Final answer:
Cyclic AMP (cAMP) and ions such as Ca2+ are the second messengers in the options provided. cAMP is synthesized by adenylyl cyclase and activates A-kinase, while Ca2+ ions are released by IP3 and can alter cellular activities.
Step-by-step explanation:
Second messengers are crucial for propagating signals within cells, following the initial binding of a signaling molecule to a receptor on the cell surface. These messengers play a key role in altering the behavior of cellular proteins. An example of this is cyclic AMP (cAMP), which is produced by the enzyme adenylyl cyclase from ATP and serves to activate the enzyme cAMP-dependent kinase (A-kinase). This kinase is involved in the phosphorylation of serine and threonine residues on target proteins, thus regulating various metabolic pathways.
Apart from cAMP, ions such as Ca2+ also act as second messengers. Upon activation by a G protein, the enzyme phospholipase C (PLC) can split a membrane-bound phospholipid to form two molecules: diacylglycerol (DAG) and inositol triphosphate (IP3). While DAG activates protein kinases leading to a phosphorylation cascade, IP3 triggers the release of calcium ions from intracellular stores, which then act as second messengers to modulate enzymatic and cellular activities either directly or by binding to calcium-binding proteins like calmodulin.
As a result of the aforementioned details, the options acting as second messengers in cells include A) Cyclic AMP and C) Ions. Neurotransmitters and hormones typically do not act as second messengers; rather they are the primary signals that trigger the second messenger system.