Monomers, polymers & dehydration synthesis

Small pieces, long chains

Life runs on four big families of molecules: carbohydrates, proteins, nucleic acids and lipids. Because many of them are huge, they're called macromolecules ("macro" means large).

Three of these families are built the same way. A small building block, called a monomer, is joined to another, and another, until you have a long chain called a polymer ("many parts"). Think of beads on a necklace: each bead is a monomer, and the finished necklace is the polymer.

• Carbohydrates: the monomers are simple sugars (monosaccharides) such as glucose. Polymers include starch, glycogen and cellulose.
• Proteins: the monomers are amino acids. A chain of them is a polypeptide.
• Nucleic acids: the monomers are nucleotides. Polymers are DNA and RNA.

Building up: dehydration synthesis

To link two monomers, the cell removes a water molecule from between them. One monomer gives up a hydroxyl group (–OH) and the other gives up a hydrogen atom (–H). Those pieces leave together as H₂O, and the two monomers are left sharing a new covalent bond. Because water is removed, this is called dehydration synthesis (also known as a condensation reaction).

1. 1Two monomers sit side by side, each with reactive groups on its ends (for example, an –OH on one and an –H on the other).
2. 2An enzyme holds them in position and removes the –OH from one monomer and the –H from the other.
3. 3The removed atoms combine to form one molecule of water, which is released.
4. 4The two monomers are now joined by a covalent bond. The new chain still has free ends, so the process can repeat.

Every new link releases one water molecule. So a chain of n monomers made this way has released n − 1 water molecules.

Breaking down: hydrolysis

Taking a polymer apart is the same reaction run backwards. A water molecule is split: its H goes to one side of a bond and its OH goes to the other, and the bond breaks. This is hydrolysis ("splitting with water"). It's how your digestive system turns the starch in bread into glucose, and the proteins in food into amino acids.

Specific enzymes speed up each hydrolysis. Amylase breaks down starch, proteases such as pepsin break down proteins, and lipases break down fats. Each enzyme fits a particular kind of bond, which is why no single enzyme digests everything.

Four kinds of link

The bond formed depends on which groups react. Each family has its own name for its link:

• Glycosidic bond (carbohydrates): joins two sugars through an oxygen atom, from an –OH on one sugar and an –OH on the next.
• Peptide bond (proteins): joins the carboxyl group (–COOH) of one amino acid to the amino group (–NH₂) of the next.
• Phosphodiester bond (nucleic acids): a phosphate group links the sugar of one nucleotide to the sugar of the next, forming the sugar–phosphate backbone.
• Ester bond (fats): joins the –COOH of a fatty acid to an –OH of glycerol. A fat has three of these, so making it releases three water molecules.

The energy side of the story

Building a polymer is an uphill reaction: it takes an input of energy. Breaking one by hydrolysis releases energy. That sounds as if polymers should fall apart on their own, and in a sense they do, but very slowly. Without an enzyme, a single peptide bond in water at 25 °C has an estimated half-life of several hundred years. Enzymes make hydrolysis fast enough to be useful.

So how do cells push the uphill building reactions? They pay for them with energy from ATP or similar molecules. In practice cells rarely just pull water out of two bare monomers. Instead they first "activate" each monomer by attaching an energy-rich group. Glycogen is built from glucose that has been attached to UDP (UDP-glucose). Amino acids are loaded onto transfer RNA using ATP before they're added to a protein. DNA and RNA are built from nucleoside triphosphates, which carry their own energy in their phosphate groups.

The monomer decides what the polymer can do

A polymer's properties come from its monomers and how they're linked. Starch and cellulose are both chains of glucose, but the glucose units are linked with a different geometry. Starch coils into an easy-to-digest store of energy, while cellulose forms stiff fibres in plant cell walls.

In proteins and nucleic acids, the order of monomers matters most. Twenty kinds of amino acid, each with a different side chain, can be arranged in an almost endless number of sequences, and each sequence folds into its own shape. DNA uses only four kinds of nucleotide, but their order along the chain stores genetic information, like letters spelling out words.

Key terms

Macromolecule

A very large biological molecule, such as a polysaccharide, protein or nucleic acid.

Monomer

A small building block that can be joined to others to make a polymer.

Polymer

A long chain of similar monomers joined by covalent bonds.

Dehydration synthesis

A reaction that joins two molecules by removing a water molecule from between them.

Hydrolysis

A reaction that breaks a bond by adding a water molecule across it.

Glycosidic bond

The covalent link between two sugars.

Peptide bond

The covalent link between the carboxyl group of one amino acid and the amino group of the next.

Phosphodiester bond

The phosphate-containing link between the sugars of neighbouring nucleotides.

Ester bond

The link formed between a carboxyl group (such as a fatty acid's) and an alcohol group (such as glycerol's).

Check yourself

Try answering in your head before you open each answer.

• 1.A protein is 300 amino acids long in a single chain. How many water molecules were released when it was made, and how many will be used when it is fully digested?Show answerHide

  299 in each case. There are 299 peptide bonds between 300 amino acids. Making each one released a water molecule, and breaking each one by hydrolysis uses a water molecule.

• 2.A student says that a triglyceride is a polymer because it's built by dehydration synthesis. What's wrong with this argument?Show answerHide

  Being built by dehydration doesn't make something a polymer. A polymer is a chain of repeating monomers. A triglyceride is one glycerol plus three fatty acids joined by three ester bonds, with no repeating chain, so it isn't a polymer.

• 3.Hydrolysis of a peptide bond releases energy, yet proteins in a test tube of pure water survive for a very long time. Why don't they fall apart quickly, and what changes in your stomach?Show answerHide

  Releasing energy doesn't make a reaction fast. Uncatalysed peptide-bond hydrolysis is extremely slow. In your stomach and small intestine, protease enzymes speed up the hydrolysis enormously, so proteins are broken into amino acids within hours.