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Domenii: Industria Alimentara, Chimie Generala, Engleza

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Macromolecules are giant molecules with many atoms and very large masses for a molecule. Nearly all macromolecules include the element carbon as a building block, because it is the only element that readily forms giant chains or networks by bonding to other carbon atoms and other elements. Chemists can create macromolecules in laboratories or in factories. Most of the synthetic (laboratory-made) macromolecules are polymers, molecules created by linking together many identical units, called monomers. Living organisms build polymers and other complex macromolecules through natural processes.

A polymer’s properties depend on its size, its monomers, the strength of its bonds, and whether links form between different parts of the molecule. Larger molecules tend to have higher melting points, so macromolecules tend to be solid at room temperature. The type of monomer or monomers affects the polymer structure and its properties. The repeating monomer unit may be polar or nonpolar, depending on the types of atoms it contains and whether they form polar bonds. If the monomers are polar, attractions can form between different parts of the molecule or between the monomers and other molecules. The bonds between the units may be stable, or they may break easily in water or in other substances. Hydrogen bonds linking two parts of a polymer can make it hold a special shape or strengthen it.

Synthetic polymers include the plastics polystyrene, polyester, nylon (a polyamide), and polyvinyl chloride. These polymers differ in their repeating monomer units. Scientists build polymers from different monomer units to create plastics with different properties. For example, polyvinyl chloride is tough and nylon is silklike. Synthetic polymers usually do not dissolve in water or react with other chemicals. Strong synthetic polymers form fibers for clothing and other materials. Synthetic fibers usually last longer than natural fibers do.

Living organisms produce three main types of biological polymers: polysaccharides, proteins, and nucleic acids. Polysaccharides are made of linked sugar molecules, such as fructose and glucose. Plants use sugars to make polysaccharides, such as starch and cellulose, to store energy and form cell walls. Animals eat plants to gain energy from the plants’ sugars and polysaccharides. These molecules are important sources of energy for both plants and animals.

Proteins consist of amino acids linked together. There are 20 different amino acids, which can combine in a myriad of ways to form the protein molecules an organism needs. Protein chains can curl or twist in upon themselves and hold a special form because of hydrogen bonds and other bonds between parts of the molecule. Proteins perform a variety of functions in a living organism. They form the enzymes that make chemical reactions possible in the human body. The protein hemoglobin carries oxygen to cells. Other proteins in the cells use the oxygen to break down the sugar glucose to create the energy the body needs. Proteins also form important bodily structures. Proteins are, for instance, the important part of muscles that enables limbs to bend and the heart to pump. They also form fingernails and hair to protect the skin.

Nucleic acids are macromolecules found in the cell’s nucleus and cytoplasm. There are two classes of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA forms an organism’s genetic code—the set of hereditary instructions that govern the activities of every cell. The DNA Important sources of energy.instructions serve as “blueprints” for all the proteins a cell needs to make. RNA enables a cell to use the DNA blueprints to build proteins. In nucleic acids, sugars link together with phosphorous and oxygen atoms (which together form the phosphate group) to form the macromolecule’s backbone. Nitrogen-containing side chains, called bases, link to the sugars of the backbone. The sequence of the bases forms the code that the cell uses to make proteins. During cellular replication—when a cell divides into two “daughter” cells—the DNA code is copied so that each daughter has a complete set of the original genetic code. (Donald K. Brandvold, Encarta 2000)

1. Answer the following questions:

a) What is the difference between a monomer and a polymer?

b) What are synthetic polymers used for?

c) What is the role of nucleic acids?

d) What is the role of enzymes?

2. Make up questions for the following short answers:

a) As a building block.

b) Twenty.

c) Living organisms.

d) Its size, monomers, the type of bonds, etc.

e) The cell nucleus and cytoplasm.

3. Reinsert the following words in the text:

accurate when differentiate through element between himself chloride table why received split electricity while ions.

Discovery of Molecules

Until the 1800s chemists did not understand the difference ionic and molecular compounds. They considered anything that contained more than one to be a compound. Investigators, such as British scientists Michael Faraday and Henry Cavendish, began to the two when they realized that some compounds, dissolved in water, made the water conduct electricity more easily, while others did not. Cavendish gave electric shocks to measure the conductivity of these water solutions. His results were surprisingly

Dutch chemist Jacobus Hendricus Van’t Hoff (who the first Nobel Prize in chemistry in 1901) and Swedish chemist Svante August Arrhenius explained different water solutions conduct electricity differently. Van’t Hoff determined that salts—such as sodium chloride (NaCl), or salt, and potassium (KCl)—split into two particles when they dissolve in water, substances such as glucose do not split apart when they dissolve. Arrhenius realized that the dissolved salts not only split, but they split into two electrically charged particles, or The ions move the water to conduct electricity. Substances such as glucose do not and thus dissolve into uncharged compound particles that do not conduct , that is, into molecules.

4. Each line contains a mistake. Correct it in the space provided. A number of 6 lines are correct.

When chemists understand the relationships among a molecule’s structure and the properties of the substance containing the molecule, he can create new molecules with best properties or molecules that copy natural substances. For example, pharmaceutical chemists study molecular structures to develop new drugs. Some drugs that dull pain work by fiting into slots on nerves in the body. A scientist can examine the structure of molecules that fit the slot to develop a similarly shaped molecule that works better. Scientists have used their understanding of molecules and molecular structure to make many useful materials, such as the plastics nylon and Teflon, vitamins, pharmaceuticals, and artificial skin and bones. Scientists can also determine weather a substance is likely to be harmfull by comparing its molecular structure with the structures of other molecules that are know to be harmful. Chemists use many tools to study molecules, including lasers, nuclear magnetic resonance (NMR), X-Ray systems , spectroscopes, and computers.

5. Rephrase the following sentences so as to preserve the initial meaning:

a) Scientists have recently developed devices that allow them to study a single molecule at a time.

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