CH. 2-5 TEST

Which of the following best describes the formation of the bond shown in Figure 1 ?

A peptide bond is a covalent bond, as shown by the straight line in Figure 1. There are no ions or pairs of charges shown in Figure 1.

Which of the following best describes the structures of carbohydrates?

They occur as monomers, chains of monomers, and branched structures. Carbohydrates occur in many forms. Glucose is an example of a monomer, sucrose is a chain of two carbohydrate monomers, and starch is an example of a carbohydrate with a branched structure.

Which of the following is responsible for the cohesive property of water?

Hydrogen bonds between the oxygen atom of one water molecule and a hydrogen atom of another water molecule.

Which of the following correctly illustrates a dipeptide and an amino acid in the optimal position to form a tripeptide?

It demonstrates an ability to refine and/or interpret a representation to explain the synthesis of a biological polymer. Peptides or proteins are polymers of amino acid monomers arranged in a unique linear sequence. Each of the 20 amino acids consists of a carbon atom surrounded by an amine group (NH2), a carboxyl group (COOH), a hydrogen, and an R (variable) group. When two amino acids are positioned so that the carboxyl group of one amino acid backbone is adjacent to the amine group of another amino acid backbone, they can join by a dehydration reaction.

Polypeptides are continuously being formed and degraded. One of these processes is shown. *Figure 1. Polypeptide reaction* Which statement is the most accurate description of the reaction shown in Figure 1?

It represents a polypeptide chain that is broken down through a hydrolysis reaction. The reaction shows the polypeptide being split into two smaller units with the addition of a hydroxide ion and a hydrogen ion from water. This facilitates the breaking of the bond holding the polypeptide together.

Which of the following conclusions is most clearly supported by the representations of nucleic acid #1 and nucleic acid #2 ?

Nucleic acid #1 contains adenine-thymine base pairs, whereas nucleic acid #2 does not. The representation of nucleic acid #1 shows adenine-thymine base pairs, whereas the representation of nucleic acid #2 shows only uracil in the nucleotide chain and does not show any base pairing.

Researchers compared similar proteins from related organisms in different habitats. They found that the proteins from organisms living in harsh environments had a greater number of cysteine amino acids than did proteins from organisms not living in harsh environments. The structure of cysteine is shown. Bonds can form between the sulfur atom of different cysteine amino acids (S-S bonds). *Figure 1. Chemical structure of cysteine* Which of the following best describes the effect of a greater number of cysteine amino acids on the stability of the proteins?

The change leads to increased protein stability because of an increased number of S-S bonds in the tertiary structure of the proteins. An increased number of S-S bonds are possible with the addition of more cysteine in the proteins. The S-S covalent bonds should add more structural stability to the proteins.

Figure 1 represents a common process that occurs in organisms. *Figure 1. Structural formula for a common biological reaction* Which of the following is an accurate description of the process shown in Figure 1?

The formation of a covalent peptide bond in a dehydration synthesis reaction. This reaction combines two smaller amino acids to form a dipeptide (a step in the formation of a protein) with the release of water. The presence of nitrogen is consistent with both amino acids and proteins. The combining of smaller molecules to form a larger molecule with the release of water is a dehydration synthesis reaction.

A small protein is composed of 110 amino acids linked together in a chain. As shown in Figure 1, the first and last five amino acids in the chain are hydrophobic (have nonpolar and uncharged R-groups), whereas the remaining 100 amino acids are hydrophilic (have charged or polar R-groups). The nature of the R-group determines if the amino acid is hydrophobic or hydrophilic. A mutation results in the production of a version of the small protein that is only 105 amino acids long, as shown in Figure 2. Five of the hydrophobic amino acids are missing from one end of the chain. Which of the following best depicts the tertiary structures of the two proteins in water? The diagrams in the options are not drawn to the same scale as those in Figure 1 and Figure 2.

The hydrophilic middle of the protein chain would be attracted to the water molecules, and the hydrophobic ends would be repelled. Hydrophobic portions of proteins in water tend to be found near the center of the tertiary structure, as far from the water as possible.

Water and ammonia interact to form hydrogen bonds, as represented in the figure. Which statement best helps explain the formation of the hydrogen bond represented in the figure?

The nitrogen has a partial negative charge, and the hydrogen attached to the oxygen has a partial positive charge. The greater electronegativity of oxygen and nitrogen compared with hydrogen makes for unequal sharing of electrons, which results in partial negative charges associated with the oxygen and nitrogen atoms and partial positive charges associated with the hydrogen atoms in both molecules. The attraction between opposite charges results in the formation of the hydrogen bond represented in the figure.

Figure 1 is a diagram of water molecules at the air-water interface at the surface of a pond. *Figure 1. Alignment of water molecules at air-water interface* Based on Figure 1, which of the following best describes how the properties of water at an air-water interface enable an insect to walk on the water's surface?

Hydrogen bonds between molecules at the surface of the water provide surface tension, which allows the water surface to deform but not break under the insect. Hydrogen bonds form between water molecules due to the partial charges produced as a result of the polarity of the water molecule. The strength of the attraction between the water molecules allows organisms to walk across the surface without breaking the hydrogen bonds.

Which of the following is most directly responsible for water's unique properties?

It forms hydrogen bonds.

The molecular structures of linoleic acid and palmitic acid, two naturally occurring substances, are shown in the figure. Based on the molecular structures shown in the figure, which molecule is likely to be solid at room temperature?

Palmitic acid, because the absence of carbon-carbon double bonds allows the molecules to pack closely together. Linoleic acid is an unsaturated fatty acid. As shown in the figure, linoleic acid has two carbon-carbon double bonds that prevent the molecules from packing closely together, which increases the fluidity of the substance and makes it less likely to form a solid at room temperature.

*Figure 1. Nucleic acid segments* DNA and RNA are nucleic acids that can store biological information based on the sequence of their nucleotide monomers. Figure 1 shows a short segment of each of the two types of nucleic acids. Which of the following best describes a structural difference between DNA and RNA?

The backbone of DNA contains deoxyribose, whereas the backbone of RNA contains ribose. The backbones of DNA and RNA are composed of an alternating sugar and base. In DNA, the sugar is deoxyribose, whereas the sugar in RNA is ribose.

Figure 1 represents a nucleic acid fragment that is made up of four nucleotides linked together in a chain. *Figure 1. Nucleic acid fragment* Which of the following characteristics of Figure 1 best shows that the fragment is RNA and not DNA?

The identity of each nitrogenous base. RNA contains uracil, whereas DNA contains thymine.

Which of the following best describes how amino acids affect the tertiary structure of a protein?

The interactions of the different R-groups with other R-groups and with their environment determine the tertiary structure of the protein. The tertiary structure is determined by the attractions, repulsions, and bonding between different amino acids. Changing the sequence of even two amino acids may affect the tertiary shape of the protein.

The synthesis of protein or carbohydrate polymers always produces which of the following as a byproduct?

Water

A culture of Spirogyra (an autotrophic alga) is maintained in a water solution containing dissolved carbon dioxide and a source of phosphates but lacking nitrogen compounds. A researcher determines the rates of synthesis of several organic compounds found in the Spirogyra before and after several weeks in the water solution. Which of the following graphs best illustrates a likely result of the experiment?

Without a source of nitrogen, the Spirogyra cannot synthesize proteins or nucleic acids, both of which contain nitrogen. This answer reflects that the student is able to represent graphically the exchange of molecules between an organism and its environment and the use of these molecules in synthesis.

A feature of organic compounds NOT found in inorganic compounds is the presence of

carbon atoms covalently bonded to each other.