure 6 Ceramide, the basic building block of sphingolipids 14

Figure 7 The variety of LCB structures found in sphingolipids 15

Figure 8 Structures of deoxysphingosine and deoxy methyl sphingosine 16

Figure 9 Glycolipid structure 16

Figure 10 Schematic representation of a cerebroside 17

Figure 11 Schematic representation of a ganglioside 19

Figure 12 Glyceroglycolipid structure 19

Figure 13 Schematic structures of CM, VLDL, and LDL 20

Figure 14 Lipid in plasma membrane structure 24

Figure 15 Plant-based lipid and animal-based lipid structures 25

Figure 16 Animal fat 26

Figure 17 Plant lipid metabolism 27

Figure 18 Glycerolipid structure 29

Figure 19 Triglycerides in chylomicrons and VLDL are broken down by lipoprotein lipase so that fatty acids and glycerol can be used for energy—or stored for later—in cells 29

Figure 20 Common lipid signaling molecules: lysophosphatidic acid (LPA), sphingosine-1-phosphate (S1P), platelet-activating factor (PAF), anandamide or arachidonoyl ethanolamine (AEA) 30

Figure 21 Glycerophospholipid 31

Figure 22 Phospholipid bilayer 31

Figure 23 Micelle made from phospholipids' formation 32

Figure 24 Chylomicron's schematic diagram 33

Figure 25 Lipid digestion and absorption in the small intestine 33

Figure 26 Overview of lipid digestion in the human gastrointestinal tract 34

Figure 27 Lipid metabolism in liver 35

Figure 28 Interactions of Fat Metabolism Pathways 36

Figure 29 Fat Metabolism in Specific Tissues 36

Figure 30 Food pyramid for European people 43

Figure 31 Recommended food pyramid for adults by the Vietnamese Ministry of Health 44

LIST OF TABLES

Table 2 Structural variety of different diacyl-glycerophospholipids 11

Table 3 Major glycosylacylglycerols of plant and algal photosynthetic membranes 12

Table 4 Some glycosylglycerides found in bacteria 13

Table 5 Lipid composition of Lipoprotein classes 21

Table 6 Plant-based lipid and animal-based lipid comparison 25

Table 7 BMR for men 38

Table 8 BMR for women 38

Table 9 Physical activity factor 39

PART 1: LIPID

1 INTRODUCTION

Unsaturated fat is generally considered to be healthier because it contains fewer calories than an equivalent amount of saturated fat Additionally, high consumption of saturated fats is linked to an increased risk of cardiovascular disease Some examples of foods with high concentrations of saturated fats include butter, cheese, lard, and some fatty meats Foods with higher concentrations of unsaturated fats include nuts, avocado, and vegetable oils such as canola oil and olive oil

Humans need lipids for many vital functions, such as storing energy and forming cell membranes Lipids can also supply cells with energy In fact, a gram of lipids supplies more than twice as much energy as a gram of carbohydrates or proteins Lipids are necessary in the diet for most of these functions Although the human body can manufacture most of the lipids it needs, there are others, called essential fatty acids, that must be consumed in food Essential fatty acids include omega-3 and omega-6 fatty acids Both of these fatty acids are needed for important biological processes, not just for energy

Although some lipids in the diet are essential, excess dietary lipids can be harmful Because lipids are very high in energy, eating too many may lead to unhealthy weight gain A high-fat diet may also increase lipid levels in the blood This, in turn, can increase the risk for health problems such as cardiovascular disease The dietary lipids of most concern are saturated fatty acids, trans fats, and cholesterol For example, cholesterol is the lipid mainly responsible for narrowing arteries and causing the disease atherosclerosis

2 DEFINITION

Lipids are “a wide variety of natural products” including fatty acids and their derivatives, steroids, terpenes, carotenoids, and bile acids, which have in common a ready solubility in organic solvents such as diethyl ether, hexane, benzene, chloroform, or methanol [1]

Lipid general properties:

• Insoluble in water

• Soluble in organic solvents such as chloroform, ether, or benzene

• Contain long-chain hydrocarbon groups in their molecules

• Present in or derived from living organisms

They are certain components of membranes and function as energy-storage molecules and chemical messengers Together with proteins and carbohydrates, lipids are one of the principal structural components of living cells

Distribution:

• Animals: In adipose tissue, egg and milk

• Plants: plants and seeds that contain oil

3 CLASSIFICATION

The term “lipid” has been loosely defined as any of a group of organic compounds that are insoluble in water but soluble in organic solvents These chemical features are present in a broad range of molecules such as fatty acids, phospholipids, sterols, sphingolipids, terpenes, and others Since lipids comprise an extremely heterogeneous collection of molecules from a structural and functional standpoint, it is not surprising that there are significant differences concerning the scope and organization of current classification schemes Several sources are based on lipids' chemical components and segregate them into “simple” and “complex” groups Some others divide lipids into storage lipids" and "membrane lipids" or "animal-based" and "plant-based" which are based

on their biological functional roles and their sources respectively [7]

Figure 1 Classification system for lipid molecules

This figure, from the consortium’s study of the lipidome of a pooled plasma sample, shows how the six major lipid classes (fatty acids, glycerolipids, glycerophospholipids, sphingolipids, sterols, and phenols) are synthesized and interconverted as well as how they relate to other lipid metabolites

3.1 Classification by the chemical component

There are numerous specific types of lipids important to live, including fatty acids, triglycerides, glycerophospholipids, sphingolipids, and steroids These are broadly classified as simple lipids and complex lipids based on their chemical composition

3.1.1 Simple lipid

Simple lipids contain one or two different types of compounds generally neutral, except for fatty acids which are studied in this group and have non-polar properties For example, one sterol and/or one fatty acid, glycerol and fatty acid(s), one alcohol and/or one fatty acid, fatty acid, and amino acid Some classification restricts this group to molecules containing one alcohol and/or one fatty acid Lipids containing sugar like glycolipids are excluded from that list and are classified

as “complex lipids”, even if they contain two compounds

The main simple lipids are triglycerides (also known as triacylglycerols), steryl esters, and wax esters Hydrolysis of these lipids yields glycerol and fatty acids, sterols and fatty acids, and fatty alcohols plus fatty acids, respectively The most important of these simple lipids for food scientists are triglycerides They are the major components of edible oils and fats, often representing more than 95% of refined oils Triglycerides are esters of the trihydric alcohol

glycerol with three fatty acids (Fig 2) Many of the properties of triglycerides are dependent on

the component fatty acids Thus, the melting point of the triglyceride reflects the melting point of the component fatty acids, with three high-melting-point fatty acids yielding a high-melting triglyceride Unsaturation in the fatty acids makes the triglyceride susceptible to autoxidation, just

as the fatty acid itself would be Steryl esters always occur together with sterols in the plant, animal,

or microbiological tissues Wax esters may accumulate in considerable amounts in some biological tissues and this class comprises the main constituent of beeswax and jojoba oil

Figure 2 Chemical reaction to produce triglycerides

3.1.2 Complex lipid

Complex lipids contain frequently three or more chemical identities (i.e glycerol, fatty acids, and sugar, one long chain base, one nucleoside, one fatty acid, and one phosphate group…) and have polar properties Some contain only two components but include a sugar moiety These important lipids are widely distributed in plants, bacteria, and animals They are the major constituents of cell membranes but are found also in circulating fluids

Complex lipids can be classified into four main groups: phospholipids, glycolipids, lipoamino acids, and nucleolipids

Table 1 Simple and complex lipids comparison

Definition Ester of fatty acids with

Classification Fat, oils, and wax Phospholipids, sphingolipids, glycolipids, and

lipoproteins

3.1.2.1 Phospholipid

3.1.2.1.1 Glycerophospholipid: A source of bioactive molecules

In addition to their roles as structural components of biological membranes, glycerolipids are precursors of potently active regulatory or signaling molecules They can be conveniently divided into two main groups - those containing phosphorus (phosphoglycerides) and those without phosphorus but containing a sugar constituent (glycosylglycerides) Confusingly, some compounds can be classified as both

Figure 3 Glycerol structure

Figure 4 Structure of glycerophosphate-based lipids

a) Phosphoglycerides

The phosphoglycerides are a very widespread and diverse group of structures In most membranes they are the main lipid components and, indeed, the only general exceptions to this statement are the photosynthetic membranes of plants, algae, and cyanobacteria, and the archaebacterial membranes

Usually, phosphoglycerides contain fatty acids esterified at the sn-1 and sn-2 positions of glycerol They are, thus, diacylphosphoglycerides These lipids are named after the moiety which

is attached to the phosphate esterified at the sn-3 position of glycerol Thus, the compounds can

be thought of as derivatives of diacylglycerols in which the hydroxyl on carbon atom 3 is esterified with phosphoric acid which in turn is esterified with a range of molecules - organic bases, amino acids, and alcohols The simplest phosphoglyceride contains only phosphoric acid attached to diacylglycerol and is called phosphatidic acid Where additional 'X' groups are esterified to the phosphate moiety the lipids are called phosphatidyl-X

Table 2 Structural variety of different diacyl-glycerophospholipids

b) Glycosylglycerides

Glycolipids which are based on glycerol have been found in a wide variety of organisms However, whereas in animals they are only found in very small quantities they are major constituents of some microorganisms and are the main lipid components of the photosynthetic membranes of algae (including the blue-green or cyanobacteria) and plants Their structure is analogous to that of glycerophospholipids with the sugar(s) attached glycosidically to the sn-3 position of glycerol and fatty acids esterified at the other two positions

digalactosyldiacylglycerol – represent about 40% of the dry weight of photosynthetic membranes

of higher plants Whereas galactose is almost the only sugar found in the glycosylglycerides of

higher plants, other sugars such as glucose may be found in algae, particularly marine species In bacteria, several combinations of residues may be found in diglycosyldiacylglycerols Such glycosylglycerides do not form a large proportion of the total lipids in bacteria but are found more frequently in the Gram-positives or photosynthetic Gram-negatives In addition, bacteria may contain higher homologs with up to seven sugar residues

Table 3 Major glycosylacylglycerols of plant and algal photosynthetic membranes

Bacteria contain a number of phosphatidylglycolipids These compounds are confined to certain types of Gram-positive organisms such as streptococci or mycoplasmas In addition, different species of algae and bacteria (including archaebacteria) contain various sulphoglycolipids

- usually with the sulfur present in a sulfate ester attached to the carbohydrate moiety

Apart from the galactose-containing lipids, a third glycosylglyceride is found in

sulphoquinovosyldiacylglycerol and contains a sulphonate constituent on carbon 6 of a deoxyglucose residue This sulphonic acid group is very stable and also highly acidic so the plant sulpholipid is a negatively charged molecule in nature Although this sulpholipid occurs in small amounts in photosynthetic bacteria and some fungi, it is really characteristic of the photosynthetic membranes of chloroplasts and cyanobacteria

Table 4 Some glycosylglycerides found in bacteria

Figure 5 Glycosylacylglycerols of plant and algal photosynthetic membranes

3.1.2.1.2 Inositol phospholipid

Inositol phosphates are important intracellular second messengers in eukaryotic cells Inositol, a cyclic molecule with six hydroxyl groups, forms the hydrophobic head group of membrane inositol phospholipids The actions of lipid kinases and phosphatases on the inositol ring of inositol phospholipids generate a variety of phosphoinositol phospholipids As a result of

upstream signaling events, specific phospholipases are activated, cleave phosphoinositol phospholipids, and release intracellular inositol phosphate second messenger molecules

The most studied inositol phosphate second messenger is inositol 1,4,5- triphosphate (IP3) IP3 is released from phosphatidylinositol 4,5-bisphosphate (PIP2) by the action of phospholipase C-β and is responsible for the release of calcium ions from intracellular stores in the endoplasmic reticulum

The principal secondary messenger diacylglycerol can also give rise to additional messengers It can either be phosphorylated to phosphatidate or hydrolyzed by a lipase Because phosphoinositides usually have a high concentration of arachidonate at their sn-2 position, lipase action will cause a significant rise in arachidonic acid levels The latter can then be converted to eicosanoids (prostaglandins, thromboxanes, and leukotrienes)

3.1.2.1.3 Sphingolipid

Sphingolipids (SLs) are ubiquitous components of eukaryotic cell membranes and are found in species as diverse as fungi to mammals, and even in some bacteria and viruses (Merrill, 2008) Interest in SLs has blossomed over the past couple of decades due to two major discoveries, namely that in addition to their well-established roles as structural components of cell membranes, SLs also ‘turnover’ in a number of cellular signaling pathways (Hannun and Obeid, 2008; Maceyka and Spiegel, 2014), and are essential components of the so-called ‘membrane-rafts’ (Simons and Gerl, 2010) These two findings have revolutionized SL biology and stimulated research directions that could not have been foreseen 20 years ago The structural complexity of SLs is huge, with thousands of possible structures varying in the three major structural regions of the SL molecule, the sphingoid LCB, the fatty acid, and the head group

Figure 6 Ceramide, the basic building block of sphingolipids

First, is the sphingoid LCB, which can differ in length, stereochemistry, degree of saturation, branching, hydroxylation, and structure of the head group moiety The most common sphingoid LCB is d-erythrosphingosine containing 18 carbon atoms, 16 of which are donated from

palmitoyl-CoA and two from serine However, the length of the LCB can be as short as 14 carbons,

in d14:0 and d14:1-sphinganine/sphingosine, or more commonly in mammalian cells, C16- or C20-sphingosine In terms of stereochemistry, the sphingoid LCB contains two chiral carbon atoms, at carbons 2 and 3 Natural SLs occur in the d-erythro (2S, 3R) configuration, but three additional stereoisomers exist, l-erythro (2R, 3S) (the enantiomer of d-erythro-), d-threo (2R, 3R) and l-threo (2S, 3S) In the below figure, Panel (A) shows a selection of LCBs, and panel (B) shows the stereochemistry of the LCB

In some lower organisms, such as Caenorhabditis elegans, the sphingoid LCB is branched

at the 19th position Hydroxylation of the LCB, in, for instance, the C6 position (in the skin) or at C4 (in yeast), is also common Finally, the use of amino acids other than serine can produce sphingoid LCBs known as deoxysphingosines, desoxysphingosines, and others

The existence of such a wide range of LCB structures has resulted in unexpected insight into some human diseases Thus, although the generic structure of the sphingoid LCB has been known for decades, the many subtle yet important variations in LCB structure have proved to be

of unexpected biological relevance

Figure 7 The variety of LCB structures found in sphingolipids

Figure 8 Structures of deoxysphingosine and deoxy methyl sphingosine

3.1.2.2 Glycolipid

The basic structure of a glycolipid consists of a mono- or oligosaccharide group attached

to a sphingolipid or a glycerol group (can be acetylated or alkylated) with one or two fatty acids These make up the classes of glycosphingolipids and glycoglycerolipids, respectively Glycolipids interact and bind to the lipid-bilayer through the hydrophobic nature of the lipid tail which anchors

it to the surface of the plasma membrane

Figure 9 Glycolipid structure

Synthesis of glycolipids proceeds by a series of enzymes that sequentially add sugars to the lipid Glycosphingolipids are derived from lactosylceramide (LacCer; β-D-galactosyl(1→4)-β-D-glucosyl-ceramide) where the first step is the acylation and desaturation of D-erythro-sphinganine Ceramide is glucosylated then β-galactosylated extracellularly to form lactosylceramide Further elongation can occur via glycosyltransferases and sulfotransferases For example, the biosynthesis of a major glycoglycerolipid in plants involves the transfer of a galactosyl from UDP-Gal onto diacylglycerol to produce β-galactosyldiacylglycerol via galactosyltransferases An additional transfer of a galactosyl from UDP-Gal forms α-D-galactosyl-(1,6)-O-β-D-galactosyldiacylglycerol

These lipids have carbohydrates instead of phosphates in their molecule The most abundant in higher animals are glycosphingolipids, mainly cerebrosides and gangliosides They are amphipathic compounds and are found in cell membranes

3.1.2.2.1 Cerebroside

Cerebrosides are neutral compounds that consist of ceramide (sphingosine and FA) and a monosaccharide bound by a β-glycosidic bond to the C1 of esfingol Often the carbohydrate is galactose (galactocerebroside) The most common FAs are lignoceric and hydroxylignoceric or cerebronic acid, both of which have 24 carbons The cerebrosides containing lignoceric acid are called kerasin, while those having cerebronic acid are known as phrenosin

Figure 10 Schematic representation of a cerebroside

Glucocerebrosides (glucose bound to ceramide) are found in very small proportions in the body, along with galactocerebrosides Cerebrosides are abundant in brain white matter and nerve myelin sheaths and they are present in small quantities within the cell membranes of other tissues

Brain white matter and, to a lesser extent, other tissues, also have lipids that contain sulfur These compounds, formerly called sulfatides, are galactocerebrosides in which the monosaccharide is esterified with sulfate Glycosphingolipids with a more complex carbohydrate portion (di, tri, and tetrasaccharides instead of a monosaccharide) have been identified Compounds of this type containing N-acetyl-galactosamine are called globosides

3.1.2.2.2 Mucolipid (ganglioside)

This is another important group of glycosphingolipids; whose basic structure is similar to that of cerebrosides, but the carbohydrate portion is of greater complexity Linked to the ceramide, they contain an oligosaccharide composed of several hexoses and one to three acetylneuraminic acid (sialic acid) residues

Many types of gangliosides have been recognized that differ in the number of hexoses and sialic acid residues and in the relative position of these residues In virtually all gangliosides, the first hexose residue of the oligosaccharide attached to the ceramide is glucose, then galactose, N-acetyl-galactosamine, and another glucose or galactose are subsequently attached by β-glycosidic bonds Sialic acid is bound to one of the monosaccharides in the chain According to the most commonly used notation, gangliosides are designated with the letter G followed by a subscript indicating the number of sialic acid residues existing in the molecule (GM: mono-, GD: di-, and GT: trisialoganglioside) Another subscript indicates the order of migration of the compound in chromatography Gangliosides are not only a structural component of cell membranes They also play a role as cell markers For example, bacterial toxins, such as those of cholera, tetanus, botulism, and diphtheria selectively bind to specific cell surface gangliosides If the toxin is first incubated with the specific ganglioside and then placed in contact with the cell, the binding site of the toxin is blocked and it cannot bind to the cell, becoming harmless Surface gangliosides also serve as specific binding sites for other molecules, including interferons, which are potent antiviral agents

Figure 11 Schematic representation of a ganglioside

3.1.2.3 Glyceroglycolipid

Figure 12 Glyceroglycolipid structure

Glyceroglycolipid is a sub-group of glycolipids characterized by acetylated or acetylated glycerol with at least one fatty acid as the lipid complex Glyceroglycolipids are often associated with photosynthetic membranes and their functions The subcategories of glyceroglycolipids depend on the carbohydrate attached

non-• Galactolipids: defined by a galactose sugar attached to a glycerol lipid molecule They are

found in chloroplast membranes and are associated with photosynthetic properties

• Sulfolipids: have a sulfur-containing functional group in the sugar moiety attached to a

lipid An important group is the sulfoquinovosyl diacylglycerols which are associated with the sulfur cycle in plants

3.1.2.4 Lipoprotein

Lipids are carried through the blood circulation in association with proteins, which allows them to be dispersed in the aqueous plasma medium There are different types of plasma lipoproteins, which vary depending on the amount and composition of lipids that they have In the lipoprotein complex, the hydrophobic lipids (triacylglycerol and cholesterol esters) are located in the interior of the molecule, and the polar components (proteins, complex lipids, and free cholesterol) are arranged on the surface Lipoproteins are also among the compounds that make

up mitochondria, microsomes, and myelin membranes

Plasma lipoproteins are spherical microemulsions consisting of a neutral lipid core surrounded by a monolayer of phospholipids (PLs) and various proteins (termed apolipoproteins) The neutral lipid core is composed mainly of triglycerides (TAG) and cholesteryl esters (CE), and the monolayer of PL is embedded with cholesterol Under normal physiological conditions, the composition and the number of lipids and apolipoproteins in each lipoprotein particle vary significantly

Figure 13 Schematic structures of CM, VLDL, and LDL

The lipoproteins in human plasma were initially named and characterized based on their density as determined by gradient ultracentrifugation, with high-density lipoproteins (HDLs) representing the smallest and most protein-rich of these particles In contrast to lower density lipoproteins, the concentration of cholesterol in the plasma HDL fraction was found to correlate with protection against coronary heart disease

Table 5 Lipid composition of Lipoprotein classes

Lipoproteins that contain apolipoprotein (apo) B are termed apo-B containing lipoproteins With the exception of HDL, all plasma lipoproteins (VLDL, LDL, IDL, and CM) are apoB-containing lipoproteins The primary function of apoB-containing lipoproteins is to transport hydrophobic lipids, in the plasma through circulation, from the sites of synthesis to target tissues where they are used for various cellular functions The metabolism of fatty acids and esters, phospholipids, and cholesterol needs to be integrated among tissues through the transport of these hydrophobic components in the blood compartment

Lipoproteins usually contain several apolipoproteins, the exception being LDL which contains only apoB100 ApoB100 and apoB48 do not transfer between lipoprotein particles and are therefore termed ‘non-exchangeable’ In contrast, the other apolipoproteins (apoA-I, A-II, A-

IV, AV, C-I, C-II, C-III, D, E, etc.) move between lipoprotein particles as they are metabolized

When produced in excess, or when removal mechanisms are impaired, apo-B containing lipoproteins can accumulate in the plasma This represents an increased risk of developing cardiovascular diseases, including myocardial infarction, ischaemic stroke, and peripheral vascular diseases Clinically, all the circulating apoB-containing lipoproteins have been shown to be risk factors for the development of atherosclerosis Therefore, understanding the mechanisms that govern the production and clearance of apoB-containing lipoproteins is the subject of considerable medical interest Modulation of the circulating levels of apoB-containing lipoproteins can be achieved by many dietary and pharmaceutical interventions, which affect either the production or the clearance of these particles Synthesis of apoB is achieved through protein translation mechanisms similar to those for other secretory proteins Translation of the apoB messenger RNA (mRNA) occurs on the rough endoplasmic reticulum (ER), where the nascent polypeptide chain is translocated into the ER lumen During and immediately after apoB translation, various lipid constituents of CM and VLDL, mainly TG, are recruited to assemble into a primordial lipoprotein particle The intestinal enterocytes synthesize CM that carries TAG (and fat-soluble compounds such as vitamins) of dietary origin On the other hand, the liver synthesizes VLDL that carry TG derived from de novo lipogenesis and adipogenesis Thus, the metabolism of CM and VLDL are termed ‘exogenous’ and ‘endogenous’ pathways, respectively, to reflect the different origins of triacylglycerol

Both CM and VLDL are metabolized in the circulation, where lipids, primarily TG, are removed by hydrolysis to deliver fatty acids to tissue sites However, the apoB protein remains in association with the particle from which it originates until it is removed from the circulation Here

we describe how the apoB lipoproteins are assembled and secreted, with particular emphasis on recent observations that point to novel cell biological processes that regulate the assembly and secretion of apoB-containing lipoproteins

3.2 Classification by biological role

Lipids are a diverse and ubiquitous group of organic compounds that are essential for several biological functions, such as acting as structural components of cell membranes, serving

as energy storage sources, and participating in signaling pathways They are heterogeneous compounds insoluble in water but soluble in organic solvents like chloroform Biological membranes are organized assemblies of lipids and proteins with some amount of carbohydrates Lipids are mainly classified into storage lipids and membrane lipids Fats, oils, and waxes belong

to the category of storage lipids Membrane lipids include glycerophospholipids, sphingolipids, and cholesterol Fatty acids are the components of the storage and membrane lipids

3.2.1 Storage lipid

Fatty acids are the components of storage lipids Fatty acids are mainly classified into saturated (no double bond) and unsaturated fatty acids (double bond present) Some of the polyunsaturated fatty acids are called essential fatty acids They are linoleic acid, linolenic acid and arachidonic acid Fats are the esters of fatty acids with glycerol as alcohol Simple fats contain the same kinds of fatty acids in all three positions while mixed fats contain two or more different fatty acids Oils are fats but liquid at room temperature Waxes are the esters of fatty acids with long-chain mono hydroxyl alcohol

3.2.2 Membrane lipid

Membrane lipids are a group of compounds (structurally similar to fats and oils) that form the double-layered surface of all cells (lipid bilayer) The three major classes of membrane lipids are phospholipids, glycolipids, and cholesterol Lipids are amphiphilic which means they have a hydrophilic (soluble in water) or polar end and a hydrophobic (soluble in fat, insoluble in water)

or nonpolar end and that causes them to form bilayers spontaneously in aqueous environments By forming a double layer with the polar ends pointing outwards and the nonpolar ends pointing inwards membrane lipids can form a 'lipid bilayer' which keeps the watery interior of the cell separate from the watery exterior The arrangements of lipids and various proteins, acting as receptors and channel pores in the membrane, control the entry and exit of other molecules and ions as part of the cell's metabolism In order to perform physiological functions, membrane proteins are facilitated to rotate and diffuse laterally in a two-dimensional expanse of lipid bilayer

by the presence of a shell of lipids closely attached to the protein surface, called an annular lipid shell

Glycerophospholipids, sphingolipids, and cholesterol are membrane lipids Glycerophospholipids contains glycerol as the alcohol, fatty acids, a phosphate group, and a nitrogenous or non-nitrogenous group Lecithin, cardiolipin, and plasmalogens are some of the