选择答案：
Biosynthetic metabolic pathways often have a common enzyme sequence and then branch leading to more than one end-product. Microorganisms have evolved feedback mechanisms, whereby a build-up of one end-product cause a feedback effect on the first enzyme of the branch leading to that product. In addition, mechanisms exist whereby the end-product of a branched pathway causes partial feedback inhibition of the first enzymes of the common sequence so that the flux of substrate passing through this sequence is proportionately reduced. This effect is achieved by use of isoenzymes, concerted feedback regulation and cumulative feedback regulation. These regulatory effect can be of two types: inhibition of enzyme activity and repression of enzyme synthesis. Where isoenzymes (multiple enzyme forms capable of catalysing the same reaction) are involved, the synthesis or inhibition of each enzyme form may be regulated by a different end-product. With concerted feedback regulation, only one enzyme is involved, but more than one product must be present to inhibit activity or repress enzyme synthesis. With cumulative feedback regulation, each end-product causes partial inhibition or repression and all end-products are required to completely block activity or synthesis.
1. In the first sentence, ‘a common enzyme sequence’ means:
(1) the reaction sequence catalysed by the enzymes
(2) that the enzyme has the same amino acids sequence
2. In concerted feedback regulation the enzyme is inhibited and repressed by
(1) several kinds of end-products
(2) only one kind of end-product
3. The isoenzymes means:
(1) the enzymes have different functions
(2) the enzymes are capable of catalysing the same reaction
4. With cumulative feedback regulation, each end-product causes:
(1) complete inhibition and repression
(2) partial inhibition or repression
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Wide-ranging research on tooth decay has recently produced some surprising findings. one indicates that cheddar may actually inhibit the tooth-decay process. It seems to have decay-slowing effect on human teeth if it is eaten immediately after sugar. Why cheese should have such an effect is unknown. It is speculated that the food might interfere with the acid that decays teeth or with bacteria that produce the acid. If so, it would be the first common food found to have this useful property. The other suprprising research finding was that heavily sweetened cereals proved about equally potent in causing decay whether they contained eight percent sugar or almost eight times that much.
1. According to the passage, how many of the test results were unexpected?
A. One B. Two C. Three D. Eight
2. According to the passage, what effect does cheddar cheese seem to have?
A. It interferes with the function of teeth.
B. It makes sugar taste sweeter.
C. It decreases the rate at which teeth decay.
D. It helps in the digestion of food.
3. It can be inferred from the passage that the research on the relationship between cheese and tooth.
A. has been discredited
B. will be slowed considerably
C. has been found to be conclusive
D. will be continued
4. Researchers discovered that sweetened cereals were.
A. important nutritionally
B. all surprisingly heavy in sugar
C. more expensive than cheese
D. all equally harmful to teeth
