About fifty years ago, plant physiologists set out togrow roots by themselves in solutions in laboratoryflasks. The scientists found that the nutrition ofisolated roots was quite simple. They required sugarand the usual minerals and vitamins. However, theydid not require organic nitrogen compounds. Theseroots got along fine on mineral inorganic nitrogen.Roots are capable of making their own proteins andother organic compounds. These activities by roots require energy, of course: The process ofrespiration uses sugar to make the high energy compound ATP, which drives the biochemicalreactions. Respiration also requires oxygen. Highly active roots require a good deal of oxygen.
The study of isolated roots has provided an understanding of the relationship betweenshoots and roots in intact plants. The leaves of the shoots provide the roots with sugar andvitamins, and the roots provide the shoots with water and minerals. In addition, roots canprovide the shoots with organic nitrogen compounds. This comes in handy for the growth ofbuds in the early spring when leaves are not yet functioning. Once leaves beginphotosynthesizing, they produce protein, but only mature leaves can "export" protein to therest of the plant in the form of amino acids.
According to the passage, what is ATP?
A. A biochemical process.
B. The tip of a root.
C. A chemical compound.
D. A type of plant cell.