YIJC [H2] CI1.2 Biomolecules (N) vf

JC1 BIOLOGY LECTURE NOTES 1 CORE IDEA 1: THE CELL AND BIOMOLECULES OF LIFE TOPIC 1.2: BIOMOLECULES Learning Outcomes You should be able to: (g) describe the structure and properties of the following monomers: i. α-glucose and β-glucose (in carbohydrates) ii. glycerol and fatty acids (in lipids) iii. amino acids (in proteins) (knowledge of chemical formulae of specific R-groups of different amino acids is not required) (h) describe the formation and breakage of the following bonds: i. glycosidic bond ii. ester bond iii. peptide bond (i) describe the structures and properties of the following biomolecules and explain how these are related to their roles in living organisms: i. starch (including amylose and amylopectin) ii. cellulose iii. glycogen iv. triglyceride v. phospholipid (m) explain primary structure, secondary structure, tertiary structure and quaternary structure of proteins, and describe the types of bonds that hold the molecule in shape (hydrogen, ionic and disulfide bonds, and hydrophobic interactions) (n) explain the effects of temperature and pH on protein structure (o) describe the molecular structure of the following proteins and explain how the structure of each protein relates to the function it plays: i. haemoglobin (transport) ii. collagen (structural) iii. G-protein linked receptor (signalling) (knowledge of details of the number of amino acids and types of secondary structures present is not required) Textbooks and References Raven, P H, Johnson, G B, Mason, K A, Losos, J and Singer, S (2013) Biology (10th Edition) (McGraw-Hill) ISBN 007338307 Reece, J B, Urry, L A, Cain, M L, Wasserman, S A, Minorsky, P V and Jackson, R B (2011) Campbell Biology (10th Edition) (Pearson Higher Education) ISBN 0321739752 H2

1 1 INTRODUCTION Biomolecules are important components of cell structures, including membranes which are made up of phospholipids, cholesterol, carbohydrates and proteins. The different classes of biomolecules (carbohydrates, lipids, proteins and nucleic acids) function as molecular building blocks for macromolecules to be assembled. • In living organisms, large biological mo lecules can be classified into four major classes based on molecular structure: o Carbohydrates o Lipids o Proteins o Nucleic Acids (covered in Core Idea 2: Topic 2.1) • Carbohydrates, proteins and nucleic acids are polymers ( polys = many ). Polymers are long molecules made from smaller repeating units called monomers ( monos = single) in a process known as condensation. Prior Knowledge You should be able to: A. [syllabus 6093] list the chemical elements which make up • carbohydrates • fats • proteins B. describe and carry out tests for • starch (iodine in potassium iodide solution) • reducing sugars (Benedict’s solution) • protein (biuret test) • fats (ethanol emulsion) C. state that large molecules are synthesised from smaller basic units • glycogen from glucose • polypeptides and proteins from amino acids • lipids such as fats from glycerol and fatty acids

2 2 CARBOHYDRATES • Carbohydrates are generally known as ‘sugars” and are the main substrate used for cellular respiration to synthesise ATPs (which can be hydrolysed to release energy). • They are a group of organic compounds c onsisting mainly of carbon (C), hydrogen (H) and oxygen (O). The general formula for carbohydrates is Cx(H2O)y. Fig. 2 Classification of carbohydrates 2.1 MONOSACCHARIDES Learning Outcome (g)(i): Describe the structure and properties of α-glucose and β-glucose (in carbohydrates). • Monosaccharides are simple sugars (monomers) that cannot be further hydrolysed. They have a general formula of (CH2O)n, where n = 3 to 7. • Monosaccharides can exist as linear chains or cyclic structures. When linear glucose cyclizes, it may form 2 different isomers: o -glucose: the hydroxyl group (-OH) on C1 is on the different plane as C6. o β-glucose: the hydroxyl group (-OH) on C1 is on the same plane as its C6. Fig. 2.1.1 Linear and ring forms of glucose Carbohydrates Monosaccharides Disaccharides Polysaccharides Glucose Galactose Fructose Maltose Lactose Sucrose Starch Glycogen Cellulose e.g. e.g. e.g. consists

3 2.2 DISACCHARIDES Learning Outcome (h)(i): Describe the formation and breakage of the glycosidic bond. • Disaccharides are sugars made up of 2 monosaccharides. Commonly occurring disaccharides include maltose, lactose and sucrose. • A disaccharide is formed when 2 monosaccharides are linked by a glycosidic bond that is formed via condensation between OH groups, with the elimination of one water molecule. • The glycosidic bond formed will be named according to: o the glucose isomers used o the C atoms involved Fig. 2.2 Condensation reaction between 2 glucose monomers to form maltose and water What are the following disaccharides made up of? 1. Maltose (commonly known as malt) Glucose + Glucose 2. Sucrose (commonly known as table sugar) Glucose + Fructose 3. Lactose (commonly known as milk sugar) Glucose + Galactose

4 2.3 POLYSACCHARIDES Learning Outcome (i): Describe the structures and properties of the following biomolecules and explain how these are related to their roles in living organisms: (i) starch (including amylose and amylopectin), (ii) cellulose and (iii) glycogen. • Polysaccharides are polymers made up of hundreds to thousands of monosaccharides. The monosaccharides are connected through glycosidic bonds, formed through condensation reactions where multiple water molecules are eliminated. Fig. 2.3 Formation of a segment of a polysaccharide • Polysaccharides can be divided into 2 groups based on function: o Storage polysaccharides (e.g. starch and glycogen) o Structural polysaccharides (e.g. cellulose) 2.3.1 STARCH • Starch is a polysaccharide that serves a storage function in plants. It consists of thousands of -glucose monomers and functions as a compact energy reserve formed from excess glucose synthesized during photosynthesis. • Starch consists of 2 components: amylose and amylopectin.

5 Fig. 2.3.1.1 Components of starch – amylose and amylopectin • Amylose consists of thousands of -glucose residues linked by -1,4- glycosidic bonds. • It is helical in shape. Its spiral configuration is stabilised by hydrogen bonds between hydroxyl (-OH) groups. Fig. 2.3.1.2 Structure of amylose • Amylopectin consists of -glucose residues linked by -1,4-glycosidic bonds and -1,6- glycosidic bonds. It has a branched structure. Branch points are due to the presence of - 1,6-glycosidic bonds. Fig. 2.3.1.3 Structure of amylopectin

6 Table 1: Structure to function adaptations of starch Structural Features Function Adaptation • Helical structure of amylose and branched structure of amylopectin makes starch compact. • Allows efficient storage of glucose molecules i.e. more molecules of starch can be stored within less space in plant cells. • -glycosidic bonds between glucose monomers. • Can be hydrolysed by amylase to release glucose molecules for respiration. • Large molecular weight. • Folding of the starch molecule such that the hydroxyl groups (-OH) are in the interior of the molecule, i.e. prevented from interacting with water. • Makes starch insoluble (inability to form hydrogen bonds with water molecules) , allowing it to be stored without changing the osmotic potential within the plant cell. • Branched structure of amylopectin. • Allows multiple action sites for amylase, increasing the rate of hydrolysis to release glucose monomers for respira