The Different Roles of Macromolecules in Biology
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There are four types of macromolecules that I am going to describe:
Proteins, carbohydrates, lipids and nucleic acid. I will also describe
the functions and why they are important in our bodies.
Proteins are polymers of amino acids that are joined head-to-tail in a
long chain that is then folded into a three-dimensional structure
unique to each type of protein. The covalent linkage between two
adjacent amino acids in a protein (or polypeptide) chain is called a
There are twenty amino acids that make up proteins. Each amino acid
has a typical generic structure as depicted in the diagram 1, the only
variance in each amino acid lies in a unique side chain (R group).
Diagram 1: [IMAGE]
Most of the amino acids have a carboxyl group and an amino group as
shown above. At physiological pH the natural amino acids exist as
zwitterions, with a negatively charged carboxyl group and a positively
charged amino group. The side chains vary greatly in their complexity
and properties. Amino acids are classified by the chemical nature of
their side chains. Five of the 20 amino acids have side chains that
can form ions in solution and thereby can carry a charge. The others
are uncharged: some are polar and hydrophilic and some are non-polar
Proteins are not linear molecules as suggested when we write out a
"string" of amino acid sequence, -Lys-Ala-Pro-Met-Gly- etc., for
example. Rather, this "string" folds into an intricate
three-dimensional structure that is unique to each protein. It is this
three-dimensional structure that allows proteins to function. Thus in
order to understand the details of protein function, one must
understand protein structure.
Protein structure is broken down into four levels. Primary structure
refers to the "linear" sequence of amino acids. Proteins are large
polypeptides of defined amino acid sequence (diagram 2). The sequence
of amino acids in each protein is determined by the gene that encodes
it. The gene is transcribed into a messenger RNA (mRNA) and the mRNA
is translated into a protein by the ribosome.
1. Primary structure is sometimes called the "covalent structure" of
proteins because, with the exception of disulfide bonds (see
below), all of the covalent bonding within proteins defines the
primary structure. In contrast, the higher orders of protein
structure (i.e. secondary, tertiary and quartenary) involve mainly
2. Secondary structure is the "local" ordered structure brought
about via hydrogen bonding mainly within the peptide backbone. The
most common secondary structure elements in proteins are the alpha
(ï¡) helix and the beta (ï¢) sheet.
3. Tertiary structure is the "global" folding of a single
polypeptide chain. A major driving force in determining the
tertiary structure of globular proteins is the hydrophobic effect.
The polypeptide chain folds such that the side chains of the
non-polar amino acids are "hidden" within the structure and the
side chains of the polar residues are exposed on the outer
surface. Hydrogen bonding involving groups from both the peptide
backbone and the side chains are important in stabilizing tertiary
structure. The teriary structure of some proteins is stabilized by
disulfide bonds between cysteine residues.
4. Quartenary structure involves the association of two or more
polypeptide chains into a multi-subunit structure. Quartenary
structure is the stable association of multiple polypeptide chains
resulting in an active unit. Not all proteins exhibit quartenary
structure. Usually, each polypeptide within a multisubunit protein
folds more-or-less independently into a stable tertiary structure
and the folded subunits then associate with each other to form the
final structure. Quartenary structures are stabilised mainly by
non-covalent interactions; all types of non-colvalent
interactions: hydrogen bonding, van der Walls interactions and
ionic bonding, are involved in the interactions between subunits.
In rare instances, disulfide bonds between cysteine residues in
different polypeptide chains are involved in stabilizing
Proteins are associated with many functions in order for a cell to
sustain its life. The following is a list of functions that are
carried out by proteins:
* Proteins are important structural components in cells: actin,
myosin and tubulin are proteins found in the cytoskeleton.
* Tubulin is a spherical protein which is built up into long threads
called microtubules. Microtubules help form the spindle apparatus
used to separate chromosomes during nuclear division. Microtubules
are found in plant and animal cells.
* Proteins can act as receptors on cell membranes.
* Haemoglobin is an important transport protein responsible for
carrying oxygen in red blood cells.
* Plasma proteins such as albumin are important as buffers in the
* Some proteins have hormone action, e.g., insulin.
* Proteins are rarely used in animals as food stores, the exception
being the large bulk of albumen which is used to provide raw
material for the growth and development of embryos.
* Some plants, particularly legumes, use proteins are a food store
in their seed (peas, beans).
* Some proteins have catalytic properties, these are termed enzymes.
Enzymes are imperative for many chemical reactions in cells.
We now focus on a second family of macromolecules that are essential
in the make up of cells, carbohydrates. Carbohydrates contain only
three elements: carbon hydrogen and oxygen.
The simplest carbohydrates are sugars. Sugars are classified into
monosaccharides (these contain only one sugar molecule as shown in
Disaccharides (which contain two sugar molecules) complex
Carbohydrates and so on. Disaccharides consist of two monosaccharides
linked together by a dehydration synthesis (a chemical reaction that
is associated with the release of water molecules) Diagram 4. Sucrose
is common disaccharide which functions as a transport sugar in plants.
Lactose is another disaccharides which is commonly called milk sugar
(energy, constituent of nucleotides).
This diagram shows the synthesis of sucrose from glucose and fructose
via a dehydration synthesis. Notice that a hydrogen is removed from
the glucose and a hydroxyl (OH) from the fructose leaving an oxygen to
link the two molecules together.
A polysaccharide is a large molecule containing many sugar molecules.
Sugars tend to be sweet and easily soluble in water. Polysaccharides,
such as starch, are not sweet and are usually insoluble in water.
Lipids are comprised of just carbon, hydrogen and oxygen but not in
the same proportions as carbohydrates. There are various types of
lipids and they are categorised according to their physical
Oils - liquid at room temperature.
Fats - liquid or solid at room temperature.
Waxes - solids at room temperature
The human body contains all three types of lipids. Most lipids are
stored in the body as triglycerides. A triglyceride molecule contains
glycerol and three fatty acid: Glycerol, Fatty acids and Phosphate
Nucleic acids include deoxyribonucleic acid and ribonucleic acid
Nucleic acids are employed in cells as both polymers and as monomers
Polymers of nucleic acids are the stuff of genes
Nucleic acid monomers are called nucleotides
Nucleotides consist of three parts: a sugar (deoxyribose in DNA,
ribose in RNA), a phosphate group attached to the sugar, and a nitrogenous
base, also attached to the sugar
This is the ribonucleic acid known as adenosine mono-phosphate (AMP)
As you can see above, the carbon that the phosphate group is attached
to; this carbon is called the 5' (five-prime) carbon
Note the carbon the nitrogenous base is attached to (in this case
adenine); this carbon is called the 1' (one-prime) carbon
Also that in ribose the 2' carbon is has a hydroxyl group attached to
it whereas in the deoxyribose the 2' carbon is bound instead by two
Ribonucleic acid (RNA)
Two distinct types of nucleic acids are found in cells, ribonucleic
acid (or RNA) and deoxyribonucleic acid (or DNA. Ribonucleic acid is
distinct in that its backbone sugar is ribose rather than deoxyribose.
Ribonucleic acid additional employs a different nitrogenous base than
DNA. Finally, RNA is employed for different functions in the cell than
DNA. RNA serves roles particularly in the conversion of genotype
information to phenotype information.
Deoxyribonucleic acid (DNA)
The backbone sugar of deoxyribonucleic acid, or DNA, called
deoxyribose, possesses one less hydroxyl group than the backbone sugar
of RNA, called ribose (this hydroxyl group is missing in
deoxyribose-hence, by the way, the name-from the 2' carbon)
DNA is the molecule of chromosomes and genotype in most organisms
Functions of the four given macromolecules
Proteins: catalysis; transport support, growth and repair, nutrients,
immunity and coordination
Carbohydrates: energy storage, cell walls and structural support
Lipids: storage, high-energy store, hormone production
Nucleic acids: Encodes genes and needed for gene expression
From the above information you can see that macromolecules play a big
part in our life. The Interactions of Macromolecules In living cells
certain macromolecules may be covalently bonded to one another. Some
proteins are capable of recognizing and affecting other
macromolecules, such as proteins, carbohydrates, lipids, and DNA.
Raven and Johnstone (2004) Biology, McGraw-Hill