Each bond gives off one water
- The idea
- Predict
- See it
- Why
- Check
The idea
“Each monomer gives off one water, so 6 monomers joined together means 6 waters.”
One water per monomer sounds right. Each water forms between two neighbors, though, and the end monomers only have one.
The short version
The mistake: Each monomer gives off one water, so 6 monomers joined together means 6 waters.
What's actually true: Each bond gives off one water. A linear chain of n monomers has n - 1 bonds, so it releases n - 1 waters.
- Dehydration synthesis removes an -OH from one monomer and an -H from the next. They leave as one water, and a covalent bond forms.
- Each end of a chain has a neighbor on only one side, so n monomers in a row are linked by n - 1 bonds.
- Hydrolysis runs it backward. Breaking each bond uses one water, so splitting that chain apart takes n - 1 waters.
- For mass, add up the free monomers and subtract 18 daltons per bond. Three glucoses at 180 daltons each make 540. Two bonds remove 36, leaving 504 daltons.
On the exam: Count bonds to count waters. A chain of n monomers in a row has n - 1 of each.