AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/1/1-3)
Unit 1 · Topic 1.3
1.3 Introduction to Macromolecules
The big molecules of life are polymers: long chains of small repeating units called monomers. Cells link monomers together with dehydration synthesis, which removes the equivalent of a water molecule for each bond, and break polymers apart with hydrolysis, which adds water back. These two reactions are how you digest food and how your cells rebuild it into your own molecules.
Key terms
- monomer
- polymer
- covalent bond
- dehydration synthesis
- hydrolysis
Monomers and polymers
A monomer is a small building-block molecule. A polymer is a large molecule made of many monomers joined by covalent bonds. Joining many monomers together is called polymerization.
Three of the four macromolecule families are true polymers. Lipids are the exception: they're large and important, but they aren't long chains of one repeating unit.
| Macromolecule | Monomer | Example polymer |
|---|---|---|
| Carbohydrates | Monosaccharide (simple sugar), such as glucose | Starch, glycogen, cellulose |
| Proteins | Amino acid | Enzymes, hemoglobin, keratin |
| Nucleic acids | Nucleotide | DNA, RNA |
| Lipids | Not a true polymer (built from parts such as glycerol and fatty acids) | Fats, phospholipids |
Dehydration synthesis: building up
In dehydration synthesis, two monomers are joined by a new covalent bond. To make that bond, one monomer gives up a hydroxyl group (−OH) and the other gives up a hydrogen atom (−H). Together, the −OH and −H leave as one molecule of water (H₂O). 'Dehydration' means 'losing water,' and 'synthesis' means 'making.'
Each new bond releases one water molecule. So a chain of n monomers has n − 1 bonds and was built by releasing n − 1 water molecules. Building polymers requires energy and enzymes.
Hydrolysis: breaking down
Hydrolysis is the reverse. 'Hydro' means water and 'lysis' means splitting. A water molecule is added across the bond between two monomers. The water splits: its hydrogen attaches to one monomer and its −OH attaches to the other, and the covalent bond between them breaks.
Digestion is mostly hydrolysis. Enzymes in your gut hydrolyze starch into sugars, proteins into amino acids, and fats into fatty acids and glycerol. Your cells then absorb these small pieces and use dehydration synthesis to build their own polymers. Inside cells, hydrolysis also recycles worn-out molecules, for example in lysosomes (2.1).
Energy and enzymes
Neither reaction happens quickly on its own inside a cell. Enzymes (3.1) speed up both the building and the breaking of these bonds, and each enzyme usually works on only one kind of bond. The enzyme that digests starch, for example, doesn't digest proteins.
In general, building a polymer by dehydration synthesis takes an input of energy, and breaking one apart by hydrolysis releases energy. Cells pay for building reactions by pairing them with energy-releasing reactions, often the hydrolysis of ATP, which is itself a hydrolysis reaction (3.3).
Why structure matters
The specific monomers and the order they're joined in decide a polymer's shape and job. The same glucose monomer can build energy-storing starch or rigid cellulose, depending on how the units are linked (1.4). Twenty kinds of amino acids, arranged in different orders, build thousands of different proteins (1.7). This idea, that structure determines function, runs through the whole course.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Counting water molecules
A protein chain contains 150 amino acids. How many water molecules were released when it was built? How many water molecules are needed to completely hydrolyze it back into amino acids?
Show the solutionHide the solution
- Step 1: Picture the chain: the amino acids are linked in a row, so the number of bonds is one less than the number of monomers.
- Step 2: Count the bonds: 150 − 1 = 149 peptide bonds.
- Step 3: Each bond formed by dehydration synthesis released one H₂O, so 149 water molecules were released.
- Step 4: Hydrolysis breaks each bond by adding one H₂O back, so 149 water molecules are used.
Answer: 149 water molecules released when it was built, and 149 water molecules used to hydrolyze it.
- Example 2
Predicting a polymer's formula (classic trap)
Glucose has the formula C₆H₁₂O₆. A short chain of 10 glucose units is built by dehydration synthesis. A student says its formula is C₆₀H₁₂₀O₆₀. What is the correct formula?
Show the solutionHide the solution
- Step 1: The student just multiplied glucose by 10, which ignores the water lost when each bond forms. That's the trap.
- Step 2: Add up 10 glucose molecules: C₆₀H₁₂₀O₆₀.
- Step 3: Count the bonds: 10 − 1 = 9, so 9 molecules of H₂O were removed. That's 18 H atoms and 9 O atoms.
- Step 4: Subtract: H: 120 − 18 = 102. O: 60 − 9 = 51. Carbon doesn't change.
Answer: C₆₀H₁₀₂O₅₁
Common mistakes
- Swapping the two reactions. Dehydration synthesis removes water to build; hydrolysis adds water to break.
- Saying n water molecules are released for n monomers. It's n − 1, because the number of bonds is one less than the number of monomers.
- Calling a fat a polymer of fatty acids. Lipids are built by dehydration synthesis but are not long chains of one repeating monomer.
On the exam
- You may be shown two monomers with their −OH and −H groups and asked to show or identify the products of dehydration synthesis, or to show where water enters during hydrolysis.
- Questions on digestion are hydrolysis questions: name the reaction, say that water is added, and connect it to the monomers the body absorbs.
Connected topics
Videos
Check yourself
4 questions on 1.3 Introduction to Macromolecules. Pick an answer to see if you got it, and why.
A cell links 12 glucose molecules into a single unbranched chain. How many water molecules are released during this process?
Which of the following best describes what happens when digestive enzymes break a protein into individual amino acids?
A glucose molecule has a mass of 180 daltons, and a water molecule has a mass of 18 daltons. A cell joins 10 glucose molecules into one unbranched chain by dehydration synthesis. What is the mass of the chain?
A fat (triglyceride) molecule is made of one glycerol joined to three fatty acids. How many water molecules are needed to completely hydrolyze four fat molecules into glycerol and fatty acids?
0 of 4 answered