Lesson 1.3 · Unit 1 · CED topic 1.3
Introduction to macromolecules
Cells need molecules far larger than water or glucose: enzymes, DNA, cell walls, and energy stores. Instead of inventing a new giant structure for each job, cells build large molecules by linking many small, similar ones, the way a long necklace is strung from beads. The small units are monomers. A chain of many of them is a polymer, and a very large polymer is a macromolecule. Joining and splitting monomers follows the same two chemical reactions every time, and this lesson is about those reactions.
Describe the chemical reactions that build and break biological macromolecules.
Four classes, one set of reactions
| Class | Monomer | Polymer | Bond that joins them |
|---|---|---|---|
| Carbohydrates | Monosaccharide (simple sugar) | Polysaccharide | Glycosidic linkage |
| Proteins | Amino acid | Polypeptide | Peptide bond |
| Nucleic acids | Nucleotide | DNA or RNA strand | Phosphodiester bond |
| Lipids | Not true polymers: built from smaller parts such as glycerol and fatty acids | Ester linkage (in fats) | |
Notice that lipids do not fit the monomer and polymer pattern. A fat is assembled from one glycerol and three fatty acids, which are not repeating copies of one unit, so a fat is not a polymer. The reaction that builds it is still the same one.
Dehydration synthesis and hydrolysis
Dehydration synthesis joins two smaller molecules by a covalent bond. A hydroxyl group (−OH) is removed from one monomer and a hydrogen (−H) from the other. Together they form a water molecule, so the reaction releases H2O, and the two monomers are now connected. Repeating it many times is polymerization. The name says what happens: de-hydration means water is taken out.
Hydrolysis is the reverse: it breaks a covalent bond between monomers by adding water. The hydrogen of the water molecule goes to one monomer and the hydroxyl to the other, restoring both. The name means water (hydro) is used to split (lysis) the bond. Digestion is hydrolysis: enzymes in your gut break the polymers in food into monomers small enough to absorb.
| Dehydration synthesis | Hydrolysis | |
|---|---|---|
| Water | Released (a product) | Added (a reactant) |
| Covalent bond | Formed | Broken |
| Size of molecules | Smaller molecules become larger | Larger molecules become smaller |
| Energy | Needs an input of energy (cells use ATP) | Releases energy |
| Example | Amino acids → polypeptide; glucose → starch | Digesting protein in the stomach; breaking down glycogen |
Both directions need enzymes (Unit 3) to proceed at a useful rate in a cell. Hydrolysis releases energy, but the bonds are stable, so without an enzyme the reaction would take far too long.
Counting water and mass
Each bond formed in a linear polymer releases one water, so joining \(n\) monomers by \(n-1\) bonds releases \(n-1\) water molecules. That gives the mass of the polymer:
\[ M_{\text{polymer}} = n\,M_{\text{monomer}} - (n-1)(18.02) \] \(M\) is molar mass in g/mol and 18.02 g/mol is the molar mass of water. The polymer is lighter than the sum of its monomers because \(n-1\) waters were removed. Hydrolysis of the same linear polymer back into \(n\) monomers consumes \(n-1\) water molecules. (A circular polymer, such as a bacterial chromosome, has \(n\) bonds instead.)
A cell links 20 glycine monomers (\(M = 75.07\) g/mol) into one linear polypeptide. Find the number of waters released and the molar mass of the polypeptide.
Bonds and water. 20 monomers joined in a line make \(20 - 1 = 19\) peptide bonds, so 19 water molecules are released.
Mass. \[ M = (20)(75.07) - (19)(18.02) = 1501.4 - 342.38 = 1159.02 \approx 1.16 \times 10^{3}\ \mathrm{g/mol} \]
The polymer is 22.8 percent lighter than the 20 free glycines (1501.4 g/mol). If it were hydrolyzed completely, 19 water molecules would be added back and the result would be 20 free glycine molecules.
A triglyceride forms from one glycerol (\(M = 92.09\) g/mol) and three palmitic acid molecules (\(M = 256.43\) g/mol each). How many waters are released, and what is the molar mass of the fat?
Each fatty acid joins the glycerol through its own bond, so \(3\) waters are released. \[ M = 92.09 + 3(256.43) - 3(18.02) = 92.09 + 769.29 - 54.06 = 807.32\ \mathrm{g/mol} \] This matches the known molar mass of tripalmitin, 807.3 g/mol. The same arithmetic worked even though a fat is not a polymer, because the reaction is the same.
- Say where the water goes. Dehydration synthesis releases water; hydrolysis uses water. A common error is to say a water molecule is “lost” in hydrolysis.
- Say which atoms are involved. The H and the OH come from the two monomers being joined, not from somewhere else. In hydrolysis the H and the OH of the added water go to the two new ends.
- Reading a diagram (the “describe the visual representation” skill): look at the arrow's direction, whether H2O is on the left or right, and whether the monomers end up joined or separated.
- Do not call a fat a polymer.
- Two glucose molecules react to form maltose and one water molecule. Name the reaction, say which atoms form the water, and state what happens to the covalent bonds.
Show answer
The reaction is dehydration synthesis. The water is formed from the −OH of one glucose and a −H from the other. A new covalent bond forms between the two glucose units (a glycosidic linkage), which makes the disaccharide maltose. The reverse reaction, hydrolysis, would add one water and break that bond.
- A cell assembles a linear polypeptide from 50 alanine monomers (\(M = 89.09\) g/mol). How many peptide bonds form, and what is the molar mass of the polypeptide?
Show answer
50 monomers make \(50 - 1 = 49\) peptide bonds, releasing 49 waters. \(M = (50)(89.09) - (49)(18.02) = 4454.5 - 882.98 = 3571.52 \approx 3.57 \times 10^{3}\) g/mol.
- A linear strand of DNA contains 240 nucleotides. How many water molecules are needed to hydrolyze it completely into free nucleotides?
Show answer
The strand has \(240 - 1 = 239\) bonds between nucleotides, and hydrolysis breaks each one by adding one water molecule, so 239 waters are needed.
- A researcher blocks the enzyme that catalyzes hydrolysis of glycogen in liver cells. Predict the effect on the release of glucose into the blood, and justify your prediction.
Show answer
Prediction: less glucose is released, and blood glucose would fall during fasting. Justification: glycogen is a polymer of glucose, and glucose is freed from it only by hydrolysis of the bonds between units. With the enzyme blocked, the bonds are not broken at a useful rate, so few glucose monomers are produced.
- Fats are made by dehydration synthesis, yet lipids are not called polymers. Explain.
Show answer
Fats are built by dehydration synthesis: each of three fatty acids bonds to glycerol, with a water released at each ester linkage. But a polymer is a chain of many repeating, similar monomers, and a fat is only one glycerol and three fatty acids, which are different kinds of molecule and are not repeated into a long chain. So the reaction is the same, but a fat is not a polymer.