BIOC2600 1.5

• Lipids are a chemically diverse group of compounds that are insoluble in water.
• Fats and oils are major stored forms of energy in many organisms.
• Phospholipids and steroids are major structural elements of biological membranes.
• Other lipids are present in relatively small quantities and play crucial roles as enzyme cofactors, electron carriers, emulsifying agents in the digestive tract, hormone and intracellular messengers.
• Fats and oils universally used as stored forms of energy in living organisms are derivatives of fatty acids.
• Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons long (C 4 to C 36 ).
• In some fatty acids, the chain is unbranched and fully saturated (contains no double bonds).
• Others contain one or more double bonds.
• The properties of fatty acids depend on the length of the hydrocarbon chain and the degree of saturation (number of double bonds in the hydrocarbon chain).
• Saturated fatty acids have no more hydrogens that can be added to the molecule.
• Triacylglycerols with 3 saturated fatty acids are called saturated lipids.
• Unsaturated fatty acids are fatty acids with one or more double bonds.
• Glycosphingolipids are determinants of blood groups.
• Gangliosides have complex oligosaccharides as polar head groups and one or more residues of N-acetylneuraminic acid at the termini.
• The lipid composition of membranes varies by organisms, tissues, and organelles, with a ratio of lipid to protein, type of phospholipid, and abundance and type of sterols varying.
• Cholesterol is the major sterol in animal tissues, amphipatic with a polar head and a nonpolar body, and modulates the fluidity of animal cell membrane.
• Integral membrane proteins are embedded within the lipid bilayer, either monotopic (interacting with one leaflet of the bilayer) or polytopic (the polypeptide chain transverses the membrane).
• Peripheral membrane proteins associate with the membrane through electrostatic interactions and H-bonds with domains of integral proteins and membrane lipids.
• Biological membranes are complex lipid-based structures forming pliable sheets, composed of a variety of lipids and proteins, and define cellular boundaries, divide cells into compartments, organise complex reaction sequences, and play a role in signal reception and energy transformation.
• Sterols are structural lipids present in membranes of most eukaryotic cells, with a steroid nucleus consisting of 4 fused carbon rings, 3 with 6 carbons, 1 with 5 carbons.
• Amphitropic proteins associate reversibly with membranes and can be found in both in the membrane and in the cytosol.
• Steroids derived from cholesterol are biologically active compounds.
• Membrane bilayers are asymmetric, with two leaflets having different lipid compositions, and the outer leaflet often more positively charged.
• Seals, walruses, penguins and other polar mammals are padded with triacylglycerols.
• Cerebrosides have a single sugar linked to ceramide.
• Globosides have 2 or more sugars, usually glucose, galactose or N - acetyl - D - galactosamine.
• Biological membranes are made of a double layer of lipids.
• Sphingolipids contain one molecule of sphingosine or derivative, one molecule of fatty acid, and a polar head joined by a glycosidic or phosphodiester linkage.
• Sphingolipids are subdivided into sphingomyelins, which contain phosphocholine or phosphoethanolamine, and are present in the plasma membrane of animal cells, and especially prominent in myelin sheath that insulates the axons of some neurons.
• C1, C2 and C3 of sphingosine molecule is analogous to the 3 carbons of glycerol in glycerophospholipid.
• Membrane lipids are amphipathic, with one end hydrophilic and one end hydrophobic.
• Glycosphingolipids are found largely in the outer surface of the membrane and have head groups with one or more sugars connected directly to C1 of the ceramide.
• The hydrophobic interactions with each other and hydrophilic interactions with water make them pack into sheets called membrane bilayer.
• Glycerophospholipids are named as derivatives of the parent compound, phosphatidic acid, according to the polar alcohol in the head group.
• Sphingolipids also have a polar head group and two nonpolar tails, but they contain no glycerol.
• Glycerophospholipids are membrane lipids with 2 fatty acids linked to C1 and C2 of glycerol and a highly polar or charged group linked to C3 via a phosphodiester linkage.
• The positions of any double bond in fatty acids are specified by Δ followed by superscript numbers.
• The position of double bonds in fatty acids can be cis or trans.
• Nearly all naturally occurring unsaturated fatty acids are in the cis-configuration.
• Trans fatty acids are obtained from dairy products and meat, and are also produced during hydrogenation of fish or vegetable oils.