Carbohydrates

1
Carbohydrates
2
Functions

• Carbohydrates serve as
• Energy stores
• Liver and muscle glycogen
• Fuels
• Catabolised in glycolysis to produce ATP
• Structural molecules
• eg. Ribose forms part of structure of ATP and
  nucleic acids (DNA and RNA)
• Molecules on surface of cell membranes involved
  in
• Cell recognition
• Cell adherence
• Hydrophilic properties attract water layer as
  lubricant

From Elliott WH Elliott DC. (1997)
Biochemistry and Molecular Biology. New York
Oxford University Press. p170
Adapted from Bettelheim FA and March J (1990)
Introduction to Organic and Biochemistry
(International Edition). Philadelphia Saunders
College Publishing p383.
From Tortora GJ Grabowski SR (2000) Principles
of Anatomy and Physiology (9th Edition). New
York John Wiley Sons, p62
3
Structures

• Carbohydrates exist as
• Monosaccharides
• Single structure which can contain up to 8 carbon
  atoms
• Not degraded by hydrolysis
• Disaccharides
• Two monosaccharides bonded together
• Polysaccharides
• More than two monosaccharides bonded together

4
Monosaccharides

• Empirical formula (CH20)n
• n indicates number of carbons in structure
• Trioses (3 carbons)
• All CHO degraded to trioses during metabolism
• Tetroses (4 carbons)
• Intermediaries during metabolism
• Pentoses (5 carbons)
• Form ring structures
• Most important is ribose
• Hexoses (6 carbons)
• Most common monosaccharides
• Glucose (Dextrose) most important
• All CHO converted to glucose during digestion
• Heptoses (7 carbons)
• Octoses (8 carbons)

From Tortora GJ Grabowski SR (2000) Principles
of Anatomy and Physiology (9th Edition). New
York John Wiley Sons, p43
5
Disaccharides

• Composed of two monosaccharides joined by
  glycosidic bond
• Produces water
• Degraded to monosaccharides by hydrolysis
• Uses water

Formation of a glycosidic bond. From Summerlin
LR (1981) Chemistry for the Life Sciences. New
York Random House, p395.
6
Disaccharides

• Only important disaccharides are
• Maltose
• Two glucose units
• Sucrose
• Glucose and fructose
• Lactose
• Glucose and galactose
• Maltase, sucrase, lactase in brush border of GI
  tract hydrolyse disaccharides
• Converted to glucose and absorbed
• Lactose intolerance - lack of lactase

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House, p339.
7
Polysaccharides

• Large complex CHO that liberate many
  monosaccharies on hydrolysis
• Glycogen one of most important
• Storage form of CHO in animals
• Liver and muscle
• Formed by strings of glucose (1-4 bonds)
• Strings joined approx. every 10 glucose units
  (1-6 bonds)
• Glycogenesis - formation of glycogen
• Glycogenolysis - hydrolysis of glycogen

From Summerlin LR (1981) Chemistry for the Life
Sciences. New York Random House, p339.
8
Phosphorylated sugars

• Important class of CHO due to charge of phosphate
  groups
• ATP
• Phosphorylation gives high potential energy
• Glucose-6-phosphate
• Phosphorylation by hexokinase traps glucose in
  skeletal muscle

From Stryer LS (1988) Biochemistry (3rd Ed.).
New York WH Freeman Co, p338.