Cells build huge molecules (starch, proteins, DNA) the way you build a train: link small cars, one at a time. The small units are monomers ("one part"); the chains are polymers ("many parts"). The same two reactions build and break all of them.
Key idea
Dehydration synthesis joins two monomers and releases a water molecule. Hydrolysis splits a polymer by adding a water molecule to each bond.
Why it works
To link two monomers, the cell takes an –OH from one and an –H from the other. Those leave together as H₂O ("dehydration": water removed), and the two monomers share a new covalent bond. Hydrolysis ("water splitting") runs it backward: a water molecule is split, its –H and –OH go back on, and the bond breaks. Digestion is hydrolysis: enzymes in your gut add water to break starch and proteins into sugars and amino acids.
Worked example: Count the water
A cell joins 5 glucose molecules into a short chain. How many water molecules are released?
- Each new bond between two neighbors releases one water.
- A chain of 5 has 4 bonds between neighbors (like 5 train cars have 4 couplings).
- So $5 - 1 = 4$ water molecules are released.
Answer: $4$ water molecules
Worked example: Follow the atoms
Glucose is C₆H₁₂O₆. What is the formula of the disaccharide made by joining two glucose molecules?
- Add two glucose molecules: C₁₂H₂₄O₁₂.
- Dehydration synthesis removes one H₂O.
- C₁₂H₂₄O₁₂ − H₂O = C₁₂H₂₂O₁₁ (maltose).
Answer: C₁₂H₂₂O₁₁
- Carbohydrates: monomers are simple sugars (Carbohydrates).
- Proteins: monomers are amino acids (Proteins).
- Nucleic acids: monomers are nucleotides (Nucleic Acids).
- Lipids are the exception: they are not long chains of one repeating monomer (Lipids).
Common mistake
Reversing the two reactions, thinking water is added to build a chain. Read the names: dehydration means water is taken out, and it happens when a bond is made; hydro-lysis means water splits something, and it happens when a bond is broken. Quick check: counting atoms, the chain is lighter than its monomers added up, by one water per bond.