Intro Biology  Biological Molecules       (updated 9/25/05)      Return to Science Standards

Biology: The Building Blocks of Life                  A&P I  The Molecules of Life

Organic compounds in cells, their functions and examples.

Carbohydrates - sugars and starches: (general formula: CH2O) - Example- glucose C6H12O6

The simple sugars that we will study in this course contain five or six carbons (pentose sugars and hexose sugars, respectively), bonded to a hydrogen and a hydroxyl group.  A monosaccharide is a single sugar, such as glucose, fructose, and galactose, while disaccharides contain two monosaccharides, such as maltose (composed of two glucose molecules), sucrose (table sugar, composed of a glucose and a fructose), and lactose (composed of glucose and galactose).

Glucose usually forms a ring structure, which is written in 'shorthand' by the second illustration.

The 'shorthand' for disaccharides is shown on the right. 

When many sugars are joined together, polysaccharides (starches) are created.

Lipids: fats, oils, waxes, phospholipids, steroids.  Mostly carbon and hydrogen with few oxygens. Some contain phosphorus.

Synthesis of large molecules occurs by removing a molecule of water from between adjacent monomers (single sugars or other organic compounds), in a dehydration reaction.  In a similar fashion, polymers (such as the starch shown above) can be broken down (as in digestion) by a hydrolysis reaction (hydro = water, lysis = breakdown).  These processes occur in all organic molecules, as illustrated below for lipids, proteins, and nucleic acids.

Saturated fats are found in animals, and are usually solid at room temperature (such as butter and the fat in beef, pork, chicken, people etc.), while unsaturated fats are liquid at room temperature (such as plant oils).


To produce a fat, or triglyceride, three fatty acids must be combined with a molecule of glycerol, by removing three molecules of water through dehydration.  Specific enzymes (catalytic proteins) are required for each reaction, and these enzymes must be synthesized by genetic instructions in the DNA of each cell.  What would be the structure of a mono- and diglyceride?  Notice that two glycerol molecules joined together would resemble a hexose monosaccharide.

Phospholipids are especially important components of membranes within and surrounding cells. However, the phosphate portion is located on the left side of the glycerol, away from the fatty acids. It is hydrophilic, since it contains electrical charges on the phosphate and nitrogen atoms. The fatty acids are hydrophobic, and face away from water.  For this reason, fat can form small droplets in water (coacervates).

Steroids are listed with lipids, since they can be synthesized by the liver from fatty acids.

Cholesterol can be changed into Vitamin D and several hormones that are necessary for normal health.

Proteins - contain C, H, O, N, and some S. Composed of amino acids, which consist of a central C attached to an amino group (NH2), an organic acid COOH, a H atom, and an R (radical) group (which varies in each amino acid).

Proteins and protein-like compounds are synthesized from amino acids (by dehydration), each of which has a similar structure. The R (radical) group differs for each amino acid.  Genetic instructions (in DNA and RNA) are a biochemical code for assembling amino acids in a specific sequence.

From eight essential amino acids required in the diet (nine in infants), we can synthesize the other 12  amino acids, and form  all human proteins.  Notice the sulfur in Cysteine; it is also present in Methionine. 

When amino acids are combined, a peptide bond (between carbon and nitrogen) is formed.  Thus, a protein is often referred to as a polypeptide.  Four levels of structural complexity exist in proteins. The primary structure is simply the sequence of amino acids.

The red balls below represent cysteines, which often form disulfide bonds, in which two  sulfur atoms join together. These bonds are important in giving the protein a particular shape, which determines its function.  Secondary structure involves hydrogen bonding between different loops of strands of amino acids, forming either a helix (a coiled structure), or a beta-pleated sheet, which appears to be folded (notice the shape of silk fibers). Tertiary structure involves both the alpha helix structure and beta folds, to give the molecule a three-dimensional structure, such as in myoglobin (a pigment found in muscle) that attracts oxygen from hemoglobin, a quaternary structure (which transports oxygen within red blood cells).  Hemoglobin is known as a conjugated protein (one composed of more than just a protein, i.e., the heme group). The heme group (not a protein) is called a prosthetic group, and is also present in chlorophyll and other blood pigments, each of which contains a different metallic ion in the center (iron, magnesium, or copper).   


Nucleic acids (DNA and RNA) are composed of nucleotides, which contain a phosphate group, a five carbon sugar (ribose or deoxyribose) and a nitrogenous base.

Chemical composition of Ribose  and Deoxyribose (pentose sugars).

A polynucleotide chain, the basis of both RNA and DNA.

DNA forms a double helix, as illustrated below in an untwisted ladder-like representation.  Note that one side is arranged in the reverse direction, a point that is important in certain functional aspects.

The Building Blocks of Life                                    A&P I  The Molecules of Life

Intro Biology  Biological Molecules         Return to Science Standards