Condensation and Hydrolysis
To the right of this page (and in the menu bar above) I have put a number of links to other files on this website showing animations of condensation and hydrolysis.
At the bottom of the page there are also links to related topics at this level on the BioTopics website.
Condensation -
basic principles
The term monomer is given to single (smaller, simple) sub-units from which (larger, complex) molecular stuctures called polymers are built up by condensation.
Condensation
is a chemical
process by which 2 molecules are joined
together to make a larger, more complex, molecule, with the loss of water.
It is the basis for the synthesis of all the important biological
macromolecules (carbohydrates, proteins, lipids, nucleic acids) from
their simpler sub-units.
It is important not to
get
condensation and hydrolysis muddled up, as they are in fact opposite
processes! Condensation is so called because the product is
drawn
together from two other substances, in effect getting smaller by losing
water. It does not give off water to condense and run down
the
window!
In all cases of condensation, molecules with projecting
-H atoms are linked to other molecules with
projecting -OH
groups, producing H2O,
( H.OH )
also known as water,
which then moves away from the original molecules.
A-H
+ B-OH
-->
A-B
+ H2O
More details:
In each of the cases
below, some
details are included in order to show the similarities which explain
the basic processes of condensation and hydrolysis, as well as some
other features which are specific to the categories of
compounds,
and which may be covered in the later sections of the syllabus
specification. In the cases of condensation reactions below, you are
not expected to know all the background details. For example, in each
case specific enzymes are involved ( names unimportant - mostly
synthetases), as well as activation energy in the form of ATP (because
simple molecules do not spontaneously join together to become more
complex or ordered).
In carbohydrates,
the
sub-units to be joined are monosaccharides like glucose.
Both of the groups
which combine are -OH
groups,
(even though there are many single -H
atoms on a glucose molecule). Joining two -OH groups with the
removal of
H2O results in a disaccharide
containing an -O-
bridge between
the 2 monosaccharide units. Between glucose units, these bonds are
usually between carbon 1 of one glucose molecule and carbon 4 or 6 on
the other. Depending on the direction of the -OH group at carbon 1, it may be called an alpha or a beta linkage. The bond so formed is called a glycosidic bond or
link. These links
can be extended many times, resulting in the production of polysaccharides.
In proteins,
the sub-units to
be joined are amino-acids.
The -H
comes from -NH2
(the amino, or amine, group) and the -OH
comes from -COOH
(the
carboxylic acid group) at the end of another amino acid molecule. As
a result, a peptide
bond (- CONH-)
is formed between the two
amino acids, and the product is called a dipeptide. These
links can also be
extended many times, resulting in the production of polypeptides. A
protein is
effectively formed when a polypeptide chain re-organises itself into a
specific shape.
In lipids
(fats and oils) glycerol provides up
to three -OH
groups (it is actually a triple
alcohol) to react with -COOH
(carboxylic acid groups) on so-called fatty
acids. Once again this results in -O- bridges forming
between the
glycerol and each fatty acid chain. The links so formed are called ester bonds.
Glycerol attached to one fatty acid chain is called a monoglyceride;
glycerol molecules
with two fatty acid chains are diglycerides,
glycerol with three attached fatty acid chains are triglycerides. A
similar ester bond
can be used to attach a single group containing phosphate, resulting in
a phospholipid.
Since the maximum number of fatty acid chains or other groups
which can be attached to glycerol is 3, there is no way that
lipids can be continually added to like polysaccharides and
polypeptides.
In nucleic acids,
there is a
similar but somewhat more complex situation.
Different nucleotides
(each
composed of a base, pentose sugar and a phosphate group) are joined by
condensation reactions to form DNA
and RNA.
The outer sections of
the DNA double helix are composed of the pentose sugar deoxyribose
alternating with phosphate groups held together with special ester
links (phosphodiester bonds). In DNA replication, each new strand that
forms is built up from nucleotides according to base pairing rules and
joined by repeated condensation reactions.
In RNA there is a single strand, this time of the pentose
sugar
ribose (each attached to a base) alternating with phosphate groups but also held together by
phosphodiester bonds. In transcription, messenger RNA is built up from
individual RNA nucleotides corresponding (by slightly
different
base pairing rules) to a section of the DNA
template molecule. These nucleotides become joined together in a row by
condensation reactions.
The linkages between nucleotides can effectively be extended to a large
extent, resulting in polynucleotide
chains.
The covalent and fairly permanent covalent linking of pentose sugars
to bases also involves a condensation reaction, but this does not
regularly become made and unmade by condensation and hydrolysis in the
same way as links in the side units.
The much weaker bonding between base pairs (caused by hydrogen bonds) does
not involve the sort of condensation reaction already mentioned.
Hydrolysis - basic principles
Hydrolysis is the opposite to condensation. A large molecule is split
into smaller sections by breaking a bond, adding -H to one section
and -OH to
the other.
The products are simpler substances. Since it
involves the addition of water, this explains why it is called
hydrolysis, meaning splitting
by water.
A-B
+ H2O --> A-H
+ B-OH
Do not confuse this
involvement of water in hydrolysis with making a
solution, in which the role of water is to act as a solvent, rather
than taking part in a chemical reaction. (Hydration is yet another completely different process, involving the addition of water, but not breaking of bonds.)
Also do not assume that this breakdown releases energy, which is
usually produced when the simpler substances are oxidised in
respiration.
Incidentally, all (food) digestion reactions are examples of
hydrolysis, and the involvement of water is often not appreciated.
Generally these reactions are controlled by enzymes such as
carbohydrases, proteases, lipases, nucleases, more specific examples of
which are fairly well known.
In fact the main information which is expected to be known is the type
of enzyme and the bonds which it opens.
However, hydrolysis can also be achieved as a result of the action of
acids (usually accompanied by heat) on complex carbohydrates
(polysaccharides, etc) and proteins (including polypeptides, etc).
More details
Carbohydrases
Amylase breaks down starch (both amylose and the straight chain sections
of amylopectin) into maltose (a
disaccharide composed of 2 glucose units linked by 1 alpha 4 glycosidic
bonds) by
adding -H and -OH from water across the glycosidic bonds.
Maltase then
completes the hydrolysis process, splitting each maltose molecule into
two glucose molecules.
Sucrase (also known as invertase) breaks down sucrose, producing
glucose and fructose.
Lactase (beta
galactosidase) breaks down lactose, breaking bonds between galactose
and glucose.
Proteases
Exopeptidases break open peptide bonds between amino acids, starting at
the end of the polypeptide chain ( and working along the polypeptide
chain to the next ).
Endopeptidases break open bonds between amino acids, starting somewhere
in the middle of the polypeptide chain (perhaps at a specific amino
acid sequence).
Lipases
Lipases break ester bonds between glycerol and fatty acids, or
phosphate groups.
This topic has connections with other BioTopics units :-
Monomers and polymers,
Carbohydrates
Lipids
Proteins
[Originally]