DNA and Natural Polymers
Natural Polymers
Many important biological molecules are polymers — very large molecules made by joining many smaller monomers. Unlike most synthetic polymers, natural polymers are biodegradable because enzymes can break them down.
The three main types of natural polymer you need to know are carbohydrates (polysaccharides), proteins and DNA.
Carbohydrates: Sugars and Polysaccharides
Simple sugars (monosaccharides) such as glucose (C₆H₁₂O₆) are monomers that can join together by condensation polymerisation to form polysaccharides, releasing water.
Starch
- Made of many glucose monomers joined together
- The storage carbohydrate in plants
- Found in potatoes, rice, pasta, bread
- Can be broken down by the enzyme amylase back into glucose
- Insoluble in water, making it good for storage (does not affect osmosis)
Cellulose
- Also made of glucose monomers, but with a different arrangement of bonds
- Forms the cell walls of plant cells
- Very strong and rigid
- Humans cannot digest cellulose (we lack the enzyme) — it passes through as dietary fibre
Glycogen
- The storage carbohydrate in animals
- Stored in the liver and muscles
- Similar to starch but more highly branched
Proteins
Proteins are condensation polymers made of amino acid monomers. There are about 20 different amino acids used by living organisms.
Structure
Each amino acid has:
- An amino group (−NH₂)
- A carboxyl group (−COOH)
- A variable side chain (R group) — this is what makes each amino acid different
When two amino acids join by condensation polymerisation:
- The −NH₂ of one reacts with the −COOH of the other
- A peptide bond (amide link, −CONH−) is formed
- A molecule of water is released
A long chain of amino acids is called a polypeptide. One or more polypeptides fold into a specific 3D shape to form a protein.
Importance of Proteins
Proteins have many functions:
- Enzymes — biological catalysts (e.g. amylase, protease)
- Structural — keratin (hair, nails), collagen (skin, tendons)
- Hormones — insulin, growth hormone
- Antibodies — immune defence
- Transport — haemoglobin carries oxygen
The specific function depends on the protein's shape, which is determined by the sequence of amino acids.
DNA (Deoxyribonucleic Acid)
DNA is a natural polymer that carries the genetic code — the instructions for building proteins and controlling every cell in an organism.
Structure of DNA
DNA is made of nucleotide monomers. Each nucleotide consists of:
- A sugar (deoxyribose)
- A phosphate group
- A base (one of four: adenine (A), thymine (T), guanine (G), cytosine (C))
The nucleotides join by condensation polymerisation to form a long strand. Two strands wind around each other to form the famous double helix structure, discovered by Watson and Crick (with crucial data from Franklin and Wilkins).
Base Pairing
The two strands are held together by complementary base pairing:
- A always pairs with T
- G always pairs with C
These pairs are held by weak hydrogen bonds between the bases.
The Genetic Code
The sequence of bases along the DNA strand provides the code. Each group of three bases (a codon) codes for one amino acid. The order of codons determines the order of amino acids in a protein.
DNA → mRNA (transcription) → protein (translation)
Why DNA Matters for Chemistry
DNA is a chemistry example of how polymers carry information. The polymer structure — sugar-phosphate backbone with bases — shows condensation polymerisation in biology. The base pairing shows how molecular shape determines function.
Comparing Natural Polymers
| Polymer | Monomer | Bond type | Formed by | Function |
|---|---|---|---|---|
| Starch/glycogen/cellulose | Glucose | Glycosidic | Condensation | Energy storage / structure |
| Proteins | Amino acids | Peptide (amide) | Condensation | Enzymes, structure, transport |
| DNA | Nucleotides | Phosphodiester | Condensation | Genetic information |
All three are formed by condensation polymerisation — small molecules (usually water) are released as the monomers join.
Hydrolysis: Breaking Down Natural Polymers
The reverse of condensation is hydrolysis — breaking the polymer by adding water. Enzymes catalyse hydrolysis in digestion:
- Amylase hydrolyses starch → glucose
- Protease hydrolyses proteins → amino acids
- Lipase hydrolyses fats → fatty acids + glycerol
Exam Tips
- All natural polymers are made by condensation polymerisation — water is released
- Know the monomers: glucose for polysaccharides, amino acids for proteins, nucleotides for DNA
- Proteins are held in their 3D shape by bonds between amino acids — if these are disrupted (by heat or pH), the protein is denatured
- DNA base pairing: A-T and G-C — this is essential for accurate copying of genetic information
- Be able to identify the ester link in polyesters, the amide/peptide link in proteins, and the glycosidic link in carbohydrates