Dehydration builds; hydrolysis breaks. One water per bond: n monomers in a line → n − 1 waters.
Cells build big molecules by linking small ones, one covalent bond at a time. Each new link gives off a water molecule, and each break takes one back in. What the chain does depends on which subunits it holds and their order.
Key terms
- Monomer — a small building block that can link into a chain: a monosaccharide, an amino acid, or a nucleotide.
- Polymer — a long chain of monomers joined by covalent bonds, like starch, a protein, or DNA.
- Dehydration synthesis — joining two monomers by removing an H from one and an OH from the other. Those leave as water.
- Hydrolysis — the reverse. Adding a water molecule across a bond splits two monomers apart. Digestion is hydrolysis.
- Hydroxyl group — an OH bonded to a carbon. It is polar, so it hydrogen bonds with water and makes a molecule more water-soluble.
How it works
- An enzyme brings two monomers together. One gives up an OH, the other gives up an H.
- The H and OH leave as one water molecule. A covalent bond now joins the two monomers.
- In a protein, the bond links the carboxyl group of one amino acid to the amino group of the next. That is a peptide bond.
- Repeat down the chain. One water leaves per bond, so a straight chain of n monomers releases n − 1 waters.
- Hydrolysis runs it backward. A water molecule goes into each bond, and the monomers come free.
- In proteins and nucleic acids, the order of monomers is information. Same parts, different order → a different molecule with a different job.
Why it matters
This one reaction pair runs the rest of the unit. Carbohydrates, proteins, and nucleic acids all grow by dehydration synthesis and come apart by hydrolysis, each with its own bond: glycosidic, peptide, or phosphodiester. Exam questions ask you to count waters, name the bond, predict what an enzyme does to a polymer, or explain why sequence matters.
Exam traps
| Trap | Actually |
|---|
| Hydrolysis joins monomers; dehydration splits them. | Swap them. Dehydration synthesis removes water to build a bond. Hydrolysis adds water to break one. |
| Fats are polymers built from fatty acids. | Fats are not polymers. A fat is one glycerol joined to three fatty acids, not a long chain of repeating units. |
| Molecules with more oxygen hold more energy. | Oxygen-rich molecules are already partly oxidized. Fats, low in oxygen, release more than twice the energy per gram of carbohydrates. |
| The enzyme gets used up as the reaction runs. | Enzymes come out unchanged and work again and again. A slowing reaction usually means its starting material is running out. |
Worked example
A cell joins 101 amino acids into one straight polypeptide. Before joining, the free amino acids have a combined mass of 12,928 daltons (atomic mass units). A water molecule has a mass of 18 daltons. What is the mass of the finished chain?
- Count the bonds, not the monomers. 101 amino acids in a line are held together by 100 peptide bonds.
- Each peptide bond forms by dehydration synthesis, so 100 water molecules leave: 100 × 18 = 1,800 daltons.
- Subtract the lost water: 12,928 − 1,800 = 11,128 daltons.
11,128 daltons.
The chain loses one water per bond, and a straight chain always has one fewer bond than it has monomers. The two ends never bond, so the chain keeps a free amino group at one end and a free carboxyl group at the other. The near-miss is 11,110 daltons, from subtracting 101 waters, one per monomer.