B2.1B2. Biological Molecules and Enzymes

Biological molecules — Combined Science 0653 - Biology Revision Notes & Topical Questions

5 min read 9 parts Lesson 2 of 6 1 quiz

📚 Combined Science 0653 - Biology Hub / Topical Revision

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Lesson summary

B2.1 Biological molecules — COMBINED SCIENCE notes: List the elements in carbohydrates, proteins and lipids, state how large molecules are built, and describe the food tests.MoleculeElementsCarbohydrateCarbon, hydrogen, oxygenLipid (fat or oil)Carbon, hydrogen, oxygenProteinCarbo…

Organic molecules
Building units
Polysaccharides
Triglyceride
Fats and oils
Amino acids

Part 1 of 7Chemical elements

Molecule	Elements
Carbohydrate	Carbon, hydrogen, oxygen
Lipid (fat or oil)	Carbon, hydrogen, oxygen
Protein	Carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur

Organic molecules
- carbohydrates, proteins and lipids all contain carbon, so they are described as organic molecules.

Part 2 of 7Carbohydrates

Building units

Building units long chains of simple sugars such as glucose.
- (monosaccharides)

Maltose

Maltose two glucose molecules join to form this disaccharide.

Polysaccharides

Polysaccharides starch, glycogen and cellulose are all made from long chains of glucose molecules.

Part 3 of 7Fats and oils

Triglyceride

Triglyceride most fats in the body are triglycerides, each made of one glycerol molecule bonded to three fatty acid chains.

Fats and oils

Fats and oils fats are solid at room temperature and oils are liquid.
The fatty acids vary in size and structure.

Part 4 of 7Proteins

Amino acids

Amino acids proteins are long chains of amino acids.
There are about 20 different amino acids, all with the same basic structure but a different R group.

Variety

Variety the amino acids can be arranged in any order, which gives hundreds of thousands of different proteins.
A small change in the order gives a different protein.

Part 5 of 7Large molecules from smaller molecules

Large molecule	Made from
Starch, glycogen and cellulose	Glucose
Proteins	Amino acids
Fats and oils	Fatty acids and glycerol

Part 6 of 7Food tests

Test for	Reagent and method	Positive result
Glucose (reducing sugar)	Add Benedict's solution and heat at 80 °C in a water bath for 5 minutes	Blue to orange or brick red
Starch	Add drops of iodine solution	Orange-brown to blue-black
Protein	Add drops of biuret solution	Blue to violet or purple
Lipid	Mix the sample with 2 cm³ of ethanol, shake, then add to an equal volume of cold water	Cloudy white emulsion
Vitamin C	Add food sample to 1 cm³ of DCPIP solution	Blue colour of the dye disappears

Part 7 of 7DNA structure

DNA structure

DNA

DNA deoxyribonucleic acid carries the instructions for the growth and development of all organisms.

Double helix

Double helix two strands of DNA wind around each other.

Nucleotide

Nucleotide the repeating unit, made of a phosphate, a deoxyribose sugar and one of four bases.

Bases

Bases adenine (A), cytosine (C), thymine (T) and guanine (G).
Adenine always pairs with thymine and cytosine always pairs with guanine.

Backbone

Backbone the phosphate and sugar form the sides of the ladder and the paired bases form the rungs.
Hydrogen bonds hold the two strands together.

Key terms

Term	Meaning
Carbohydrate	A molecule of carbon, hydrogen and oxygen, such as sugar or starch.
Protein	A long chain of amino acids.
Lipid	A fat or oil made of glycerol and fatty acids.
Amino acid	The unit from which proteins are made.
Triglyceride	Glycerol joined to three fatty acids.
Benedict's solution	A reagent that turns brick red with reducing sugar when heated.
Biuret reagent	A reagent that turns purple with protein.
DNA	The molecule that carries genetic instructions as a double helix.

Avoid these mistakes

Giving only the result colour for a food test — give the starting colour as well.

Saying Benedict's solution is added cold — the mixture is heated.

Saying proteins contain only carbon, hydrogen and oxygen — they also contain nitrogen.

Pairing A with C in DNA — A pairs with T and C pairs with G.

Last updated: 12 October 2026