receptor molecules have been identified in preparations of fossilized organisms billions of years old
	simple forms of life, such as prokaryotic cells, utilize cell signaling
	cell signaling is observed in organisms that do not utilize sexual reproduction
	the molecular details of cell signaling are quite similar in organisms whose last common ancestor was a billion years ago
	the pathways in primitive organisms, such as bacteria, are short, while those in advanced forms, such as mammals, are long

	neurotransmitter
	paracrine signal
	local regulator
	hormone
	synaptic signal

	the target cell of a signal molecule
	any small molecule that can bind in a specific manner to a larger one
	the bond that forms between a signal molecule and its receptor
	the change in shape that occurs when a signal molecule binds to its receptor
	a molecule that can occupy a receptor site while not activating the receptor

	growth factor receptors
	DNA polymerase
	transcription factors
	enhancers
	nucleosomes

	enzymatic phosphorylation of tyrosine in the receptor protein
	enzymatic degrading of GTP to GDP
	allowing specific ions to enter the cell after ligand binding
	binding to nonpolar signal molecules such as nitric oxide or the steroid hormones
	enzymatic hydrolyzing of the signal molecule shortly after its arrival

	calcium ion and cAMP
	GTP and GDP
	kinase and phosphate groups
	G proteins and GTP
	kinase and phosphatase

	diffusion of a signaling molecule across the plasma membrane ® binding of the signaling molecule to its receptor ® movement of the signal molecule-receptor complex into the nucleus ® transcription
	binding of a growth factor to its receptor ® phosphorylation cascade ® activation of transcription factor ® transcription
	binding of a signaling molecule to its receptor ® G-protein activation ® adenylyl cyclase activation ® cAMP production ® protein phosphorylation
	binding of a signaling molecule to its receptor ® G-protein activation ® phospholipase C activation ® IP3 production ® increase in cytoplasmic calcium concentration
	Binding of a signaling molecule to its receptor ® G-protein activation ® phosphodiesterase activation ® increase in cytoplasmic cAMP levels ® protein kinase A activation

	Each transduction is a checkpoint.
	Each step in a cascade produces a large number of activated products, causing signal amplification as the cascade progresses.
	Long, highly specific pathways minimize the possibility that a relay molecule accidentally could activate a pathway leading to a secondary response.
	The accumulation of genetic mutations over time has added redundant steps to the pathway.
	Lengthy pathways provide the opportunity for the initial pathway molecules to recycle back to inactive forms should additional signal molecules be present.

	different cells possess different enzymes, which modify the signal molecule into different molecules after it has arrived
	the transduction process is unique to each cell type; to respond to a signal, different cells require only a similar membrane receptor
	different cells have membrane receptors that bind to different sides of the signal molecule
	the transduction pathway in cells has a variable length
	all of the above are correct

	When a ligand binds a G-protein-linked receptor, the G protein associated with the receptor is phosphorylated and becomes active.
	Increased cytoplasmic levels of IP3 cause calcium ions to move rapidly out of the cell.
	Local regulators bind their receptors and are internalized by endocytosis, triggering the cell's response
	Steroid hormone receptors have kinase activity
	In most cases, signaling molecules never actually enter the cytoplasm, but interact only with the outer surface of the cell.