The Water Ledger

AP Biology · Unit 1 · Topic 1.3

The Water Ledger

Two reactions build and break every macromolecule in you. One releases water, one consumes it. Run both and watch the count.

Learning objective 1.3.A Describe the chemical reactions that build and break biological macromolecules.

Build it, then break it

Pick a family of macromolecule. Drag two monomers together until they touch — a bond forms and a water molecule is pushed out. To break a bond, drag a water molecule onto it and the two monomers drift apart. Watch the water ledger.

Which macromolecule

monomer −H available to leave −OH available to leave covalent bond formed water — drag it onto a bond to break it

Drag two monomers together until they touch.
Monomers0
Bonds formed0
Water on the bench0
Water released0
Water used up0

What just happened

Dehydration synthesis builds the bond

A hydrogen ion is removed from one monomer and a hydroxyl group from the other. Those two pieces are the equivalent of one water molecule, which leaves. The two monomers are now joined by a covalent bond.

The count that matters: joining n monomers into one unbranched chain takes n − 1 bonds and releases n − 1 water molecules. Six monomers, five bonds, five waters. Connecting many monomers this way is polymerization.

Hydrolysis breaks the bond

Hydrolysis runs the same chemistry backwards. Water is a reactant, not a product. One water molecule is split across the bond: the hydrogen ion goes to one monomer and the hydroxyl group goes to the other, and the covalent bond is cleaved.

This is what a digestive enzyme does. A protein-digesting enzyme does not tear a polypeptide apart by force — it enables the hydrolysis of peptide bonds, using water as a reactant.

Breaking a chain of n monomers all the way back to free monomers consumes exactly the n − 1 water molecules that building it released. The water sitting on the bench after you build a chain is the same water you need to take it apart — drag it back onto a bond and watch the ledger balance.

The same two reactions, four families

The monomers and the bond names change, but the reaction does not. Switch families on the bench and watch the ledger behave identically.

Learning check

Answer these in your own words before you open the answers.

  1. A polypeptide is 40 amino acids long. Determine how many peptide bonds it contains and how many water molecules were released while it was assembled.
  2. An enzyme in pineapple breaks proteins down into smaller molecules. Explain how the reaction that breaks up those proteins occurs, and state the role water plays in it.
  3. Describe what happens to the hydrogen ion and the hydroxyl group of a water molecule during hydrolysis.
  4. A student says “dehydration synthesis needs water because water is in the name.” Explain what is wrong with that statement.
Show answers

1. 39 peptide bonds and 39 water molecules. An unbranched chain of n monomers has n − 1 bonds, and each bond formed released one water.

2. The enzyme enables hydrolysis of the peptide bonds holding the amino acids together. Water is a reactant: a water molecule is split across each bond, breaking it and freeing shorter chains or individual amino acids.

3. The water molecule splits. Its hydrogen ion is added to one monomer and its hydroxyl group is added to the other, so each fragment ends up with a complete end again and the covalent bond between them is gone.

4. Backwards. Dehydration synthesis produces water rather than consuming it — “dehydration” means water is removed. The reaction that consumes water is hydrolysis.