Question

phosphorylation cascades involving a series of protein kinases are useful for cellular signal transduction because ...

they are species specific.

they always lead to the same cell response.

they amplify the original signal many fold.

they counter harmful effects of phosphatases.

the number of molecules used is small and fixed.

Explanation:

In phosphorylation cascades, each protein - kinase molecule in the pathway can activate multiple downstream target molecules. This leads to a significant increase in the number of responding molecules at each step, amplifying the original signal many - fold for efficient cellular signal transduction. They are not species - specific, don't always lead to the same cell response, are not mainly about countering phosphatases' effects, and the molecule number isn't small and fixed.

Answer:

they amplify the original signal many fold.