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First teaching 2023

First exams 2025

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Biological Molecules: Key Terms (CIE A Level Biology)

Revision Note

Cara Head

Author

Cara Head

Last updated

The Two Forms of Glucose

  • The most well-known carbohydrate monomer is glucose
  • Glucose has the molecular formula C6H12O6
  • Glucose is the most common monosaccharide and is of central importance to most forms of life
  • There are different types of monosaccharide formed from molecules with varying numbers of carbon atom, for example:
    • Trioses (3C) e.g. glyceraldehyde
    • Pentoses (5C) e.g. ribose
    • Hexoses (6C) e.g. glucose
  • Glucose exists in two structurally different forms – alpha (α) glucose and beta (β) glucose and is therefore known as an isomer
    • This structural variety results in different functions between carbohydrates

Alpha and Beta Glucose DiagramThe Two Forms of Glucose, downloadable AS & A Level Biology revision notes

Straight chain and ring structural formula of alpha & beta glucose

  • Different polysaccharides are formed from the two isomers of glucose

Structure of polysaccharides table

Polysaccharide Alpha Glucose Beta Glucose
Starch X
Glycogen X
Cellulose X

Examiner Tip

You must be able to recognise and draw the isomer rings of α and β glucose.

Biological Molecules: Key Terms

  • The key molecules that are required to build structures that enable organisms to function are:
    • Carbohydrates
    • Proteins
    • Lipids
    • Nucleic Acids
    • Water

Biological Molecules Diagram

5 Biological molecules (1), downloadable IGCSE & GCSE Biology revision notes

inorganic-and-organic-molecules

The key biological molecules for living organisms

  • Carbohydrates, proteins, lipids and nucleic acids contain the elements carbon (C) and hydrogen (H) making them organic compounds
  • Carbon atoms are key to the organic compounds because:
    • Each carbon atom can form four covalent bonds – this makes the compounds very stable (as covalent bonds are so strong they require a large input of energy to break them)
    • Carbon atoms can form covalent bonds with oxygen, nitrogen and sulfur
    • Carbon atoms can bond to form straight chains, branched chains or rings
  • Carbon compounds can form small single subunits called monomers, that bond with many repeating subunits to form large molecules called polymers, by a process called polymerisation
  • Macromolecules are very large molecules
    • That contain 1000 or more atoms therefore having a high molecular mass
    • Polymers can be macromolecules, however not all macromolecules are polymers as the subunits of polymers have to be the same repeating units

Carbohydrates

  • Carbohydrates are one of the main carbon-based compounds in living organisms
  • All molecules in this group contain C, H and O
  • As H and O atoms are always present in the ratio of 2:1 (e.g. water H2O, which is where ‘hydrate’ comes from) they can be represented by the formula Cx (H2O)y
  • The three types of carbohydrates are monosaccharides, disaccharides and polysaccharides

Types of carbohydrate table

  Monosaccharide Disaccharide Polysaccharide
Definition A single sugar monomer, all of which are reducing sugars A sugar formed from two monosaccharides joined by a glycosidic bond in a condensation reaction A polymer formed by many monosaccharides joined by a glycosidic bond in a condensation reaction
Examples
  • Glyceraldehyde (3C)
  • Ribose (5C)
  • Glucose (6C)
  • Maltose (α glucose + α glucose)
  • Sucrose (α glucose + fructose)
  • Lactose (α glucose + β galactose)
  • Cellulose (monomer is β glucose)
  • Starch (monomer is α glucose)
  • Glycogen (monomer is α glucose)
Function
  • Energy in respiration
  • Building blocks of polymers
  • Sugar in germinating seeds (maltose)
  • Mammal milk sugar (lactose)
  • Sugar stored in cane sugar (sucrose)
  • Energy storage (plants - starch; animals - glycogen)
  • Structural cell wall (cellulose)

Examiner Tip

When discussing monomers and polymers, give the definition but also name specific examples e.g. nucleic acids – the monomer is a nucleotide.

You need to be able to define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide.

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding