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
Lichens can be spectacular for anyone who cares to look ,but few people take the trouble. Often modestly colored and seemingly two-dimensional as they cling to whatever surface they find, they grow in the background; as though designed to be ignored. Yet they hold a special fascination for botanists, partly because they present mysteries still to be solved and partly because they do so many things so well.
No casual observer of a lichen would ever suspect that it was a composite of interacting life forms. The seemingly uncomplicated lichen is actually composed of a fungus and a colony of algae (or blue green algae, which some scientists now consider to be bacteria). A few species even include all three of these diverse forms of life. A complete lichen is strikingly different from its separated partners in both appearance and biochemistry; many produce unique compounds which cannot be made by the component organisms alone.
Lichens grow in almost every natural habitat imaginable, from deserts to tropical rain forests—even on the back of certain beetles in New Guinea and inside rocks(along with algae) in the otherwise barren dry valleys of Antarctica.
Many species can not tolerate extreme heat, cold or dryness. Very few, however, can survive heavy air pollution, and many live only where the air is very clean. The disappearance of lichens from an area gives warning of a threatened environment.
1. Which of the following would be the best title for the passage?
A. The versatility and complexity of the lowly lichen
B. The hidden characteristics of algae colonies
C. The disappearance of the lichen species
D. The habitats of spectacular fungi
2. The author states that lichens grow “as though designed to be ignored” because they are
A. not totally understood by botanists
B. troublesome to collect for the purposes of study
C. uncomplicated in their internal structure
D. not easily noticed by observers
3. 3.According to the author, most people are unaware that lichen is a
A. leafy plant
B. class of simple bacteria
C. two-dimensional life form
D. Combination of organisms
4. 4.The “unique” compounds mentioned in the second paragraph are produced
A. through the cooperative efforts of the lichen’s parts
B. only under laboratory conditions
C. through one of the three possible processes
D. once in the lichen’s life cycle
5.5.The author implies that lichens might be used to.
A. find water sources
B. destroy unwanted plant life
C. test for air purity
D. provide food in remote areas
