CHAPTER 4 CARBON AND MOLECULAR DIVERSITY

Aside from water, most biologically important molecules are carbon-based (organic).

The structural and functional diversity of organic molecules emerges from the ability of carbon
to form large, complex and diverse molecules by bonding to itself and to other elements such
as H, O, N, S, and P.

I. The Importance of Carbon

A. Organic chemistry is the study of carbon compounds

    Organic chemistry = The branch of chemistry that specializes in the study of carbon
    compounds.
    Organic molecules = Molecules that contain carbon

 B. Carbon atoms are the most versatile building blocks of molecules

    The carbon atom:
        - Usually has an atomic number of 6; therefore, it has 4 valence electrons.
        - Usually completes its outer energy shell by sharing valence electrons in four covalent
        bonds. (Not likely to form ionic bonds.)

    Emergent properties, such as the kinds and number of bonds carbon will form, are
    determined by their tetravalent electron configuration.

        - It makes large, complex molecules possible. The carbon atom is a central point from
        which the molecule branches off into four directions.

        - It gives carbon covalent compatibility with many different elements. The four major
        atomic components of organic molecules are as follows:
            Hydrogen
            Oxygen
            Nitrogen
            Carbon
        - It determines an organic molecule's three-dimensional shape, which may affect
        molecular function. For example, when carbon forms four single covalent bonds, the
        four valence orbitals hybridize into teardrop-shaped orbitals that angle from the
        carbon atoms toward the corners of an imaginary tetrahedron.
 
 
 
 
 
 

C. Variation in carbon skeletons contributes to the diversity of organic molecules

    Covalent bonds link carbon atoms together in long chains that form the skeletal
    framework for organic molecules. These carbon skeletons may vary in:
        - Length
        - Shape (straight chain, branched, ring)
        - Number and location of double bonds
        - Other elements covalently bonded to available sites
    This variation in carbon skeletons contributes to the complexity and diversity of organic
    molecules.

    Hydrocarbons = Molecules containing only carbon and hydrogen
        - Are major components of fossil fuels produced from the organic remains of organisms
        living millions of years ago, though they are not prevalent in living organisms.
        - Have a diversity of carbon skeletons which produce molecules of various lengths and
        shapes.
        - As in hydrocarbons, a carbon skeleton is the framework for the large diverse organic
        molecules found in living organisms. Also, some biologically important molecules may have
        regions consisting of hydrocarbon chains (e.g. fats).
        - Hydrocarbon chains are hydrophobic because the C-C and C-H bonds are nonpolar.

 II. Functional Groups

A. Functional groups also contribute to the molecular diversity of life

    Small characteristic groups of atoms (functional groups) are frequently bonded to the carbon
    skeleton of organic molecules. These functional groups:
        - Have specific chemical and physical properties.
        - Are the regions of organic molecules which are commonly chemically reactive.
        - Behave consistently from one organic molecule to another.
        - Depending upon their number and arrangement, determine unique chemical
        properties of organic molecules in which they occur. As with hydrocarbons, diverse organic
        molecules found in living organisms have carbon skeletons. In fact, these molecules can be
        viewed as hydrocarbon derivatives with functional groups in place of H, bonded to carbon
        at various sites alongthe molecule.

1. The hydroxyl group

    Hydroxyl group = A functional group that consists of a hydrogen atom bonded to
    an oxygen atom, which in turn is bonded to carbon (-OH).
        - Is a polar group; the bond between the oxygen and hydrogen is a polar covalent bond.
        - Makes the molecule to which it is attached water soluble. Polar water molecules are
        attracted to the polar hydroxyl group which can form hydrogen bonds.
        - Organic compounds with hydroxyl groups are called alcohols.
            Example:  Ethyl alcohol
 
 
 

2. The carbonyl group

    Carbonyl group = Functional group that consists of a carbon atom double-bonded
    to oxygen (-CO).
        - Is a polar group. The oxygen can be involved in hydrogen bonding, and molecules with
        this functional group are water soluble.

        - If the carbonyl is at the end off the carbon skeleton, the compound is an
       aldehyde.
            Example:   Glyceraldehyde
 
 
 
 

        - If the carbonyl is at the end of the carbon skeleton, the compound is a
       ketone.
            Example:  Acetone
 
 
 
 

        -  The carbonyl group is the functional group found in sugars.
            Example: glucose
 
 
 
 
 
 
 
 
 
 
 

3. The carboxyl group

    Carboxyl group = Functional group that consists of a carbon atom which is both
    double-bonded to an oxygen and single-bonded to the oxygen of a hydroxyl group
    (-COOH).
 
 
 

        - Is a polar group and water soluble. The covalent bond between oxygen and hydrogen
        is so polar, that the hydrogen reversibly dissociates as H+. This polarity results from the
        combined effect of the two electronegative oxygen atoms bonded to the same carbon.
        Example:  Acetic acid
 
 
 
 
 
 

        - Since it donates protons, this group has acidic properties. Compounds with this
        functional group are called carboxylic acids.

4. The amino group

    Amino group = Functional group that consists of a nitrogen atom bonded to two hydrogens
    and to the carbon skeleton (-NH₂).
        - Is a polar group and soluble in water.
        - Acts as a weak base. The unshared pair of electrons on the nitrogen can accept a
        proton, giving the amino group a +1 charge.
        Example:
 
 
 
 
 

        - Organic compounds with this function group are called amines.

5. The phosphate group

    Phosphate group = Functional group which is the dissociated form of phosphoric acid
    (H₃P0₄).
        - Loss of two protons by dissociation leaves the phosphate group with a negative
        charge.
        Example:
 
 
 
 
 

        - Has acid properties since it loses protons.
        - Polar group and soluble in water.
        - Organic phosphates are important in cellular energy storage and transfer.
            (Example: ATP.)

6. The methyl group

    Methyl group = Functional group which consists of a single carbon and three attached
    hydrogens (CH₃-).

    - Hydrocarbons would only have methyl groups.
    - Fatty acids, oils and waxes would be examples.
    - would be nonpolar and hydrophobic molecules.