XII. Reproduce in your own words:

Витамины — пищевые вещества, необходимые для поддержания жизненных функций. Организм человека и животных не синтезирует витамины, поэтому получать их должен в готовом виде. Недостаток витаминов в пище или изменение процессов их усвоений приводит к нарушениям обмена веществ.

Витамины должны поступать в организм постоянно в относительно определенных количествах. Однако содержание эитамйнов в пищевых продуктах подвержено значительным колебаниям и не всегда полностью обеспечивает потребность в них организма. Эти колебания связаны с сезонными изменениями состава пищевых продуктов, неравномерным употреблением ягод, фруктов, овощей.

Открытие витаминов тесно связано с изучением роли отдельных пищевых веществ. Во второй половине XIX в. считалось, что для нормального функционирований организма достаточно определенного содержания в пище белков, жиров, углеводов и минеральных солей и воды.

Однако в течение долгого времени ученым не удавалось установить причину происхождения многих заболеваний. В 1880 г. русский исследователь Н. И. Лунин установил,

что в пищевых продуктах имеются еще неизвестные факторы питания, необходимые для жизни. Выводы Лунина были в дальнейшем подтверждены С. А. Сосиным,а в 1897 г.— врачом Эйкманом. В 1906—1912 годах по предложению Гопкинса и польского ученого Функа эти вещества были названы витаминами.

XIII. Questions for discussion:

1. What are nutrients? Give examples.

2. What is the chief use of sugar and starch in the body?

3. What are the good food sources of sugar and starch?

4. What is meant by a calorie?

5. What nutrient gives the most energy?

6. What is the chief use of protein in the body?

7. What are some good food sources of proteins?

XIV. Give the main points of all the texts of the lesson. Lesson nine

A WHITE-EYED FLY

To scientists, the most important mutation that ever took place happened inside a milk bottle — in an ordinary little fruit fly. For a year, starting in 1909, Thomas Hunt Morgan, professor of zoology, at Columbia University, had been breeding this little fly called Drosophila. Drosophila is a small, ordinary looking insect the sort you often find in grape arbors.

One day ill 1910 professor Morgan noticed a very unusual sight in one of his fly-filled milk bottles. There, among all the red-eyed Drosophila was one with white eyes!

Was the white-eyed fly really something new, or would its offspring go back to the red eyes of the rest of the flies? Professor Morgan bred his white-eyed fly and waited to see what colour the eyes of the breed would be. Some were white! He had discovered a real mutation.

This single white-eyed fly started professor Morgan and his co-workers off on eighteen years scientific work. They are known to have studied 15 million flies and found about 500 mutations. The mutations affected the development of every part of the flies' bodies, their legs, their shape and colour, their internal organs. Through the long years of work in the now famous at Columbia, professor Morgan and his colleagues were able to show that the genes were arranged on the chromosomes like beads on a string. Drosophila seems almost made to order for scientists to study mutation. The flies are known to grow very easily on bananas or other simple food. They are hardly little creatures and will stand up under all kinds of treatment. They are known to have a great many clear, easy to recognize features. They have a very small number of chromosomes - only 8 (man has 48 and the crayfish has 200). Most important of all, these flies breed very rapidly. It takes Drosophila only 12 days from the time an egg is laid to grow into mature fly ready to lay eggs in its turn.

And under the right conditions, a single fly may lay over a thousand eggs. Drosophila, trees, bacteria, molds, and every other living thing —all have genes which pass along from parents to offspring generation after generation. What we learn about genes in one living thing tells us a lot about genes in all living things. In 1933, twenty-three years after the first white-eyed fly appeared in the milk bottle, professor Morgan was awarded the Nobel Prize in medicine for his part in showing how characteristics pass from one generation to another.

A few mutations are very striking and most mutations have little influence on development. Their results are so tiny that we never notice them. Occasionally, however, a mutation may have an important effect because it occurs under just the right set of circumstances. Mutations occur rarely, but over the years they begin to pile tip. Remember that in evolution we deal with many thousands even millions of years. After a million years, offsprings begin to have quite a few genes that are different from their ancestors. But what causes mutations? Unfortunately scientists still know very little about what actually,does make genes mutate. We do know that mutations can pite up in any direction. By X-rays or other treatments, scientists have made mutations take place much more often than they do naturally. So far, they have not been able to control the direction in which mutations take place. But even this may become possible before long. By treating such things as bread, mold and bacteria with certain chemicals scientists are known to make them mutate in direct way. The possibility of directing mutations in more complicated plants and animals will certainly increase as we learn more about the gene's chemistry and understand better what causes mutations in nature.

Notes to the text:

1. for his part — с его стороны

2. hard — твердый, упорно

3. hardly — едва

4. so far — пока

5. before long — скоро

6. under just the right set of circumstances — при удачном стечении обстоятельств

7. to stand up to all kinds of treatment — выдержать все виды обработки

8. to deal with — иметь дело.

EXERCISES