How Do We Smell and Taste Substances?
Although the link between taste and smell is widely observed, as when you can almost taste "blood" when you smell iron-rich compounds, the sense of taste and smell are still different systems.
The Sense of Taste
It is widely believed among evolutionary psychologists that the sense of taste comes from the need to select the right foods and to properly regulate food consumption. We have around 100,000 taste buds or taste sensory receptors scattered in the papillae of our tongue. Papillae are tiny bumps that make up the texture of the tongue.
The tongue is divided into sections that detect different taste qualities. The tip of the tongue detects sweetness and saltiness; the sides of the tongue detect sourness; and the rear of the tongue detects bitter tastes. The brain integrates the different flavors present in complex tastes.
Update: The tongue map was debunked by research.
The Sense of Smell
Evolutionary psychologists, likewise, believe that the sense of smell (or the olfactory sense) developed from the need to select the right foods and to track the odor of predators and preys. The olfactory epithelium located at the roof of the nasal passage, like the retina in the visual system, is considered as the primary mechanism for smell. The hair-like antennae projecting through the nasal passage serve as olfactory sensory receptors, where olfactory sensory information is detected. This information is then transduced by the olfactory bulbs, and then transmitted to the brain by olfactory nerves connected to the bulbs. The olfactory sensory information then moves up to the thalamus, then to the temporal lobes, and finally to the limbic system. This special olfactory pathway, where sensory information passes through the limbic system, results to strong memory ties for olfactory sensory information. This is also the reason why smells bring a lot of memories.