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Biomolecules (Molecules of Life) Reading
Carbohydrates, proteins, and lipids (fats) 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, but proteins also contain nitrogen. However, each class has
different properties, different ratios of carbon, hydrogen, and oxygen atoms, and different
structures. Most of the biomolecules in our body are macromolecules, known as polymers, that
are made up smaller molecules, known as monomers, chemically bonded together.
Carbohydrates
Carbohydrates are molecules that mostly provide energy to animals. See below how plants use
carbohydrates in other ways than animals. Carbohydrates are classified into three main types:
 monosaccharides
 disaccharides
 polysaccharides.
Monosaccharides and disaccharides are grouped together and called simple carbohydrates,
simple sugars or just “sugars.”
Polysaccharides are also called complex carbohydrates.
Monosaccharides are the simplest carbohydrate molecules and contain 3 to 8 carbon atoms.
They also contain hydrogen and oxygen atoms. Glucose, galactose, and fructose are the most
common examples of 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.
Disaccharides are two monosaccharides monomers
chemically bonded. The prefix “di—“ means “two.” Sucrose
is an example of a disaccharide and is what we refer to as
table sugar. It is formed from the combination of fructose and
glucose (two of the monomers above).
Polysaccharides are three or more monosaccharides bonded
together. The prefix “poly—“ means “many.” There are three
common polysaccharides in nature:
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
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Glycogen - glycogen consists of hundreds of glucose
molecules bonded together in a chain and is how animals
store glucose. Some of the glucose that comes from food
is stored in your liver and muscles as glycogen and is
ready to be used for quick energy.
Starch – starch also consists of hundreds of glucose
molecules bonded together in a chain but is how plants
store glucose. Starch molecules have two basic forms –
highly branched chains that are similar to glycogen and
long, unbranched chains that coil like a telephone cord.
Cellulose - plants also make the large polysaccharide
cellulose. The function of cellulose is to make plant cells
strong and rigid. 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.
Proteins
Except for water, protein is the most abundant class of molecules in most living organisms.
There are several types of proteins. Some of these include:
 Structural – Examples of structural proteins include collagen, keratin, and glycoproteins.
(Collagen holds tissues together; keratin gives strength to hair, skin, nails, horns, and
feathers; and glycoproteins lubricate joints.)
 Transport – Transport proteins, as their name implies, move other molecules around an
organism. One example of a transport protein is hemoglobin, which carries oxygen in red
blood cells from the lungs to cells and also carries carbon dioxide from the cells back to
the lungs.
 Enzymatic – Enzymes are proteins that are responsible for most of the chemical
reactions that take place in cells. They typically either break down large molecules
(biomolecules) into smaller molecules (monomers) or build biomolecules from monomers.
They will also convert monomers of one biomolecule into monomers of another. One
example is lactase, an enzyme that breaks down lactose into glucose and galactose.
 Hormonal – Hormone proteins send are chemical messengers that are sent throughout
the body to signal wide-spread changes. One example of a hormonal protein is growth
hormone, which when released, stimulates bone and muscle cells to grow. Another
example is oxytocin, which when released during pregnancy, stimulates the uterus to
contract, resulting in childbirth.
Proteins are made of monomers 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 (NH2). These ends
are joined by a carbon atom to complete the backbone.
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.
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.
Lipids
Lipids are made up of a variety of molecules that do not dissolve in water, and they 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. The three main types of lipids are:
 Fats
 Phospholipids
 Steroids
Fats are large molecules (macromolecules) that are
composed of three fatty acids bonded to a glycerol
molecule. 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).
Two different types of fatty acids exist, saturated and
unsaturated. Saturated fatty acids have exactly two
hydrogen atoms bonded to each carbon atom, except on the
end, and only have single bonds between each carbon atom.
Unsaturated fatty acids have one or more carbon atoms
bonded to only one hydrogen atom, and one or more double
bonds between their carbon atoms.
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.
Structure of a phospholipid
Steroids are a class of lipids with a quite different structure. They all have intricate ring
structures that join a chain of carbon and hydrogen atoms. Cholesterol, show below, is an
example of a steroid. Testosterone is another example of a steroid.
Structure of cholesterol