Categories Of Lipids

Lipids are a group of naturally occurring molecules that include fats and waxes, sterols, fat soluble vitamins such as vitamin A, D, E, K. , mono-glycerides, di-glycerides, triglycerides, phospholipids and others. They are soluble in non polar solvents such as ether, chloroform, but are relatively insoluble in water. They have these properties because they consist mainly of carbons. Even though lipids are hydrophobic, they can also be amphiphilic. Their amphiphilic nature allows them to form structures such as vesicles, liposome, or membranes in an aqueous environment. Although the term lipid is often used as a synonym of fat, fats are in fact a subgroup of lipids called triglycerides. 

Although humans and other mammals use various biosynthetic pathways to break down and synthetise lipids, the truth is that some essential lipids must be obtained from the diet because they cannot be made this way. Some lipids are used for energy storage. Others serve as structural components of cell membranes and some are important hormones.

Categories of Lipids

Fatty acids: These are diverse group of molecules that are long chains of lipid-carboxylic acid found in fats and oils and in cell membranes as a component of phospholipids and glycolipids. (Carboxylic acid is an organic acid containing the functional group -COOH.).Fatty acids are the building blocks of the fat in our bodies and in the food we eat. During digestion, the body breaks down fats into fatty acids, which can then be absorbed into the blood. Fatty acid molecules are usually joined together in groups of three, forming a molecule called a triglyceride. Triglycerides are also made in our bodies from the carbohydrates that we eat.

Fatty acids are made up of carbon and hydrogen molecules. There are three types of fatty acids: saturated, monounsaturated, and polyunsaturated. The basic difference between each of these is the number of carbon atoms with or without two hydrogen atoms bonded to them. Here's the difference.

Saturated fatty acids. In a saturated fatty acid, each carbon atom has bonded with two hydrogen atoms. In other words, it's "saturated" with hydrogen. This saturation makes the fatty acid very stable, which means it can withstand more heat before it becomes rancid. Common examples of saturated fats are butter and coconut oil. An easy way to know if a fat is saturated is if it's solid at room temperature. Saturated fats are ideal for cooking because of their natural ability to withstand heat without being damaged.

Monounsaturated fatty acids. In a monounsaturated fatty acid, one pair of carbon atoms forms a double bond with each other that replaces the bond each would have with one hydrogen atom. So it is unsaturated, but only by one bond. This means the fatty acid is less stable than a saturated fatty acid molecule. The classic example of monounsaturated fat is olive oil. The oil in almonds, hazelnuts, and avocados is also monounsaturated. Monounsaturated fats are liquid at room temperature, but they solidify in the refrigerator. These are okay for cooking, but only at very low temperatures.

Polyunsaturated fatty acids. A polyunsaturated fatty acid has two or more carbon pairs that have bonded together rather than with a hydrogen atom. This means the fatty acid is quite unstable. Examples of polyunsaturated fats include most vegetable oils, most seed oils, soybean oil, flaxseed oil, and sunflower oil. These fats are liquid both at room temperature and in the refrigerator, and they should never be used for cooking. This might be surprising, given that so many of our standard cooking oils are exactly these unstable vegetable oils.
There is a fourth type of fatty acid, but I haven't
Fatty acids come from animal and vegetable fats and oils. Fatty acids play roles outside the body; they are used as lubricants, in cooking and food engineering, and in the production of soaps, detergents, and cosmetics.

Related terms include the following:

Essential fatty acid: An essential fatty acid is a polyunsaturated fatty acid needed by the body that is synthesized by plants but not by the human body and is therefore a dietary requirement.
Free fatty acids: By-products of the metabolism of fat in adipose tissues.
Omega-3 fatty acids: Omega-3 fatty acids are a class of fatty acids found in fish oils, especially in salmon and other cold-water fish, that lowers the levels of cholesterol and LDL (low-density lipoproteins) in the blood. (LDL cholesterol is the "bad" cholesterol.)
Trans fatty acid: Trans fatty acids (trans fats) are made through hydrogenation to solidify liquid oils. They increase the shelf life of oils and are found in vegetable shortenings and in some margarines, crackers, cookies, and snack foods. Intake of trans fatty acids increases blood LDL-cholesterol ("bad" cholesterol) levels and raises the risk of coronary heart disease.

Glycerolipids: The most well known glycerolipids are the fatty acid trimesters of glycerol called triglycerides. The term glycerides are sometimes used as a synonym of tri-glycerol. The triglycerides contain no hydroxyl group unlike the tri-glycerol, rather the three hydroxyl groups of the glycerol are each esterified typically by different fatty acids. These lipids comprise the bulk of storage fat in animal tissue.

Glycerophospholipids: Glycerophospholipids are derivatives of sn‐glycero‐3‐phosphoric acid. They contain an O ‐acyl or O ‐alkyl or O ‐alk‐1′‐enyl residue at the sn‐1 position and an O ‐acyl residue at the sn‐2 position of the glycerol moiety and are defined on the basis of the substituents on the phosphoric acid at the sn‐3 position. Glycerophospholipids are asymmetrically distributed between the two bilayers membranes, which also contain cholesterol and proteins. Glycerophospholipids not only constitute the backbone of cellular membranes, but also provide the membrane with a suitable environment, fluidity and ion permeability. They are synthesised at the endoplasmic reticulum and are transported to other membranous structures by phospholipid exchange and transfer proteins. Once glycerophospholipids are laid down in a biomembrane, they undergo interconversion reactions. These reactions and activities of phospholipases may be responsible for the turnover, compositional maintenance and rearrangements of glycerophospholipids in membranes. This process results in the modulation of membrane function. Glycerophospholipids are precursors for lipid mediators, which play important roles in internal and external communication and modulate cellular responses. In addition, glycerophospholipids and their lipid mediators may be involved in membrane fusion, apoptosis and regulation of the activities of membrane‐bound enzymes and ion channels.

Sphingolipids: These are a complicated family of compounds that share a common structural feature, a sphingolipid base backbone that is synthetised from the amino acid serine and a long chain fatty acylcoA , then converted into ceramides, phosphosphingolipids and other compounds. The major sphingolipids of mammals are shingomyelins.s phingolipid, any member of a class of lipids (fat-soluble constituents of living cells) containing the organic aliphatic amino alcohol sphingosine or a substance structurally similar to it. Among the most simple sphingolipids are the ceramides (sphingosine plus a fatty acid), widely distributed in small amounts in plant and animal tissues. The other sphingolipids are derivatives of ceramides.
Glycolipids , a large group of sphingolipids, are so called because they contain one or more molecules of sugar (glucose or galactose). Glycolipids, a general property of which is immunological activity, include the
cerebrosides, gangliosides, and ceramide oligosaccharides. Of limited distribution in nature, cerebrosides are most abundant in the
myelin sheath surrounding nerves. Sulfate-containing cerebrosides, known as sulfatides, occur in the white matter of brain. Gangliosides , most abundant in nerve tissue (especially the gray matter of brain) and certain other tissues ( e.g., spleen) are similar to cerebrosides except that, in addition to the sugar component, they contain several other molecules of carbohydrate ( N-acetylglucosamine or N-acetylgalactosamine and N-acetylneuramine). Ceramide oligosaccharides also contain several molecules of carbohydrate; an example is globoside from red blood cells.
Sphingomyelins, which are the only phosphorus-containing sphingolipids, are most abundant in nervous tissue, but they also occur in the blood.

Sterol lipids: These are important components of membrane lipids and they include cholesterol and its derivatives. The steroids, all derived from the same fused four ring core structure have different biological roles as hormones and signaling molecules.Sterols may be found either as free sterols, acylated (sterol esters ), alkylated (steryl alkyl ethers), sulfated (sterol sulfate ), or linked to a glycoside moiety ( steryl glycosides) which can be itself acylated ( acylated sterol glycosides ).
Sterol biosynthesis is nearly ubiquitous among eukaryotes, it is almost completely absent in prokaryotes. They are not found in Archaea and the proven occurrences in bacteria are sparsely distributed and yield a limited array of products. The proteobacterium Methylococcus capsulatus and the planctomycete Gemmata obscuriglobus and some members of the myxobacteria are proven steroid-producing bacteria. As a result, the presence of diverse steranes (saturated 4-cycle skeleton) in ancient rocks is used as evidence for eukaryotic evolution 2.7 billion years ago.

Structure of sterols with carbon numbers
Sterols are derived from the same squalene precursor as hopanoids but, in marked contrast, they are known to have an oxygen-dependent biosynthesis beginning with the formation of the first intermediate, 2,3- oxidosqualene. There is a close connection between modern-day biosynthesis of particular triterpenoid biomarkers and presence of molecular oxygen in the environment. Thus, the detection of steroid and triterpenoid hydrocarbons far back in Earth history has been used to infer the antiquity of oxygenic photosynthesis. It has been hypothesized that increased levels of O2 in the atmosphere not only made the evolution of sterols possible, but that these sterols may in turn have facilitated the birth of complex organisms (the eukaryotes) likely in providing them with an early defense mechanism against O2 

Sterols form an important group among the
steroids. Unsaturated steroids with most of the skeleton of cholestane containing a 3b-hydroxyl group and an aliphatic side chain of 8 or more carbon atoms attached to position 17 form the group of sterols.
They are lipids resistant to saponification and are found in an appreciable quantity in all animal and vegetal tissues. Furthermore, cholestane may be considered as a biological marker compound valuable in the assessment of marine sediment maturity, even after hundreds of millions of years. Sterols may include one or more of a variety of molecules belonging to 3-hydroxysteroids, they are C27-C30 crystalline alcohols (in Greek,
stereos , solid). These lipids can be classed also as triterpenes , as they derive from squalene which gives directly by cyclization, unsaturation and 3b-hydroxylation, lanosterol in animals or cycloartenol in plants.
In the tissues of vertebrates, the main sterol is the C27 alcohol cholesterol (Greek, chole, bile), particularly abundant in adrenals (10%, w/w), nervous tissues (2%,w/w), liver (0.2%,w/w) and gall stones. The vertebrate brain is the most cholesterol-rich organ, containing roughly 25% of the total free cholesterol present in the whole body. Its fundamental carbon structure is a cyclopentanoperhydrophenanthrene ring (also called sterane). 

Cholesterol is found in high concentrations in animal cell membranes, typical concentrations (expressed as molar percentage of total lipids) being about 30 mol%, ranging up to 50mol% in red blood cells and as high as 80 mol% in the ocular lens membranes. Consequently, cholesterol has numerous functions in membranes ranging from metabolism, as a precursor to hormones and vitamins, to providing mechanical strength and a control of the phase behavior of membrane. It became clear that the key role of cholesterol in the lateral organization of membranes and its free volume distribution seems to be involved in controlling membrane protein activity and "raft" formation .At the cellular level, cholesterol may be replaced to some extent by some other sterols with minor modifications of the side chain. Cholesterol is abundant in the femoral gland of the male lizard Acanthodactylus boskianus which uses it as a scent marking pheromone to establish dominance hierachies.
In addition to these roles, cholesterol can form ester linkages with a class of secreted polypeptide signaling molecules encoded by the hedgehog gene family. These proteins function in several patterning events during metazoan development.
Sponges, a primitive group of multicellular organisms (Poriphera), represent the richest source of bizarre sterols found in nature. Most sponges have the general sterol structure found in animals, plants, and fungi., i.e. cholesterol and sterols, but bearing one to three extra carbon atoms at C24. These side chains have been isolated with such unusual features as quaternary alkyl groups, cyclopropane and cyclopropene rings, allenes, and even acetylenes
24-Isopropylcholesterol is abundant and characteristic (with its analogue unsaturated at C22-C23) of the class Demospongiae. This sterol is absent in "true animals", the eumetazoans (cnidarians and bilaterian animals).
This demosponge sterane is abundant in sediment dating from the Neoproterozoic era (1,000-542 million years) and is the oldest evidence for animals in the fossil record.
Among the large list of sterols with cyclopropane ring, Nicasterol was identified in a Demospongiae,
Calyx nicaensis .

While cholesterol was considered to be nearly absent in vegetal organisms, its presence is now largely accepted in higher plants. It can be detected in vegetal oils in a small proportion (up to 5% of the total sterols) but remains frequently present in trace amounts. An unusual relatively high content of cholesterol was described in
camelina oil (about 200 mg per kg). However, several studies have revealed the existence of cholesterol as a major component sterol in chloroplasts, shoots and pollens. Furthermore, cholesterol has been detected as one of the major sterols in the surface lipids of higher plant leaves (rape) where he may amount to about 72% of the total sterols in that fraction. Cholesterol is also dominant in most all Rhodophyceae algae, it is the only sterol presesnt in Laurencia paniculata.
In late-step synthesis of cholesterol, discrete oxidoreductive and/or demethylation reactions occur, which start with the common precursor
lanosterol . Lanosterol is also found as a major constituent of the unsaponifiable portion of wool fat (lanoline) : about 15%. It has been shown that the bacterium (planctomycete), Gemmata obscuriglobus , is able to synthesize lanosterol and its uncommon isomer, parkerol.