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.
Adolescent white lion in Kruger National Park, South Africa.
Wagga Wagga, New South Wales, Australia
The liver in this photograph was from a deceased patient with an advanced state of cirrhosis.
Mosaic image of the lunar south pole of the Moon
Percentage of Americans Living in Communities with a Fluoridated Water Supply, by State. 2012