Biomolecules
The chemistry of life — sugars for energy, proteins for structure and catalysis, vitamins for regulation, and the nucleic acids that carry the code
- How carbohydrates are classified, and the difference between reducing and non-reducing sugars.
- The open-chain and cyclic structure of glucose and its key reactions.
- Amino acids, the zwitterion, the peptide bond, and the four levels of protein structure.
- What enzymes are and why they are such efficient, specific catalysts.
- The fat- and water-soluble vitamins and their deficiency diseases.
- The structure of nucleic acids — nucleotides, the double helix and base pairing.
What Are Biomolecules?
Biomolecules are the organic compounds that build and run living cells. Four great families dominate: carbohydrates (energy and structure), proteins (structure and catalysis), nucleic acids (information), and smaller regulators such as vitamins and hormones. All are built from the functional groups met earlier in Part 3 — they are organic chemistry doing the work of life.
Carbohydrates: Classification
Carbohydrates are polyhydroxy aldehydes or ketones, or compounds that give these on hydrolysis. They are classified by how many sugar units they yield.
| Class | Hydrolysis | Examples |
|---|---|---|
| Monosaccharide | cannot be hydrolysed | glucose, fructose, ribose |
| Disaccharide | gives 2 monosaccharides | sucrose, maltose, lactose |
| Polysaccharide | gives many units | starch, cellulose, glycogen |
Glucose: Structure & Reactions
Glucose (\(\ce{C6H12O6}\)) is an aldohexose — a six-carbon sugar with an aldehyde group. It exists in an open-chain form and, predominantly, as a cyclic six-membered (pyranose) ring with two anomers, \(\alpha\) and \(\beta\), which interconvert in solution (mutarotation).
| Reagent | Product | Reveals |
|---|---|---|
| \(\ce{HI}\), heat | \(n\)-hexane | straight C₆ chain |
| \(\ce{Br2}\) water | gluconic acid | \(\ce{-CHO}\) group (oxidised) |
| conc. \(\ce{HNO3}\) | saccharic acid | \(\ce{-CHO}\) and \(\ce{-CH2OH}\) (both ends) |
| \(\ce{NH2OH}\) | oxime | \(\ce{-CHO}\) group |
| acetic anhydride | pentaacetate | five \(\ce{-OH}\) groups |
Di- & Polysaccharides
Sugar units join through a glycosidic linkage (an ether-like C–O–C bond). The same glucose monomer, linked differently, builds molecules with utterly different roles.
| Carbohydrate | Built from | Role |
|---|---|---|
| Sucrose | glucose + fructose | transport sugar (non-reducing) |
| Maltose | glucose + glucose | reducing disaccharide |
| Lactose | glucose + galactose | milk sugar (reducing) |
| Starch | α-glucose (amylose + amylopectin) | energy store in plants |
| Cellulose | β-glucose | structural fibre in plants |
| Glycogen | α-glucose (branched) | energy store in animals |
Amino Acids & the Zwitterion
Proteins are built from α-amino acids — molecules with both an amino (\(\ce{-NH2}\)) and a carboxyl (\(\ce{-COOH}\)) group on the same carbon. Twenty standard amino acids occur; those the body cannot make are essential. Because each carries an acid and a base, an amino acid exists largely as a dipolar zwitterion.
Being both acid and base, an amino acid reacts with either, and at a specific pH — its isoelectric point — it carries no net charge and is least soluble. Amino acids join through the peptide bond (\(\ce{-CO-NH-}\)), an amide link, to build proteins.
Proteins & Their Structure
A protein is a polymer of amino acids folded into a precise shape. Chemists describe that shape at four levels, each held together by different forces.
| Level | What it is | Held by |
|---|---|---|
| Primary | sequence of amino acids | peptide (covalent) bonds |
| Secondary | α-helix, β-pleated sheet | hydrogen bonds |
| Tertiary | overall 3-D fold | various side-chain interactions |
| Quaternary | assembly of subunits | same interactions between chains |
Enzymes
Enzymes are proteins that act as biological catalysts. Like all catalysts they speed reactions by lowering the activation energy, but they do so with extraordinary specificity and efficiency — a single enzyme often acts on just one substrate, fitting it like a lock and key. Most enzymes are named after their substrate with the suffix -ase (maltase, urease).
Vitamins
Vitamins are organic compounds needed in tiny amounts to keep metabolism running; the body cannot make most of them. They split into fat-soluble (A, D, E, K) and water-soluble (B-complex, C). A shortage causes a characteristic deficiency disease.
| Vitamin | Solubility | Deficiency disease |
|---|---|---|
| A | fat | night blindness, xerophthalmia |
| \(\ce{B1}\) (thiamine) | water | beri-beri |
| C (ascorbic acid) | water | scurvy |
| D | fat | rickets, osteomalacia |
| K | fat | poor blood clotting |
Nucleic Acids
Nucleic acids — DNA and RNA — store and transmit genetic information. They are polymers of nucleotides, each a unit of a nitrogenous base, a pentose sugar, and a phosphate. (A base plus sugar alone is a nucleoside.)
In DNA the bases pair specifically — adenine with thymine, guanine with cytosine — held by hydrogen bonds, giving the two strands their complementary double helix. DNA stores the genetic code and is copied during replication; RNA (mRNA, tRNA, rRNA) carries that code into protein synthesis.
Putting It to Work
Problem. Classify glucose, maltose and sucrose as reducing or non-reducing sugars.
Solution. A free carbonyl makes a sugar reducing; sucrose has none:
Problem. What does the reaction of glucose with \(\ce{HI}\) prove about its structure?
Solution. It gives \(n\)-hexane:
Problem. Why is an amino acid amphoteric, and what is its isoelectric point?
Solution. It bears both \(\ce{-COO-}\) and \(\ce{-NH3+}\):
Problem. When egg white is boiled it sets solid. Which protein structures are lost, and which survives?
Solution. Heat disrupts the higher-order folding only:
Problem. A DNA strand has 30% adenine. What percentage of cytosine does the molecule contain?
Solution. A = T = 30%, so G + C = 40%, split equally:
Problem. Name the disease caused by lack of vitamin C and its solubility class.
Solution. Vitamin C is water-soluble:
Chapter Summary
Mono/di/polysaccharides; reducing sugars (free carbonyl) vs non-reducing sucrose.
Aldohexose; open-chain ⇌ pyranose; reactions reveal \(\ce{-CHO}\), 5 \(\ce{-OH}\), C₆ chain.
Amphoteric zwitterions; isoelectric point; peptide bond joins them.
Four structural levels; denaturation loses shape but not sequence.
Fat-soluble (A,D,E,K) vs water-soluble (B,C); each deficiency has a disease.
Nucleotides; DNA double helix; A-T, G-C pairing; DNA stores, RNA translates.
Problems
For each item, first decide which family it concerns — carbohydrate, protein, vitamin or nucleic acid — then apply the relevant structure–function idea. Difficulty rises down the list.
- Define a carbohydrate and classify mono-, di- and polysaccharides with examples.
- What makes a sugar reducing? Why is sucrose non-reducing?
- List three reactions of glucose and state what each reveals about its structure.
- Compare starch and cellulose in terms of linkage and biological role.
- Explain why an amino acid exists as a zwitterion and define the isoelectric point.
- What is a peptide bond? Name the four levels of protein structure.
- Describe denaturation and say which structural level survives it.
- Why are enzymes described as highly specific catalysts?
- Classify vitamins by solubility and match A, C and D to their deficiency diseases.
- Describe the components of a nucleotide and distinguish it from a nucleoside.
- State the base-pairing rules in DNA and the forces that hold the pairs.
- Give three differences between DNA and RNA.