Cell Communication Practice Test

A neuron releases neurotransmitter Y into a synaptic cleft. Postsynaptic Cell P responds, but only when Y is released locally at the synapse. When Y is injected into the bloodstream at the same concentration, Cell P does not respond. Which of the following best explains why synaptic release is effective but bloodstream injection is not?

In a plant root, Cell M releases signal Z into the apoplast. Cells within 2–3 cell diameters respond, but cells farther away do not. When the same amount of Z is injected directly into the xylem, distant cells begin responding. Which of the following best explains the change in responding cells?