REGULATION OF METABOLISM
The fact that cells and tissues retain their dynamic equilibrium throughout the life of an organism clearly shows that metabolic processes are under fine control. Cells and entire tissues are constantly dying, yet all the chemical ingredients that replenish and form new cells and their products are supplied by metabolism, striking a nearly perfect balance. Although much remains to be revealed about metabolic processes, biochemists now agree that regulatory, or rate-limiting, enzymes figure largely in the reactions involved. Affecting metabolic pathways at the earliest steps, each enzyme molecule has a specific, or active, site that matches, or "fits," its particular substrate—the compound with which the enzyme forms a product. The precision with which rate-limiting enzymes and substrates join to set off a particular reaction inhibits reactions from occurring indiscriminately in cells, where so many diverse chemical compounds are in flux. Tiny amounts of a rate-limiting enzyme can cause profound changes in the metabolism of a cell. Another way in which metabolic pathways are controlled is through negative feedback. Thus, once a cell synthesizes the correct balance of a product, such as ATP, the accumulation of that product will inhibit the enzymes that trigger its production. Metabolism, especially in higher animals, is also regulated by the nervous system and by the pancreas and the pituitary and adrenal glands of the endocrine system. Hormones, secreted into the bloodstream, reach target tissues, often altering the permeability of cell membranes and thereby altering the amounts of substances that get into and out of cells. Hormones, which also affect plant metabolism, change metabolic pathways by altering the catalytic sites of rate-limiting enzymes. Notes. To set off = to start Within = in Therefore = so, =» Versatile = diverse, various Exercise 1. Match the words in A with the words in B, translate the combinations and memorise them.
Exercise 2. Fill in the gaps with a preposition, translate and memorise the obtained phrases. 1. point____view, 2. 9.3 calories_____gram, 3. source _____energy, 4. to be ______control, 5. secreted _______the bloodstream, 6. ingested ______herbivores, 7. ______fact, 8._______the earliest steps, 9. to derive the energy______the sun. Exercise 3. Sentences in italics have construction Complex Subject, explained in Unit 1. Underline the construction and translate the sentences. Exercise 4. Now read and translate the whole text. Exercise 5. Make 8 short sentences in English based on the text. Work with your partner. Take turns to translate each other's sentences. Exercise 6. Make 8 short sentences in your native language. Work with your partner. Take turns to translate each other's sentences. Exercise 7. Make a short talk on one of the following topics. 1. Anabolism. 2. Catabolism. 3. Food and energy. Exercise 8. Topics for presentation. 1. Metabolic disorders. 2. Diabetes mellitus as a metabolic disorder. 3. Diabetes mellitus. Situation in Ukraine.
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