{"id":160,"date":"2019-04-10T10:58:36","date_gmt":"2019-04-10T10:58:36","guid":{"rendered":"http:\/\/biocp03.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=160"},"modified":"2019-04-12T05:49:30","modified_gmt":"2019-04-12T05:49:30","slug":"lipids-an-introduction","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/biocp03\/chapter\/lipids-an-introduction\/","title":{"rendered":"Lipids: An introduction"},"content":{"raw":"
    \r\n \t
  1. Objectives<\/strong><\/li>\r\n<\/ol>\r\nTo understand the general importance of lipids in biology and Biotechnology<\/em><\/strong>\r\n
      \r\n \t
    1. Concept Map<\/strong><\/li>\r\n<\/ol>\r\n\"\"\r\n
        \r\n \t
      1. Description<\/strong><\/li>\r\n<\/ol>\r\n

        For quite sometime lipids were considered not as exciting a class of biological molecules as proteins and nucleic acids! Nucleic acids were genetic material and so they attracted the attention of mainstream biochemists even prior to rDNA era.<\/p>\r\n \r\n

        Enzymes were proteins and as catalysis of all the reactions in living organisms is carried out by enzymes, they were the focus of microbiologists, biochemists and many others.<\/p>\r\n \r\n

        Lipids were of interest to nutritionists. Now ofcourse the general population is also interested in fats\/oils and \u1f60-fatty acids in view of massive advertisement by oil industry in the media.<\/p>\r\n \r\n

        The role of lipids goes beyond nutritional aspects. Let us look at a brief overview and we will cover their various biological roles in this set of modules exclusively devoted to lipids.<\/p>\r\n \r\n

        3.1 Definition<\/strong><\/p>\r\n \r\n

        All the classes of biological molecules discussed so far had a structural basis. Carbohydrates are sugars or their dimmers\/oligomers or polymers. Proteins and nucleic acids are made up of amino acids and nucleotides as monomers respectively.<\/p>\r\n \r\n

        Lipids differ in their structures widely. As we will see, structure of a fatty acid has little resemblance to a cholesterol molecule. Both nevertheless are classified as lipids.<\/p>\r\n \r\n

        To start with, both show higher solubility in organic solvents than in water. So, it is a physical property which was responsible for their being classified together as lipids.<\/p>\r\n \r\n

        Interesting enough, later work showed that fortuitously, this turned out to be a judicious choice.<\/p>\r\n \r\n

        Cholesterol and lipoproteins have a close relationship in metabolism. Look at the clinical reports on the blood of the people. You would find triglyceride content, cholesterol, LDL and HDL all mentioned together. So, it turns out that biologically, it is not a bad idea to classify these different kinds of molecules as lipids.<\/p>\r\n\"\"\r\n

        Historically, the class of lipids emerged as any compound which was part of a fraction which was extracted from a biological source (e.g.: animal or plant) by a so called \u201cfat solvent\u201d or non-polar solvent (e.g.: benzene, ether, chloroform or petroleum ether etc.)<\/p>\r\n \r\n\r\nSo, the basis for compounds being classified together is operational rather than structural.\r\n\r\n\"\"\"\"\"\"\"\"\r\n\r\nThe simplest sub-classes of lipids are fatty acids and lipids which are derivatives of glycerol.\r\n\r\n \r\n

          \r\n \t
        1. Fatty Acids<\/strong><\/li>\r\n<\/ol>\r\n\"\"\r\n\r\n \r\n\r\nII. Lipids which are derivatives of glycerol<\/strong>\r\n\r\n \r\n\r\n\"\"\r\n

          Glycerol is a trihydric alcohol. It is used as moisturizer\/solvent and commonly called glycerin.<\/p>\r\n \r\n

          Glycerol is freely soluble in water, esterifying its alcoholic groups with long chain fatty acids turns this into fats\/oils.<\/p>\r\n \r\n

          During soap making, a process which has been called saponification for a long time, glycerol is produced as a by product.<\/p>\r\n \r\n

          Currently, most of the industries which were major suppliers of glycerol in the world are shut down, it is mostly available as a by product during biodiesel production!<\/p>\r\n \r\n\r\n(A) <\/strong>Neutral Fats<\/strong>\r\n

            \r\n \t
          1. Mono-, Di-, Triglycerides<\/strong><\/li>\r\n<\/ol>\r\n
              \r\n \t
            1. Glyceryl Ethers<\/strong><\/li>\r\n<\/ol>\r\n
                \r\n \t
              1. Glycosyl Glycerides<\/strong><\/li>\r\n<\/ol>\r\n

                \"\"<\/p>\r\n

                Neutral fats are actually what we generally call fats or oils. Structurally, both fats and oils are identical.<\/p>\r\n \r\n

                If triglycerides are present as a free flowing liquid, these are called oils. If present as solid, we call these fats.<\/p>\r\n

                While something like lard will be solid at wide range of temperatures which will be prevalent as room temperature, in a country like India where temperature vary widely in different seasons, this may be confusing.<\/p>\r\n \r\n

                In Delhi winter, \u201ccoconut oil\u201d is generally solid! In other seasons, it may be semi solid. As we will learn in a later module, there is a process in oleochemical industry which is called \u201cwinterization\u201d. Nevertheless, we should not get confused.<\/p>\r\n\"\"\r\n

                While trimesters of glycerol, fats and oils are well known compounds, glycerol also forms ethers. Here are examples of one of its terminal C-atom as a part of ether linkage.<\/p>\r\n \r\n

                As the other two \u2013OH groups are free in these compounds, these compounds are called alcohols.<\/p>\r\n \r\n

                The long chain R of the other component of the ether glycerol \u2013OR ensures poor solubility of these compounds in water and solubility in organic solvents. Hence glycerol ethers are lipids.<\/p>\r\n \r\n\r\n(B) <\/strong>Phospholipids<\/strong>\r\n

                  \r\n \t
                1. Phosphatides or glycerophospholipids<\/strong><\/li>\r\n<\/ol>\r\n

                  The nitrogeneous compounds choline, ethanolamine, L-serine etc. containing the alcohol group forms ester bonds with the free acidic group of the phosphoric acid of the L-\u03b1-phosphatidic acid.<\/strong><\/p>\r\n \r\n\r\nThese are now generally called glycerophospholipids.<\/strong>\r\n

                  \"\"<\/p>\r\n

                  The isomer is called L-form because of its stereochemical relationship with L-glyceryl phosphate<\/p>\r\n \r\n

                  One way to look at phosphatide structure is as monophosphate ester of 1,2 diglycerides.<\/p>\r\n \r\n

                  When the phosphate groups form another ester bond with these nitrogeneous bases containing alcohol, many important phospholipids are formed. As we will see in a later module, glycophospholipids are the phospholipids which are generally present in biological membranes.<\/p>\r\n\r\n

                    \r\n \t
                  1. Phosphoglycerides<\/strong><\/li>\r\n<\/ol>\r\n

                    These are derivatives of glycerol phosphate and do not contain nitrogen. All these are, as we just learnt, also called phosphatidic acids.<\/p>\r\n \r\n

                    The common phosphoglycerides present in animals and plants are diphosphatidyl glycerols and mono phosphatidyl glycerols respectively.<\/p>\r\n \r\n

                    So, It is necessary to realise that phosphatidic acid is not a single component but a class of phosphoglycerides, different members have different fatty acids esterifying the \u2013OH on the glycerol chain.<\/p>\r\n\"\"\r\n

                      \r\n \t
                    1. Phosphoinositides<\/strong><\/li>\r\n<\/ol>\r\n

                      These are also derivatives of \u03b1-phosphatidic acid without nitrogen but contains inositol.<\/p>\r\n

                      Inositol is a sugar alcohol.<\/p>\r\n

                      Myo-inositol is the best known member of this group as it is widely distributed among micro-organisms, plants and animals, often as a phosphorylated form. In animals, myo-inositol also occurs in free form in tissues like muscle, heart, lung and liver.<\/strong><\/p>\r\n\"\"\r\n\r\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Figure 7: Phosphoinositides<\/em><\/strong>\r\n\r\n \r\n

                      Just to clarify again, currently phosphotides, phosphoglycerides and phosphoinositides are all considered members of a bigger class glycerophospholipids.<\/p>\r\n \r\n

                      Glycerophospholipids are also excellent surfactants. Dipalmitoyl phosphatidyl choline (DPCC), as we will see is an important lung surfactant required for good pulmonary performances.<\/p>\r\n \r\n\r\nIII. Sphingolipids<\/strong>\r\n

                        \r\n \t
                      1. Lipids containing sphingosine or dihydrosphingosine are called sphingolipids<\/strong><\/li>\r\n<\/ol>\r\n\"\"\r\n\r\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0Figure 8B: Dihydrophingosine<\/em><\/strong>\r\n

                        Sphingolipids are also part of membranes.<\/p>\r\n \r\n

                        In most sphingolipids, sphingosine, the C18 amino alcohol has trans configuration around its single double bond.<\/p>\r\n \r\n

                        In dihydrosphingosine, ofcourse, this double bond is abolished.<\/p>\r\n \r\n

                        Sphingosine has two \u2013OH groups and one \u2013NH2 group as main functional groups.<\/p>\r\n \r\n\r\nIn fact the biosynthesis of sphingosine is as follows:<\/strong>\r\n\r\n \r\n\r\n\"\"\r\n

                        Apart from these most common C18 sphingosines, less common C16, C17, C19 and C20 sphingosines also occur as a part of some naturally occurring sphingolipids.<\/p>\r\n \r\n\r\n2. Ceramides<\/strong>\r\n\r\n \r\n

                        These are sphingolipids in which free amino group of sphingosine is aylated by a C16, C18, C20 or C22 fatty acid.<\/p>\r\n\"\"\r\n

                        Hence ceramides are essentially N-acylsphingosine. Ceramides differ from each other in this constituent fatty acid.<\/p>\r\n \r\n

                        Ceramides are the basic structural units of sphingolipids Ceramides are part of both sphingomyelin and gangliosides<\/p>\r\n \r\n\r\n3. Sphingomyelins<\/strong>\r\n\r\n\"\"\r\n

                        Sphingomyelins are important constituents of brain lipids and are also found in blood.<\/p>\r\n \r\n

                        Plasma membrane of the animal cells contain sphingomyelins.<\/p>\r\n \r\n\r\nSphingomyelins are phosphodiesters of a ceramide and choline.\r\n\r\n \r\n

                        Cerebrosides, on the other hand, are ceramides which can be called glycosphingolipids. The common cerebrosides are galactocerebroside and glucocerebroside. Here, the galactose or glucose is linked to the terminal \u2013OH group of sphingosine unit of ceramides.<\/p>\r\n \r\n

                        Please note that cerebrosides do not contain phosphate group and hence are neither phospholipids nor ionic in nature!<\/p>\r\n \r\n

                        So, sphingomyelins are phospholipids. Ceramides and cerebrosides are not!<\/p>\r\n

                        \"\"<\/p>\r\nSome important cerabrosides are:<\/strong>\r\n\r\n \r\n

                        Kerasin:\u00a0\u00a0 Contains saturated C24 lignoceric acid in the ceramide part.<\/p>\r\n \r\n

                        Phrenosin: Contains cerabronic acid (2-hydroxy derivative of lignoceric acid) instead.<\/p>\r\n \r\n

                        Sulfatides: These are group of S-containing cerabrosides. Often, S is present as sulfoesters in the sugar part.<\/p>\r\n \r\n\r\n \r\n

                        Ceramide oligosaccharide: Contains more than one sugar residue and hence are called cerabroside disaccharide etc. Some examples are Ceramide lactoside and cytolipin K (has a tetrasaccharide moiety).<\/p>\r\n\"\"\r\n\r\nGangliosides<\/strong>\r\n\r\n \r\n

                        As mentioned earlier, gangliosides are also formed from ceramides.<\/strong><\/p>\r\n\"\"\r\n

                        Please note that just like cerebrosides and gangliosides do not contain phosphorous. So, these are not phospholipids but glycolipids.<\/p>\r\n \r\n

                        Most complex sphingolipids are also ceramide oligosaccharides but deserve to constitute a class of its own. The oligosaccharide chain in gangliosides contains an acidic sugar called sialic acid or N-acetyl neuraminate.<\/p>\r\n

                        Myelin sheath are membranes wrapped around each nerve axon. About 10-15 layers thick, it has high lipid content and hence an electrical insulator.<\/p>\r\n \r\n

                        Myelin sheath is rich in sphingomyelins. The name sphingomyelin, in fact, owes its origin to this fact.<\/p>\r\n\"\"\r\n

                        The so called grey matter in the brain is rich in gangliosides.<\/p>\r\n \r\n

                        Tay Sach disease, generally fatal for infants is an inherited disorder associated with the deficient hydrolase which breaks down ganglioside GH2<\/p>\r\n \r\n\r\nWaxes<\/strong>\r\n\r\n \r\n

                        Waxes are esters of long chain fatty acids with long chain alcohols\u00a0So, while fats\/oils are esters of glycerol, waxes contain long chain alcohols such as cetyl alcohol.<\/p>\r\n\"\"\r\n

                        Other important classes of lipids are terpenes and steroids.<\/p>\r\n \r\n

                        Many essential oils are terpenoids. Vitamin A, the visual pigment and many plant pigments are terpenoids.<\/p>\r\n

                        Steroids are compounds having Perhydrocyclopentanophenanthrene fused ring system.<\/p>\r\n\"\"\r\n

                        Cholesterol, bile acids and sex hormones are important examples of steroids.<\/p>\r\n \r\n\r\nSummary<\/strong>\r\n\r\n \r\n

                          \r\n \t
                        • All compounds soluble in organic solvents and mostly insoluble in aqueous buffers are called lipids<\/li>\r\n \t
                        • Structurally, this is a very diverse class<\/li>\r\n \t
                        • This, as we shall see, results in lipids having diverse roles and functions in biological systems<\/li>\r\n<\/ul>\r\nBroadly we have gained preliminary knowledge about:\r\n
                            \r\n \t
                          • Fats\/oils<\/li>\r\n \t
                          • Phospholipids<\/li>\r\n \t
                          • Sphingolipids<\/li>\r\n \t
                          • Waxes, terpenes and steroids<\/li>\r\n<\/ul>\r\n

                            Lipids also combine with other classes of biological compounds to form lipoproteins and lipopolysaccharides.<\/p>\r\n \r\n\r\n \r\n\r\n ","rendered":"

                              \n
                            1. Objectives<\/strong><\/li>\n<\/ol>\n

                              To understand the general importance of lipids in biology and Biotechnology<\/em><\/strong><\/p>\n

                                \n
                              1. Concept Map<\/strong><\/li>\n<\/ol>\n

                                \"\"<\/p>\n

                                  \n
                                1. Description<\/strong><\/li>\n<\/ol>\n

                                  For quite sometime lipids were considered not as exciting a class of biological molecules as proteins and nucleic acids! Nucleic acids were genetic material and so they attracted the attention of mainstream biochemists even prior to rDNA era.<\/p>\n

                                   <\/p>\n

                                  Enzymes were proteins and as catalysis of all the reactions in living organisms is carried out by enzymes, they were the focus of microbiologists, biochemists and many others.<\/p>\n

                                   <\/p>\n

                                  Lipids were of interest to nutritionists. Now ofcourse the general population is also interested in fats\/oils and \u1f60-fatty acids in view of massive advertisement by oil industry in the media.<\/p>\n

                                   <\/p>\n

                                  The role of lipids goes beyond nutritional aspects. Let us look at a brief overview and we will cover their various biological roles in this set of modules exclusively devoted to lipids.<\/p>\n

                                   <\/p>\n

                                  3.1 Definition<\/strong><\/p>\n

                                   <\/p>\n

                                  All the classes of biological molecules discussed so far had a structural basis. Carbohydrates are sugars or their dimmers\/oligomers or polymers. Proteins and nucleic acids are made up of amino acids and nucleotides as monomers respectively.<\/p>\n

                                   <\/p>\n

                                  Lipids differ in their structures widely. As we will see, structure of a fatty acid has little resemblance to a cholesterol molecule. Both nevertheless are classified as lipids.<\/p>\n

                                   <\/p>\n

                                  To start with, both show higher solubility in organic solvents than in water. So, it is a physical property which was responsible for their being classified together as lipids.<\/p>\n

                                   <\/p>\n

                                  Interesting enough, later work showed that fortuitously, this turned out to be a judicious choice.<\/p>\n

                                   <\/p>\n

                                  Cholesterol and lipoproteins have a close relationship in metabolism. Look at the clinical reports on the blood of the people. You would find triglyceride content, cholesterol, LDL and HDL all mentioned together. So, it turns out that biologically, it is not a bad idea to classify these different kinds of molecules as lipids.<\/p>\n

                                  \"\"<\/p>\n

                                  Historically, the class of lipids emerged as any compound which was part of a fraction which was extracted from a biological source (e.g.: animal or plant) by a so called \u201cfat solvent\u201d or non-polar solvent (e.g.: benzene, ether, chloroform or petroleum ether etc.)<\/p>\n

                                   <\/p>\n

                                  So, the basis for compounds being classified together is operational rather than structural.<\/p>\n

                                  \"\"\"\"\"\"\"\"<\/p>\n

                                  The simplest sub-classes of lipids are fatty acids and lipids which are derivatives of glycerol.<\/p>\n

                                   <\/p>\n

                                    \n
                                  1. Fatty Acids<\/strong><\/li>\n<\/ol>\n

                                    \"\"<\/p>\n

                                     <\/p>\n

                                    II. Lipids which are derivatives of glycerol<\/strong><\/p>\n

                                     <\/p>\n

                                    \"\"<\/p>\n

                                    Glycerol is a trihydric alcohol. It is used as moisturizer\/solvent and commonly called glycerin.<\/p>\n

                                     <\/p>\n

                                    Glycerol is freely soluble in water, esterifying its alcoholic groups with long chain fatty acids turns this into fats\/oils.<\/p>\n

                                     <\/p>\n

                                    During soap making, a process which has been called saponification for a long time, glycerol is produced as a by product.<\/p>\n

                                     <\/p>\n

                                    Currently, most of the industries which were major suppliers of glycerol in the world are shut down, it is mostly available as a by product during biodiesel production!<\/p>\n

                                     <\/p>\n

                                    (A) <\/strong>Neutral Fats<\/strong><\/p>\n

                                      \n
                                    1. Mono-, Di-, Triglycerides<\/strong><\/li>\n<\/ol>\n
                                        \n
                                      1. Glyceryl Ethers<\/strong><\/li>\n<\/ol>\n
                                          \n
                                        1. Glycosyl Glycerides<\/strong><\/li>\n<\/ol>\n

                                          \"\"<\/p>\n

                                          Neutral fats are actually what we generally call fats or oils. Structurally, both fats and oils are identical.<\/p>\n

                                           <\/p>\n

                                          If triglycerides are present as a free flowing liquid, these are called oils. If present as solid, we call these fats.<\/p>\n

                                          While something like lard will be solid at wide range of temperatures which will be prevalent as room temperature, in a country like India where temperature vary widely in different seasons, this may be confusing.<\/p>\n

                                           <\/p>\n

                                          In Delhi winter, \u201ccoconut oil\u201d is generally solid! In other seasons, it may be semi solid. As we will learn in a later module, there is a process in oleochemical industry which is called \u201cwinterization\u201d. Nevertheless, we should not get confused.<\/p>\n

                                          \"\"<\/p>\n

                                          While trimesters of glycerol, fats and oils are well known compounds, glycerol also forms ethers. Here are examples of one of its terminal C-atom as a part of ether linkage.<\/p>\n

                                           <\/p>\n

                                          As the other two \u2013OH groups are free in these compounds, these compounds are called alcohols.<\/p>\n

                                           <\/p>\n

                                          The long chain R of the other component of the ether glycerol \u2013OR ensures poor solubility of these compounds in water and solubility in organic solvents. Hence glycerol ethers are lipids.<\/p>\n

                                           <\/p>\n

                                          (B) <\/strong>Phospholipids<\/strong><\/p>\n

                                            \n
                                          1. Phosphatides or glycerophospholipids<\/strong><\/li>\n<\/ol>\n

                                            The nitrogeneous compounds choline, ethanolamine, L-serine etc. containing the alcohol group forms ester bonds with the free acidic group of the phosphoric acid of the L-\u03b1-phosphatidic acid.<\/strong><\/p>\n

                                             <\/p>\n

                                            These are now generally called glycerophospholipids.<\/strong><\/p>\n

                                            \"\"<\/p>\n

                                            The isomer is called L-form because of its stereochemical relationship with L-glyceryl phosphate<\/p>\n

                                             <\/p>\n

                                            One way to look at phosphatide structure is as monophosphate ester of 1,2 diglycerides.<\/p>\n

                                             <\/p>\n

                                            When the phosphate groups form another ester bond with these nitrogeneous bases containing alcohol, many important phospholipids are formed. As we will see in a later module, glycophospholipids are the phospholipids which are generally present in biological membranes.<\/p>\n

                                              \n
                                            1. Phosphoglycerides<\/strong><\/li>\n<\/ol>\n

                                              These are derivatives of glycerol phosphate and do not contain nitrogen. All these are, as we just learnt, also called phosphatidic acids.<\/p>\n

                                               <\/p>\n

                                              The common phosphoglycerides present in animals and plants are diphosphatidyl glycerols and mono phosphatidyl glycerols respectively.<\/p>\n

                                               <\/p>\n

                                              So, It is necessary to realise that phosphatidic acid is not a single component but a class of phosphoglycerides, different members have different fatty acids esterifying the \u2013OH on the glycerol chain.<\/p>\n

                                              \"\"<\/p>\n

                                                \n
                                              1. Phosphoinositides<\/strong><\/li>\n<\/ol>\n

                                                These are also derivatives of \u03b1-phosphatidic acid without nitrogen but contains inositol.<\/p>\n

                                                Inositol is a sugar alcohol.<\/p>\n

                                                Myo-inositol is the best known member of this group as it is widely distributed among micro-organisms, plants and animals, often as a phosphorylated form. In animals, myo-inositol also occurs in free form in tissues like muscle, heart, lung and liver.<\/strong><\/p>\n

                                                \"\"<\/p>\n

                                                \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Figure 7: Phosphoinositides<\/em><\/strong><\/p>\n

                                                 <\/p>\n

                                                Just to clarify again, currently phosphotides, phosphoglycerides and phosphoinositides are all considered members of a bigger class glycerophospholipids.<\/p>\n

                                                 <\/p>\n

                                                Glycerophospholipids are also excellent surfactants. Dipalmitoyl phosphatidyl choline (DPCC), as we will see is an important lung surfactant required for good pulmonary performances.<\/p>\n

                                                 <\/p>\n

                                                III. Sphingolipids<\/strong><\/p>\n

                                                  \n
                                                1. Lipids containing sphingosine or dihydrosphingosine are called sphingolipids<\/strong><\/li>\n<\/ol>\n

                                                  \"\"<\/p>\n

                                                  \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0Figure 8B: Dihydrophingosine<\/em><\/strong><\/p>\n

                                                  Sphingolipids are also part of membranes.<\/p>\n

                                                   <\/p>\n

                                                  In most sphingolipids, sphingosine, the C18 amino alcohol has trans configuration around its single double bond.<\/p>\n

                                                   <\/p>\n

                                                  In dihydrosphingosine, ofcourse, this double bond is abolished.<\/p>\n

                                                   <\/p>\n

                                                  Sphingosine has two \u2013OH groups and one \u2013NH2 group as main functional groups.<\/p>\n

                                                   <\/p>\n

                                                  In fact the biosynthesis of sphingosine is as follows:<\/strong><\/p>\n

                                                   <\/p>\n

                                                  \"\"<\/p>\n

                                                  Apart from these most common C18 sphingosines, less common C16, C17, C19 and C20 sphingosines also occur as a part of some naturally occurring sphingolipids.<\/p>\n

                                                   <\/p>\n

                                                  2. Ceramides<\/strong><\/p>\n

                                                   <\/p>\n

                                                  These are sphingolipids in which free amino group of sphingosine is aylated by a C16, C18, C20 or C22 fatty acid.<\/p>\n

                                                  \"\"<\/p>\n

                                                  Hence ceramides are essentially N-acylsphingosine. Ceramides differ from each other in this constituent fatty acid.<\/p>\n

                                                   <\/p>\n

                                                  Ceramides are the basic structural units of sphingolipids Ceramides are part of both sphingomyelin and gangliosides<\/p>\n

                                                   <\/p>\n

                                                  3. Sphingomyelins<\/strong><\/p>\n

                                                  \"\"<\/p>\n

                                                  Sphingomyelins are important constituents of brain lipids and are also found in blood.<\/p>\n

                                                   <\/p>\n

                                                  Plasma membrane of the animal cells contain sphingomyelins.<\/p>\n

                                                   <\/p>\n

                                                  Sphingomyelins are phosphodiesters of a ceramide and choline.<\/p>\n

                                                   <\/p>\n

                                                  Cerebrosides, on the other hand, are ceramides which can be called glycosphingolipids. The common cerebrosides are galactocerebroside and glucocerebroside. Here, the galactose or glucose is linked to the terminal \u2013OH group of sphingosine unit of ceramides.<\/p>\n

                                                   <\/p>\n

                                                  Please note that cerebrosides do not contain phosphate group and hence are neither phospholipids nor ionic in nature!<\/p>\n

                                                   <\/p>\n

                                                  So, sphingomyelins are phospholipids. Ceramides and cerebrosides are not!<\/p>\n

                                                  \"\"<\/p>\n

                                                  Some important cerabrosides are:<\/strong><\/p>\n

                                                   <\/p>\n

                                                  Kerasin:\u00a0\u00a0 Contains saturated C24 lignoceric acid in the ceramide part.<\/p>\n

                                                   <\/p>\n

                                                  Phrenosin: Contains cerabronic acid (2-hydroxy derivative of lignoceric acid) instead.<\/p>\n

                                                   <\/p>\n

                                                  Sulfatides: These are group of S-containing cerabrosides. Often, S is present as sulfoesters in the sugar part.<\/p>\n

                                                   <\/p>\n

                                                   <\/p>\n

                                                  Ceramide oligosaccharide: Contains more than one sugar residue and hence are called cerabroside disaccharide etc. Some examples are Ceramide lactoside and cytolipin K (has a tetrasaccharide moiety).<\/p>\n

                                                  \"\"<\/p>\n

                                                  Gangliosides<\/strong><\/p>\n

                                                   <\/p>\n

                                                  As mentioned earlier, gangliosides are also formed from ceramides.<\/strong><\/p>\n

                                                  \"\"<\/p>\n

                                                  Please note that just like cerebrosides and gangliosides do not contain phosphorous. So, these are not phospholipids but glycolipids.<\/p>\n

                                                   <\/p>\n

                                                  Most complex sphingolipids are also ceramide oligosaccharides but deserve to constitute a class of its own. The oligosaccharide chain in gangliosides contains an acidic sugar called sialic acid or N-acetyl neuraminate.<\/p>\n

                                                  Myelin sheath are membranes wrapped around each nerve axon. About 10-15 layers thick, it has high lipid content and hence an electrical insulator.<\/p>\n

                                                   <\/p>\n

                                                  Myelin sheath is rich in sphingomyelins. The name sphingomyelin, in fact, owes its origin to this fact.<\/p>\n

                                                  \"\"<\/p>\n

                                                  The so called grey matter in the brain is rich in gangliosides.<\/p>\n

                                                   <\/p>\n

                                                  Tay Sach disease, generally fatal for infants is an inherited disorder associated with the deficient hydrolase which breaks down ganglioside GH2<\/p>\n

                                                   <\/p>\n

                                                  Waxes<\/strong><\/p>\n

                                                   <\/p>\n

                                                  Waxes are esters of long chain fatty acids with long chain alcohols\u00a0So, while fats\/oils are esters of glycerol, waxes contain long chain alcohols such as cetyl alcohol.<\/p>\n

                                                  \"\"<\/p>\n

                                                  Other important classes of lipids are terpenes and steroids.<\/p>\n

                                                   <\/p>\n

                                                  Many essential oils are terpenoids. Vitamin A, the visual pigment and many plant pigments are terpenoids.<\/p>\n

                                                  Steroids are compounds having Perhydrocyclopentanophenanthrene fused ring system.<\/p>\n

                                                  \"\"<\/p>\n

                                                  Cholesterol, bile acids and sex hormones are important examples of steroids.<\/p>\n

                                                   <\/p>\n

                                                  Summary<\/strong><\/p>\n

                                                   <\/p>\n

                                                    \n
                                                  • All compounds soluble in organic solvents and mostly insoluble in aqueous buffers are called lipids<\/li>\n
                                                  • Structurally, this is a very diverse class<\/li>\n
                                                  • This, as we shall see, results in lipids having diverse roles and functions in biological systems<\/li>\n<\/ul>\n

                                                    Broadly we have gained preliminary knowledge about:<\/p>\n

                                                      \n
                                                    • Fats\/oils<\/li>\n
                                                    • Phospholipids<\/li>\n
                                                    • Sphingolipids<\/li>\n
                                                    • Waxes, terpenes and steroids<\/li>\n<\/ul>\n

                                                      Lipids also combine with other classes of biological compounds to form lipoproteins and lipopolysaccharides.<\/p>\n

                                                       <\/p>\n

                                                       <\/p>\n

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