Chapter 2 | The Chemical Foundation of Life

 

  1. Figure 2.4 How many neutrons do carbon-12 and carbon-13 have, respectively?

    Visual Connection

     

    Carbon is indicated by its atomic symbol, a capital C. Carbon has the atomic number six and two stable isotopes, carbon-12 and carbon-13.
    Figure 2.4 Carbon has an atomic number of six, and two stable isotopes with mass numbers of twelve and thirteen, respectively. Its relative atomic mass is 12.011

     

  2. Figure 2.8 An atom may give, take, or share electrons with another atom to achieve a full valence shell, the most stable electron configuration. Looking at this figure, how many electrons do elements in group 1 need to lose in order to achieve a stable electron configuration? How many electrons do elements in groups 14 and 17 need to gain to achieve a stable configuration?

    Visual Connection

     

    Bohr diagrams of elements from groups 1, 14, 17 and 18, and periods 1, 2 and 3 are shown. Period 1, in which the 1n shell is filling, contains hydrogen and helium. Hydrogen, in group 1, has one valence electron. Helium, in group 18, has two valence electrons. The 1n shell holds a maximum of two electrons, so the shell is full and the electron configuration is stable. Period 2, in which the 2n shell is filling, contains lithium, carbon, fluorine, and neon. Lithium, in group 1, has 1 valence electron. Carbon, in group 14, has 4 valence electrons. Fluorine, in group 17, has 7 valence electrons. Neon, in group 18, has 8 valence electrons, a full octet. Period 3, in which the 3n shell is filling, contains sodium, silicon, chlorine, and argon. Sodium, in group 1, has 1 valence electron. Silicon, in group 14, has 4 valence electrons. Chlorine, in group 17, has 7 valence electrons. Argon, in group 18, has 8 valence electrons, a full octet.
    Figure 2.8 Bohr diagrams indicate how many electrons fill each principal shell. Group 18 elements (helium, neon, and argon are shown) have a full outer, or valence, shell. A full valence shell is the most stable electron configuration. Elements in other groups have partially filled valence shells and gain or lose electrons to achieve a stable electron configuration.

     

  3. Figure 2.25 Which of the following statements is false?
    1. Molecules with the formulas CH3CH2COOH and C3H6O2 could be structural isomers.
    2. Molecules must have a double bond to be cis–trans isomers.
    3. To be enantiomers, a molecule must have at least three different atoms or groups connected to a central carbon.
    4. To be enantiomers, a molecule must have at least four different atoms or groups connected to a central carbon.

      Visual Connection

       

      Part A shows butane and isobutane are structural isomers. Both molecules have four carbons and ten hydrogens, but in butane the carbons form a single chain, while in isobutane the carbons form a branched chain. Part B shows cis dash 2 butene and trans dash 2 butene each consist of a four-carbon chain. The two central carbons are connected by a double bond resulting in a planar, or flat shape. In the cis isomer, both terminal upper case C upper case H subscript 3 baseline groups are on the same side of the plane, and two hydrogen atoms are on the opposite side. Imagine a person with arms stretched out and upwards and legs spread apart, with a glove on the left hand and a sock on the left foot: this represents a cis configuration. In cis-butene the terminal upper C upper H subscript 3 baseline groups are on opposite sides of the plane. Now, imagine a person with outstretched arms and legs, but this time with a glove on the left hand and a sock on the right foot: this is what a trans configuration looks like. Part C shows two enantiomers, each with different arrangement of hydrogen, bromine, chlorine and fluorine around a central carbon. The molecules are mirror images of one another.
      Figure 2.25 We call molecules that have the same number and type of atoms arranged differently isomers. (a) Structural isomers have a different covalent arrangement of atoms. (b) Geometric isomers have a different arrangement of atoms around a double bond. (c) Enantiomers are mirror images of each other.

       

  4. Figure 2.33 What kind of sugars are these, aldose or ketose?

    Visual Connection

     

    The molecular structures of the linear forms of glucose, galactose, and fructose are shown. Glucose and galactose are both aldoses with a carbonyl group (carbon double-bonded to oxygen) at one end of the molecule. A hydroxyl (OH) group is attached to each of the other residues. In glucose, the hydroxyl group attached to the second carbon is on the left side of the molecular structure and all other hydroxyl groups are on the right. In galactose, the hydroxyl groups attached to the third and fourth carbons are on the left, and the hydroxyl groups attached to the second, fifth and sixth carbon are on the right. Frucose is a ketose with C doubled bonded to O at the second carbon. All other carbons have hydroxyl groups associated with them. The hydroxyl group associated with the third carbon is on the left, and all the other hydroxyl groups are on the right.
    Figure 2.33 Glucose, galactose, and fructose are all hexoses. They are structural isomers, meaning they have the same chemical formula (C6H12O6) but a different atom arrangement.

     

  5. Figure 2.51 Which categories of amino acid would you expect to find on the surface of a soluble protein, and which would you expect to find in the interior? What distribution of amino acids would you expect to find in a protein embedded in a lipid bilayer?

    Visual Connection

     

    The molecular structures of the twenty amino acids commonly found in proteins are given. These are divided into five categories: nonpolar aliphatic, polar uncharged, positively charged, negatively charged, and aromatic. Nonpolar aliphatic amino acids include glycine, alanine, valine, leucine, methionine, isoleucine, and proline. Polar uncharged amino acids include serine, threonine, cysteine, asparagine, and glutamine. Positively charged amino acids include lysine, arginine, and histidine. Negatively charged amino acids include aspartate and glutamate. Aromatic amino acids include phenylalanine, tyrosine, and tryptophan. For example, in the amino acid glycine, the R group is a single hydrogen; but in alanine the R group is upper C upper H subscript 3 baseline.
    Figure 2.51 There are 20 common amino acids commonly found in proteins, each with a different R group (variant group) that determines its chemical nature.

     

  6. Figure 2.61 A mutation occurs, and cytosine is replaced with adenine. What impact do you think this will have on the DNA structure?

    Visual Connection

     

    Hydrogen bonding between thymine and adenine and between guanine and cytosine is shown. Thymine forms two hydrogen bonds with adenine, and guanine forms three hydrogen bonds with cytosine. The phosphate backbones of each strand are on the outside and run in opposite directions.
    Figure 2.61 In a double stranded DNA molecule, the two strands run antiparallel to one another so that one strand runs 5′ to 3′ and the other 3′ to 5′. The phosphate backbone is located on the outside, and the bases are in the middle. Adenine forms hydrogen bonds (or base pairs) with thymine, and guanine base pairs with cytosine.

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