3.2 Carbohydrates, Lipids and Proteins

3.2.1  Distinguish between organic and inorganic compounds

  • Organic compounds are compounds containing carbon that are found in living things - except hydrogen carbonates (HCO3-), carbonates (CO32-) and oxides of carbon (CO, CO2)
  • Inorganic compounds are all other compounds (there are less different inorganic compounds than organic compounds)

Carbohydrates are organic compounds consisting of one or more simple sugars that as monomers follow the general basic formula of (CH2O)x

Note:  Exceptions to this basic formula and the inclusion of other atoms (e.g. N) can occur

3.2.2  Identify glucose and ribose from diagrams showing their structure

                                                               Glucose (C6H12O6)                                                                         Ribose (C5H10O5)

3.2.3  List three examples each of monosaccharides, disaccharides and polysaccharides

Monosaccharides:  Glucose, galactose, fructose

Disaccharides:  Lactose, maltose, sucrose

Polysaccharides:  Cellulose, glycogen, starch

3.2.4  State one function of glucose, lactose and glycogen in animals and of fructose, sucrose and cellulose in plants


Glucose:  A source of energy which can be broken down to form ATP via cellular respiration

Lactose:  A sugar found in the milk of mammals, providing energy for suckling infants

Glycogen:  Used by animals for short term energy storage (between meals) in the liver


Fructose:  Found in honey and onions, it is very sweet and a good source of energy

Sucrose:  Used primarily as a transportable energy form (e.g. sugar beets and sugar cane)

Cellulose:  Used by plant cells as a strengthening component of the cell wall

3.2.5  Outline the role of condensation and hydrolysis in the relationship between monosaccharides, disaccharides and polysaccharides

  • Condensation (dehydration) reactions occur when molecules are covalently joined together and water is formed as a by-product
  • In carbohydrates, the bond that is formed is called a glycosidic linkage
  • The opposite of a condensation reaction is a hydrolysis reaction, which requires a water molecule to break a covalent bond between two subunits
  • Monosaccharides are single monomers that are joined to form disaccharides, while sugars containing multiple subunits (more than 10) are called polysaccharides

A Condensation Reaction between Two Monosaccharides

Lipids are a group of organic molecules that are insoluble in water but soluble in non-polar organic solvents

Common lipids include triglycerides (fats and oils), phospholipids and steroids

3.2.2  Identify fatty acids from diagrams showing their structure

          General Structure                                                   Saturated (no double bonds)                                                                Unsaturated (double bonds)

3.2.5  Outline the role of condensation and hydrolysis in the relationship between fatty acids, glycerol and triglycerides

  • A condensation reaction occurs between the three hydroxyl groups of glycerol and the carboxyl groups of three fatty acids
  • This reaction forms a triglyceride (and three molecules of water)
  • The bond between the glycerol and the fatty acids is an ester linkage
  • When one of the fatty acids is replaced by a phosphate group and phospholipid is formed
  • Hydrolysis reactions will, in the presence of water, break these molecules down into their constituent subunits

Formation of a Triglyceride

3.2.6  State three functions of lipids

Structure:  Phospholipids are a main component of cell membranes

Hormonal signalling:  Steroids are involved in hormonal signalling (e.g. estrogen, progesterone, testosterone)

Insulation:  Fats in animals can serve as heat insulators while sphingolipids in the myelin sheath (of neurons) can serve as electrical insulators 

Protection:  Triglycerides may form a tissue layer around many key internal organs and provide protection against physical injury

Storage of energy:  Triglycerides can be used as a long-term energy storage source

3.2.7  Compare the use of carbohydrates and lipids in energy storage


  • Complex carbohydrates (e.g. polysaccharides) and lipids both contain a lot of chemical energy and can be used for energy storage
  • Complex carbohydrates and lipids are both insoluble in water - they are not easily transported
  • Carbohydrates and lipids both burn cleaner than proteins (they do not yield nitrogenous wastes)


  • Lipid molecules contain more energy per gram than carbohydrates (about twice as much)
  • Carbohydrates are more readily digested than lipids and release their energy more rapidly
  • Monosaccharides and disaccharides are water soluble and easier to transport to and from storage sites than lipids
  • Animals tend to use carbohydrates primarily for short-term energy storage, while lipids are used more for long-term energy storage
  • Carbohydrates are stored as glycogen in animals while lipids are stored as fats (in plants carbohydrates are stored as cellulose and lipids as oils)
  • Lipids have less effect on osmotic pressure within a cell than complex carbohydrates

Proteins are large organic compounds made of amino acids arranged in a linear chain

The sequence of amino acids in a protein is defined by a gene and encoded in the genetic code

3.2.2  Identify amino acids from diagrams showing their structure

Generalised Structure of an Amino Acid

Types of Amino Acids

3.2.5  Outline the role of condensation and hydrolysis in the relationship between amino acids and polypeptides

  • A condensation reaction occurs between the amino group (NH2) of one amino acid and the carboxylic acid group (COOH) of another amino acid
  • This reaction forms a dipeptide (plus a molecule of water) that is held together by a peptide bond
  • Multiple amino acids can be joined together to form a polypeptide chain
  • In the presence of water, polypeptides can be broken down into individual amino acids via hydrolysis reactions

Formation of a Dipeptide