Lipids Essay – This is one of the best essays on ‘Lipids and Its Classification’ especially written for school and college students.
Essay # 1. Introduction to Lipids:
Lipids are the biomolecules of structural and functional importance. They are the important component of membrane composition and function with a role in long-term energy storage. One gram of fat stores more than twice as much energy as one gram of carbohydrate. Lipids have multiple functions within the body. Phospholipids are important part of cell membranes, which encase each and every cell.
The triglycerides are very important for fat metabolism and energy production. There is recent evidence that lipids are also important as signaling molecules within the body, sometimes serving as a marker for programmed cell death. Finally, some lipids take the form of vitamins, which can be important for mitochondria and creating other compounds. Chemically, they are made up of long chain hydrocarbons and are non-polar in nature; hence they are insoluble in water but soluble in non-polar solvents (Fig. 3.7). This group of molecules include fats and oils, waxes, phospholipids, steroids (like cholesterol), and some other related compounds.
Essay # 2. Classification of Lipids:
Classification of fatty acids is based on the length of the carbon chain (short, medium, or long); the number of double bonds (unsaturated, mono-, or polyunsaturated); and or essentiality in the diet (essential or non-essential). A current designation is based on the position of the end most double bond, counting from the methyl (CH3) carbon, called the omega end.
The most important omega fatty acids are:
Omega 6 – linolein and arachidonic acids and
Omega 3 – linolenic, icosapentaenoic, and docosahexaenoic acids.
(a) Triglycerides (Neutral Fats):
Triglycerides consist of three long hydrocarbon chains known as fatty acids, attached to each other by a molecule called glycerol, are joined to each other by dehydration synthesis. They are the esters of three molecules of fatty acids plus one molecule of glycerol; the fatty acid may be all different. As they include three fatty acids, fats and oils are also known as triglycerides. They are found in adipose tissue, butterfat, lard, suet, fish oils, olive oil, corn oil, etc. Some fats are saturated, while others are unsaturated.
These terms refer to the presence or absence of double bonds in the fatty acids of fats. Saturated fats have no double bonds, whereas unsaturated fats contain one or more such bonds. In general, plant fats are unsaturated and animal fats are saturated. Saturated fats are generally solid at room temperature, while unsaturated fats are typically liquid.
(b) Phospholipids (Phosphatides):
Phospholipids are major components of cell membranes. Like all fats, the hydrocarbon tails of phospholipids do not dissolve in water. However, phosphate groups are polar and soluble in water. The different solubilities of the two ends of phospholipid molecules allow them to form the bilayers that make up the cell membrane (Fig. 3.8). Their structure is similar to that of triglycerides, but they contain only two fatty acids.
The third molecule attached to the glycerol is a phosphatidylcholine molecule (choline is one of the vitamin B). Certain phospholipids also contain inositol (another vitamin B) as phosphatidylinositol, as well as phosphatidylethanolamine, another phospholipid that has several functions, such as being a precursor to choline and acetylcholine. Most common phospholipids are lecithin, found in soybeans and egg yolks in highest concentration. It is also found in brain and other organ tissue.
(c) Sphingolipids:
Sphingolipids are derived from the aliphatic amino alcohol sphingosine. These compounds play important role in signal transmission and cell recognition. They impart the coating on nerve axons and myelin. The sphingolipid is not a derivative of glycerol. The structure of the sphingosine is given below. Once a fatty acid and a phosphate with an attached choline are bonded to the sphingosine, forming sphingomyelin (Fig. 3.9).
(d) Glycolipids:
Glycolipids are complex lipids that contain carbohydrates. Cerebrosides are examples which contain the sphingosine backbone attached to a fatty acid and a carbohydrate. The carbohydrates are most often glucose or galactose. Those that contain several carbohydrates are called gangliosides. Glucocerebroside has the specific function in the cell membranes of macrophages (cells that protect the body by destroying foreign microorganisms). Galactocerebroside is found almost exclusively in the membranes of brain cells.
(e) Steroids:
Steroids, the primary structure in hormones are substances that play important signaling role in the body. The general structure of cholesterol consists of two six-membered rings side-by-side and sharing one side in common, a third six-membered ring off the top corner of the right ring, and a five-membered ring attached to the right side of that.
Cholesterol is the central steroid from which other steroids such as the sex hormones are synthesized. Cholesterol is only found in animal cells. Examples of steroids include estrogen, progesterone, cortisone, aldosterone, testosterone, and Vitamin D. Steroids differ in the functional groups that are attached to the ring’s edges. Cholesterol is the precursor of sex hormones and Vitamin D.
Vitamin D is formed by the action of UV radiations in sunlight on cholesterol molecules that have “risen” to near the surface of the skin. Our cell membranes contain a lot of cholesterol, in between the phospholipids, to keep them fluid.
Bile salts which assist in the digestion of lipids and other non-soluble molecules are synthesized in liver from cholesterol. Hormones are another derivative of steroids. Testosterone and estrogen are the hormones which give males and females their unique physical characteristics.
(f) Lipoproteins:
These are the complexes of lipids with proteins, which act as a means of carrying lipids, including cholesterol in our blood. LDL or low density lipoprotein is the “bad lipid,” being associated with deposition of “cholesterol” on the walls of arteries. HDL or high density lipoprotein is the “good lipid,” being associated with carrying “cholesterol” out of the blood system and is more dense/ compact than LDL. The ratio of these two, LDL: HDL is the blood test currently being used to evaluate our risk of cardiovascular disease.
The different classes of lipoproteins are:
(i) VLDLs (very-low-density lipoproteins) are made in the intestines and the liver to carry fats throughout the body. VLDLs carry mostly triglycerides, about 5-15% of the cholesterol to the body tissues.
(ii) LDLs (low-density lipoproteins) are made by the liver (and possibly by transformation of VLDLs in the blood) and are the primary molecular complexes that carry cholesterol in the blood to the organs and cells.
(iii) HDLs (high-density lipoproteins) pick up already used or unused cholesterol and cholesterol esters and take them back to the liver as part of a recycling process and the most protective form of lipoprotein in preventing buildup of cholesterol. The site of synthesis of HDL is not certain (probably in the liver). Low HDL levels are indications of risk of cardiovascular diseases. It also appears that HDL may be able to collect cholesterol from artery plaque, thus reversing the atherosclerotic process that leads to heart attacks. HDLs deliver cholesterol to the VLDL, converting them to LDL, which has more density; the liver then removes the LDLs from the blood and converts their cholesterol into bile acids, which are then eliminated.
(g) Waxes:
They are composed of esters of long chain fatty acids with alcohol other than glycerol and are of industrial and medicinal importance. Examples are beeswax and lanolin.