During a marathon, runners draw heavily on their internal reserves of glycogen (carbohydrate) and triglycerides (fat) to fuel muscle contraction. Initially, energy is derived mostly from carbohydrates, with increasing amounts of fat being used as the race progresses. If runners use up their muscle glycogen reserves before they finish the race, they hit what is known as "the wall.," a point of diminished performance that arises because fatty acids from triglyceride breakdown cannot be delivered to the muscles quickly enough to sustain maximum effort. One trick that marathon runners use to avoid the wall is to drink a cup of strong black coffee an hour or so before the race begins. Coffee contains caffeine, which is an inhibitor of cyclic AMP phosphodiesterase. How do you suppose inhibition of this enzyme helps them avoid the wall

The activity of cyclic AMP phosphodiesterase is prevented by caffeine. The caffeine prevents further dissociation of cAMP, which eventually increases the response within the body. The activity of neurotransmitters, that is, of noradrenaline or epinephrine, gets increased due to enhancement in response.  

This further enhances the activity of the heart, the rate of muscle contractions, blood pressure that further helps in delivering more oxygen to the brain, and other parts of the body. Thus, the inhibition of the enzyme cyclic AMP phosphodiesterase helps the athletes to prevent the wall.  

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.