Biological molecules

Biological molecules are the molecules living things are made of and run on: carbohydrates, lipids, proteins and nucleic acids, plus ATP, water and inorganic ions. Most of the big ones are polymers, built from small repeating monomers joined by condensation reactions and broken apart by hydrolysis. It is the first topic in AQA's A-level Biology specification.

Monomers and polymers

AQA defines monomers as the smaller units from which larger molecules are made, and polymers as molecules made from a large number of monomers joined together. Monosaccharides, amino acids and nucleotides are its examples of monomers.

• A condensation reaction joins two molecules with a new chemical bond and removes a molecule of water.
• A hydrolysis reaction breaks the bond between two molecules, using a molecule of water.

The same two reactions build and break carbohydrates, lipids, proteins and nucleic acids; only the bond that forms changes.

The four main groups at a glance

The bonds and tests are all from AQA's specification. How to carry out each test is in food tests.

Carbohydrates

AQA names glucose, galactose and fructose as common monosaccharides. Two joined by a glycosidic bond make a disaccharide: maltose (glucose + glucose), sucrose (glucose + fructose) or lactose (glucose + galactose). Glucose has two isomers, α and β, and that difference decides the polysaccharide: glycogen and starch are made of α-glucose and cellulose of β-glucose. See alpha and beta glucose.

Lipids

AQA's two groups of lipid are triglycerides and phospholipids. A triglyceride forms by condensation of one glycerol molecule with three fatty acids, making three ester bonds. A fatty acid's R group can be saturated or unsaturated. In a phospholipid, one fatty acid is replaced by a phosphate-containing group, which gives it a water-attracting head: see phospholipid structure and the fluid mosaic model.

Proteins

Amino acids are the monomers, and AQA notes that the twenty common in all organisms differ only in their side group. Two make a dipeptide and many make a polypeptide; a functional protein may contain one or more polypeptides. Hydrogen bonds, ionic bonds and disulfide bridges hold the shape, which links structure to function. See the primary structure of a protein.

Many proteins are enzymes, which lower the activation energy of the reactions they catalyse: see enzymes and the induced fit model.

Nucleic acids

DNA and RNA are polymers of nucleotides. In AQA's description, every nucleotide has three parts: a phosphate group, a five-carbon sugar and an organic base. DNA nucleotides use deoxyribose and the bases A, C, G or T; RNA nucleotides use ribose and A, C, G or U. Nucleotides join by condensation to form phosphodiester bonds. DNA is a double helix whose two chains are held together by hydrogen bonds; the bases pair up in a fixed way, adenine to thymine and cytosine to guanine. See what is DNA and DNA replication.

ATP

AQA describes ATP (adenosine triphosphate) as a nucleotide derivative made of ribose, adenine and three phosphate groups. The enzyme ATP hydrolase splits it into ADP and an inorganic phosphate (Pi), releasing energy that can be coupled to energy-requiring reactions; the phosphate can also make other compounds more reactive. ATP synthase rebuilds ATP from ADP and Pi by condensation, during respiration and photosynthesis.

Water

Inorganic ions

For sodium's role, see co-transport.

Worked examples

Example 1: the mass of a polymer

A short chain of 500 glucose units (C₆H₁₂O₆, Mr 180) forms by condensation. What is its relative molecular mass?

1. A chain of 500 units has 499 glycosidic bonds, so 499 water molecules (Mr 18) are removed.
2. Mr = 500 × 180 − 499 × 18 = 90 000 − 8982 = 81 018.

Example 2: base frequencies in DNA

AQA suggests using incomplete information about base frequencies to find the others. In a sample of double-stranded DNA, 31% of the bases are cytosine. Find the percentages of the other three.

1. C pairs with G, so guanine is also 31%.
2. C and G together make 62%, so A and T make 100 − 62 = 38%.
3. A pairs with T, so each is 38 ÷ 2 = 19%.

Example 3: building a triglyceride

How many water molecules are released when one triglyceride forms, and how many are needed to hydrolyse it completely? One glycerol joins three fatty acids with three ester bonds, so 3 waters are released, and the same 3 are needed to break it back down.

Common mistakes

• Calling lipids polymers. Triglycerides aren't made of repeating monomers; they are glycerol plus three fatty acids.
• Mixing up condensation and hydrolysis. Condensation joins and releases water; hydrolysis breaks and uses water.
• Pairing the wrong bases. A with T (or U in RNA), C with G.
• Naming the wrong bond. Glycosidic for sugars, ester for lipids, peptide for proteins, phosphodiester for nucleic acids.
• Saying water has a high specific heat "so it heats up fast". It is the opposite: a high heat capacity means its temperature changes slowly.

Biological molecules start the first year of A-level Biology, so practise them on today's A-level Biology: Year 1 short timed paper. The cell biology course shows where these molecules work inside the cell.

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