CHARLESWARDLE.COM

Part of the ‘Learn Biology’ series

Reaction = process of changing one chemical substance into another

Sometimes biological systems require more of something; sometimes they require less of something.

If you know something of economics you could think of it as a supply-and-demand economy.

For example:

When you are exercising your muscle cells require more energy.

This energy is obtained from breaking down sugar molecules to obtain ATP (a molecule that provides chemical energy).

If a cell couldn’t regulate itself to slow production of ATP when enough has been produced then it would not be able to function properly.

So what happens in a cell when there is enough ATP available and there is a need to slow down the production of more?

Well, ATP is manufactured through a series of steps, just like something in a factory.

When there is more ATP in the cell than can be used the extra ATP molecules themselves slow production of more ATP.

This is accomplished through negative feedback.

Like most of the cell’s chemical processes molecules called enzymes speed up reactions.

Each step in the production pathway of ATP is sped up by its own specific enzyme.

So, what happens is that the extra ATP molecules in the cell bind to the first enzyme in the production pathway and decrease its ability to speed up the reaction. The first enzyme in the production pathway that was causing ATP to be produced so fast has been prevented from continuing to do so.

The final manufactured product, when not otherwise occupied, is used to slow down production of more of itself!

In negative feedback it is usually the first stage of production that is slowed through the inhibition of enzymes as it would be wasting energy to produce more of an unneeded intermediate product.

Positive feedback, less common than negative feedback, also occurs where an end product speeds up its own production by stimulating, rather than inhibiting, enzymes that speed up one of the steps in its production.

An example of positive feedback in a biological system:

In child birth when contractions occur a signal is sent to the brain to release a hormone called oxytocin. Oxytocin stimulates further contractions which in turn signal the release of more of the hormone, thus speeding up its own release.

(Campbell, Reece, et al, 2008, “Biology”)