Condensation and Hydrolysis


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.
See example?


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.
See example?


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.
See example?
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 ections 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.
See example?

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).
See example?

Lipases

Lipases break ester bonds between glycerol and fatty acids, or phosphate groups.
See example?




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