In research on aging (both cellular aging and organismal aging), it has been found that aged cells do not progress through the cell cycle as they had previously. Which of the following, if found in cells or organisms as the age would provide evidence that this is related to cell signaling?

a) Growth factor ligands do not bind as efficiently to receptors.

b) cAMP levels change very frequently.

c) Enzymatic activity declines.

d) Hormone concentrations decrease.

The correct answer is option a) "Growth factor ligands do not bind as efficiently to receptors".

Explanation:

Growth factors are proteins that have a key role on cell's growth, survival and differentiation. Growth factors' activity is achieved by cell signaling processes, by interacting with receptors that transmit information to the intracellular space. Therefore, if it is found on aged cells that growth factor ligands do not bind as efficiently to receptors, it will be evidence that cell's signaling has been compromised.