Identifying pathway components in cell signaling

Steps:

• Signal molecule binds receptor on cell surface, activating cytoplasmic proteins.
• P represents an inactive protein in cytoplasm that receives the signal.
• This protein activates an enzyme in cytoplasm, identified as Q.
• Activated enzyme triggers response like secretion via phosphorylation cascade.

Why C is correct:

• In signal transduction, cytoplasmic proteins are activated as enzymes to amplify signals, per second messenger pathways.

Why the others are wrong:

• A: Targets are intracellular, not on membrane; misplaces pathway location.
• B: Lipids and extracellular enzymes don't fit intracellular signaling steps.
• D: Both as targets ignores activation sequence from protein to enzyme.

Final answer: C