سؤال قراءة ستيب محلول — فهم التفاصيل (تدريب 5243)

Passage A: Taste (1) Our sense of taste involves four basic sensations - sweet, sour, salty, and bitter. All other tastes are mixtures of these. Investigators have, however, been frustrated in their search for specialized nerve fibers for each of the four basic taste sensations. (2) Taste is a chemical sense. Inside the little bumps on the top and sides of your tongue are 200 or more taste buds. Each contains a pore that catches food chemicals. These molecules are sensed by 50 taste receptor cells that project antenna-like hairs into the pore. Some of these receptors respond mostly to sweet-tasting molecules, others to salty, sour, or bitter-tasting ones. (3) Although taste buds are essential for taste, there is more to taste than meets the tongue. Hold your nose, close your eyes, and have someone feed you various foods. A piece of apple may then be indistinguishable from a piece of raw potato; a piece of steak may taste like cardboard. To savor a taste, we normally breathe the aroma through our nose-which is why eating is not much fun when you have a bad cold, and why people who lose their sense of smell may think they have also lost their sense of taste. Smell not only adds to our perception of taste, it also changes it. A drink's strawberry odor enhances our perception of its sweetness. This is sensory interaction at work, the principle that one sense may influence another. Smell plus taste equals flavor. Passage B: Smell (1) Breaths come in pairs-inhale, exhale-except at two moments: birth and death. Each day, as you inhale and exhale nearly 20,000 breaths of life sustaining air, you bathe your nostrils in a stream of molecules full of scents. (2) Like taste, smell is a chemical sense. We smell something when air-carried molecules of a substance reach a tiny cluster of 5 million receptor cells at the top of each nasal cavity. These olfactory receptor cells respond selectively to the aroma of cake baking, to a wisp of smoke, to a friend's fragrance, and they instantly alert the brain through their axon fibers. (3) Even nursing infants and mothers have a literal chemistry to their relationship, as they quickly learn to recognize each other's scents. Aided by smell, a mother fur seal returning to a beach crowded with pups will find her own. Our own sense of smell is less impressive than the acuteness of our seeing and hearing. Looking out across a garden we see its forms and colors in exquisite detail and hear its singing birds, yet smell little of it without jamming our nose into the flowers. (4) Precisely how olfactory receptors work is a mystery. Unlike light, which can be separated into its spectral colors, an odor cannot be separated into more elemental odors. Thus, the olfaction system has no parallel to the retina, which detects myriad colors with sensory cells dedicated to red, green, or blue. Olfactory receptors recognize odors individually.