Feedback mechanisms regulate biological systems

A kind of supply-and-demand economy applies to many biological systems. Consider your muscles, for instance. When your muscle cells require more energy during exercise, they increase their consumption of the sugar molecules that provide fuel. In contrast, when you rest, a different set of chemical reactions converts surplus sugar to storage molecules.
Like most of the cell's chemical processes, those that decompose or store sugar are accelerated, or catalyzed, by the specialized proteins called enzymes. Each type ofenzyme catalyzes a specific chemical reaction. In many cases, these reactions are linked into chemical pathways, each step with its own enzyme. How does the cell coordinate its various chemical pathways? In our example ofsugar management, how does the cell match fuel supply to demand, regulating its opposing pathways of sugar consumption and storage? The key is the ability ofmany biological processes to self-regulate by a mechanism called feedback.
In feedback regulation, the output, or product, of a process regulates that very process. In life, the mostcommon form of regulation is negative feedback, in which accumulation of an end product of a process slows that process. For example, the cell's breakdown of sugar generates chemical energy in the form of a substance called ATP. When a cell makes more ATP than it can use, the excess ATP "feeds back" and inhibits an enzyme near the beginning of the pathway (figure 1a).
Though less common than processes regulated by negative feedback, there are also many biological processes regulated by positive feedback, in which an end product speeds up its production (figure 1b). The clotting of your blood in response to injury is an example. When a blood vessel is damaged, structures in the blood called platelets begin to aggregate at the site. Positive feedback occurs as chemicals released by the platelets attract more platelets. The platelet pile then initiates a complex process that seals the wound with a clot.
Feedback is a regulatory motifcommon to life at all levels, from the molecular level to ecosystems and the biosphere. Such regulation is an example of the integration that makes living systems much greater than the sum of their parts.


Negative feedback. ThiS three-step chemICal pathway converts substance A to substance D. A specific enzyme catalyzes each chemical reaction. Accumulation of the final product (OJ inhibits the first enzyme in the sequence. thus slowing down production of moreD.


Positive feedback. In a biochemical pathway regulated by positive feedback, a product stimulates an enzyme in the reaction sequence, incteasing the tate of production of the product.


source: Campbell and Reece book