1 Carbohydrates

• Polysaccharides

  • Structure & Formation of Polysaccharides
  • Properties of Polysaccharides
  • Starch (including Amylose and Amylopectin)
  • Glycogen
  • Cellulose
  • Structure-Function Relationships
• Core Idea 1: Biomolecules of Life explores three classes of biomolecules – carbohydrates, lipids and proteins
• Learning Outcomes

  • Describe the structure and properties of α-glucose and β-glucose (in carbohydrates)
  • Describe the formation and breakage of glycosidic bond
  • Describe the structures and properties of starch (including amylose and amylopectin), cellulose, and glycogen
• Monosaccharides

  • Structure of Monosaccharides
  • Properties of Monosaccharides
• References: Raven, P., Johnson, G., Mason, K., Losos, J. and Duncan, T. (2022) Biology (13th Edition) (McGraw-Hill) ISBN-10 1264097859
• The specific structures of biomolecules give rise to certain properties that allow these biomolecules to carry out their specific functions
• Four major classes of biomolecules function as molecular building blocks for macromolecules to be assembled
• Biomolecules discussed

  • Starch
  • Glycogen
  • Cellulose
  • Triglycerides
  • Phospholipids
  • Haemoglobin
  • DNA
  • RNA
• Nucleic acids will be covered in Core Idea 2: Genetics and Inheritance
• References: Alberts, B., Heald, R., Johnson, A., Morgan, D., Raff, M., Roberts, K. and Walter, P. (2022) Molecular Biology of the Cell (7th Edition) (W. W. Norton & Company) ISBN-10 0393884821
• References: Reece, K., Jane, B. Taylor, Martha, R., Simon, Eric, J., Dickey, Jean, L., Hogan (2020) Campbell Biology (12th Edition) (Pearson Higher Education) ISBN-10 0135188741
• Disaccharides

  • Glycosidic Bond – Formation and Hydrolysis
  • Properties of Disaccharides
• Structure of Monosaccharides

  The general formula for monosaccharides is represented by a simple formula (CH2O)n where the ratio of C:H:O is 1:2:1. Monosaccharides can be classified according to the number of carbon atoms present. All monosaccharides have a carbonyl group (C=O) and multiple hydroxyl groups (–OH) attached. The position of the carbonyl group also determines if the sugar is considered an aldose or a ketose sugar
• If the carbonyl group is at the end of the carbon skeleton, it forms an aldehyde group (–CHO), in which case the monosaccharide is called an aldose (aldehyde sugar). If the carbonyl group is located within the carbon skeleton, it is a ketone, and the monosaccharide is called a ketose (ketone sugar)
• Classification of carbohydrates

  • Complexity: Simple carbohydrate - Monosaccharides containing one monomer, Disaccharides containing two monomers; Complex carbohydrate - Oligosaccharides containing ~ 3-10 monomers, Polysaccharides containing >100 monomers; Number of carbon atoms: Triose (3-carbon sugars), Tetrose (4-carbon sugars), Pentose (5-carbon sugars), Hexose (6-carbon sugars), Heptose (7-carbon sugars); Reducing properties: Reducing sugars (e.g. all monosaccharides), Non-reducing sugars (e.g. sucrose); Position of carbonyl group (C=O): Aldose (sugars having an aldehyde group), Ketose (sugars having a ketone group)
• Properties of Monosaccharides

  • Physical properties: Sweet and have crystalline structures, Soluble in water due to its small size and presence of numerous polar –OH groups that can form hydrogen bonds with water, Pentoses and hexoses can exist as rings which are more stable as building blocks for the synthesis of disaccharides and polysaccharides; Chemical Properties: Monosaccharides are reducing sugars, They contain a carbonyl group (C=O) (aldehyde or ketone group) that is able to
• Monosaccharides, particularly glucose, are major nutrients for cells and serve as a major fuel source for cells through respiration. Their carbon skeletons also serve as raw material for the synthesis of amino acids and fatty acids
• Two simplest monosaccharides

  • Triose sugars: glyceraldehyde, dihydroxyacetone
• Ring forms are the usual forms of monosaccharides, with only a small proportion of the molecules existing in the 'linear or open chain' form at any one time
• Glucose


  • In aqueous solutions, glucose molecules, as well as most other five- and six-carbon sugars, form stable ring structures
• Monosaccharides


  Changing the position of any one of the three hydroxyl groups attached to carbons 2, 3, or 4 would result in a different molecule
• Pentoses and hexoses


  Can exist as rings which are more stable as building blocks for the synthesis of disaccharides and polysaccharides
• Sucrose is a common non-reducing sugar
• Disaccharides consist of two monosaccharides joined together
• Condensation reaction involves the loss of one water molecule
• Ring structure of glucose

  • Can exist in two forms, the α- form and the β- form, depending on the position of the hydroxyl (–OH) group at carbon 1
• Glucose is the main source of energy for cellular respiration
• Glycosidic Bond – Formation


  A glycosidic bond is formed during a condensation reaction involving the loss of one water molecule
• Monosaccharides


  • Reducing sugars
  • Contain a carbonyl group (C=O) (aldehyde or ketone group) that is able to donate electrons and reduce Cu2+ to Cu+ in the Benedict’s test
• Common disaccharides

  • Maltose: Glucose + Glucose
  • Lactose: Glucose + Galactose
  • Sucrose: Glucose + Fructose
• Ways hydrolysis reactions can occur

  • Enzymatic
  • Acid hydrolysis
• Disaccharides, like monosaccharides, are sweet, have crystalline structure, and are soluble in water
• A larger sugar molecule such as a disaccharide can be broken into smaller molecules via hydrolysis
• Some polysaccharides serve as food and energy storage material and are readily hydrolysed into sugars when required to provide an energy source
• Most disaccharides are reducing sugars, except sucrose which is non-reducing
• Polysaccharides are macromolecules made of hundreds to thousands of monosaccharides joined by glycosidic linkages
• Acid hydrolysis of lactose


  Yields one galactose and one α-glucose
• Polysaccharides can be divided into two groups based on their functions: Energy storage polysaccharides and Structural polysaccharides
• Hydrolysis


  The covalent glycosidic bond can be broken with the addition of one molecule of water
• Disaccharides can be hydrolysed to monosaccharides under acid hydrolysis or via enzymatic reactions