Chapter 7 · Macromolecules
Building, Breaking and Carbohydrates
Relate monomers to polymers, choose the right reaction for building or breaking, and classify sugars by size and job.
Monomers and polymers
Think of language. Monomers are letters, polymers are words, and biomolecules are sentences. A monomer is a small unit, and many monomers link to make a polymer. Every living cell has thousands of different kinds of polymers.
Two reactions, one water molecule
| Dehydration builds | Hydrolysis breaks |
|---|---|
Joins a monomer to the chain | Splits a bond in the polymer |
A water molecule comes OUT | A water molecule goes IN |
De = remove, hydration = water | Hydro = water, lysis = break |
Makes disaccharides, starch, polypeptides | Happens in digestion |
Worked example
Counting water
A cell links 5 glucose monomers into one straight chain. How many water molecules are released?
Sketch 5 circles in a row. Count the links between neighbors: 4.
Every link is one dehydration reaction, and each one releases one water.
So 4 bonds means 4 waters. The rule: waters = bonds = monomers − 1.
Answer
4 water molecules. Breaking the same chain back into 5 monomers would use 4 waters.
Quick check
Saliva turns a cracker's starch into smaller sugars while you chew. Which reaction is this?
Carbohydrates
Sugars have a fixed ratio of 1 carbon : 2 hydrogen : 1 oxygen and a carbon skeleton that often forms a ring. Glucose, C₆H₁₂O₆, shows the ratio. They come in three sizes.
| Type | Size | Examples |
|---|---|---|
Monosaccharide | 1 sugar unit | Glucose, fructose, galactose, ribose |
Disaccharide | 2 linked units | Sucrose (glucose + fructose), lactose (galactose + glucose), maltose (glucose + glucose) |
Polysaccharide | Many units | Starch, glycogen, cellulose |
Glucose is the main fuel for cellular work. Cells break it down and pull out the stored energy.
Three polysaccharides, one monomer
| Starch | Glycogen | Cellulose |
|---|---|---|
Plant cells | Animal cells | Plant cells |
Sugar stockpile | Sugar stockpile in liver and muscle | Building material: protection, stiffness |
Chains coil up like phone cords | Highly branched | Straight chains |
Humans digest it | Humans make and use it | Humans cannot digest it (fiber) |
Key concept
Same letters, different words
All three are made of glucose monomers. The way the units are linked and arranged gives each one a different job, and it decides whether your enzymes can take it apart.
Carbohydrates and water
All carbohydrates are hydrophilic because of their hydroxyl groups. Mono- and disaccharides dissolve easily. Cellulose absorbs water well but does not really dissolve. Attracting water and dissolving are not the same thing.
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Lipids
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