The large molecules that make up living cells — proteins, carbohydrates, and lipids — are built from small repeating units joined together. Understanding monomers, polymers, and the reactions that link and unlink them explains everything from muscle contraction to energy storage to why digestion is necessary.
What are monomers and polymers?
A monomer is a small molecule that can join to other monomers to form a larger molecule. A polymer is the large molecule made from many monomers linked in a chain.
The linking reaction is a condensation reaction: each time two monomers join, a water molecule is released. This is why the reaction is also called dehydration synthesis.
The reverse reaction — breaking polymers into monomers by adding water — is hydrolysis. Digestive enzymes catalyse hydrolysis reactions in the gut.
| Monomer | Polymer | Bond formed |
|---|---|---|
| Amino acids | Proteins | Peptide bond |
| Glucose (and other monosaccharides) | Polysaccharides (starch, glycogen, cellulose) | Glycosidic bond |
| Fatty acids + glycerol | Triglycerides (lipids) | Ester bond |
Note: lipids are not true polymers because the fatty acid chains are not repeated units of the same monomer — but they are still formed by condensation reactions.
What are proteins made from?
Proteins are polymers of amino acids. There are 20 different amino acids used in living organisms. Each has the same basic structure — a central carbon atom bonded to an amino group (−NH₂), a carboxyl group (−COOH), a hydrogen atom, and a variable R group that differs between amino acids and determines the amino acid's properties.
Amino acids join by a condensation reaction between the amino group of one and the carboxyl group of the next, releasing water and forming a peptide bond (−CO−NH−). A chain of two amino acids is a dipeptide; many amino acids form a polypeptide, which folds into a functional protein.
Protein diversity comes from:
- The sequence of amino acids (determined by DNA)
- The three-dimensional shape the chain folds into
Different sequences produce proteins with completely different shapes and functions: structural proteins (collagen, keratin), enzymes (amylase, catalase), hormones (insulin), antibodies, and transport proteins (haemoglobin).
What are carbohydrates made from?
Carbohydrates are compounds of carbon, hydrogen, and oxygen with a hydrogen-to-oxygen ratio of approximately 2:1.
The basic monomer unit is a monosaccharide (a simple sugar). The most important at GCSE is glucose (C₆H₁₂O₆). Two glucose units join in a condensation reaction to form a disaccharide; many glucose units form a polysaccharide.
Important polysaccharides:
| Polysaccharide | Monomer | Function | Where found |
|---|---|---|---|
| Starch | Glucose (α) | Energy storage in plants | Plant cells (starchy foods) |
| Glycogen | Glucose (α) | Energy storage in animals | Liver, muscle cells |
| Cellulose | Glucose (β) | Structural — cell wall rigidity | Plant cell walls |
Starch and glycogen are coiled and branched, allowing rapid addition and removal of glucose units. Cellulose is unbranched, forming straight chains that hydrogen-bond together into strong fibres — this is why plant cell walls are rigid and cellulose is difficult to digest (few animals produce cellulase, the enzyme needed to break it down).
What are lipids made from?
Lipids include fats and oils. The most common form, a triglyceride, is made from one glycerol molecule and three fatty acid chains joined by ester bonds (formed in condensation reactions releasing 3 water molecules).
- Saturated fats — all single bonds in the fatty acid carbon chains; found mainly in animal products; solid at room temperature
- Unsaturated fats — contain one or more C=C double bonds; found in plant oils; liquid at room temperature
Lipids are used for:
- Long-term energy storage (more energy per gram than carbohydrates — 37 kJ/g vs 17 kJ/g)
- Insulation and protection around organs
- Making cell membranes (phospholipids)
- Fat-soluble vitamins (A, D, E, K) are carried in lipids
How are polymers broken down in digestion?
Digestive enzymes catalyse hydrolysis — the addition of water across bonds:
- Amylase breaks starch into maltose (by hydrolysing glycosidic bonds)
- Protease breaks proteins into amino acids (by hydrolysing peptide bonds)
- Lipase breaks triglycerides into fatty acids and glycerol (by hydrolysing ester bonds)
The small monomers produced (glucose, amino acids, fatty acids, glycerol) are small enough to be absorbed across the gut wall into the blood.
Frequently asked questions
Why does the sequence of amino acids determine protein shape?
The sequence of amino acids determines which R groups are adjacent along the chain. R groups interact with each other through hydrogen bonds, disulfide bridges, ionic interactions, and hydrophobic (water-avoiding) forces. These interactions cause the chain to fold into a specific three-dimensional shape. The shape of the active site of an enzyme, the binding site of an antibody, or the oxygen-carrying pocket of haemoglobin all depend critically on the exact folding pattern, which is determined entirely by the amino acid sequence.
Why can't humans digest cellulose?
Humans lack the enzyme cellulase, which would be needed to break the glycosidic bonds in cellulose. The β-glucose monomers in cellulose form a different bond geometry (β-1,4-glycosidic bonds) compared with the α-glucose in starch (α-1,4 or α-1,6 bonds). Human salivary and pancreatic amylases can only break α-linkages. Cellulose therefore passes through the gut undigested — but it is not useless: dietary fibre (largely cellulose) adds bulk to faeces, helping prevent constipation and reducing the risk of bowel cancer.
Why is glycogen stored in the liver and muscles rather than as fat?
Glycogen provides faster access to glucose than fat does — breaking glycosidic bonds in glycogen is quicker than hydrolysing fat and then converting fatty acids back to glucose. The liver uses glycogen to regulate blood glucose levels (releasing glucose into the blood when needed), while muscles use local glycogen stores for rapid energy during exercise. Fat stores provide much larger amounts of energy overall, but mobilising fat is slower and cannot sustain sudden intense effort as quickly as glycogen.
How are condensation and hydrolysis reactions related to energy?
Condensation (building polymers) requires energy input — the cell uses ATP to drive peptide bond formation during protein synthesis on ribosomes. Hydrolysis (breaking polymers) releases some energy, but much less than full aerobic respiration of the monomers. Digestion itself releases very little energy; the energy locked in molecules like glucose is released in cell respiration, not during the hydrolysis step. This is why digestive enzymes do not power the cell directly — they simply convert insoluble polymers into absorbable monomers that cells can then respire.
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