VIII. Retell the text “Molecules of Life” using the questions after it as a plan.

IX. Read the text using a dictionary and speak on it:

Amino Acids

All proteins, whether from the most ancient lines of bacteria or from the most complex forms of life, are constructed from the same set of 20 amino acids, covalently linked in characteristic linear sequences. What is most remarkable is that cells can produce proteins with strikingly different properties and activities by joining the same 20 amino acids in many different combinations and sequences. From these building blocks different organisms can make such widely diverse products as enzymes, hormones, antibodies, muscle fibers, feathers, spider webs, rhinoceros horn, milk proteins, antibiotics, mushroom poisons, and thousand other substances that have distinct biological activities. Among these protein products, the enzymes are the most varied and specialized.

All amino acids share a common structure. They are organic molecules that possess both carboxyl and amino groups. At the center of the amino acid there is an asymmetric carbon atom called the alpha carbon. Its four different partners are an amino group, a carboxyl group, a hydrogen atom, and the side chain that differs with each amino acid.

The physical and chemical properties of the side chain determine the unique characteristics of a particular amino acid, thus affecting its functional role in a polypeptide. The amino acids are grouped according to the properties of their side chains. One group consists of amino acids with non-polar side chains, which are hydrophobic. Another group consists of amino acids with polar side chains, which are hydrophilic. Acidic amino acids are those with side chains that are generally negative in charge owing to the presence of a carboxyl group, which is usually dissociated (ionized) at cellular pH. Basic amino acids have amino groups in their side chains that are generally positive in charge. Because they are charged, acidic and basic side chains are also hydrophilic.

When two amino acids are positioned so that the carboxyl group of one is adjacent to the amino group of the other, they can become joined by a dehydration reaction, with the removal of a water molecule. The resulting covalent bond is called a peptide bond. This process produces a polypeptide, a polymer of many amino acids linked by peptide bonds. At one end of the polypeptide chain there is a free amino group; at the opposite end there is a free carboxyl group. Thus, the chain has an amino end and a carboxyl end. Polypeptides range in length from a few monomers to a thousand or more. Each specific polypeptide has a unique linear sequence of amino acids. The immense variety of polypeptides in nature illustrates an important concept – that cells can make many different polymers by linking a limited set of monomers into diverse sequences.

Animals require 20 amino acids to make proteins. The majority of animal species can synthesize about half of these amino acids, as long as their diet includes organic nitrogen. The remaining amino acids must be obtained from food in prefabricated form and are therefore called essential amino acids. Most animals, including adult humans, require eight amino acids in their diet. A diet that provides insufficient amounts of one or more essential amino acids causes protein deficiency, the most common type of malnutrition among humans. The proteins in animal products such as meat, eggs, and cheese are 'complete', which means that they provide all the essential amino acids in their proper proportions. In contrast, most plant proteins are 'incomplete' – they are deficient in one or more essential amino acids. To prevent protein deficiency, vegetarian diets must include combinations of plant products that together provide all of the essential amino acids.

Notes to the texts:

muscle fibers – м’язові волокна

owing to – завдяки, внаслідок

dissociated – роздвоєний

dehydration – зневоднювання, дегідратація

as long as – доки, до тих пір

X. Look through the text and decide whether the following sentences are true or false:

1. Muscle fibers are the most diverse and specialized of protein products.

2. Amino acids are organic molecules that possess either carboxyl or amino groups.

3. The alpha carbon is at the periphery of the amino acid.

4. The amino acids are grouped according to the features of their side chains.

5. Acidic and basic side chains are hydrophobic.

6. Cells can make many different polymers by linking a limitless set of monomers into varied sequences.

7. If there is organic nitrogen in their diet, the majority of animals can synthesize about ten amino acids.

Дата добавления: 2015-10-26