How do we smell and taste?

When we smell or taste something, how does our nose or tongue know to send good or bad signals to the brain? Specifically, I want to know whether or not it has to do with the atomic arrangement of the molecules or molecular structure of the given substance that we smell or taste, and (if applicable) any mathematical ratios therein that cause the perception of sweetness, bitterness, sourness, spiciness, etc. for smell AND taste, if possible.