The diagram above depicts the signal transduction pathway of the sweet taste. 
Object A is a taste bud, object B is one taste cell of the taste bud, and object C is the neuron attached to the taste cell. 
I. Part I shows the reception of a molecule. 
1. Sugar, the first messenger, binds to a protein receptor on the cell membrane. 
II. Part II shows the transduction of the relay molecules. 
2. G Protein-coupled receptors, second messengers, are activated. 
3. G Proteins activate adenylate cyclase, an enzyme, which increases the cAMP concentration. Depolarization occurs. 
4. The energy, from step 3, is given to activate the K+, potassium, protein channels.
III. Part III shows the response of the taste cell. 
5. Ca2+, calcium, protein channels is activated.
6. The increased Ca2+ concentration activates neurotransmitter vesicles. 

7. The neuron connected to the taste bud is stimulated by the neurotransmitters.
Own work &amp; Purves, Dale. Taste Receptors and the Transduction of Taste Signals. U.S. National Library of Medicine, n.d. Web. May-June 2015.
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