A monomer is a single molecule of any compound but is most commonly associated with large organic molecules.
Biomolecules can be extremely massive, including hundreds to thousands of distinct molecules. To make things easier, they’re classified as monomers, which are repeating units of smaller molecules.
BIOMOLECULES AND THEIR RESPECTIVE MONOMERS:
|Carbohydrates||Monosaccharides (C:H:O) in the ratio 1:2:1|
|Lipids||Fatty acids + glycerol (C:H:O)in the ratio greater than 2:1 H:O (carboxyl group)|
|Nucleic acids||Nucleotides (CHONP) pentose(sugar)+nitrogenous base+ phosphate|
|Proteins||Amino acids (CHON) −NH2 + −COOH +R group|
Some common monomer examples are listed below:
Monosaccharides (Carbohydrate monomers):
Unlike most other molecules carbohydrates have a large variety of monomers as they come in a variety of forms. These monomers can be differentiated on whether they have ketose groups or aldose groups or if their chain has 5C or 6C atoms (called pentoses and hexoses respectively).
- GLUCOSE: The simplest and most abundantly found hexose sugar. Glucose is the monomer for most commonly known and studied carbohydrate polymers like- starch, cellulose and glycogen.
- GALACTOSE: Although not as commonly known it is one of the most components of the lactose disaccharide which is the main sugar present in milk.
- FRUCTOSE: Fructoses are the monomers of all fruit sugars that naturally make fruit taste sweet and tart.
- DEXTROSE: Dextrose is another hexose sugar that is a component of honey.
Some carbohydrate monomers can also be disaccharides i.e when the monomer itself is made up of 2 sugars itself.
Amino acids(Protein monomers):
Protein monomers are called amino acids- meaning an acid having an amine group. NH2-C(R)-COOH is how we commonly represent amino acids where the amine group and the COOH group are attached to the same carbon atom called the α(alpha)C. The R is any group attached to the C atom and the nature of the amino acids depends on how long or short the R group is.
The human body requires a total of 20 amino acids, which are employed in protein synthesis. They can be classified according to the R group, which refers to the presence of a side chain.
- Aliphatic side chains: When the amino acid side chain contains only H and C in them. These include Glycine, Alanine, Valine, Leucine, Isoleucine and Proline.
- Neutral side chains: These amino acids do not have any polarizing capability due to the presence of alcohol sid-chains. Hence they do not ionize easily. Eg. Serine and Threonine.
- Amide side chains: Asparagine and Glutamine are two such amino av=cids that have an amide group or -NH2 in their side chain.
- Sulfurated side chains: Amino acids that have -S- in their side chains. Eg, Cysteine and Methionine.
- Aromatic side chains: These amino acids have side chain aromatic rings. They include Phenylalanine, Tyrosine and Tryptophan.
- Anionic side chains: These amino acids due to the presence of Carboxylic groups in their side chains are anions at ordinary pH and hence act as Bronsted Bases. These are Aspartate and Glutamate.
- Cationic side chains: Some amino acids like Histidine, Lysine and Arginine contain side chains that are cationic at neutral pH.
Fatty Acids (Lipid monomers):
Carboxylic acids with saturated or unsaturated aliphatic chains are called fatty acids. These are the molecules that combine to form lipids or what we commonly call fats. They are mainly based on length or usually based on saturation as it is more health-related.
Based on the length of the aliphatic chain they can be classified as:
- Short-chain fatty acids or SCFA for short are those with aliphatic tails of five or fewer carbon atoms (e.g. butyric acid).
- MCFA or Medium-chain fatty acids have aliphatic tails of 6 to 12 carbon atoms. They can form medium-chain triglycerides.
- Long-chain fatty acids or LCFA are fatty acids with aliphatic tails of 13 to 21 carbon atoms.
- Very long-chain fatty acids or VLCFA these fatty acids have aliphatic tails of 22 carbon atoms or more.
Based on the presence or absence of hydrolysable bonds they can be also be classified into:
- Saturated fatty acids: This means that they do not have any C=C bonds or Carbon-Carbon double bonds in their aliphatic chains. They have the same chemical formula of CH3-(CH2)n -COOH with variation in the mummer represented by “n”.
- Unsaturated fatty acids: The aliphatic chain of these fatty acids has one or more C=C linkages. Unsaturated fatty acids are classed as cis or trans depending on whether the two H atoms close to the double bond protrude on the same or opposite sides of the bond.
The most biologically significant fatty acids include Palmitoleic acid, Oleic acid, Linoleic acid, Arachidonic acid etc.
Nucleotides (Nucleic Acid monomers):
Nucleic acid monomers are called nucleotides. They are composed of 2 main parts, namely- the nucleoside and a phosphate group. The nucleoside part of the monomer consists of 2 different parts- a pentose sugar and a nitrogen base. These bases are of 2 types- purine and pyrimidine. Purine bases include Adenine and Guanine. Pyrimidine bases include Cytosine, Thymine and Uracil.
Nucleoside= Nitrogen base+ pentose sugar
Nucletide = Nuceloside+phosphate group
There are 2 main nucleic acids- DNA and RNA that can be differentiated based on their sugars or the nitrogen bases in their nucleotide.
COMPARISON BETWEEN DNA AND RNA NUCLEOTIDES:
|DEOXYRIBONUCLEIC ACID(DNA)||RIBONUCLEIC ACID(RNA)|
|Pentose sugar in DNA is deoxyribose||Pentose sugar in RNA is ribose|
|Nitrogen bases are Adenine, Guanine, Cytosine and Thymine.||Nitrogen bases are Adenine, Guanine, Cytosine and Uracil|
All biomolecules that are present in living systems are composed of a chain or coagulation of monomeric units. This makes it easier for the molecule to be broken down and return to its smallest atomic form after the organism dies. This also makes the biomolecule to be more easily bioavailable i.e it increases their ability to be absorbed by living organisms and systems easier.
So all biomolecules are composed of their specific type of monomers that differ in chemical and structural nature which also determines the polymer’s nature. So technically monomers are the building units of the large biomolecules. The monomers come together to make carbohydrates, proteins, lipids, and nucleic acids which are the key physiologically relevant substances in nature.