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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Insects´ lives are very short and they have many enemies, but they must survive long enough to breed and perpetuate their kind. The less insect like they look, the better their chance of survival. To look “inedible” by resembling or imitating plants, is a deception widely practiced by insects. Mammals rarely use this type of camouflage, but many fishes and invertebrates do.

The stick caterpillar is well named. It is hardly distinguishable from a brown or green twig. This caterpillar is quite common and can be found almost anywhere in North America. It is also called “measuring worm” of “inchworm”. It walks by arching its body, then stretching out and grasping the branch with its front feet, then looping its body again to bring the hind feet forward. When danger threatens, the stick caterpillar stretches its body away from the branch at an angle and remains rigid and still, like a twig, until the danger has passed.

Walkingsticks, or stick insects, do not have to assume a rigid, twiglike pose to find protection, they look like inedible twigs in any position. There are many kinds of walkingsticks, ranging in size from the few inches of the North American variety to some tropical species that may be over a foot long. When at rest their front legs are stretched out. Some of the tropical species are adorned with spines or ridges, imitating the thorny bushes or trees in which they live.

Leaves also seem to be a favorite object for insects to imitate. Many butter flies can suddenly disappear from view by folding their wings and sitting quietly among the foliage that they resemble.

1. What is the main subject of the passage?

A. Catepillars that live in trees

B. The feeding habits of insects

C. How some insects camouflage themselves

D. Insects that are threatened with extinction

2. In lines 1, the word “enemies” refers to.

A. other creatures competing for space

B. extreme weather conditions

C. creatures that eat insects

D. inedible insects

3. According to the passage, how does the stick caterpillar make itself look like a twig?

A. By holding its body stiff and motionless

B. By looping itself around a stick

C. By changing the color of its skin

D. By laying its body flat against a branch

4. Which of the following is true of stick insects?

A. They resemble their surroundings all the time.

B. They make themselves look like other insects.

C. They are camouflaged only when walking.

D. They change color to make themselves invisible.

5. Which of the following are not mentioned in the passage as objects that are imitated as a means of protection?

A. Thorns B. Flowers C. Leaves D. Sticks

6.In which paragraph does the author describe the way in which stick caterpillars move?

A. Paragraph one B. Paragraph two

C. Paragraph three D. Paragraph four

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