Answer to Question 1
a
Answer to Question 2
Vitamin A plays two indispensable roles in the eye. It helps maintain a healthy, crystal-clear outer window, the cornea; and it participates in light detection at the retina. Some of the photosensitive cells of the retina contain pigment molecules called rhodopsin; each rhodopsin molecule is composed of a protein called opsin bonded to a molecule of retinal, which plays a central role in vision. When light passes through the cornea of the eye and strikes the retina, rhodopsin responds by changing shape and becoming bleached. In turn, this initiates the signal that conveys the sensation of sight to the optic center in the brain. When vitamin A is lacking, the eye has difficulty adapting to changing light levels. At night, after the eye has adapted to darkness, a flash of bright light is followed by a brief delay before the eye can see again. This lag in the recovery of night vision is known as night blindness. Because night blindness is easy to test, it aids in the diagnosis of vitamin A deficiency. Night blindness is only a symptom, however, and may indicate a condition other than vitamin A deficiency.
(b) Espeletia in the Andes of South America.
Gecarcinus quadratus, a land crab from Central and South America
The endangered West African giraffe
Mars sketched as observed by Lowell sometime before 1914. (South top)
Generally, the total tax burden in the United States is much lower than that of other industrialize
Population Losses in the South, 1920–1930