Chemistry of the Cell
Key Elements
An element is a type of matter composed of only one kind of atom which
cannot be broken down into a simpler structure. There are six elements
commonly found in living cells, sulfur, phosphorous, oxygen,
nitrogen, carbon and hydrogen (easily remembered as SPONCH).
These elements make up 99% of a living tissue and combine to form the
molecules that are the basis of cellular function. Carbon is especially
important because one carbon atom can make covalent bonds with four
other atoms, resulting in the formation of very stable and complex
structures. Carbon is in all living things, as well as in the remains of
living things. Molecules containing carbon are called organic
molecules, while those without carbon are called inorganic molecules.
Water is the most important inorganic molecule for living things and
serves as the medium in which cellular reactions take place.
Those cellular reactions occur in great part between biological
molecules, often called biomolecules. The four primary classes of
cellular biomolecules are carbohydrates, lipids, proteins and nucleic
acids. Each of these is a polymer-that is, a long chain of small repeating
units called monomers. Another term related to molecules is isomer.
Isomers have the same molecular formula but different chemical
structure. The example below has a chemical formula of C3H8O
Carbohydrates
Carbohydrates are often called sugars and are an energy source.
Structurally, they are chains of carbon units with hydroxyl groups (-OH)
attached. They are composed of carbon, hydrogen and oxygen usually
in a 1:2:1 ration. Exp. C6H12O6. The simplest carbohydrates are
monosaccharides. The ends of these sugars bond and unbond
continuously so that the straight-chain and cyclic (ring-like) forms are
in equilibrium.
These monosaccharides may join together to form disaccharides (2),
oligosaccharides (3-10) or polysaccharides (10+), depending on how
many monosaccharides make up the polymeric carbohydrate.
Disaccharides consist of two monosaccharide units. Common table
sugar, or sucrose, and disaccharide formed from the bound
monosaccharides fructose and glucose. Oligosaccharides are made up
of 3-10 monosaccharide units. Oligosaccharides are sugars that are
either being assembled or broken down, so there aren’t any well-known
common names for them. Polysaccharides consist of ten or more
monosaccharide units. Complex carbohydrates such as starch and
cellulose are classified as polysaccharides. Common carbohydrates are
glucose (the sugar produced by photosynthesis) and glycogen
(produced in the liver).
Lipids
Lipids are fats: they are made up of chains of methyl (-CH) units. The
chain may be long or short. They may be straight or fused into ring
(cyclic). Lipids are composed of carbon, hydrogen and oxygen in no
particular ratio. Saturated fats have the maximum number of hydrogen
atoms per carbon, unsaturated fats have at least one carbon to carbon
double bond. They have several functions but are most well known as
fat molecules that store energy. They are also the structural
components of the cell membrane. Several important lipids have names
that you may recognize: waxes, steroids, fatty acids and triglycerides.
Triglycerides are made up of a glycerol and three fatty acid chains. The
figure below depicts a triglyeride.
H2
. .I
. .C - O - CO - CH2 - CH2 ------- CH3
.. I
HC - O - CO - CH2 - CH2 ------- CH3
..I
. .C - O - CO - CH2 - CH2 ------- CH3
..I
. .H2
Nucleic Acids
Nucleic acids are found in the nucleus of a cell. The nucleic acid
polymer is made up of nucleotide monomers. The nucleotide monomer
consists of a sugar, a phosphate group and a nitrogenous base. Nucleic
acids are the backbone of the following genetic material:
A. DNA (deoxyribonucleic acid) directs the activities of the cell
and contains that sugar deoxyribose.
B. RNA (ribonucleic acid) is involved in protein synthesis and
contains the sugar ribose.
Proteins
Proteins consist of long linear chains of polypeptides. The polypeptide
is itself a chain of amino acid monomers. There are 20 standard amino
acids which combine to form every single protein needed by the human
body; protein synthesis will be discussed later.
There are many different types of proteins, all of which have different
biological functions. They include structural proteins, regulatory
proteins, contractile proteins, transport proteins, storage proteins,
protective proteins, membrane proteins and enzymes. Despite the wide
variation in function shape and size, all proteins are made from the
same 20 amino acids. The chemical makeup of the amino acid’s unique
side chain ultimately determines its final bonding structure within the
larger protein molecule. Some side chains contain various elements like
carbon, sulfur or nitrogen. They can be linear, branched or ring-shaped.
Side chains are classified based on how they distribute their molecular
charge: nonpolar, polar or charged. The interaction between the side
chains of the individual amino acids eventually determines the 3dimensional structure of the protein. The structure of the protein
establishes how it will function for the organism. Since mammals
cannot make all 20 amino acids themselves, they must eat protein in
order to maintain a healthy diet. Protein may be eaten in animal (meat)
or vegetable (beans) form, but most organisms must have protein to
survive.
H O
NH2
C C OH
CH
H3C CH3
Amino Acid
Review Questions
1. Define the following terms:
a. Organic molecule
b. Inorganic molecule
c. Monomer
d. Biomolecule
e. Nucleic acid
f. DNA
g. RNA
h. Carbohydrate
i. Lipid
j. Protein
k. Amino acid
l. Polymer
m. polypeptide
2. Carbon chains are principal features of both carbohydrates and
lipids. What is the primary difference between these two types of
biomolecules?
a. Lipids always have longer carbon chains than
carbohydrates
b. Carbohydrates carry hydroxyl groups on their carbon
backbone
c. Carbohydrates cannot form rings and lipids can
d. Lipids provide energy, but carbohydrates do not
3. What molecules make up the bulk of the cell?
a. Carbohydrates
b. Lipids
c. Proteins
d. water
4. Carbon is important to living things because
a. It metabolizes easily, creating a quick energy source
b. It is abundant on the earth’s surface
c. It can form four covalent bonds with other atoms
d. It ahs twelve protons and neutrons
5. Nucleotides are to nucleic acids as amino acids are to
a. DNA
b. Water
c. Proteins
d. carbohydrates
6. All living things have a common tie with the Earth on which we
live. Explain why this is true.
7. What are the six elements commonly found in living things?
8. Why is carbon important to living things?