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Molecules of Life
Carbohydrates, proteins, and lipids are classes of organic molecules that are essential to
the life processes of all living things. All three classes of molecules are built from carbon,
hydrogen, and oxygen atoms. However, each class has different properties, different ratios
of carbon, hydrogen, and oxygen atoms, and different structures.
Carbohydrates
Carbohydrates usually contain carbon, hydrogen, and oxygen atoms in a ratio of 1:2:1. This
means that for each carbon atom a carbohydrate molecule contains, it also contains twice
as many hydrogen atoms and the same number of oxygen atoms. Carbohydrates are
important because they contain a great deal of chemical energy. When the chemical bonds
in carbohydrate molecules are broken, energy is released.
Monosaccharides, also known as simple sugars, are the simplest carbohydrates and can
contain 3 to 8 carbon atoms. Glucose, galactose, and fructose are the most common
monosaccharides. These three monosaccharides have the same molecular formula,
C6H12O6, but their atoms are arranged differently (see their structure below), giving them
slightly different properties. Glucose is a main source of energy for cells, and plants
produce it during photosynthesis. Galactose is found in milk and is usually combined with
glucose or fructose. Fructose is found in fruits and is the sweetest of the monosaccharides.
Figure 1. Although glucose, fructose, and galactose have the same chemical formula, their
structural differences result in different properties among the three compounds.
Complex carbohydrates are made when two ore more monosaccharides combine to form
larger molecules. Two monosaccharides can combine in a condensation reaction to form a
disaccharide. For example, fructose and glucose combine to form sucrose, which is
common table sugar. Maltose (malt sugar) and lactose (milk sugar) are other
disaccharides.
Figure 2. The dehydration synthesis of a molecule of glucose and a molecule of
fructose produces a molecule of sucrose shown above and a molecule of water.
Three or more monosaccharides can combine to form a polysaccharide. Polysaccharides
are the form in which living things store excess sugar. Animals store glucose in the form of
the polysaccharide glycogen. Glycogen consists of hundreds of glucose molecules strung
together in a highly branched chain. Much of the glucose that comes from food is ultimately
stored in your liver and muscles as glycogen and is ready to be used for quick energy.
In plants, glucose molecules are linked in the polysaccharide starch. Starch molecules have
two basic forms – highly branched chains that are similar to glycogen and long, unbranched
chains that coil like a telephone cord. Plants also make the large polysaccharide cellulose.
The function of cellulose is to make plant cells strong and rigid, and it is the major
component of wood. A single cellulose molecule is composed of thousands of glucose
molecules linked in long, straight chains. These chains tend to form hydrogen bonds with
each other, providing strength to the molecule.
Figure 3. Both starches and cellulose are carbohydrates that are classified as
polysaccharides since they are composed of chains of glucose molecules. While they are
similar, starches can be used as energy sources by the human body while cellulose
cannot.
--------------------------------------------------------BUILD CARBOHYDRATES
1. Build glucose and fructose, two monosaccharides. Their structures can be found in
Figure 1.
 Get signed off on your glucose and fructose models.
2. Build sucrose, a disaccharide, by joining glucose and fructose. This is done through
a dehydration synthesis reaction.
a. Remove an OH from glucose and an H from fructose to build water.
b. Now join the glucose to fructose.
 Get signed off on your sucrose model.
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Proteins
Except for water, protein is the most abundant class of molecules in most living organisms.
Proteins make up many of the structural components of organisms. Examples include
collagen, keratin, and glycoproteins. (Collagen holds tissues together; keratin gives
strength to hair, skin, nails, horns, and feathers; and glycoproteins lubricate joints.) Other
important proteins called enzymes are molecules that enable vital chemical reactions in
living organisms.
Proteins are made of subunits called amino acids. There are 20 different amino acids that
make up the proteins of most organisms. All amino acids have a characteristic backbone
structure that is made up of carbon, hydrogen, nitrogen, and oxygen. One end of this
backbone structure is the carboxyl group (COOH). The other end is the amino group (NH 2).
These ends are joined by a carbon atom to complete the backbone. See this backbone
structure in Figure 4.
Figure 4. An amino acid backbone has an R group branching
from its central carbon atom.
The main difference among different amino acids is found in their R groups. The R group
can be made up of different arrangements of carbon, hydrogen, oxygen, sulfur, and
nitrogen, and these differences allow proteins to perform many different roles in the
chemistry of living things. Glycine has a simple R group, just one hydrogen attached to its
central carbon, while that of alanine is slightly more complex with a CH3 group attached to
its central carbon.
Figure 5. Glycine has a simpler R group than alanine.
Two amino acids bond to form a dipeptide in a condensation reaction. A bond between the
two amino acids is called a peptide bond.
Figure 6. Glycine and alanine bond together in a condensation reaction to
form glycylalanine and a molecule of water.
Amino acids can bond to each other one at a time, forming a very long chain called a
polypeptide. A protein, then, is a large biomolecule formed by linking amino acids together
through peptide bonds. The type of protein, and thus its function, is determined by the kind
of amino acids joined together and the order in which those amino acids are arranged.
Some proteins are very large molecules, containing hundreds of amino acids. Often these
long proteins are bent and folded upon themselves as a result of interactions – such as
hydrogen bonding – among individual amino acids. Protein shape can also be influenced
by conditions such as temperature or the type of solvent in which a protein is dissolved.
When you cook an egg, heat changes the shape of proteins in the egg white. The firm,
opaque result is very different from the clear, runny material with which you began.
------------------------------ -------------------------Lipids
Lipids are made up of a variety of molecules that do not dissolve in water, and they are
commonly known as fats, oils, and waxes. Lipids serve three major roles in living
organisms. They can be used to store energy; they form biological membranes; and they
can be used as chemical messengers.
Lipids are formed from combinations of fatty acids and glycerol. Fatty acids are long
chains of hydrogen and carbon atoms that have a carboxyl group attached at one end. A
carboxyl group is a chemical group consisting of one carbon atom, one hydrogen atom, and
two oxygen atoms (--COOH).
Figure 7. Palmitic acid is represented in the two illustrations above. Like other fatty acids, it has a polar
carboxyl head (right end above) and a nonpolar hydrocarbon tail (left end above).
The two ends of the fatty-acid molecule have very different properties. The carboxyl end of
the fatty-acid molecule is polar and is thus attracted to water molecules. Because of this
attraction, the carboxyl end of the fatty-acid molecule is said to by hydrophilic, which
means “water loving.” In contrast, the hydrocarbon end of the fatty-acid molecule is
nonpolar. This end tends not to interact with water molecules and is said to by
hydrophobic, meaning “water fearing.”
Some chains of fatty-acid molecules consist entirely of carbons that are each bonded to two
hydrogen atoms. In these chains, the fatty acid is described as saturated. This means that
the carbon atoms are each bonded to four other atoms. Some fatty acids contain one ore
more carbon atoms that have only three neighboring atoms (two carbon atoms and one
hydrogen atom). In this fatty acid, a double bond forms between the two carbons. This fatty
acid is described as unsaturated. Fatty acids differ from one another primarily in length,
having different numbers of carbons joined together.
Figure 8. Saturated fatty acids have each carbon atom bonded to four other atoms.
Unsaturated fatty acids have one or more carbons bonded to only three other
atoms.
Some other lipids contain fatty acids, such as phospholipids. Phospholipids form an
important component of the cell membrane. They are made up of fatty acids joined to a
phosphate group through the glycerol molecule. Cholesterol is a lipid with a quite different
structure. It has an intricate ring structure that joins a chain of carbon and hydrogen atoms.
Figure 9. Structure of a phospholipid
Figure 10. Structure of cholesterol