Introduction:
Cellulose is a complex carbohydrate found in the cell walls of plants. While humans are capable of digesting many types of carbohydrates, such as starch, cellulose is one that the human body cannot break down and digest. This inability to digest cellulose is due to several reasons which will be discussed in detail below.

Lack of necessary enzymes:
Digestion of carbohydrates in the human body primarily occurs in the small intestine. However, humans lack the necessary enzymes, such as cellulase, to break down the cellulose molecule. Cellulase is an enzyme produced by some animals, such as cows and termites, as well as certain bacteria and fungi. These organisms possess the ability to break down cellulose and derive energy from it. In contrast, the human body does not produce cellulase, therefore making it incapable of digesting cellulose.

Structural complexity:
Cellulose is made up of long chains of glucose molecules that are linked together by beta-glycosidic bonds. The arrangement of these bonds creates a rigid and complex structure, forming a major component of plant cell walls. This structural complexity makes it difficult for the human digestive system to break down the cellulose molecule.

Lack of necessary gut microbiota:
While humans lack the necessary enzymes to digest cellulose, some animals, such as herbivores, can break down cellulose with the help of specialized gut microbiota. These microorganisms, including certain bacteria and protozoa, reside in the digestive system of these animals and produce cellulase enzymes to break down cellulose. However, the human gut microbiota is not adapted to digest cellulose efficiently, further contributing to the inability of humans to digest cellulose.

Conclusion:
In summary, humans cannot digest cellulose due to the lack of necessary enzymes like cellulase, the structural complexity of cellulose, and the absence of specialized gut microbiota. As a result, cellulose passes through the human digestive system largely undigested, providing little to no nutritional value.