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BIOLOGY 1A03 (168)
Xudong Zhu (12)
Chapter 5

Textbook and Class Notes Collaborated - Week 2 - Unit 1 - Chapter 5 Bio 1A03

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McMaster University
Xudong Zhu

Bio 1A03 Unit One: The Molecules of Life Chapter 5: An Introduction to Carbohydrates  The term carbohydrates encompasses o Monosacharides, “one sugar” (monomers) o Oligosaccharides, “few sugars” (small polymers) o Polysaccharides, “many sugars” (large polymers)  Carbohydrates are molecules with a carbonyl and several hydroxyl functional groups, along with many several many carbon-hydrogen (C-H) bonds  Functions of Carbohydrates o Energy Source – glucose is rapidly metabolized o Energy Storage – starch and glycogen o Structural Support – cellulose, chitin, cartilage o Transport of energy source – sucrose (plants), lactose (milk) o Cell Surface Signals – cell communication and cell-cell recognition 5.1 Sugars as Monomers  Monosaccharide’s – single sugars (simple sugars) o Figure 5.1, page 86 o Carbonyl group  On the end of the molecule – forms an aldehyde sugar (aldose)  Within the carbon chain – forms a ketone sugar (ketose) o Large amount of functional groups in sugars – provided by the presence of carbonyl group along with multiple hydroxyl groups o Able to participate in many chemical reactions o Carbons are numbered starting with the end nearest the carbonyl group o Ribose – present in RNA; acts as a building block for nucleotides; is a pentose sugar (5 carbons) o Deoxyribose – present in DNA o Glucose – present in starch, cellulose, glycogen; used as a source of chemical energy that sustains life; is a hexose sugar o Galactose – present in cartilage; must be converted to glucose in order to be a source of energy  Differences in monosaccharides o Varying locations of carbonyl groups (aldose, ketose) o Total number of carbon atoms present o Spatial arrangement of atoms (optical isomers, different mirror images)  Eg/ Glucose and Galactose are hexose sugar6(C126O ) Optical Isomers – differ in spatial arrangement of one hydroxyl group at the C#4  8 different hexoses exist, and two forms of each (mirror images) = 16 distinct structures within the molecular formula 6 12O6  Alternative ring strcutres  In solutions, simple sugars tend to form a ring structure  Glucose – C#1 bonds with an oxygen atom and with a hydroxyl group that can be oriented in two distinct ways – creating α glucose and β glucose o α glucose – hydroxyl lies below the plane of the ring o β glucose – hydroxyl lies above the plane of the ring o Figure 5.3, page 87; Ring Form of Sugars 5.2 The Structure of Polysaccharides  Glycosidic linkage o Simple sugars polymerize when a condensation reaction occurs between two hydroxyl groups, resulting in a covalent bond called a glycosidic linkage o Analogous to the peptide bonds that holds proteins together and to the phosphodiester bonds that connect the nucleotides in nucleic acids Bio 1A03  Peptide bonds and phosphodiester bonds always form at the same location between monomers, but glycosidic linkages do not o Glycosidic linkages form between hydroxyl groups – location and geometry can vary widely among polysaccharides (since every monosaccharide has at least 2 hydroxyls) o Figure 5.4, page 88  Disaccharides – double sugars that consist of two monosaccharide’s, which are joined by a condensation (dehydration) reaction o Glucose +Glucose  Maltose o Glucose + Fructose  Sucrose o Glucose + Galactose  Lactose  Real World Example: Lactose Intolerance o Symptoms: Flatulent/gassy, stomach pains and cramps, diarrhea, nausea o Adults do not produce lactase (enzyme) that splits lactose into glucose and Galactose  Some cannot produce lactase at all  Some can produce only insufficient amounts o Treatment: Pills, “lactaid”, which supplies the body with lactase o Autosomal recessive disease  Real World Example: Galactosemia o Relatively rare o Symptoms: Enlarged livers, renal and kidney failure, develop cataracts (eyes) o If not treated, brain damage occurs o Lacking the enzyme that converts galactose to glucose o Found by genetic testing o Treatment: Avoid certain foods, gene therapy possibly in the future o Cause  not enough data to know, possibly stress, genetic, spontaneous mutation, point mutation etc  Polysaccharides – are polymers that consist of many sugars monomers; formed when monosaccharide link together; also known as complex carbohydrates o Starch – storage polysaccharides in plants  Consists entirely of α-glucose monomers that are joined by glycosidic linkages  Angle of linkages between C#1 and C#4 causes the chain of glucose subunits to coil into a helix  Mix of two polysaccharides – amylose and amylopectic  Amylose- unbranched molecule, contains only α-1,4-glycosidic linkages  Amylopectin – branches when glycosidic linkages form between C#1 of one glucose monomer and C#6 of another monomer of glucose on another strand; branches occur about every 30 monomers o Glycogen – highly branches storach polysaccharide in animals  Stored in liver and muscles in humans  Enzymes break glycogen into glucose monomers which are processed in muscle cells to supply energy when exercising  α-glucose polymer and is nearly identical to amylopectin – but instead of α-1,6-glycosidic linkages occurring every 30 monomers, a branch occurs one out of every 10 glucose subunits o Cellulose – structural polysaccharides in plants  Major component of the cell wall  Cell wall: A protective sheet that occurs outside the membrane (plants)  Polymer of β-glucose monomers, joined by β-1,4-glycosidic linkages  Geometry of the bond is such that each glucose monomer in the chain is flipped in relation to the adjacent monomer – increases the stability of cellulose strands because the flipped orientation makes it possible for multiple hydrogen bonds to form between adjacent and parallel strands of cellulose  Often occurs in long, parallel strands that are joined by hydrogen bonds – strong and provide the cell with structural support o Chitin – structural polysaccharide in fungi and animals  Stiffens the cell walls of fungi  Most important component of the external skeleton of insects and crustaceans Bio 1A03  Similar to c
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