2.3: Carbon Chemistry

Citrus College General Biology Textbook · LibreTexts

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    From LibreTexts, under CC BY 4.0, as the source states it; its equations are written in TeX, which the PDF shows typeset. This page at LibreTexts.

    Learning Objectives

    By the end of this section, you will be able to do the following:

    • Explain why carbon is important for life
    • Describe the bonding characteristics of carbon and how they contribute to its prominent role in the chemistry of living things.
    • Describe the role of functional groups in biological molecules

    The large molecules necessary for life that are built from smaller organic molecules are called biological macromolecules. There are four major classes of biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids), and each is an important component of the cell and performs a wide array of functions. Combined, these molecules make up the majority of a cell’s mass. Biological macromolecules are organic, meaning that they contain carbon (with some exceptions, like carbon dioxide). In addition, they may contain hydrogen, oxygen, nitrogen, phosphorus, sulfur, and additional minor elements.

    Carbon

    It is often said that life is “carbon-based.” This means that carbon atoms, bonded to other carbon atoms or other elements, form the fundamental components of many, if not most, of the molecules found uniquely in living things. Other elements play important roles in biological molecules, but carbon certainly qualifies as the “foundation” element for molecules in living things. It is the bonding properties of carbon atoms that are responsible for its important role.

    Carbon Bonding

    Carbon contains four electrons in its outer shell. Therefore, it can form four covalent bonds with other atoms or molecules. The simplest organic carbon molecule is methane (CH4), in which four hydrogen atoms bind to a carbon atom (Figure \(\PageIndex{1}\)).

    Diagram of a methane molecule.
    Figure \(\PageIndex{1}\): Carbon can form four covalent bonds to create an organic molecule. The simplest carbon molecule is methane (CH4), depicted here.

    However, structures that are more complex are made using carbon. Any of the hydrogen atoms can be replaced with another carbon atom covalently bonded to the first carbon atom. In this way, long and branching chains of carbon compounds can be made (Figure \(\PageIndex{2}\)a). The carbon atoms may bond with atoms of other elements, such as nitrogen, oxygen, and phosphorus (Figure \(\PageIndex{2}\)b). The molecules may also form rings, which themselves can link with other rings (Figure \(\PageIndex{2}\)c). This diversity of molecular forms accounts for the diversity of functions of the biological macromolecules and is based to a large degree on the ability of carbon to form multiple bonds with itself and other atoms.

    Examples of three different carbon-containing molecules.
    Figure \(\PageIndex{2}\): These examples show three molecules (found in living organisms) that contain carbon atoms bonded in various ways to other carbon atoms and the atoms of other elements. (a) This molecule of stearic acid has a long chain of carbon atoms. (b) Glycine, a component of proteins, contains carbon, nitrogen, oxygen, and hydrogen atoms. (c) Glucose, a sugar, has a ring of carbon atoms and one oxygen atom.

    Functional Groups

    Functional groups are groups of atoms that occur within molecules and confer specific chemical properties to those molecules. We find them along the “carbon backbone” of macromolecules. Chains and/or rings of carbon atoms with the occasional substitution of an element such as nitrogen or oxygen form this carbon backbone. Molecules with other elements in their carbon backbone are substituted hydrocarbons.

    The functional groups in a macromolecule are usually attached to the carbon backbone at one or several different places along its chain and/or ring structure. Each of the four types of macromolecules—proteins, lipids, carbohydrates, and nucleic acids—has its own characteristic set of functional groups that contributes greatly to its differing chemical properties and its function in living organisms.

    A functional group can participate in specific chemical reactions. Figure \(\PageIndex{3}\) shows some of the important functional groups in biological molecules. They include: hydroxyl, methyl, carbonyl, carboxyl, amino, phosphate, and sulfhydryl. These groups play an important role in forming molecules like DNA, proteins, carbohydrates, and lipids. We usually classify functional groups as hydrophobic or hydrophilic depending on their charge or polarity characteristics. An example of a hydrophobic group is the nonpolar methyl molecule. Among the hydrophilic functional groups is the carboxyl group in amino acids, some amino acid side chains, and the fatty acids that form triglycerides and phospholipids. This carboxyl group ionizes to release hydrogen ions (H+) from the COOH group resulting in the negatively charged COO- group. This contributes to the hydrophilic nature of whatever molecule on which it is found. Other functional groups, such as the carbonyl group, have a partially negatively charged oxygen atom that may form hydrogen bonds with water molecules, again making the molecule more hydrophilic.

    Illustration
    Figure \(\PageIndex{3}\): These functional groups are in many different biological molecules. R, also known as R-group, is an abbreviation for any group in which a carbon or hydrogen atom is attached to the rest of the molecule.
    Table shows the structure and properties of different functional groups. Hydroxyl groups, which consist of upper case O upper case H attached to a carbon chain, are polar. Methyl groups, which consist of three hydrogens attached to a carbon chain, are nonpolar. Carbonyl groups, which consist of an oxygen double bonded to a carbon in the middle of a hydrocarbon chain, are polar. Carboxyl groups, which consist of a carbon with a double bonded oxygen and an upper O upper H group attached to a carbon chain, are able to ionize, releasing H positive ions into solution. Carboxyl groups are considered acidic. Amino groups, which consist of two hydrogens attached to a nitrogen, are able to accept H positive ions from solution, forming H subscript 3 baseline positive. Amino groups are considered basic. Phosphate groups consist of a phosphorous with one double bonded oxygen and two upper O upper H groups. Another oxygen forms a link from the phosphorous to a carbon chain. Both upper O upper H groups in phosphorous can lose an H positive ion, and phosphate groups are considered acidic.

    Hydrogen bonds between functional groups (within the same molecule or between different molecules) are important to the function of many macromolecules and help them to fold properly into and maintain the appropriate shape for functioning. Hydrogen bonds are also involved in various recognition processes, such as DNA complementary base pairing and the binding of an enzyme to its substrate, as Figure \(\PageIndex{4}\) illustrates.

    Illustration
    Figure \(\PageIndex{4}\): Hydrogen bonds connect two strands of DNA together to create the double-helix structure
    Molecular models show hydrogen bonding between thymine and adenine, and between cytosine and guanine. These four DNA bases are organic molecules containing carbon, nitrogen, oxygen, and hydrogen in complex ring structures. Hydrogen bonds between the bases hold them together.

    Contributors and Attributions

    Remixed and/or curated from the following works:

    Fowler, S., Roush, R., & Wise, J. (2013). 2.3 Biological Molecules. In Concepts of Biology. OpenStax (CC BY 4.0; Access for free at https://openstax.org/books/concepts-biology/pages/1-introduction(opens in new window)).

    Clark, M. A., Douglas, M., & Choi, J. (2018). 1.2.4 Carbon. In Biology 2e. OpenStax (CC BY 4.0; Access for free at https://openstax.org/books/biology-2e/pages/1-introduction(opens in new window)).

    2.3: Carbon Chemistry is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Katherine Harker (Citrus College).