Course 6.2Lesson 2 of 3
To join two monomers, a cell removes a molecule of water. Amino acids join by peptide bonds, sugars by glycosidic bonds, fatty acids to glycerol by ester bonds, and nucleotides by phosphodiester bonds. A chain of n monomers has n minus 1 bonds. Adding water breaks a bond: this is hydrolysis, as in digestion.
Try the code and the simulations beside the lesson.
Cells build large molecules from small ones, much as a train is built from coaches. The small molecules are monomers. The large ones are polymers.
To join two monomers, the cell removes a molecule of water. This kind of reaction is called dehydration, or condensation. The link that forms is a covalent bond, which is strong.
The cell has four main kinds of link, one for each family of biomolecule.
Amino acids join to make a protein. The carboxyl group, , of one amino acid meets the amino group, , of the next. One molecule of water is given off, and the two amino acids are joined by a peptide bond.
A chain of amino acids is a polypeptide. A chain of amino acids, joined end to end without branching, has peptide bonds.
Monosaccharides, the single sugars, join to make disaccharides and polysaccharides. A bond forms between two carbon atoms of two neighbouring sugars, and water is given off. This is the glycosidic bond.
Two glucose units joined this way make the double sugar maltose. Starch, glycogen and cellulose are long chains of glucose joined by glycosidic bonds.
A fat is built from glycerol and fatty acids. The acid group of a fatty acid meets a hydroxyl group of glycerol, and water is given off. The new link is an ester bond. Joining a fatty acid to glycerol in this way is called esterification.
Glycerol has room for three fatty acids. A fat made with all three, a triglyceride, has three ester bonds. Three molecules of water are given off when it forms.
Nucleotides join to make a nucleic acid. The phosphate group of one nucleotide links the carbon of its own sugar to the carbon of the sugar of the next nucleotide. Each of these two links, between the phosphate and a hydroxyl group of a sugar, is an ester bond.
The phosphate group makes two ester bonds, so the link is called a phosphodiester bond. A strand of nucleotides has of them. They make the sugar and phosphate backbone that you met in the double helix.
The two ends of a strand are different. One end is the end and the other is the end. This is why the two strands of DNA run in opposite directions.
You should find that a chain of monomers has bonds, and molecules of water are given off. With monomers of daltons, the chain has a mass of daltons. The chain is lighter than the monomers added together, because each bond takes away one water molecule of daltons.
Add water and the reaction runs backwards. This is hydrolysis. It is how the body digests food. Enzymes called proteases break peptide bonds. Amylases break glycosidic bonds. Lipases break ester bonds. Nucleases break phosphodiester bonds.
The two strands of DNA are held together by hydrogen bonds between bases, which are much weaker than the covalent bonds of the backbone. So the strands can be pulled apart without breaking either backbone.
The program joins ten monomers of three kinds and works out the bonds and the mass of each chain.
You should see bonds and water molecules in each case. The mass of the chain of ten glucose units is daltons. With units there are bonds.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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