Ribonucleotides serve as precursors for synthesis of deoxyribonucleotides and the reaction is catalyzed by enzyme ribonucleotide reductase. The reduction of a ribonucleotide into deoxyribonucleotide requires a pair of hydrogen atoms.
These hydrogen atoms are supplied from NADPH via ubiquitous protein thioredoxin. The oxidized form of thioredoxin is reduced back in a reaction catalyzed by thioredoxin reductase.
C. Upward; downward. As the hot rock rises, cooler rock flows downward to take its place next to the core, where it is heated and becomes buoyant enough to rise again later
