DHP YR 5 Biology: Carbohydrates 2024 PDF

DUNMAN HIGH SCHOOL DHP YR 5 BIOLOGY CORE IDEA 1: BIOMOLECULES OF LIFE PART 1: CARBOHYDRATES CONTENT The structure of carbohydrates, lipids...

DUNMAN HIGH SCHOOL DHP YR 5 BIOLOGY CORE IDEA 1: BIOMOLECULES OF LIFE PART 1: CARBOHYDRATES CONTENT The structure of carbohydrates, lipids and proteins and their roles in living organisms LEARNING OBJECTIVES: g) Describe the structure and properties of the following monomers: i. α-glucose and β-glucose (in carbohydrates) h) Describe the formation and breakage of the following bonds: i. glycosidic 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 Outline 1. Carbohydrates A. Monosaccharides (i) Glucose (ii) Galactose (iii) Fructose B. Disaccharides (i) Maltose (ii) Sucrose (iii) Lactose C. Polysaccharides (i) Starch (ii) Glycogen (iii) Cellulose References: Clegg & Mackene (2000). Advanced Biology (2nd edition). Hoh Yin Kiong (2003). Longman A-Level course in Biology Core Syllabus Taylor, Green & Stout (1997). Biological Science 1 (3rd Edition). Toole & Toole (1999) Understanding Biology for Advanced Level (4th Edition). Useful websites: Straight chain to ring form, alpha and beta isomers https://www.youtube.com/watch?v=L14m74Pt_6g Formation of disaccharides https://www.youtube.com/watch?v=0gBB4jquYz0 DHS 2024 1. CARBOHYDRATES  Carbohydrates are organic compounds made up of carbon, hydrogen and oxygen atoms.  There are usually twice as many hydrogen atoms as oxygen atoms, in the ratio of 2:1. General formula Cx(H2O)y where x and y are variable numbers.  Carbohydrates (Carbo-hydrates) receive their name because they are hydrates of carbon.  All contain several –OH groups, one attached to every carbon in the skeleton except for the carbonyl group (C=O). MAJOR CLASSES OF CARBOHYDRATES  Different classes of carbohydrates are distinguished by different number of sugar units / monomers. Class of carbohydrates No. of sugar units Monosaccharides (reducing) 1 Sugars Disaccharide 2 Oligosaccharides 3 – 10 Polysaccharides > 10 – 1000s includes starch, glycogen and cellulose A. MONOSACCHARIDES  General formula is (CH2O)n, where 3  n  9.  Classified according to the number of carbon atoms:  triose (3C) e.g. glyceraldehyde  tetrose (4C) rare  pentose (5C) e.g. ribose, deoxyribose  hexose (6C) e.g. glucose, fructose, galactose  heptose (7C) *Simple sugars with 3, 5 or 6 carbon atoms are more common.  All the carbon atoms of monosaccharides, except one with a carbonyl group (C=O), have a hydroxyl group (-OH) attached. DHS 2024 2 Hexoses (6C sugars)  These have a molecular formula of C6H12O6.  Some common hexose sugars include: (i) Glucose (in detail) (ii) Galactose (iii) Fructose (i) Glucose  For glucose, 1C contains an aldehyde group ( ), while 2C – 5 C each possess a hydroxyl group ( -OH).  Glucose molecules may exist in the open chain form, but like most hexoses and pentoses, easily forms the stable ring structure.  1 C (with the aldehyde group) reacts favourably with the oxygen atom on 5C to form a 6-sided ring structure called a pyranose ring. Isomers of glucose There are 2 possible ring forms (isomers) of glucose – alpha () and beta (β).  glucose: β glucose: OH group on OH group on C1 projects C1 projects below C1. above C1.  A glucose molecule can switch spontaneously from the open chain form to either of the two ring forms and back again. Overall equilibrium is reached where the proportions of the different forms remain constant. DHS 2024 3  α glucose β glucose  The existence of - and β-isomers leads to greater chemical variety and is important in, e.g. the formation of polymers such as starch and cellulose B. DISACCHARIDES  General formula C12H22O11.  Some common disaccharides include: (i) Maltose (ii) Lactose (iii) Sucrose  A disaccharide consists of two monosaccharides joined by a glycosidic bond.  Formation of a glycosidic bond: A glycosidic bond is formed by a polymerisation reaction called condensation reaction between two monosaccharide units (usually hexoses) combining with the elimination of a molecule of water. When it is formed between carbon 1 of one monosaccharide and carbon 4 of the other, it is called a α(14) glycosidic bond. Formation of maltose: α glucose α glucose condensation maltose α(14) glycosidic bond DHS 2024 4  Formation of sucrose: condensation sucrose α(14) glycosidic bond  Breakage of a glycosidic bond: The addition of water, under suitable conditions, is necessary if the disaccharide is to be split into its constituent monosaccharides; this process is known as hydrolysis. maltose α(14) glycosidic bond α glucose α glucose hydrolysis C12H22O11 + H2O C6H12O6 + C6H12O6 condensation Summary of the three common disaccharides Maltose Lactose Sucrose Location Animals and plants Animals Plants Monosaccharide 2 glucose Glucose and Glucose and constituents molecules galactose fructose Reducing sugar Yes Yes No DHS 2024 5 C. POLYSACCHARIDES  General formula: (C6H10O5)n where n = no. of hexose units linked together in the polysaccharides.  Some common polysaccharides include: (i) Starch (ii) Glycogen (iii) Cellulose  Polysaccharides are polymers of a few hundred or thousand monosaccharides.  These monosaccharides can be linked by α-glycosidic bonds or β-glycosidic bonds depending on which form of monosaccharide is present (α or β). A polymer is a substance of large, relative molecular mass and is formed as a result of joining together a large number of basically similar smaller molecules (monomers) in an enzyme-mediated condensation reaction. 4 6 3 5 2 1 α(16) glycosidic bond 6 6 6 6 5 5 5 5 4 1 4 1 4 1 4 1 3 2 3 2 3 2 3 2 α(14) glycosidic bond Figure showing part of amylopectin (in starch) molecule.  In the above figure, two types of bonds can be observed between the monosaccharide units: 1. α (14) glycosidic bonds for linear chain linkages 2. α (16) glycosidic bonds for branch chain linkages of its neighbour monomer. DHS 2024 6 (i) Starch  Starch is a polysaccharide found in most parts of the plant and deposited in the form of insoluble starch granules.  These are visible in plant cells notably in chloroplasts of leaves, in storage organs such as potato tubers and in seeds of legumes and cereals where it forms food supply for germination.  Function as energy storage, formed from excess glucose produced during photosynthesis. Structure  A polymer of  glucose units.  It has two main components, namely amylose and amylopectin  Starches differ from one plant species to the next, but on the whole they comprise 15 – 30 % amylose and 70 – 85 % amylopectin and 1 % of other substances such as phosphates and fatty acids. Amylose Amylopectin DHS 2024 7 The two pictures below are to show you how the concentric circles are formed. the concentric circles in starch granules arise from the arrangement and packing of both amylose and amylopectin molecules. Amylose contributes with its helical structures, while the branching and clustering of amylopectin further enhance the organization within the granule. DHS 2024 8 Summary: You need to pick, choose and rephrase the appropriate points depending on the question. Amylose Amylopectin Monomers  Both are made up of α glucoses. Bonds  (14) glycosidic bonds  (14) glycosidic bonds  (16) glycosidic bonds Structure  Is unbranched helical chain  Is helical chains which are highly with six glucose residues for branched and thus more every complete turn of the helix. compact. [Significance]  Helical chain is formed as a  branch points are formed by How result of intra-chain hydrogen (16) glycosidic bonds. bonding between hydroxyl structure is groups of glucose. achieved Hydrolysis  (14) glycosidic bonds break  (16) glycosidic bonds break by by amylase. de-branching enzyme. Solubility Insoluble in water due to  its bulky size (macromolecule)  many -OH groups are protected within the helical regions of the molecule and are unavailable for water molecules to hydrogen bond with. Why is positive starch test blue-black?  Observation: In the presence of starch, the iodine forms a complex with the starch molecules, resulting in a colour change from brown to blue-black.  Explanation: Iodine molecules fit into the helical structure of starch, causing the starch structure to change, thus the refractive index is changed, and iodine turns from brown to blue-black. DHS 2024 9 Relate Structure and Properties to Roles in living organisms Starch functions as a good food reserve in plants because of its structure. Structure Property* / Significance (role) Composed of thousands of  ꞏ Stores large amount of energy glucose linked by (14) ꞏ (14) glycosidic bonds can be broken by glycosidic bonds amylase, hydrolysing starch into glucose  glucose for respiration since it is the main respiratory substrate Amylose chains are helical in ꞏ compact, ideal for storage shape, with six glucose per turn. Helical structure is maintained by intra-chain hydrogen bonding between hydroxyl groups of glucoses. Amylopectin are highly ꞏ compact, ideal for storage branched due to branch points ꞏ Debranching enzymes break (16) maintained by (16) glycosidic bonds, this converts the glycosidic bonds branched structure of amylopectin into a more linear structure, increases the accessibility of the remaining linear chains to amylase, thus increases the rate of hydrolysis. ꞏ -OH groups occupied in intra- ꞏ unavailable for interaction with water chain hydrogen bonding. molecules ꞏ *insoluble, therefore osmotically inactive  Will not affect osmotic concentration in cells  water cannot be absorbed into the cells, and the cells will not swell. DHS 2024 10 (ii) Glycogen  Branched chain polymer of  glucose.  Molecular structure is similar to amylopectin. o However, it is larger and much more highly branched; o Thus more easily hydrolysed to  glucose by  Debranching enzymes which break (16) glycosidic bonds.  Glycogen phosphorylase which breaks (14) glycosidic bonds.  Function as energy storage in animal.  This picture shows cross-sectional view of glycogen. There is a core protein of glycogenin (enzyme) surrounded by branches of glucose. Try on your own: Relate structure of glycogen to its function. Clue: Similar to that of starch. Refer to P.12. DHS 2024 11 (iii) Cellulose  comprises up to 50% of plant cell walls.  primary function is to provide strength, rigidity, and structural support to plant cells.  also function to protect plant cells and cytoplasm from damage and mechanical injuries.  Despite its strength, the cellulose cell wall is fully permeable to water and solutes, an important property in the functioning of plant cells. Structure  Cellulose is an unbranched polysaccharide of  glucose linked by β(14) glycosidic bonds. Each cellulose chain consists of 1000 or more  glucose residue.  Formation of β(14) glycosidic bonds in cellulose requires the 180º rotation of alternating glucose residues, resulting in straight chains of cellulose molecules. DHS 2024 12  Cross-linking between chains: Hydroxyl groups (-OH) (at carbon atom 2) project outwards, alternately from both sides of each chain, allowing for the formation of hydrogen bonds between adjacent chains, thus establishing a rigid cross-linking between the chains. Thus many unbranched linear chains run parallel to each other.  Numerous cellulose chains associate to form microfibrils. Microfibrils are held together by hydrogen bonding between adjacent cellulose chains. These structures provide strength and rigidity to plant cell walls.  Microfibrils can be further associated into macrofibril which further coiled into larger bundles to form fibers.  These fibers contribute great tensile strength to the plant cell walls. DHS 2024 13 Relate Structure and Properties to Roles in living organisms Cellulose functions as a good structural support in plants because of its structure. Structure Property* / Significance (role) Composed of thousands of β ꞏ Stores large amount of energy glucose linked by β(14) ꞏ ꞵ(14) glycosidic bonds can be broken by glycosidic bonds, cellulase, hydrolysing cellulose into glucose  glucose for respiration since it is the main respiratory substrate 180◦ rotation of alternate β ꞏ Result in long straight chains of cellulose glucose residues. which allow for the formation of stable microfibrils. ꞏ Microfibrils further bundle ꞏ The bundling of chains and inter-chain together to form macrofibrils and hydrogen bonding between adjacent then fibers cellulose chains contribute to the strength and rigidity of these microfibrils. ꞏ Resulting in *high tensile strength and mechanical strength in the cell wall ꞏ Thus protect the plant cell from mechanical damage. ꞏ Due to the rotation of every ꞏ Allow inter-chain hydrogen bond to be alternate glucose, -OH groups formed between adjacent chains are projected outward of ꞏ Such cross-linkages increase tensile strength. cellulose chains ꞏ The lack of free –OH groups in this large molecule mean that it does not interact with water molecules, hence it is *insoluble in water. ꞏ Being *insoluble make cellulose a good material for forming structural support. Spaces between the macrofibrils ꞏ Makes it a *fully permeable structure, hence cell wall does not restrict movement of substance in and out of cell. DHS 2024 14 SUMMARY TABLE OF STARCH, GLYCOGEN AND CELLULOSE Starch Glycogen Cellulose Location Plant Animal Plant cell wall Structural Function Storage of energy Storage of energy support Monomers α-glucose α-glucose β-glucose α(14) in amylose Glycosidic bonds α(14) & α(16) in α(14) & α(16) β(14) amylopectin α-amlyases glycogen β-amlyases & phosphorylase Enzymes involved Cellulase debranching & debranching enzymes enzymes Size Large polysaccharides Solubility in water Insoluble in cold water DHS 2024 15 ANNEX DHS 2024 16 DHS 2024 17  Table for Structural Comparison between the different polysaccharides Starch Glycogen Cellulose Amylose Amylopectin Element C, H, O C, H, O C, H, O C, H, O Monomer glucose glucose glucose -glucose i.e. i.e. hydroxyl i.e. hydroxyl i.e. hydroxyl hydroxyl group of carbon group of carbon group of group of 1 1 carbon 1 carbon 1 is below is below is below is above the ring the ring the ring the ring Type(s) of bond 1->4 1->4 1->4  1->4 between glycosidic bond glycosidic bond glycosidic glycosidic monomers bond bond 1->6 1->6 glycosidic bond glycosidic at branched bond at points branched points Bond None none none Hydrogen between bonds adjacent forming chains cross- linkages between neighbouring glucose chains General structure Unbranched Branched Similar to unbranched linear - amylopectin linear - glucose chains Has up to twice but more glucose Chains forms a as many branching chains helix, with 6 glucose Rotation of glucose residues as every other - residues for amylose glucose every complete residue turn of helix -OH groups -OH groups project project inwards outwards in in each chain each chain (so allowing (so allowing no cross-linking cross-linking between between chains) chains) -glucose chains associated into groups to form microfibrils Microfibrils combines to form macrofibrils. Occurrence Component of Component of Found in Plant starch, which is starch, which is liver & cell wall a storage a storage muscle in polysaccharide polysaccharide animal in plant in plant DHS 2024 18