Although tastes are not relegated to different regions on the tongue, it is important to note that there are slight differences between the major taste sensations. “The chemicals that produce salty and sour tastes act directly through ion channels, whereas those responsible for sweet and bitter tastes bind to surface receptors that trigger a bucket brigade of signals to the cells interiors that ultimately results in the opening and closing of ion channels” (Smith & Margolskee 2001, p.2). At least one of the receptors for salty substances “is an ion channel that allows sodium ions (Na+) to enter directly into the cell” (Kimball 2009). The hormone aldosterone is thought to increase the number of salt receptors to better enable humans to maintain a normal sodium level in the body. Sour ion channels are liberated by the release of acids into the taste cell (Kimball 2009).
Sweet substances bind to G-protein-coupled receptors in the cell and the hormone leptin inhibits sweet cells by opening their K+ channel (Kimball 2009). Leptin thus could be a way for the body to signal enough for a sweet sensation, and leptin abnormalities have been linked to some manifestations of obesity. “Humans have genes encoding 25 different bitter receptors,” although every single taste cell seems to respond to certain bitter-tasting molecules in preference to other bitter-tasting molecules (Kimball 2009).
Finally, some researchers have called umami, the response to glutamic acid — like processed foods monosodium glutamate (MSG) to be a fifth taste. Like sweet and bitter flavors, this response is linked to G-protein-coupled receptors.
Every individuals taste is unique. However, some generalizations can be made in terms of the evolution of taste over the lifespan. Children tend to be more averse to strong flavors. Babies have taste buds on the sides and roof of their mouths and are thus hyper-sensitive to tastes. As people age, these taste buds begin to disappear, which is why adults can tolerate strong cheeses and coffee and other bitter flavors unpalatable to them when they were young (Your sense of taste, Think Quest, 2009).
Kimball, J. “The sense of taste. Biology Pages. Updated November 1, 2009.
November 3, 2009. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/T/Taste.html
Smith, David V. & Robert F. Margolskee. Making sense of taste. Scientific American. March 2001. November 3, 2009.
“Your sense of taste.” Think Quest. November 3, 2009.