Transcript File
Biomolecules Part II: Carbohydrates Carbohydrates • Are made from just three elements: carbon, hydrogen and oxygen • Have a characteristic ratio of these atoms – for every one carbon atom there are two hydrogen atoms and one oxygen atom. • Carbohydrates perform many important biological functions. Functions of Carbohydrates • Source of energy and a means of storing chemical energy. • Involved in many chemical reactions, as components of molecules such as DNA and RNA, and can be chemically bonded to lipids and proteins. • Form structural elements in the cell walls of plants, fungi and bacteria. Also make up the exoskeleton of insects and crustaceans. Classes of Carbohydrates • Carbohydrates are classified into classes depending upon the number of linked sugar molecules they contain. • Classes of Carbohydrates – – – – Monosaccharides Disaccharides Oligosaccharides Polysaccharides • Carbohydrates containing one or two sugar units are often referred to as simple carbohydrates; those containing many sugar molecules are called complex carbohydrates Simple Carbohydrates Monosaccharides General formula: (CH2O)n n=3, triose n=5, pentose n=6, hexose Glucose Fuel molecule Fructose Fuel molecule Ribose Component of the nucleotide for RNA Disaccharides (common dietary components) Sucrose (a glucose + b fructose) Transport sugar in vascular plants Lactose Component of milk (b galactose + a glucose) Maltose (a glucose + a glucose) Obtained in the breakdown of starch Complex Carbohydrates Polysaccharides (insoluble macromolecules) Starch Storage molecule in plants Glycogen Storage molecule in animals Cellulose Component of the plant cell wall Chitin Component of the exoskeletons of insects and crustaceans Peptidoglycan Component of bacterial cell wall More about monosaccharides • They are white crystalline solids that readily dissolve in water. • Solubility is due to their OH groups which readily form hydrogen bonds with water molecules. • All monosaccharides are sweet tasting to varying degrees. • The most common and biologically important simple sugar is glucose. What’s so good about glucose? • Fuel molecule for the cell. • Building block for many other important carbohydrates. • Has a large number of isomeric forms. (e.g. L and D forms) • When in solution it forms a ring structure. • There are two forms called alpha and beta glucose depending on the position of the OH group on carbon 1. • The properties of many biologically important polysaccharides depend upon whether the alpha or beta form is present. Linking monosaccharides • Linking two monosaccharides results in a disaccharide molecule. • Repetitive linking of many monosaccharides results in a polysaccharide. • The process of linking monosaccharide monomers is referred to as condensation polymerisation. Condensation polymerisation • The polymerisation reaction involves an OH group at C1 with another OH group on an adjacent monosaccharide. • During the reaction a water molecule is eliminated, hence the name condensation polymerisation. • The bond formed between the monomers is referred to as a glycosidic bond. Polysaccharides • The different properties of polysaccharides can be explained by: – The length of the polymer chain – The extent of any branching chains – Whether the polymer chain is straight, or folded, or coiled. More about Polysaccharides • Polysaccharides are divided into two categories: – Storage polysaccharides •Starch •Glycogen – Structural polysaccharides •Cellulose •Chitin •peptidoglycan More about storage polysaccharides • Starch – Fuel storage polysaccharide in plants. – Exists in two forms: amylose and amylopectin. – Both are made by linking alpha glucose monomers. – Amylose is an unbranched polymer that coils into a helical structure and can form a suspension in hot water. – Amylopectin is a branched polymer that is completely insoluble in water. More about storage polysaccharides • Glycogen – Fuel storage polysaccharide in animals. – Found in the liver and muscle cells. – Excess glucose will be polymerised to glycogen for storage. – It is amylopectin with short distances between the branching side chains. – Glycogen molecules tend to be larger and more branched than starch molecules. – Glycogen is insoluble in water. More about structural polysaccharides • Cellulose – – – – Major component of plant cell walls. Most abundant organic substance on Earth. Made from linking of b-glucose monomers In the plant cell wall, hydrogen bonding between the long parallel polymer chins holds them together to form strong microfibrils. More about structural polysaccharides • Chitin – Formed by linking glucose monomers that have a nitrogen-containing group of atoms attached. – Translucent, pliable but tough compound. – Found in the exoskeletons of insects and crustaceans. – Also in the cell walls of many fungi, moulds and yeasts. More about structural polysaccharides • Peptidoglycan – Made up of two alternatively repeating monomers are Nacetylglucosamine (the monomer for chitin) and N-acetylmuramic acid. – Both monomers based upon glucosamine – a glucose molecule with an NH2 (amino group) substituted for an OH group on C2.