Which one of the following exemplifies exocytosis?

Steps 1 and 5Step 2Step 3Steps 2 and 4

1. On the plasma membrane of the signaling neuron, voltage-gated Ca2* channels open. Ca2* ions diffuse into the cell. 2. In the signaling neuron, secretory vesicles fuse with the plasma membrane, releasing neurotransmitters. 3. Neurotransmitters diffuse over to the receiving neuron. 4. Gated ion channels on the receiving neuron open in response to binding neurotransmitters, allowing ions to flow into the receiving neuron. 5. Neurotransmitters are transported back through the plasma membrane through membrane proteins. Proton pumps in the secretory vesicle membrane use ATP to move protons into the vesicle. A proton is allowed out of the vesicle in exchange for a neurotransmitter being allowed into the vesicle.