单选题
{{B}}
Importance of
Vitamins{{/B}}
Vitamins, which come in many different types all of which are quite diverse in chemical configuration and function, can be any of several organic substances that are separated into water-soluble and fat-soluble groups. Originally defined as organic compounds obtainable in a normal diet and capable of maintaining life and promoting growth, vitamins are differrent from carbohydrates, fats, and proteins in function, as well as in the quantities in which organisms require them. So critical are vitamins to a body's essential strength and health that if they are absent from the diet or not properly absorbed by an organism, a specific deficiency disease may develop.
The term vitamin originated from "vitamine," a word first used in the early 19th century to designate a group of compounds considered vital for life (though the term "accessory food factor" sometimes is used interchangeably to refer to these substances). Like other nutrients, vitamin consumption is imperative to keep our bodies functioning properly, and if there is a lack of vitamin consumption, the body will fail to react in a way that's considered healthy. Lack of vitamin A will result in various disorders that most commonly involve the eye and the tissues around it. One of the earliest symptoms of vitamin A deficiency is nyctalopia, the medical term for night blindness, which causes a visual failure to adapt quickly from light to darkness and an inability to see in the dark. This aspect of vision is normally dependent on rhodopsin, a protein found in the eye that maintains itself only in the presence of vitamin A; in the lack of vitamin A, rhodopsin will malfunction.
These earlier symptoms are quite harmless but the side effects can become increasingly serious if not treated early on. If the deficiency is severe and persists, especially in malnourished infants and children, a condition known as xerophthalmia--when the eyes are sensitive to light, the secretion of lubricating tears is stopped, and the eyelids become swollen and develop pus -- may develop. Furthermore, the mucous surfaces of the eye may become eroded, allowing infection to set in, thus leading to ulceration and other destructive changes of the cornea and other structures of the eye, resulting eventually in blindness.
Early signs of
vitamin A deficiency may also be reflected in changes in the membranes of the
mouth, throat, and respiratory and genito-urinary passages where the lining
membranes become malnourished and dry and lose their cilia, the tiny hairlike
projections that normally help in clearing away foreign particles. 
The natural immune system is weakened and if
insufficient intake of vitamin A is prolonged, the skin may become dry and
rough. 
Vitamin A deficiency may also result
in defective bone and teeth formation and in poor general growth.
Vitamins, which come in many different types all of which are quite diverse in chemical configuration and function, can be any of several organic substances that are separated into water-soluble and fat-soluble groups. Originally defined as organic compounds obtainable in a normal diet and capable of maintaining life and promoting growth, vitamins are differrent from carbohydrates, fats, and proteins in function, as well as in the quantities in which organisms require them. So critical are vitamins to a body's essential strength and health that if they are absent from the diet or not properly absorbed by an organism, a specific deficiency disease may develop.
The term vitamin originated from "vitamine," a word first used in the early 19th century to designate a group of compounds considered vital for life (though the term "accessory food factor" sometimes is used interchangeably to refer to these substances). Like other nutrients, vitamin consumption is imperative to keep our bodies functioning properly, and if there is a lack of vitamin consumption, the body will fail to react in a way that's considered healthy. Lack of vitamin A will result in various disorders that most commonly involve the eye and the tissues around it. One of the earliest symptoms of vitamin A deficiency is nyctalopia, the medical term for night blindness, which causes a visual failure to adapt quickly from light to darkness and an inability to see in the dark. This aspect of vision is normally dependent on rhodopsin, a protein found in the eye that maintains itself only in the presence of vitamin A; in the lack of vitamin A, rhodopsin will malfunction.
These earlier symptoms are quite harmless but the side effects can become increasingly serious if not treated early on. If the deficiency is severe and persists, especially in malnourished infants and children, a condition known as xerophthalmia--when the eyes are sensitive to light, the secretion of lubricating tears is stopped, and the eyelids become swollen and develop pus -- may develop. Furthermore, the mucous surfaces of the eye may become eroded, allowing infection to set in, thus leading to ulceration and other destructive changes of the cornea and other structures of the eye, resulting eventually in blindness.
Early signs of
vitamin A deficiency may also be reflected in changes in the membranes of the
mouth, throat, and respiratory and genito-urinary passages where the lining
membranes become malnourished and dry and lose their cilia, the tiny hairlike
projections that normally help in clearing away foreign particles. The natural immune system is weakened and if
insufficient intake of vitamin A is prolonged, the skin may become dry and
rough. Vitamin A deficiency may also result
in defective bone and teeth formation and in poor general growth.
单选题
According to the paragraph 2, which of the following is true of
vitamins?
【正确答案】
C
【答案解析】
单选题
The word imperative in paragraph 2 is closest in meaning to
【正确答案】
B
【答案解析】
单选题
Which of the sentences below best expresses the essential information
in the highlighted sentence in the passage?
Incorrect choices change the meaning in important ways or leave out essential information.
Incorrect choices change the meaning in important ways or leave out essential information.
【正确答案】
D
【答案解析】
单选题
The phrase this aspect of vision refers to
【正确答案】
B
【答案解析】
单选题
The author's description of symptoms of vitamin A deficiency mentions
all of the following EXCEPT
【正确答案】
A
【答案解析】
单选题
According to the paragraph 4, which of the following is true of cilia?
【正确答案】
C
【答案解析】
单选题
The word malnourished in the passage is closest in meaning to
【正确答案】
A
【答案解析】
单选题
Look at the four squares [■] that indicate where the following sentence
can be added to the passage.
The defective mucous surfaces will have weakened resistance to bacterial invasion, and their susceptibility to various infections increases.
Where would the sentence best fit?
The defective mucous surfaces will have weakened resistance to bacterial invasion, and their susceptibility to various infections increases.
Where would the sentence best fit?
【正确答案】
B
【答案解析】
单选题
Which of the following can be inferred from paragraph 5 about vitamin
A?
【正确答案】
D
【答案解析】
单选题
The author uses nausea, blurred vision, and drowsiness as examples of
【正确答案】
D
【答案解析】
单选题
The word excessive in the passage is closest in meaning to
【正确答案】
C
【答案解析】
单选题
According to the paragraph 6, which of the following is true of
carotene?
【正确答案】
C
【答案解析】