CHAPTER 12
REVIEW QUESTIONS
12.1It has an equal number of positively charged protons and negatively charged electrons.
12.2It becomes filled with electrons and thus the molecule becomes stable.
12.3In both ionic and molecular compounds, the outer energy levels of the constituent atoms become filled and so the molecule is stable. Ionic compounds are formed when atoms lose (and become positively charged) or gain (and become negatively charged) electrons to form ions. The ions are then attracted to each other by electrostatic forces to form a stable compound. In molecular compounds atoms share electrons in the formation of the compound.
12.4There are three molecules of water. Thus there are 6 hydrogen atoms and 3 water atoms making a total of 9 atoms.
12.5Both mixtures are composed of two or more chemicals. In homogenous mixtures the different types of molecules are evenly dispersed, forming a solution, whereas in heterogeneous mixtures the molecules are unevenly dispersed and form either a colloidal or insoluble suspension.
12.6In a solution the solute particles are small and easily dispersed by Brownian motion and so form a homogenous mixture which is transparent. The solute particles in a colloidal suspension are too large to be evenly dispersed by Brownian motion and tend to be suspended in clumps within the solvent (heterogenous mixture) and have a cloudy to opaque appearance. Insoluble suspensions are formed when the particles of one substance are so large that they cannot be dispersed by Brownian motion and they settle to the bottom of the liquid.
12.7Brownian motion is caused by the buffeting action of freely moving fluid molecules on solid particles suspended in them.
12.8Hydrogen chloride is an ionic compound which separates into its component ions in solution. Since there are more hydrogen ions than hydroxide ions present in the solution, it is acidic.
12.9The solution with pH 2 is more acidic than that with a pH of 6 since the lower the pH value the greater the hydrogen ion concentration.
12.10The ability of substances to dissociate to release hydrogen or hydroxide ions in solution.
12.11There is an uneven distribution of charge on the molecule. Because oxygen is a larger atom than hydrogen, its force of attraction for the shared electrons is greater than that of hydrogen. Thus electrons are pulled towards oxygen giving it a slightly negative charge and leaving a slightly positive charge on the two hydrogen atoms.
12.12Although they exert a force of attraction between charged molecules, hydrogen bonds are not as strong as ionic bonds.
12.13Water is polar, producing strong forces of cohesion.
It is a good solvent.
Water has a very high heat capacity.
Water has a high latent heat of vaporisation and fusion.
Water molecules have a high surface tension and display cohesion and adhesion.
Water is neutral.
Water increases in volume and decreases in density between 4o and 0oC.
12.14The polarity of water is responsible for its forces of cohesion resulting in a high surface tension and increasing the dispersal of solutes by Brownian motion, making water a good solvent. The high heat capacity of water and its high latent heat of vaporisation is related to the amount of energy required to break the hydrogen bonds between water molecules.
12.15An antifreeze is a chemical which depresses the freezing point of a liquid.
12.16Birds and mammals are endothermic and able to control their body temperature by physiological means. Thus they are able to remain active under very cold conditions so long as they have adequate food and water to produce energy from respiration, Fish are ectothermic and cannot control their body temperature in this way. Thus their body temperature fluctuates with environmental temperature. Since the waters of Antarctica are extremely cold, few fish have adaptations which allow them to remain active at these temperatures.
12.17They are in very cold water and any body heat, required for maintenance of life, can readily be lost to the surroundings by conduction.
12.18Their body fluids contain antifreeze substances – salts and glycoproteins – which depress the freezing point to -2.2°C.
12.19Since it can form covalent bonds with up to four other elements, including another carbon, a large number of macromolecules, with different structures and functions and which store large amounts of chemical bond energy, can be formed.
12.20Carbon can form covalent bonds with other carbon atoms.
12.21An unsaturated carbon compound contains one or more multiple carbon-carbon bonds which can be changed by the addition of further atoms, whereas a saturated carbon compound only has single carbon-carbon bonds and so cannot add further atoms to its structure.
12.22A macromolecule is a large carbon compound composed of repeating units or monomers of that substance.
12.23When two monomers join, a hydrogen atom from one molecule and a hydroxide group from the other are removed, creating a bond between the two monomers and thereby releasing a water molecule. This process is called dehydration synthesis or condensation.
12.24Hydrolysis: water and an enzyme specific for the reaction are required.
12.25Monomers: CnH2nOnPolysaccharides: (C6H10O5)n + H2O
12.26a.Disaccharide.
b.Dehydration synthesis.
12.27They are isomers and have different structural formulae. Thus different atoms or groups of atoms will be exposed, giving them different physical and chemical properties.
12.28 R
HO
NCC
H HOH
amino groupcarboxyl groupR is a variable group
12.29They all contain an amino group at one end of the molecule and a carboxyl group at the other end. The central group contains a variable R group which may consist of one or more atoms. This R group gives the amino acid its specific properties. Thus each of the twenty different amino acids has a characteristic R group.
12.30A polypeptide chain is a straight chain of many amino acids each linked by a peptide bond. A protein is formed when this polypeptide chain folds due to interactions between different amino acids along its length.
12.31Heating the protein has caused it to irreversibly lose its specific three-dimensional shape. The protein has become denatured.
12.32a.The protein but not lipid has an amino group, although both have a carboxyl group and variable R groups.
b.There is no set proportion of carbon, hydrogen and oxygen in a lipid but there is in a carbohydrate.
12.33
Three fatty acids independently form an ester bond with an OH group on the glycerol molecule, thus releasing three water molecules.
12.34B: an amino acid or fatty acid. Both amino acids and fatty acids contain a carboxyl group.
12.35Phosphate, a sugar and a base.
12.36A strand of DNA is composed of two chains whilst that of RNA is a single chain.
The sugar in DNA is deoxyribose and that of RNA is ribose.
The base thymine of DNA is replaced by uracil in RNA.
12.37DNA is involved in the transmission of genetic information from one generation to the next and in controlling the action of the cell through protein synthesis. RNA is involved in the translation of the genetic code into protein production by the cell.
12.38a.
b.A: base. B: sugar. C: phosphate.
c.Adenine.
d.By hydrogen bonds.
12.39First Law of Thermodynamics: energy cannot be created or destroyed but can be changed from one form to another. Thus radiant energy can be converted to chemical bond energy in photosynthesis. This energy can be released during respiration to provide energy for life processes. Some of the energy, however, is converted to heat energy and released into the atmosphere.
The Second Law of Thermodynamics: every energy transformation results in a reduction of the useable or free energy of the system, and this results in a general tendency towards greater disorder. Thus, in order for living systems to retain their ‘order’ they must constantly replenish their energy supplies.
12.40All chemical reactions are initiated by an input of energy (activation energy), the amount determined by the type of reaction, the temperature of the reaction, concentration of substrates and presence or absence of a catalyst.
12.41Chemical reactions only take place if the reactants collide, thus any factor which increases the chance of collision will determine if, and at what rate, the reaction will occur.
12.42pH and substrate concentration.
12.43a.They are globular proteins which increase the rate of chemical reactions without being used up in the reaction. Their presence does not alter the nature of the end products of the reaction. A small amount of enzyme catalyses a large quantity of substrate. They are specific in their action which they can catalyse in either direction depending on substrate concentration. They reduce the amount of activation level required for the reaction. Each enzyme operates at an optimum temperature and pH. As the concentration of either substrate or enzyme increases, so does the rate of reaction up to a maximum level. Enzymes can be inhibited by poisons.
b.Each enzyme has an active site, the shape and size of which allows it to interact with a specific substrate. At the active site the substrate molecules are rearranged (chemical reaction) to form products, which now have a different shape and no longer fit the active site. They are therefore released by the enzyme. Thus the enzyme acts as a lock and the substrate a key.
12.44In competitive inhibition the poison either competes for the substrate or the active site on the enzyme. In non-competitive inhibition the poison permanently blocks the active site of the enzyme.
12.45The amino acids.
12.46The amino acids.
12.47The reaction can take place in either direction.
12.48Enzyme A acts as the lock for the amino acids (the key). The enzyme activates the formation of the dipeptide and water which are then released. In a similar way Enzyme B activates the reaction between the dipeptide and water to release the two amino acids.
12.49Each enzyme has an active sites, the shape and side of which only allows it to interact with specific molecules.
12.50B: differ in composition and shape.
12.51ATP is composed of an adenosine unit and three phosphate groups arranged end to end. The last two phosphate groups are attached by bonds which are very weak and unstable. When each is broken the free phosphate can react with another molecule and form more stable bonds, releasing large amounts of energy. Thus removal of one or two phosphates from ATP results in the release of energy which can be used by the cell to do work. Addition of phosphate to AMP or ADP results in the storage of energy. Since ATP is a small molecule it is easily transported from a site of formation to a site where energy is required.
12.52A metabolic reaction is any chemical reaction occurring in the body. A metabolic pathway is a series of small chemical reactions involved in the anabolism or catabolism of a substance. Since a large amount of energy may be required or released in the formation of the substance, a series of small steps, in which energy levels can be controlled, is more efficient than one single reaction.