which molecule decides whether the cell will perform catabolism or anabolism?

Answer 1

The processes which control a cell's metabolism are very complex, so many molecules play a role in the regulation of catabolism and anabolism. A lot of the time, metabolic pathways are controlled by feedback inhibition. If a pathway builds molecule A, then a high concentration of molecule A inhibits the enzymes that build molecule A. Therefore, when molecule A is present in sufficient amounts, the cell won't make any more molecule A. However, when molecule A is lacking, then the enzymes to build molecule A will be active and make more of molecule A. Furthermore, an excess of molecule A can sometimes activate pathways which break down or use molecule A.

Other times, some signaling molecules can regulate metabolism. For example, cyclic adenosine monophosphate (cAMP) is an important regulator of glycogen metabolism. cAMP is produced in response to glucagon, a hormone produced by the pancreas. cAMP tells liver cell to slow down glycolysis, to slow down glycogen synthesis, and to increase glycogen breakdown, resulting in producing a lot of glucose for export to other cells. In contrast, the hormone insulin will keep cAMP levels low and tell liver cells to take up glucose from the blood, to stop glycogen breakdown, to increase glycogen synthesis, and to increase glycolysis. The net result is that the liver helps to get rid of excess glucose by storing it and breaking it down.

Answer 2

The chemical reactions in cells (cell metabolism) are organised into metabolic pathways, where one chemical is transformed to another by a sequence of enzymes. Enzymes are crucial to metabolism because they allow cells to drive unfavourable reactions by coupling them to favourable ones. Enzymes also allow the regulation of metabolic pathways in response to changes in the cell's environment or signals from other cells.

Enzymes are proteins that catalyze (i.e. accelerate) chemical reactions. In these reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, the products. Almost all processes in the cell need enzymes in order to occur at significant rates. Since enzymes are extremely selective for their substrates and speed up only a few reactions from among many possibilities, the set of enzymes made in a cell determines which metabolic pathways occur in that cell.