Molecules can be grouped as either inorganic or
organic based on various properties.
We will be looking closer at the structure of
various organic molecules important to living organisms. We will see repeating
combination of atoms associated with the various organic molecules. These
repeating units are known as functional groups and they give the organic
molecule certain chemical and physical properties. If you go on to study organic
chemistry in more detail you will need to learn these properties, however for
biology all we need to know is the functional groupís structure and which
types of organic molecules you would find them in.
The R component of the functional group is
not the symbol for an atom; it refers to the rest or remainder of the organic
molecule. Recall that functional groups are attached to organic molecules.
When studying the structure of these functional groups keep things
simple. The phosphate is the only one with phosphorus in it. Sulfhydryl is the
only one with sulfur in it. Amine is the only one with nitrogen in it. By
thinking of simple ways to remember the unique looking functional groups, you
will have an easier time learning the more confusing ones.
The organic molecules that help form cells are
described as macromolecules. Macromolecules are large organic molecules.
These large organic molecules can also be described as polymers. A
polymer is made by the joining of many smaller units known as monomers. Poly
means many and Mono means one.
We will be studying four groups of macromolecules;
we need to know their polymer name and the monomers that joined to form them.
We will be looking at each group of macromolecule in some
detail in this unit.
Good news, the chemical reaction that joins each
group of monomers to form the polymer is the same. So we only need to understand
this one type of reaction to make polysaccharides, or proteins, or
triglycerides, or nucleic acids.
In order to join monomers together to make a
polymer you remove an OH from one monomer and an H from the other. OH + H =
water so you are removing water in order to join the two monomers together. This
type of reactions is called a dehydration synthesis reaction or a condensation
reaction. You can figure out what is happening in the reaction by just
looking at the reaction name. Dehydration means to remove water and synthesis
means to make. So you are removing water to make a polymer. Condensation is
another name for forming water as well.
Often our cells need to break the large polymers
down to obtain monomers. This is just the opposite of dehydration synthesis.
This type of reaction is called a hydrolysis reaction. Water will be
split putting an OH back onto one monomer and the H onto the other. In doing
this the bond holding the two monomers together is broken separating them.
The first group of organic molecules we will look
at are the carbohydrates. They are used as a quick energy source for our
cells. Some carbohydrates have
structural roles in the cell as well. We will learn more about this later.
The basic unit of a carbohydrate is known as a
monosaccharide. The basic formula of a monosaccharide is (CH2O)n.
This means there are always twice as many hydrogens as carbons and oxygens.
You need to be able to look at a structure of a
carbohydrate and identify functional groups.
looking at the stick (linear) figure of glucose functional groups can easily be
seen. You should be able to pick out many hydroxyl groups as well as an aldehyde
group on the molecule. If you count up the number of carbons, hydrogens, and
oxygens you should come up with the molecular formula as C6H12O6
for glucose. This formula fits the profile of a monosaccharide.
looking at the linear figure of fructose you should see several hydroxyl
functional groups as well as a ketone. If you count up the number of carbons,
hydrogens, and oxygens you should come up with the molecular formula as C6H12O6
for fructose. This formula fits the profile of a monosaccharide.
Why do glucose and fructose have the same molecular
formula but different names? Well it all depends on the way the atoms are
arranged in the molecule and the functional groups present. When molecules have
the same molecular formula but are put together and function differently they
are said to be isomers.
The molecules are shown in their ring structure form that
is how they occur naturally. As the figure clearly shows, one monomer, fructose,
will lose an OH while the other monomer, glucose, will lose a H. This forms
water that is removed leaving the two molecules free to bond. You can see that
an oxygen atom bonds the two molecules together.
The joining of two
monosaccharides together forms what is known as a disaccharide. Sucrose,
which is our common table sugar, is an example of a disaccharide. Two other
disaccharides are lactose and maltose. Lactose is formed by joining glucose and galactose and is called a
milk sugar because it is found in dairy products. Maltose is formed by joining
two glucose molecules together. It is the first product produced when starch (a
polysaccharides of glucose) is digested.
Polysaccharides are formed when many
monosaccharides link up. We will consider four polysaccharides all based on the
linkage of the same monosaccharide, glucose.
Starch vs. Glycogen:
Both of these molecules are formed by joining many many glucose units together.
They differ structurally with the fact that starch has few glucose side branch
chains while glycogen has many. The function of each molecule is basically the
same however they differ in the type of organism they are found in. Starch is
the storage form of glucose in plants while glycogen is the storage form of
glucose in animals. Both types of cells will store the excess glucose in the
polysaccharides form that can quickly be broken down if the glucose monomers are
Cellulose vs. Chitin:
Both of the molecules are formed by joining many many glucose units together.
They arrangement of the bond holding each glucose together is different then the
type of bond in starch and glycogen. We do not have enzymes to break this bond
so we cannot digest cellulose or chitin. Cellulose is the structural form of
glucose in plants, it helps form their cell wall and is what we call fiber in
our diet. It is formed when many long glucose chains hydrogen bond together.
This bonding of chains produced a much stronger molecule, hence its function.
Chitin forms the exoskeleton of insects, crabs, and lobsters. It is formed from
a long chain of glucose with amino groups attached.
Lipids: This group
of organic molecules is often referred to as fats. They have many functions.
They provide a long term energy storage, can insulate organisms from extreme
heat (camelís hump) or extreme cold (whale blubber). They protect internal
organs from damage and act a cushion or shock absorber. In plants they form a
protective waxy coating to prevent water loss at the leaves.
Fats or oils are the common term for triglycerides.
Recall that a triglyceride is formed by joining a glycerol molecule with three
fatty acids. Triglyceride fats can be further categorized based on the type of
bonding that occurs between the carbon atoms in the fatty acid tails.
Waxes: Waxes are
long chain fatty acids with long chain alcohols. They are extremely hydrophobic
(water hating) and make great waterproofers because of this. Plants have a waxy
cuticle on the surface of leaves to prevent water loss. Aquatic animals, such as
ducks, have a waxy coating over their feathers to protect the underlying down
feathers from getting wet.
will learn a lot more about phospholipids later in this unit. They are unusual
molecules because they have two personalities, one part is hydrophobic (water
hating) and the other part is hydrophilic (water loving). They consist of
glycerol bonded to two fatty acid tails and one phosphate group. In water they
arrange themselves into a phospholipid bilayer which as we will see will become
the backbone of all cellís plasma membrane.
are not made from long carbon chains, they are formed by carbon rings. They play
an important role in stabilizing the animal cell plasma membrane. They also make
up a class of hormones, chemical messages, in our body. They help our various
organs communicate with each other.
have many functions in our cells. They can be structural to help the cell keep
its shape. Proteins also have many chemical roles including acting as organic
catalysts to speed up chemical reactions in our cells.
The basic subunit of a protein is the amino acid.
It consists of a central Carbon atom with an Amine group, Carboxyl group,
Hydrogen, and R group attached. The R group stands for something unique for each
amino acid. For living organisms there are 20 different kinds of amino
acids each one is different because of the R group.
When two amino acid join up to form a protein they
always react in a specific way. The amino end of one amino acid will
react with the Carboxyl end of the other amino acid. Once the water is
removed the two amino acids will be joined between the Carbon of one amino acid
and the Nitrogen of the other. This type of covalent bond is given the special
name of Peptide Bond. You should have noticed that the reaction joining
the two amino acids was a condensation reaction. We remove water to join the two
amino acids together.
Proteins are very complicated molecules. In order
for them to function properly they must take on a specific three dimensional
shape. There are various levels to the folding of the protein into its three
Due to R group interactions portions of the chain will be attracted to or
repelled from other portions. This will cause the protein to fold further into a
glob type of shape. This glob shape is not random and must be maintained
in order to the protein to function.
What happens if the proteinís shape is changed?
When environmental conditions such as temperature
or pH cause a change if protein shape to the extent that it can no longer
function; it has been denatured. I am sure we have all witnessed this,
for example the white of an egg is liquid and clear at room temperature.
However, when we cook the egg (heat) the white turns solid and white. This
change is due to changes in shape of the albumin (egg white protein) protein
brought about by heating during cooking.
The last group of organic molecules is known as the
nucleic acids. We will learn a lot more about this group of molecules in
later units so we will just briefly mention them here.
Nucleic acids are very large organic molecule made
by joining small subunits known as nucleotides. A nucleotide has three
parts to it. A nucleotide always contains a sugar, at least one phosphate group,
and a nitrogen containing base region. For our studies we will deal with only 5
possible nitrogen containing bases : adenine, guanine, cytosine, thymine, and
Some nucleotides function alone others join up to
form nucleic acids.
ATP: This is a
nucleotide named Adenosine Triphosphate. From its name you can probably guess
that adenine is the nitrogen containing base and that three phosphate groups are
present. We will learn a lot more about this molecule in unit two, it is the
energy molecule for the cell.
We will learn more about these in unit two also. They are organic molecules that
help our enzymes function. Enzymes speed up chemical reactions in our cells and
certain enzymes required helpers to get their job done. These helpers are the
vitamins in our diet and are called nucleotide coenzymes.
DNA (deoxyribonucleic acid) and RNA (ribonucleic
acid) are nucleic acids from which our genetic material is
formed. We will learn much about these two molecules in unit three and four. The
DNA is the blueprint for making all of the proteins in our cells and the RNA
functions as a disposable copy of the DNA blueprint. We will see how these two
molecules interact and function at a later date.