Final answer:
The primary role of bacteria in a horse's digestion of cellulose is to break it down completely into glucose monomers for energy. Horses rely on microorganisms in their gut to produce the enzyme cellulase needed for this process.
Step-by-step explanation:
The primary role of bacteria in a horse's digestive process regarding cellulose is to break it down completely. Horses, like other herbivores such as cows and buffalos, cannot break down cellulose on their own because they lack the enzyme cellulase. However, they harbor specific species of bacteria and protists in their digestive systems, particularly in the rumen, which produce cellulase. These microorganisms break down the cellulose into glucose monomers, which the animals can then use as a source of energy.
In the context of the question posed, the correct answer is (a) Break it down completely. Cellulose is the most common carbohydrate in plants and is a form of dietary fiber for humans, which we do not digest but plays an important role in our digestive system. The other options, (b) convert it into water, (c) help absorb it into the body, and (d) enhance its taste, do not accurately describe the role of bacteria in a horse's digestion of cellulose.
Thus, the specialized flora within the digestive system of horses is crucial for breaking down tough plant materials they consume, allowing them to derive nutritional benefits from a diet rich in cellulose.