• Unit 3: Biological Molecules

    Biological molecules are the essential molecules needed for life. These molecules can be organic or inorganic. Organic chemistry is the study of carbon, which is an element that forms strong covalent bonds essential for the foundational structures of all living things. Water, salts, acids, and bases are mostly essential inorganic molecules that facilitate many biological processes.

    All organisms contain organic biological molecules – carbohydrates, proteins, lipids, and nucleic acid – that are essential to life. This unit will help you understand the structures and functions of these organic molecules and how our body needs them to function properly.

    Completing this unit should take you approximately 5 hours.

    • 3.1: Water and Organic Molecules

      The important molecules of life include water, salts, acids, bases, and organic compounds. Water is the solvent of life. Living organisms survive because of chemical reactions that occur in the presence of water. Molecules that are dissolved by water (salts, acids, and bases) are hydrophilic, while organic molecules that are nonpolar and unable to form hydrogen bonds with water are hydrophobic. Each type of substance has an important role in the chemical reactions of life.

    • 3.2: Acids and Bases

      pH is the measure of the very reactive hydrogen ion. We use the pH scale to measure the hydrogen ion concentration of biological systems. An imbalance of the level of hydrogen ions can be damaging to life. Acids are molecules that, when dissolved by water, increase the levels of hydrogen concentration in the solution. Bases are molecules that, when dissolved by water, decrease the hydrogen concentration in the solution. Buffers work to maintain pH by regulating levels of hydrogen ions in living things.

    • 3.3: Biological Macromolecules

      Living things are mainly composed of organic or carbon-based molecules. Carbon has four valence electrons, a quality that gives it the ability to form strong covalent bonds in large, complex, and diverse molecules. Living things need this molecule diversity to provide the many structures that have so many different functions. These biological macromolecules of life are carbohydrates, lipids, proteins, and nucleic acids. All organisms have four major classes of large biological molecules (also called macromolecules):

      1. Lipids comprise a diverse set of hydrocarbon molecules (containing hydrogen and carbon). This makes them largely non-polar because the covalent bonds in hydrocarbons (between two carbon atoms or between a carbon atom and a hydrogen atom) feature equal sharing of electrons.

      2. Polysaccharides are complex carbohydrates made up of carbon, hydrogen, and oxygen in a 1:2:1 ratio, giving them an empirical formula generalized as (CH2O)n.

      3. Proteins are enormously diverse in structure and function, yet they all feature the substructure of amino acids. Each amino acid features a central carbon atom simultaneously connected to a hydrogen atom, an amino group, a carboxyl group, and a variable R group.

      4. Nucleic acids are informational molecules with a basic structure. Each of their subunits includes a five-carbon sugar (either ribose or deoxyribose) attached to a phosphate group and a nitrogenous base.


      Knowing the chemical structure that underlies these essential biomolecules not only allows you to recognize them. It allows you to understand how they are constructed within cells and how they chemically interact with each other in metabolism and to give rise to the structural component of organisms.

    • Unit 3 Assessment