Final answer:
Herbivores can't digest cellulose on their own and rely on gut bacteria and protozoa for this process, which is essential for their nutritional intake.
Step-by-step explanation:
Herbivores are unable to digest cellulose, and thus rely on the help of specialized bacteria and protozoa living in their digestive tracts. These microorganisms possess the necessary enzymes to break down cellulose, transforming it into simpler substances that can be absorbed as nutrients by the herbivores. This mutualistic relationship is vital for the survival of these herbivores, as their own bodies do not produce cellulase, the enzyme required for cellulose digestion.
For example, ruminants such as cattle and giraffes have a multi-chambered stomach, where bacteria and protozoa contribute to the breakdown of cellulose and even synthesize proteins from urea and ammonia. Similarly, termites and wood-eating cockroaches host protozoa and bacteria in their guts to aid in the digestion of lignin from wood.