Sarah Gibson
BIOL 1610
September 13, 2011
Organic Molecules
Although each and every organism on Earth is completely unique to the next, there are four
organic molecules that help make up nearly all living things. There are four classes of said molecules:
carbohydrates, proteins, nucleic acids, and lipids. All of these molecules but lipids are polymers: long
molecules that are made up of many building blocks, called monomers. These molecules are polar,
therefore hydrophilic, while lipids are not.
Polymers, as said, are long molecules that are formed of monomers. The monomers, like beads
on a string, are joined by covalent bonds. Building these chains is done via anabolic reactions, which
require energy. As a byproduct of these reactions, water is created. Through the process of hydrolysis a
type of catabolic reaction, polymers are disassembled and energy that was stored in bonds is released.
The first class of organic molecules is carbohydrates – sugars. There are three groups of these
sugars: monosaccharides, disaccharides, and polysaccharides. Monosaccharides are the major nutrient
for cell work. Glucose, a monosaccharide, is burned by all cells during respiration, and is only able to be
produced by plant cells. Disaccharides are formed by dehydration reactions between monosaccharides.
Polysaccharides consist of anywhere from three, to thousands of monomers, and are mainly used for
energy storage and structural support in cells. Some types of polysaccharides include starches, glycogen,
cellulose, and chitin.
The second class of organic molecules is proteins, whose monomers are amino acids (the fourth
class). They account for more than fifty percent of the dry mass of a cell; their main function is structural
support. There are twenty different amino acids, some charged, some polar, and some nonpolar, but all
amino acids are classified by their structure: a central carbon with a hydrogen attached, plus an amino
group on the left, a carboxyl group on the right, and an “R” group attached to the central carbon. To
form proteins, amino acids join together with peptide bonds when hydrogen atoms from the amino
groups join with the hydroxides of the carboxyl groups to form water. Because of this dehydration
reaction, proteins are easily identifiable by their beginning with an amino group and ending with a
carboxyl group. The way the amino acids line up determines the shape and function of the protein they
create. An example of this is DNA, which is a protein. Each molecule of DNA has a completely different
line-up of amino acids from the next, therefore a different function.
Lipids are the fourth class of organic molecules. They, unlike the other molecules, are not
polymers. They are broken into three groups: fats, phospholipids, and steroids. Fats are constructed
from fatty acids and glycerol. A fatty acid consists of a carboxyl group attached to a long carbon skeleton
via an ester linkage. A glycerol is a three carbon alcohol with a carboxyl group attached to each carbon.
Fats are further broken into two categories: saturated fats, and unsaturated fats. Saturated fats do not
have any double bonds, which allows for the maximum number of hydrogen atoms. Unsaturated fats
have fewer hydrogen atoms due to the occurrence of double bonds. The major function of fats is energy
storage, but it is also used for protecting bodily organs and as insulation in animals. Plants do not need
this, however, as they have a more compact alternative.
Phospholipids, the second group of lipids, contain only two fatty acids and one phosphate
group. These molecules are very interesting, as the phosphate group gives them a sort of polar “head,”
while the fatty acids give them two, nonpolar tails. When phospholipids are in water, they clump
together and form a micelle by squeezing their heads into a circle with their tails on the interior of that
circle. Because of their unique structure, phospholipids are able to form cell membranes that allow
things to pass in and out of the cells.
The third group of lipids is cholesterol, which is the precursor of all steroid hormones. It is vital
to all animals because it is used in bile, is important to metabolism and hormones, and is a component
of all animal cell membranes. Our bodies naturally produce cholesterol, but it is also acquired through
the foods we eat. An excess of cholesterol can cause cardiovascular disease. Interestingly enough, only
fifteen percent of cases of cardiovascular disease are due to an excess of cholesterol from diet; the
other eighty five percent of cases are caused by people’s bodies’ genetically over-producing cholesterol.
Although there are thousands upon thousands of molecules used to create cells, most can be
classified as carbohydrates, proteins, amino acids, or lipids. These four groups of molecules are organic
molecules, and are very important in cells. Each of the groups has a different function, and therefore
help to determine the functions of cells - the building blocks of every living thing.