• Biomolecules


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If you separate a living organism into its molecular parts, you will find about 70% water. The remainder of each organism is composed of organic molecules, which are necessary for life on Earth. Covalent bonds between carbon and hydrogen atoms form the basis of organic molecules. Other elements such as oxygen, nitrogen, and phosphorus play key roles in life processes by modifying the structure and chemical properties of organic molecules

Every living organism depends on the set of organic molecules comprising its structure and providing life’s functions. Organic molecules called biomolecules create living cells by assembling into cellular membranes, forming internal structures, and storing and relating hereditary information. Biomolecules also perform the activities required to grow, develop, and reproduce cells. Understanding the chemical structure of each of these biomolecules is required before we can begin to understand the complex processes of respiration, photosynthesis, cell division, and gene expression.

structural models for the hydrocarbons: pentane, isopentane and pentane ringAt the base of all organic molecules is the element carbon, whose versatility provides the key to all biomolecule structures. In most organic molecules, a linked chain of carbon atoms forms a backbone to which other atoms attach. The unique number and arrangement of bonds between carbon atoms creates a structurally diverse set of organic molecules in nature. These molecules may be constructed in straight, branched, or ringed carbon chains and may include varying numbers of carbon-carbon single, double, or triple bonds.

Due to carbon’s versatile bonding ability, multiple molecules may share identical molecular formulas but possess different three-dimensional structures. These isomers use the same number and kind of atoms, but arrange the chemical bonds between atoms differently. Despite a matching set of atoms, each isomer possesses unique functional characteristics because of its unique structure. The structural formulas of three isomers, each with molecular formula C5H12, are illustrated, showing how one set of atoms can create at least three different structures.

The simplest organic molecules, hydrocarbons, contain only hydrogen atoms bonded to a carbon backbone. The carbon backbone ranges from one carbon atom in methane, or natural gas, to thousands of carbon atoms, such as in polyethylene, a commonly used plastic. Forming the structural basis of many biomolecules in living cells, hydrocarbons are utilized as starting materials and released as breakdown products during many chemical reactions.

Many students mistakenly assume the “hydro” in hydrocarbons refers to water, leading to a second assumption that hydrocarbons contain oxygen as well as hydrogen and carbon. The “hydro” in hydrocarbon actually refers to hydrogen; oxygen is not present in these organic molecules. Understanding hydrocarbon structure allows us to understand more about how modifying this basic structure adds functionality to biomolecules.

oil pumpDespite containing only carbon and hydrogen atoms, hydrocarbons are a structurally diverse group of molecules, including fossil fuels, several kinds of plastic, and paraffin wax. Fossil fuels such as coal, oil, and natural gas burn readily, reacting with oxygen to release energy as the hydrocarbon molecules are broken apart, forming carbon dioxide and water. Fossil fuels are not always recognized as organic material, although the name hints at their origin. However, fossil fuels are organic, created by the decomposition of living organisms over millions of years. Because millions of years are required to create large quantities of fossil fuels, these hydrocarbon energy sources are non-renewable, at least in this millennium! One sociopolitical result of this biological fact is spirited debate, and even outright war, over the possession and use of energy-rich fossil fuels.

Decoding Structural Models

These animations demonstrate how atoms are represented in chemical depictions of organic molecules.

Comparing Structures


Abbreviated Structures I


Abbreviated Structures II


Abbreviated Structures III


Abbreviated Structures IV


Going Molecular

In this activity, you will evaluate each structural model and identify the corresponding molecular formula.


Additional Resources


Hydrocarbon Isomers
Use this online activity to build a butane isomer.