By Dr Ananya Mandal, MD
All the basic structures in animals, plants as well as microorganisms are made up primarily of three basic classes of molecules:
These are important for life and metabolism helps organisms in breaking down complex molecules in food to form these basic molecules.
Some of the most common biological polymers are listed in the table below.
| Type of molecule
|| Name of monomer forms
|| Name of polymer forms
|| Examples of polymer forms
| Amino acids
|| Amino acids
|| Proteins (also called polypeptides)
|| Fibrous proteins and globular proteins
|| Starch, glycogen and cellulose
| Nucleic acids
|| DNA and RNA
Metabolism also brings together these basic molecules and help in construction of cells and tissues. This is called anabolism. Many important biochemicals can be joined together to make polymers such as nucleic acids, DNA and RNA that carry genetic information. These macromolecules are essential parts of all living organisms.
Metabolism involves catabolism as well. This is breakdown of complex molecules to use them as source of energy in digestion and utilization of food.
Important molecules formed from the basic building blocks include:
- Amino acids – Proteins or polypeptides, fibrous proteins or globular proteins etc.
- Carbohydrates (Monosaccharides and polysaccharides) - Starch, glycogen and cellulose
- Fatty acids – Lipids, Lipoproteins etc.
- Nucleic acids – Polynucleotides, DNA and RNA
Proteins are made of amino acids. During the process of protein formation the amino acids are linked in a long chain called polypeptide chains. These are joined together by peptide bonds. The polypeptide chains undergo further modification to form proteins.
Some proteins form structure of the cells and tissues. Many proteins are enzymes that catalyze the chemical reactions in metabolism. Proteins are also important in cell signalling, immune responses, cell adhesion, active transport across membranes, and the cell cycle.
Carbohydrates provide the basic source of energy in the body. Carbohydrates are straight-chain aldehydes or ketones with many hydroxyl groups that can exist as straight chains or rings.
Carbohydrates exist most abundantly in nature and play several roles in living organisms. They can be converted to glycogens and used as storage sources of energy as structural components (cellulose in plants, chitin in animals) and as direct source of energy (glucose).
Lipids are important biochemicals that have a versatile function in the body. They form the structural part of the biological membranes, such as the cell membrane, or are used as a source of energy.
The fats are a large group of compounds that contain fatty acids and glycerol. Steroids such as cholesterol are another major class of lipids that are made in cells.
Nucleotides help in the formation of DNA and RNA. DNA and RNA are long chains of nucleotides. These molecules are critical for the storage and use of genetic information. RNA and DNA also code for protein synthesis.
RNA has several other functions in the cell as well. Nucleotides also act as coenzymes in metabolic group transfer reactions.
Coenzymes are forms of proteins that mediate several chemical reactions in the metabolic pathways of the body. These fall under a few basic types of reactions that involve the transfer of functional groups.
Coenzymes help in the transfer of energy as well. One central coenzyme is adenosine triphosphate (ATP), the universal energy currency of cells. There is only a small amount of ATP in cells, but it is continuously regenerated. Others include Nicotinamide adenine dinucleotide (NADH), a derivative of vitamin B3 that acts as a hydrogen acceptor.
Hundreds of separate types of dehydrogenases remove electrons from their substrates and reduce NAD+ into NADH. This reduced form of the coenzyme is then a substrate for any of the reductases in the cell that need to reduce their substrates. NADH exists in two related forms in the cell, NADH and NADPH. The NAD+/NADH form is more important in catabolic reactions, while NADP+/NADPH are used in anabolic reactions.
Cofactors and minerals in metabolism
There are several minerals and vitamins that play critical roles in metabolism. Common and abundant among these are sodium and potassium. Other important minerals include:
- chloride ions
The organic compounds (proteins, lipids and carbohydrates) contain the majority of the carbon and nitrogen and most of the oxygen and hydrogen is present as water.
Cations often act as cofactors that are bound tightly to a specific protein. Enzyme cofactors can be modified during catalysis but cofactors always return to their original state after catalysis has taken place. The metal micronutrients are taken up into organisms by specific transporters.
Reviewed by April Cashin-Garbutt, BA Hons (Cantab)