Monomers and Polymers

Essential Question

What are the molecules of life made of?

Your cells are mostly water, and almost all of the rest is made of four kinds of large, carbon-based molecules. Each kind is built from small units that join together like beads on a string. In this lesson, you’ll learn the four classes of large molecules, how small units join to make them, and how cells take them apart again.

Carbon-Based Molecules

• Cells are about 70% water
• Most of the rest is four classes of large molecules
• Carbon forms four bonds, so it can build chains and rings
• The four classes: carbohydrates, lipids, proteins, nucleic acids

The graph shows sample data for a bacterial cell. Water makes up most of the mass, and the largest share of what is left is made of proteins, nucleic acids, lipids, and carbohydrates. These are called macromolecules because they are very large. All four classes are built around carbon. An atom of carbon can form four covalent bonds, so carbon atoms can link into long chains, branches, and rings, and can bond to hydrogen, oxygen, nitrogen, and other elements. That flexibility is why so many different molecules can be built from so few elements.

Monomers and Polymers

• Monomer: a small building block
• Polymer: a chain of many monomers
• A few monomers can make many different polymers
• Lipids are large but are not true polymers

A monomer is a small molecule that can join with others of the same kind. A polymer is a long molecule made of many monomers linked by covalent bonds. The 26 letters of the alphabet can spell thousands of words, and in the same way a handful of monomers can be arranged in different orders to build a huge variety of polymers. Carbohydrates, proteins, and nucleic acids are polymers. Lipids are large molecules too, but they are built from a few different parts and not from a long chain of one repeating unit, so they are not true polymers.

Dehydration Synthesis and Hydrolysis

• Dehydration synthesis joins monomers and removes water
• Hydrolysis breaks polymers apart and adds water
• Each reaction is sped up by an enzyme
• n monomers joined in a chain make n − 1 bonds

Cells build polymers by dehydration synthesis. Two monomers bond, and a molecule of water is removed from them. The reverse reaction is hydrolysis: a molecule of water is added to break the bond between two monomers. When you digest food, hydrolysis breaks polymers into monomers small enough for your cells to use. In both reactions, enzymes speed up the change. A chain of n monomers has n − 1 bonds between them, so building it releases n − 1 molecules of water, and breaking it completely uses n − 1 molecules of water.

The Four Classes of Large Molecules

• Each class has its own elements and building blocks
• Proteins always contain nitrogen
• Nucleic acids always contain phosphorus
• Each class has main jobs in the cell

Class	Elements	Building block	Main jobs
Carbohydrates	C, H, O	Monosaccharides (simple sugars)	Energy, support
Lipids	C, H, O (little O)	Glycerol and fatty acids	Energy storage, membranes
Proteins	C, H, O, N (some S)	Amino acids	Enzymes, structure, transport
Nucleic acids	C, H, O, N, P	Nucleotides	Store and carry genetic information

Knowing the elements in a molecule helps you tell which class it belongs to. A molecule with only carbon, hydrogen, and oxygen is a carbohydrate or a lipid. If it also contains nitrogen, it is likely a protein, and if it contains phosphorus as well, it may be a nucleic acid. The next four lessons look at each class in turn.

Where Do the Atoms Come From?

• Producers make sugar from carbon dioxide and water
• Sugar supplies carbon, hydrogen, and oxygen
• Other elements, such as nitrogen, must be added
• Consumers get these molecules by eating

Starting materials	Large molecules built
Sugar (C, H, O)	Carbohydrates and lipids
Sugar + nitrogen (and some sulfur)	Amino acids, then proteins
Sugar + nitrogen + phosphorus	Nucleotides, then nucleic acids

The carbon, hydrogen, and oxygen in your large molecules came from sugar. Plants and other producers make sugar by photosynthesis, and consumers get sugar and other molecules by eating. To make an amino acid, a cell combines atoms from sugar with nitrogen, which producers take up from the soil as nitrate or ammonium (the nitrogen cycle, from Unit 1). To make a nucleotide, a cell also needs phosphorus. This is how atoms from the environment are built into the large molecules of living things.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Building a Protein

A protein is a polymer of 150 amino acids joined in one chain. (a) How many bonds join the amino acids? (b) How many water molecules are released when the protein is built? (c) How many water molecules are needed to break the protein down into separate amino acids?

Show the solution

1. (a) Bonds: n monomers in a chain have n − 1 bonds. 150 − 1 = 149 bonds.
2. (b) Water released: each bond forms by dehydration synthesis and releases 1 water molecule, so 149 molecules of water are released.
3. (c) Water needed: hydrolysis uses 1 water molecule for each bond broken, so 149 molecules of water are needed.
4. Answer: (a) 149 bonds. (b) 149 water molecules released. (c) 149 water molecules used.

Example 2: What Is a Cell Made Of?

Use the graph of a bacterial cell. (a) What percent of the cell’s mass is not water? (b) What percent of that non-water mass is protein? (c) What percent of it is nucleic acids?

Show the solution

1. (a) Not water: 100% − 70% = 30%.
2. (b) Protein: 15% ÷ 30% × 100 = 50%.
3. (c) Nucleic acids: 7% ÷ 30% × 100 = 23%.
4. Reasoning: half of the dry mass of the cell is protein, so proteins are the most abundant large molecules in this cell.
5. Answer: (a) 30%. (b) 50%. (c) 23%.

Example 3: Which Class Is It?

Four unknown molecules are described below. Name the class of large molecule that fits each one.

Molecule	Clues
A	Contains only C, H, and O. The atoms of H and O are in a 2 : 1 ratio. Built from sugar units.
B	Contains C, H, O, N, and S. A chain of amino acids.
C	Contains C, H, O, N, and P. A chain of nucleotides.
D	Mostly C and H. Does not mix with water. Has three fatty acid tails.

Show the solution

1. A: carbohydrate. The 2 : 1 ratio of H to O and the sugar units point to carbohydrates.
2. B: protein. Amino acids are the building blocks of proteins, and sulfur is found in some of them.
3. C: nucleic acid. Phosphorus and nucleotides point to DNA or RNA.
4. D: lipid. Mostly C and H, not soluble in water, with fatty acid tails (a triglyceride).
5. Answer: A: carbohydrate. B: protein. C: nucleic acid. D: lipid.

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Practice

Match descriptions to the four classes of large molecules, calculate the water released and used when polymers form and break, read a graph of a cell’s makeup, and label a model of dehydration synthesis and hydrolysis. Try it on your own first, then check your work with the answer key.

• Monomers and Polymers Practice Worksheet (PDF)
• Monomers and Polymers Practice Answer Key (PDF)

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Lesson Slides

• 5.1 Monomers and Polymers Slides (PDF)

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Big Ideas

• Four classes of large molecules: carbohydrates, lipids, proteins, nucleic acids
• Monomers join into polymers
• Dehydration synthesis removes water; hydrolysis adds water
• n monomers in a chain have n − 1 bonds
• Sugar supplies C, H, and O; other elements are added to make other molecules

Back to the essential question: the molecules of life are carbohydrates, lipids, proteins, and nucleic acids, built mostly from carbon, hydrogen, and oxygen, with nitrogen, phosphorus, and sulfur in some. Small monomers join into polymers by dehydration synthesis, which removes water, and hydrolysis breaks them apart again by adding water. Next, you’ll look at the first class in detail: carbohydrates.

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