Introduction
The term lipids is used to refer to a group of organic compounds that are naturally occurring and that are all soluble in non-organic solvents such as benzene , chloroform, acetone and ether but not soluble in water. The later characteristic means that lipids can also be defined as amphiphilic or hydrophobic small modules. This amphophilic nature of lipids essentially allows them to be able to form structures such as liposome’s, vesicles or membranes when subjected to aqueous environments. The biological lipids mainly originate from two major build-blocks or biochemical sub-units: isoprene and ketoacyl groups. Another distinct characteristic of lipids is that they contain a greasy, oily, waxy pr fatty texture.
Liquids are hydrocarbons and as such, they contain carbon and hydrogen in their structural composition. When these hydrocarbons are metabolized, they generally undergo oxidation to release ernomous amounts of energy and this is in fact on their functions in the human body as will be seen later. So where do lipids come from? Lipid can be found from a variety of sources. The most common sources are however plants and animals. Since they are soluble in polar solvents such as acetone, chloroform and benzene, these solvents can be used to extract the lipids from the plant and animal sources. Lipids can also be found in processed foods such as salad dressings, butter and cooking oils.
Sometimes, the word ‘lipid’ is used as a synonym for fats. However, it should be noted that fats are actually a sub-group of lipids that are known as triglycerides. Therefore, the two should not be confused. Although, the human body uses the diverse biosynthetic pathways to both synthesize and breakdown lipids, there are some essential lipids that are incapable of being manufactured this way and must therefore be gotten from the diet.
Classification of Lipids
Although they share some universal properties and characteristics, there are some lipids that however display unique feature, properties or characteristics. For instance, lipids do not display a similar molecular structure. Therefore, it is only logical that lipids are subdivided into several classifications based on these unique characteristics and properties to enable easier study of them. L
Lipids can be classified into 3 main groups and these are:
- Triglycerides
- Steroids
- Phospholipids
Triglycerides
A triglyceride is essentially an ester that is derived from the combination of a glycerol molecule and three fatty acid molecules. The triglycerides are of varying types and depending on their oil source, some of them are highly unsaturated while other are only minimally unsaturated. Saturation means that all available bonding spaces on each carbon atoms at filled with hydrogen atoms. Unsaturated compounds usually have a double bond existing between two carbon atoms which therefore lead to unsaturation.
As mentioned earlier, triglycerides are usually formed by the combination of one molecule of glycerol and three molecules of fatty acids. In alcohols, there is the presence of a hydroxyl or a OH group. Organic acids on the other hand have a carboxyl or a COOH group. The organic acid and the alcohol combine to form an ester. The glycerol group usually has a three HO- groups while each fatty acid has a COOH or a carboxyl group. In triglycerides, the glycerol’s hydroxyl groups join the fatty acids carboxyl group to form ester bonds. This can be represented by the equation below.
HOCH2CH(OH)CH2OH + RCO2H + R'CO2H + R''CO2H → RCO2CH2CH(O2CR')CH2CO2R'' + 3H2O
The diagram below is also another representation of the formation of a triglyceride where 3 fatty acids combine with a glycerol unit to form a triglyceride.
In many occasions, the three fatty acids are different therefore giving rise to different forms of triglycerides. The fatty acids chain lights usually vary in the naturally occurring triglycerides but in many cases, the number of carbon atom usually range between 20, 28 and 16. The fatty acids commonly found in animals and plants are typically only composed of an even number of carbon atoms. Examples of fatty acids that often combine glycerol to form triglycerides include stearic acids, oleic acids and palmitic acids.
Bacteria however have the ability to produce or synthesis odd numbered fatty acids. Therefore, some animal fats such as that of ruminants has fatty acids with an odd number of carbon atoms due to bacteria action. This bacterium is usually found in their rumen. Natural fats contain complex mixtures of triglycerides. As result, they exhibit varying melting points. Triglycerides are essentially the human body’s major energy storage form. Their huge presence in the human blood can be attributed to eating. It is noted that the levels of triglycerides rise dramatically after consuming simple foods such as sugars and alcohols. If these sugars are not used up immediately by the body as energy, they are often converted to triglycerides and then stored in the body fat. Hormones often regulated the release of triglycerides depending on the body’s energy needs. For instance, fasting will tend to stimulate this release of triglycerides. Together with cholesterol, triglycerides are part of the blood lipids carried in the human bloodstream and then into the arteries.
Phospholipids
This is the second major class of lipids that are usually major components of cell membranes. The phospholipids are essentially comprised of a glycerol molecule or unit, 2 fatty acids, a polar molecule and a phosphate group. This way, it can be seen that a phospholipids is closely similar to a triglyceride only that here, one of the triglyceride’s fatty acids is substituted with a phosphate group.
The phosphate groups together with the polar head of the molecule are hydrophilic or are attracted to water. On the other hand, the tail that is mainly the fatty acid is hydrophobic and is therefore repelled by water. When a phospholipids is placed in water, it will tend to orient itself into 2 layers or a bilayer whereby the fatty acid (non polar) tail will face the bilayer inner are. Consequently, the polar head region will face outward and will therefore interact with the water.
The diagram below represents the behaviour of phospholipids in water.
As mentioned earlier, phospholipids are often the cell membrane’s major components and therefore enclose the cell’s cytoplasm together with its other component. The phospholipids in the cell membrane behave in a similar manner like they would when immersed in water. They coalesce into a lipid bilayer whereby the hydrophilic head arranges itself spontaneously to face the cytosol and the extracellular fluids both which are aqueous. On the other hand, the hydrophobic tail will face away from the aqueous cytosol and the extracellular fluid. This lipid bilayer in the membrane exhibits semi-permeability characteristics, and consequently only allows only specific molecule types to diffuse though the cell membrane into and out of the cell.
- Phospholipids are of varying types and are often classified according to the type of alcohol that they possess. Some of the most common phospholipids include
- Glycerophasphatides (phosphatidic acid, phosphatidylchli or lecithin, and phosphoatdyethanolamine)
- Phospho-inositides (phosphotidylinositol phosphate, phosphotidylinositol biophosphate)
- Phospho-sphingocides
Steroids
This is another important class of lipids. The steroids are significantly different from the other major classes of lipids. Steroids are mostly recognizable through their tetracyclic skeleton. This skeleton usually consists of one five numbered ring and three fused six membered ring.
Some common examples of steroids include sex hormones like testosterone and estradiol, cholesterol which is a dietary fat and dexamethasone, an antiflammatory drug.
The steroid core is essentially composed of 17 carbon atoms that are bonded to together to form a structure of four fused rings comprised of the three cyclone rings mentioned above and one cyclopentane ring. There is a wide variation to this core of four rings and the oxidation state of the rings also varies. Below is the basic structure of the steroid skeleton
In the steroid skeleton, methyl groups can be found at carbons C13 and C10. An alkyl side may also be located at C17.
The simplest steroid is known as gonane and is usually made up of a total of 17 carbon atoms that have been effectively bonded together to form four rings that are fused together.
There are hundreds of distinct steroids that can be found in animals, plants and steroids. In animals, steroids are usually made in the cells, mainly from lanestrol in animals and fungi from cycloartenol in plants. Below is the diagrammatic representation of the formation of lanestrol.
However, one feature that is common among all steroids and that therefore makes them similar is that they possess a significantly huge number of carbon-hydrogens that therefore make all steroids non –polar, that is they tend to repel water.
The most famous as well as most abundant steroid the human body is actually cholesterol. This steroid is usually formed in the nerve tissue, blood stream and the nerve tissues. Cholesterol is one of the major compounds in bile salts as well as the galls stones. Excess cholesterol in the body is however harmful as it leads to the formation of deposits in the blood vessels’ inner walls therefore obstructing the flow of blood which may lead to several blood conditions such as strokes and hypertension.
Testosterone is a sex hormone most common in men and is also a steroid. This hormone is primarily responsible for the secondary sex characteristics development.
Apart from the three major classes of lipids that have been discussed above, that is triglycerol, phospholipids and steroids, there are still other classes or classifications of lipids in existence today. These other classifications are the glycolipids, lipoproteins 9 which are catrually combinations of lipids and proteins that are most commonly found in the cell membranes and finally the waxes.
The role of lipids in the Human Body and the Immune system
The role of lipids in the human body cannot be overemphasized. Different types of lipids play differing roles in the Human body system.
Triglycerols in addition to acting as energy reserves for the body has other functions. The subcutaneous layer in which the triglycerol is deposited in the human body function as a form of heat insulation.
Phospholipids as mentioned previously are the major components of the cell membranes. They strengthen the membrane and thus enable the cells to retain their contents and distinguish them from the external environment. Fats and oils which are both lipids contain almost twice as much energy (per unit weight) as that contained by proteins or carbohydrates. Lipids enable the storage of this energy in the human cells so that it can be broken down in case of emergencies or when the body requires the extra energy.
Some lipids especially those in form of steroids act as body hormones. The most common of these is testosterone which is a major hormone in male humans and that is responsible for the regulation of male characteristics developments. Other steroids also bring a huge benefit to the human body since they help in the determination and control of plasma membrane structure. There are also other lipids that function as vitamins, which an extremely important nutrient constituent responsible for the prevention of a wide variety of diseases in humans. Another form of lipids in the human body is the lipophilic bile juices that aid in the lipid solubilization in the human body. Wax is another form of lipids founds in humans particularly in the ear. The wax helps in the trapping of dust particles and also insects that may be making their way into the ear and that nay potentially damage the eardrum. Therefore, their main role in the human body is the role of protection.
As seen, above, there are very many types of lipids that exist. Each class of lipids has unique characteristics that distinguish it from the other. In addition, the lipids have varying roles in the human body.
References
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