We have seen how an energy currency that runs low is topped up by conversion from another currency. This is not enough to ensure a constant energy supply, however. Therefore the cell has more mechanisms that ensure that the supply of cellular energy is accelerated or slowed as appropriate. These mechanisms are of two types called feedforward and feedback. We will introduce the terms by analogy with real money. Consider a bank teller. During the day people deposit checks but draw out cash to spend. As time passes the stock of banknotes and change in the till gets low. The teller signals to the supervisor, who opens the bank vault, takes out more cash, and refills the teller's till. This is an example of negative feedback. In general negative feedback is said to occur when a change in some parameter activates a mechanism that reverses the change in that parameter. We have already met an analogous negative feedback system in the control of tryptophan biosynthesis (page 116). A downward change in the concentration of tryptophan in the bacterial cell activates the mechanism that causes the cell to make more tryptophan. Positive feedback is less common in both biology activated by AMP
l+ve fructose-6-phosphate + ATP <== => fructose-1,6-bisphosphate
Figure 13.14. Phosphofructokinase is regulated by the binding of ATP or AMP at a regulatory site that is separate from the active site. Binding of ATP inhibits while binding of AMP activates.
and banking. It is said to occur when a change in some parameter activates a mechanism that accelerates the change. In banking this occurs when a rumor starts that a bank is about to fail. The lower a bank's reserves of money get, the more its depositors rush to take their money out before it is too late. Biological examples of positive feedback are unusual. Quorum sensing in luminescent bacteria (page 116) and the action potential (page 325) are two examples of positive feedback that we describe in this book.
What about feedforward? The bank is especially busy at lunchtime, with lots of cash withdrawn between 12:30 and 2:00. During this time, everyone is rushed off their feet. The supervisors do not wait until tellers signal that they are short of cash but instead open the vaults at 12 noon and bring out enough cash to see the tellers through the lunchtime rush. They are preparing for a future drain on cash by stocking up the tills before the drain occurs—this is feedforward. Feedforward happens in biological systems too, as we will see in this section.
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