AND THEIR TRANSFORMATIONS IN THE ORGANISM

Some proteins dissolve in water (for example, egg albumin ("egg white"), forming colloidal solutions, while others dissolve only in the presence of an acid or alkali.

If some concentrated salt solution, such as (NH₄)₂S0₄, is added to a colloidal protein solution, the protein is precipitated. Such a protein can, however, be transferred back into the solution.

A protein can be precipitated irreversibly by heating. Upon heating, a protein "coagulates" (this is called denaturation of proteins) and can no longer be dissolved in water. A boiled egg and the flakes in meat broth are instances of denatured proteins.

More intense heating causes proteins to decompose with the evolution of volatile substances that have the odour of burned feathers. This is a characteristic that makes it possible to distinguish, say, woollen textiles from cotton, or natural silk from the artificial product. The property of proteins to undergo hydrolysis has already been discussed.

There are several colour tests for detecting proteins among other substances. When, for example, concentrated nitric acid is added to a protein solution, a yellow coloration appears. It is this reaction that causes yellow stains to form on the skin of the hands when nitric acid is handled carelessly in the laboratory.

Animal organisms can build up their proteins only from the proteins they get in their food.

These proteins are first decomposed in the digestive tract under the influence of enzymes; the products of this decomposition are amino acids.

The amino acids are then absorbed by the walls of the intestines, enter the blood, and are carried to all the cells of the organism. The cells synthesize proteins specific for that particular organism. These proteins are used primarily to build the organism's cells and tissues. Fats and carbohydrates, it will be remembered, serve mainly as the organism's source of energy.

Parallel with this process of synthesis in the cells, in the course of the organism's life activity, there is constant decomposition of proteins. In this case they are again hydrolyzed to amino acids, and the amino acids are ultimately broken down to carbon dioxide, water, and ammonia. Ammonia is converted in the liver to urea and is discharged from the organism with urine.

Notes on the text:

solution – раствор, растворение

to precipitate – осаждать

volatile – летучий

stain – пятно

to discharge – выпускать, выводить

Задания к тексту:

1) Прочтите текст и ответьте на следующие вопросы:

1. Do proteins dissolve in water?

2. What is meant by the word “coagulate”?

3. Can you give the examples of denatured proteins?

4. What happens upon more intense heating?

5. How is it possible to detect proteins among other substances?

6. What are the products of the protein decomposition in the digestive tract?

7. What serves as the organism’s source of energy?

Найдите в тексте английские эквиваленты следующих слов и словосочетаний:

яичный белок, в присутствии кислоты или щелочи, осаждаться, необратимо, при нагревании, больше не, вареное яйцо, хлопья, мясной бульон, с выделением летучих веществ, запах горелых перьев, шерстяная ткань, шелк, искусственный, среди других веществ, управление, желтые пятна, небрежно, пищеварительный тракт, стенки кишечника, печень, аммиак, мочевина, моча.

3) Переведите следующие предложения на английский язык:

1) Растворимые в воде белки образуют коллоидные растворы.

2) При нагревании белок коагулирует и не может больше растворяться в воде.

3) Образованные летучие вещества имеют запах горелых листьев.

4) Это дает возможность отличить шерсть от хлопка, натуральный шелк от искусственного.

5) Уравнение такой реакции дано несколькими абзацами ниже.

6) Стенки кишечника всасывают аминокислоты.

7) Кровь доставляет аминокислоты во все клетки организма.

8) Эти белки используются главным образом для строительства клеток и тканей организма.

Расскажите о свойствах белка.

Найдите в тексте предложения с эмфатической конструкцией и переведите их.

Опишите последовательно все этапы преобразования белков в организме.

Прочтите описание некоторых из тестов на определение белка и скажите, какой из них Вам знаком. Где и когда Вы его проводили или наблюдали за его проведением?

1. To 2 ml of a protein solution in a test tube add a few drops of concentrated nitric acid. A white precipitate is formed. Heat the contents of the test tube and observe that the protein precipitate turns yellow. Cool the mixture and carefully add a concentrated ammonia solution by drops (in excess); the colour turns to orange.

2. To 2 ml of a protein solution in a test tube add an equal volume of an alkali and a few drops of a weak solution of cupric sulphate. The protein turns reddish violet.

3.  Boil some thoroughly minced meat with water. Filter the liquid through cotton wool and establish by a colour test whether the solution contains any proteins.

4. Ignite a few threads of some fabric and determine by the odour whether the fabric or wool.