21. A cell is known to respond to a particular signaling molecule. Which of the following must be true of this cell?

A. It is in the heart muscle
B. It is also the site of production for the signaling molecule
C. It contains the receptor for the signaling molecule
D. It is incapable of signal transduction

22. The enzyme that catalyzes the splitting of PIP2 into two molecules of inositol triphosphate (IP3) and diacylglycerol in cell signaling, is

A. phosphokinase C
B. phospholipase C
C. phosphodiesterase C
D. lipokinase

23. The binding of ligands to many G-proteins linked receptors leads to shortlived

A. increase in the concentration of certain intracellular signaling molecules called second messenger
B. decrease in the concentration of certain intracellular signaling molecules called second messenger
C. increase in the concentration of certain extracellular signaling molecules called first messenger
D. decrease in the concentration of certain extracellular signaling molecules called first messenger

24. Which of the following processes involve the combining of a message from one signaling molecule with that of another to either enhance or inhibit a cellular effect?

A. Signal transduction
B. Signal reception
C. Signal integration
D. Signal amplification

25. A signal cascade induced by adrenaline or thyroxine

A. must begin with receipt of the signal molecule by a surface receptor
B. involves the activation of a G protein
C. results in the activation of a sequence of enzymes needed for the cell effect
D. all of the above