We know that all the living organisms are made from some basic units which repeat themselves to form the complete organism. These basic units are called as cells. You must have seen the image of plant cell and animal cell. All the living cells are basically composed of four chemical elements; C, H, O and H.
There are many chemical compounds which consist of these four elements and involve in biochemical activities of living cells. These chemical compounds or macromolecules which take part in biochemical reactions of living cells are known as biomolecules. Vitamins, enzymes, hormones, carbohydrates, lipid, proteins, nucleic acid are common examples of biomolecules.
Out of these all biomolecules, carbohydrates, lipids, proteins and nucleic acid are most important and studied. Carbohydrates are macromolecules which composed of monosaccharide units. Monosaccharides are polyhydroxy carbonyl compounds which are linked with glycosidic linkage. Carbohydrates are major source of energy for biological activities. Proteins are building blocks of living cells. They are polymers of amino acids. Amino acids are chemical compounds with two functional groups; carboxyl group and amino group. Amino acid molecules are bonded with each other through peptide bonds which form between –COOH group and –NH2 group. Nucleic acids are polymers of nucleotides which consist of phosphate group, sugar molecule and a nitrogenous base.
are not a single compound but a range of different compounds such as fats, oils, phospholipids, and steroids. These all compounds are hydrophobic in nature means water hating compounds. Triacylglycerols which are also known as fats and oils are basically composed of two types of molecules; glycerol and fatty acid. One molecule of glycerol and three molecules of fatty acids form tri-acylglycerol. Some common examples of triacylglycerols are butter, lard oils, corn oil, olive oil and margarine.
Another type of lipids is diacylglycerides which are also called as phospholipids
. They are mainly found as lipid bilayers in the plasma membrane of living cell and also in the membranes within eukaryotic cells. The monomer units of phospholipids are one glycerol, two fatty acids, and a -PO4 group. Unlike triacylglycerol, they are water soluble and hydrophilic due to presence of phosphate group in the molecule. No doubt the long hydrocarbon tail is hydrophobic in nature. Steroids
are other types of lipids. Cholesterol, steroid hormones are good examples of steroids. They are cousin of fats - and are made from lipids. They do have any fatty acid molecule like other two lipids. Like triacylglycerol, they are very hydrophobic and water insoluble in nature.
There are two major functions of fats and oils
. One is to store energy and another is to insulate the body. Compare to carbohydrates, fats are more compact fuels and contains twice of the energy compare to glucose. They can produce twice as many calories when burned. They also insulate the body and nerves. The myelin sheath over nerves is a lipid-rich layer.
Phospholipids are amphipathic in nature means they are both water-loving and water-hating parts in a molecule. These types of lipids form bilayers which shield the tail from water and forms membranes which are called as phospholipid bilayers. Cholesterol helps to maintain membrane fluidity and pliability. Steroids are also found in sex hormones such as testosterone, estrogen and progesterone. Cholesterol circulates in the bloodstream and gets linked with lipo-proteins.
The high density lipoprotein carries blood cholesterol to the liver whereas low density lipoprotein deposits blood cholesterol in the artery walls. Lipids like waxes act as a protective barriers to prevent water loss in living cells. They also protect seeds and nutrients inside fruits. The coating of wax on the surface of leaves forms a cuticle which prevents water loss. The beeswax in honeycombs also store food and protects offspring. It also prevents the dehydration from body surfaces of insects.
and glycerol are main monomers of lipids. Fatty acids are long chain carboxylic acids. The long hydrocarbon chain makes fatty acid different from other carboxylic acids. Since hydrocarbon chain is non-polar whereas carboxyl group is polar in nature therefore fatty acids have two opposite ends.
The carboxyl group of fatty acid is bonded with glycerol through dehydration reaction and form an ester linkage. These carboxylic acids can be saturated or unsaturated which depends on the presence of multiple covalent bonds in the molecules. Saturated fatty acids do not contain any double or triple bonds in the hydrocarbon chain whereas in an unsaturated fatty acid the hydrocarbon chain contains at least one triple or double bond.
The degree of saturation of fatty acid affects the structure and function of lipid. Even the geometry of fatty acids also alters the properties of lipids. The orientation of cis-fatty acids and trans-fatty acids is different. Lipids like waxes contain two monomers; one fatty acid and one alcohol. Both of these monomer units are bonded through an ester linkage. The hydrocarbon chain in the alcohol varies from simple to complex molecule.
So we can say that there are different monomer units for various types of lipids. Fatty acids and glycerol combine to form triacylglycerol whereas presence of phosphate group forms phospholipids. The presence of any polar group as monomer unit in lipids varies the chemical and physical properties of the molecule. Some common examples of fatty acids are listed below.
See the structure of triglyceride. One molecule of glycerol form ester linkage with three molecules of fatty acids. The bonding takes place between the –OH groups of glycerol and –COOH group of fatty acid. The reaction furnishes with the elimination of three molecules of water.
In phospholipids, the polar ends is arranged toward water whereas non-polar part remains soluble in non-aqueous solvents such as the formation of micelle in soaps and detergents.