Which of the following is the most abundant biomolecule on earth ?

Question

Accepted Answer

The correct option is 4:
Cellulose is unequivocally considered the most abundant organic compound, or biomolecule, on Earth. It is a polysaccharide, specifically a long-chain polymer made of thousands of D-glucose units linked by $\beta$-1,4 glycosidic bonds. Its structure allows it to form strong, rigid microfibrils, which serve as the primary structural component of the cell walls of green plants, many forms of algae, and oomycetes. Given that the entire terrestrial ecosystem is overwhelmingly dominated by plant life—from grasses and shrubs to the planet's vast forests—the sheer biomass of cellulose in plant cell walls is immense. It accounts for approximately one-third of all plant matter, and its slow rate of decomposition in many environments allows for its massive accumulation across global ecosystems, far surpassing the total quantity of any other single biomolecule. This global abundance is a direct consequence of its essential, load-bearing role in supporting the immense structure of the plant kingdom.

The incorrect option is 1:
RUBISCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) is another candidate frequently cited in biochemistry, but it holds the title of the world's most abundant *protein*, not the most abundant *biomolecule* overall. This enzyme is crucial because it catalyzes the first major step of carbon fixation, converting carbon dioxide from the atmosphere into energy-rich molecules within the chloroplasts of all photosynthetic organisms. Due to its relatively slow reaction rate, plants must produce it in enormous quantities to sustain global photosynthesis, making it highly concentrated in leaves. However, the collective mass of all proteins on Earth is significantly less than the total mass of structural carbohydrates like cellulose.

The incorrect option is 2:
Lignin is indeed one of the world's most abundant polymers, often cited as the second most abundant organic polymer after cellulose. It is a highly complex, cross-linked aromatic polymer that is deposited in the secondary cell walls of plants, particularly in wood and bark, providing them with essential rigidity, impermeability, and resistance to pests and decay. Its presence is vital for the water-conducting ability of xylem vessels and the overall structural integrity of woody plants. Nevertheless, since lignin is primarily restricted to vascular plants and is not a component of all plant cell walls, its global quantity does not reach the all-encompassing mass of cellulose, which is found in the cell walls of virtually all plant and plant-like organisms.

The incorrect option is 3:
DNA (Deoxyribonucleic acid) is the genetic material and the master blueprint for life, making it the most important informational molecule on Earth. It stores the hereditary information and is responsible for passing traits from one generation to the next. While DNA is present in every living cell on the planet, it exists in extremely minute and highly controlled amounts within the cell. It is the information carrier, not a structural component, and therefore contributes a minuscule fraction to the total dry weight of living organisms when compared to the vast quantities of structural polysaccharides (like cellulose) or even the most abundant enzymes (like RUBISCO).