A Level Biology Lesson 4:
Polysaccharides: The Structure and Function of Starch, Glycogen and Cellulose.
Task 1. Check Where this Lesson fits into your Exam Specification!
Task 2. Watch the Revision Notes Lesson in Full.
00:00 Intro Screen / Learning Outcomes
02:07 alpha 1,4 glycosidic bond of Amylose
02:27 Amylose is an Ideal Storage Molecule
03:58 alpha 1,6 Glycosidic bonds of Amylopectin
04:43 Amylopectin is a great Energy Storage Molecule
05:05 Starch is Made up of Amylose & Amylopectin
05:19 Key Features of Starch
06:10 Animal Cells Store Excess Glucose in the form of Glycogen
07:20 Structure and Function of Cellulose
08:11 Structure of Cellulose (microfibrils)
08:25 beta 1,4 glycosidic bonds
08:36 Weak hydrogen bonds link the long unbranched chains of cellulose together
08:55 Key features of Cellulose
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A-Level Biology "Polysaccharides: The Structure and Function of Starch, Glycogen and Cellulose" Lesson 4. Summary.
Polysaccharides are formed via condensation reactions.
Amylose is a compact cylindrical polysaccharide composed of many a-glucose molecules. The Glycosidic bond linking the α-glucose molecules is an α-1,4 glycosidic bond.
Amylose is a compact, energy storage molecule.
Amylopectin is a compact, branched polysaccharide composed of many a-glucose molecules. The Glycosidic bonds linking the α-glucose molecules are α-1,4 glycosidic bonds and α-1,6 glycosidic bonds.
It is the α-1,6 glycosidic bonds which give rise the branched structure of Amylopectin.
Starch is a compact “spiral” molecule composed of Amylose and Amylopectin.
Starch is an ideal energy storage molecule, found in plant cells.
Starch is insoluble in water and does not affect water potential of cells.
Animal cells store excess glucose in the form of Glycogen – a highly branched polysaccharide.
Glycogen is an excellent energy reserve molecule – due to the highly branched structure.
The α-1,6 glycosidic bonds are readily hydrolysed releasing the α-glucose molecules.
Cellulose is a long unbranched polysaccharide composed of β-glucose monosaccharides, which form plant cell walls. Cellulose chains are linked together via weak hydrogen bonds, which ‘hold’ the cellulose molecules together in structures known as microfibrils. These strong microfibrils form very strong microfibers. Cellulose is strong and provides structural support for plant cells – helping prevent cell lysis (bursting) when they fill with water via osmosis.
Check Your Spec!
★ AQA Specification Reference: - 3.1 Biological molecules. 3.1.2 Carbohydrates. Polysaccharides are formed by the condensation of many glucose units. Glycogen and starch are formed by the condensation of α-glucose. Cellulose is formed by the condensation of β-glucose. The basic structure and functions of glycogen, starch and cellulose. The relationship of structure to function of these substances in animal cells and plant cells.
★ CIE Specification Reference: - 2 Biological molecules. 2.2 Carbohydrates and lipids. b) define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide. e) describe the molecular structure of polysaccharides including starch (amylose and amylopectin), glycogen and cellulose and relate these structures to their functions in living organisms.
★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 1: Lifestyle, Health and Risk. 1.12 i) Know the difference between monosaccharides, disaccharides and polysaccharides, including glycogen and starch (amylose and amylopectin). ii) Be able to relate the structures of monosaccharides, disaccharides and polysaccharides to their roles in providing and storing energy. Topic 4: Biodiversity and Natural Resources: 4.9 Understand the structure and function of the polysaccharides starch and cellulose, including the role of hydrogen bonds between β -glucose molecules in the formation of cellulose microfibrils.
★ Edexcel (Biology B) Specification Reference: - Topic 1: Biological Molecules: 1.1 Carbohydrates. iii Understand how monosaccharides join to form polysaccharides (starch formed from amylose and amylopectin; glycogen) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions. iv Understand how the structure of glucose, starch, glycogen and cellulose relates to their function.
★ OCR (Biology A) Specification Reference: - 2.1.2 Biological molecules (f) the structure of starch (amylose and amylopectin), glycogen and cellulose molecules - how the structures and properties of glucose, starch, glycogen and cellulose molecules relate to their functions in living organisms.
★ OCR (Biology B) Specification Reference: - Module 2: Cells, chemicals for life, transport and gas exchange, 2.1.2 Water and its importance in plants and animals (d) the importance of hydrolysis and condensation of biological molecules in cell metabolism - To include the concept of monomers and polymers in a range of biological molecules. (f) i) the formation of polysaccharides by condensation - To include glycogen and starch (amylose and amylopectin) AND the formation of 1,4- and 1,6-glycosidic bonds and reference to the significance of branching on solubility.
★ WJEC Specification Reference: - Core Concepts 1. Chemical elements are joined together to form biological compounds - (c) the structure, properties and functions of carbohydrates: polysaccharides (starch, glycogen, cellulose) (d) alpha and beta structural isomerism in glucose and its polymerisation into storage and structural carbohydrates, illustrated by starch and cellulose (e) the chemical and physical properties which enable the use of starch and glycogen for storage and cellulose.