Cells perform many tasks and are very efficient. They do not waste energy and do not make substances which they do not need. Following processes keep the metabolism of different compounds under control:
Control of Anabolism
Sometimes, an amino acid is present in larger amounts. The anabolic pathway synthesizes amino acids from an intermediate in the Krebs cycle. This pathway is turned off.
Feedback inhibition is the most common mechanism for the control of metabolism. It uses the end product (feedback) of metabolic reaction. The end product of the anabolic pathway inhibits the enzyme. This enzyme catalyzes a key step in the pathway.
Control of Catabolism
Like other metabolic processes, cells can also control catabolism. Such as a cell-like muscle cell is working very hard. Therefore, its ATP concentration decreases. So aerobic respiration increases.
When ATP is sufficient to meet demand, aerobic respiration slows down. It saves organic molecules for other necessary functions. The control is based on regulating enzyme activity. This activity is present at special points in the catabolic activity.
Therefore, the metabolism can be controlled at these points.
Control of Metabolism by phosphofructokinase
One of the main controlling points in aerobic respiration is the enzyme phosphofructokinase. The speed of functioning of this enzyme is controlled. So, a cell can speed or slow the entire metabolic process.
Phosphofructokinase has specific receptor sites for specific inhibitors and activators.
- ATP inhibits it
- ADP or AMP stimulates it
Phosphofructokinase is sensitive to the energy needs of the cell and the ratio of ATP to ADP or AMP.
- If ATP begins to accumulate, this enzyme shuts down glycolysis.
- If ADP or AMP begins to accumulate, phosphofructokinase becomes active and turns on the glycolytic pathway.
- The citric acid in the cytoplasm also inhibits phosphofructokinase.
This control pathway synchronizes the rates of glycolysis and the Krebs cycle. For example, when citric acid begins to accumulate, glycolysis slows down.
It reduces the supply of acetyl-coenzyme A to the Krebs cycle. On the other hand, if citric acid consumption increases, glycolysis accelerates and meets the demand for more acetyl-coenzyme A.
The Metabolic Pool
The destructive chemical reactions (catabolism) of glycolysis and the Krebs cycle perform many functions. They constitute a metabolic pool. This pool supplies material for the synthesis (anabolism) of many important cellular components.
Therefore, the balance between catabolism and anabolism is maintained. It maintains, homeostasis in the cell and in the whole animal.
For example, glycolysis and the Krebs cycle are open systems. An open system has a two-way flow of materials into and out of it.
- Various compounds enter the pathways at different points. Thus, carbohydrates, fats, and proteins can all be oxidized.
- At the same time, some of the intermediates of these pathways can be withdrawn. They are used in synthesis reactions.
Thus, the products of glycolysis and the Krebs cycle form a metabolic pool. The materials can be added or withdrawn from this pool according to the need of the body.