1.2 Biomolecules

In brief:Living cells contain four main types of biomolecule: carbohydrates, lipids, proteins, and nucleic acids. Each has distinct structures and roles.

Living cells contain four main types of biomolecule: carbohydrates, lipids, proteins, and nucleic acids. Each is built from smaller subunits (monomers) joined together.

Carbohydrates (Cₓ(H₂O)y) - energy and structure:
• Monosaccharides: single sugars - glucose, fructose, galactose (6-carbon sugars)
• Disaccharides: two monosaccharides joined by a glycosidic bond - sucrose (glucose + fructose), maltose, lactose
• Polysaccharides: long chains - starch (plant storage), glycogen (animal storage), cellulose (plant cell walls)

Lipids - energy storage, insulation, membranes:
• Triglyceride = glycerol + 3 fatty acids (joined by ester bonds)
• Phospholipids = glycerol + 2 fatty acids + phosphate group → forms cell membrane bilayer
• Saturated fats: no C=C double bonds (solid at room temp). Unsaturated: has C=C double bonds (liquid)

Proteins - enzymes, hormones, antibodies, structural:
• Built from amino acids linked by peptide bonds (condensation reaction)
• 4 levels of structure: primary (sequence), secondary (α-helix/β-sheet), tertiary (3D fold), quaternary (multiple polypeptides)

Nucleic acids - store and transmit genetic information:
• Built from nucleotides: sugar + phosphate + nitrogenous base
• DNA: deoxyribose, double-stranded, bases A-T and G-C
• RNA: ribose, single-stranded, bases A-U and G-C

Glucose (monosaccharide) — Glucose — a monosaccharide carbohydrate
Wikimedia Commons (public domain / CC)

Sucrose (disaccharide) — Sucrose — glucose + fructose (disaccharide)
Wikimedia Commons (public domain / CC)

Amylose (starch) — Amylose — a polysaccharide (starch)
Wikimedia Commons (public domain / CC)

Triglyceride — glycerol + 3 fatty acids
Wikimedia Commons (public domain / CC)

Amino acid structure — Amino acid — building block of proteins
Wikimedia Commons (public domain / CC)

table

The Four Biomolecules

Biomolecule	Elements	Monomer	Bond	Functions
Carbohydrate	C, H, O	Monosaccharide	Glycosidic	Energy, structure (cellulose)
Lipid	C, H, O	Glycerol + fatty acids	Ester	Energy storage, membranes, insulation
Protein	C, H, O, N (S)	Amino acid	Peptide	Enzymes, hormones, transport, structure
Nucleic acid	C, H, O, N, P	Nucleotide	Phosphodiester	Genetic information (DNA, RNA)

diagram

Carbohydrate Hierarchy

Monosaccharide
glucose, fructose
1 sugar unit

Disaccharide
sucrose, maltose
2 sugar units bonded together

Polysaccharide
starch, glycogen, cellulose
Many sugar units

diagram

Protein Structure Levels

Key Points

1Carbohydrates (Cₓ(H₂O)y): monosaccharides (glucose), disaccharides (sucrose), polysaccharides (starch, glycogen, cellulose).
2Lipids: triglyceride unit - glycerol + 3 fatty acids. Include fats, oils, phospholipids.
3Proteins: chains of amino acids (peptides → polypeptides) folded into 3D shapes. Contain C, H, O, N (sometimes S, P).
4Protein roles: enzymes, hormones, channel proteins, transporters, structural proteins, receptors, antibodies.
5Nucleic acids: DNA and RNA made of nucleotide subunits.
6ATP, NAD⁺ and NADP⁺ are transfer molecules in metabolic pathways.

Learning Outcomes

Outline the structures and metabolic roles of carbohydrate, lipid and protein, and identify nutritional sources of each
Describe the basic structure and function of a DNA and RNA nucleotide
Outline the role of ATP, NAD+ and NADP+ in metabolic pathways

Vitamins, Minerals & Water

In brief:Organisms require vitamins, minerals and water for various essential biological processes.

In addition to the four main biomolecules, organisms need vitamins, minerals, and water for essential biological processes.

Minerals are inorganic elements needed in small amounts:
• Calcium (Ca): bone and teeth strength, muscle contraction, nerve impulses
• Iron (Fe): part of haemoglobin - carries oxygen in red blood cells
• Magnesium (Mg): needed for chlorophyll in plants; enzyme function

Vitamins:
• Water-soluble: Vitamin C (ascorbic acid) - needed for collagen production. Deficiency → scurvy (bleeding gums, fatigue)
• Fat-soluble: Vitamin D - needed for calcium absorption. Deficiency → rickets (soft, weak bones in children)

Water makes up ~70% of the human body and is essential as:
• A transport medium (blood plasma, sap)
• A solvent for biochemical reactions
• For temperature regulation (high specific heat capacity, sweating)
• For osmoregulation (maintaining water balance in cells)

table

Food Tests - How to Detect Nutrients

Test	Tests For	Reagent	Positive Result
Benedict's Test	Reducing sugars	Benedict's solution + heat	Blue → orange/red
Iodine Test	Starch	Iodine solution	Brown → blue-black
Biuret Test	Protein	NaOH + CuSO₄	Blue → purple/violet
Emulsion Test	Lipids	Ethanol + water	Cloudy white emulsion

Key Points

1Minerals needed in small amounts: pH balance, enzyme regulation, nerve impulses, muscle contraction.
2Vitamins: water-soluble (e.g. Vitamin C) and fat-soluble (e.g. Vitamin D).
3Vitamin C deficiency → scurvy. Vitamin D deficiency → rickets.
4Water roles: transport medium, chemical reactions, temperature regulation, pH balance, osmoregulation.
5Food tests: Benedict's test (reducing sugars), Iodine test (starch), Biuret test (protein), Emulsion test (lipids).

Learning Outcomes

Recognise the roles of minerals in biological processes
Outline the role of vitamins in biological processes, including the role of one water soluble and one fat soluble vitamin in humans, and their associated deficiency disease
Outline the main roles of water in living organisms
Investigate qualitatively the presence of nutrients in a range of food samples, use primary data to support conclusions
Investigate quantitatively the level of reducing sugars in a range of food samples, use primary data to support conclusions

3. Condensation, Hydrolysis and the Chemistry of Biomolecules

In brief:Every large biomolecule is built from small repeating units joined by condensation and broken apart by hydrolysis.

The four families of biomolecule are built the same way: small monomers are joined into large polymers.

Biomolecule	Monomer	Bond formed	Elements
Carbohydrate	Monosaccharide (glucose)	Glycosidic bond	C, H, O
Lipid	Glycerol + 3 fatty acids	Ester bond	C, H, O
Protein	Amino acid	Peptide bond	C, H, O, N (+ S)
Nucleic acid	Nucleotide	Phosphodiester bond	C, H, O, N, P

Condensation

In a condensation reaction two monomers join and one molecule of water is removed for each bond formed. Making a dipeptide removes one water; making a protein of 100 amino acids removes 99. Condensation is anabolic and needs energy, usually from ATP.

Hydrolysis

In hydrolysis a molecule of water is added to break a bond, splitting the polymer back into monomers. Hydrolysis is catabolic and releases energy. Digestion is hydrolysis: amylase hydrolyses starch to maltose, protease hydrolyses protein to amino acids, lipase hydrolyses fat to glycerol and fatty acids.

Why water matters

Water is not just the reactant in these reactions, it is the solvent life runs in. Water is polar, so it dissolves ions and other polar molecules and transports them in blood, xylem and phloem. Hydrogen bonding gives water a high specific heat capacity (temperatures change slowly, protecting enzymes), a high latent heat of vaporisation (sweating and transpiration cool effectively), and cohesion (unbroken water columns are pulled up the xylem).

Exam tip

If a question gives you the number of monomers joined, the number of water molecules removed is always one fewer.

svg

Condensation and hydrolysis

Key Points

1Condensation joins monomers and removes one water molecule per bond; it is anabolic
2Hydrolysis adds water to break a bond; digestion is hydrolysis and it is catabolic
3Bonds: glycosidic (carbohydrate), ester (lipid), peptide (protein), phosphodiester (nucleic acid)
4Proteins contain nitrogen; nucleic acids contain nitrogen and phosphorus
5Water is polar, has a high specific heat capacity and shows cohesion, all vital for life

Learning Outcomes

Describe the formation and breakdown of biomolecules by condensation and hydrolysis
Relate the properties of water to its biological roles