B io Factsheet
September 2000 Number 78
Chemical Bonding in Biological Molecules
The contents of this Factsheet are directed towards AS level By adding more alpha-glucose molecules on at positions X and Y the
candidates. By studying this factsheet students should gain a backbone molecules of starch( alpha-amylose) and glycogen may be built
knowledge and understanding of: up. Alpha-amylose can be between 300 and 3000 alpha-glucoses long. The
other component of starch is amylopectin. This is similar to alpha-amylose
• glycosidic bonds in carbohydrate structure.
but is branched about every twentieth glucose by a 1,6 glycosidic branch
• peptide bonds in polypeptide structure. link. Glycogen has a structure similar to amylopectin but branches more
• ester bonds in lipid structure. frequently at about every twelfth glucose.
• hydrogen bonding.
• sulphur bonding. The reactive – OH group on carbon 1, labelled Y, is a reducing group and so
• phosphate bonding. gives the glucose reducing properties. (the power to donate hydrogen or
electrons to other substances). In alpha-glucose this group is below the ring
Introduction structure and during polymerisation forms 1,4 alpha-glycosidic bonds.
Glycosidic, peptide and ester bonds are formed by a process called These can be hydrolysed by alpha-amylase enzymes, for example, salivary
condensation. This is the joining of molecules by the removal of water and and pancreatic amylases in mammals and diastase in seeds, and so molecules
involves removing a hydroxide group from one of the molecules and a such as starch and glycogen can be digested to maltose and then by maltase
hydrogen from the other molecule. This type of reaction is important in enzyme to glucose. The structures of alpha-amylose and amylopectin are
synthetic processes. The reverse process, involved in digestion, is hydrolysis shown in Fig 2.
which is the splitting of molecules by the addition of water.
Exam Hint - Candidates will be expected to recognise and name
Glycosidic bonds molecules and bonds but will not be expected to write down structural
These are the bonds which join single sugars (monosaccharides) together to formulae from memory. Candidates may be asked direct questions
form double sugars (disaccharides) and multiple sugars (polysaccharides). about types of bonds, or may need to refer to the different bond types
The formation of a glycosidic link is shown in Fig 1. in essay answers.
Fig 1. Formation of a glycosidic link
carbon 1 carbon 4 CH2OH CH2OH
CH2OH CH2OH
H C O C O
H H H
C O C O hydrolysis
H H H H H H
C C C C + H2O
H H OH H OH H
C C C C
OH H OH H HO O OH
C C C C
HO C C OH + HO C C OH
condensation H OH H OH
H OH H OH X alpha-maltose Y
alpha-glucose alpha glucose 1,4 alpha-glycosidic link
Fig 2. Structure of alpha-amylose and anmylopectin
α - amylose
O O O O O
O O O O O O
1,4 glycosidic α - link
O O O
amylopectin
O O O O 1,6 glycosidic branch link
CH2
O O O O O
O O O O O O
1
September 2000 Number 78
Chemical Bonding in Biological Molecules
The contents of this Factsheet are directed towards AS level By adding more alpha-glucose molecules on at positions X and Y the
candidates. By studying this factsheet students should gain a backbone molecules of starch( alpha-amylose) and glycogen may be built
knowledge and understanding of: up. Alpha-amylose can be between 300 and 3000 alpha-glucoses long. The
other component of starch is amylopectin. This is similar to alpha-amylose
• glycosidic bonds in carbohydrate structure.
but is branched about every twentieth glucose by a 1,6 glycosidic branch
• peptide bonds in polypeptide structure. link. Glycogen has a structure similar to amylopectin but branches more
• ester bonds in lipid structure. frequently at about every twelfth glucose.
• hydrogen bonding.
• sulphur bonding. The reactive – OH group on carbon 1, labelled Y, is a reducing group and so
• phosphate bonding. gives the glucose reducing properties. (the power to donate hydrogen or
electrons to other substances). In alpha-glucose this group is below the ring
Introduction structure and during polymerisation forms 1,4 alpha-glycosidic bonds.
Glycosidic, peptide and ester bonds are formed by a process called These can be hydrolysed by alpha-amylase enzymes, for example, salivary
condensation. This is the joining of molecules by the removal of water and and pancreatic amylases in mammals and diastase in seeds, and so molecules
involves removing a hydroxide group from one of the molecules and a such as starch and glycogen can be digested to maltose and then by maltase
hydrogen from the other molecule. This type of reaction is important in enzyme to glucose. The structures of alpha-amylose and amylopectin are
synthetic processes. The reverse process, involved in digestion, is hydrolysis shown in Fig 2.
which is the splitting of molecules by the addition of water.
Exam Hint - Candidates will be expected to recognise and name
Glycosidic bonds molecules and bonds but will not be expected to write down structural
These are the bonds which join single sugars (monosaccharides) together to formulae from memory. Candidates may be asked direct questions
form double sugars (disaccharides) and multiple sugars (polysaccharides). about types of bonds, or may need to refer to the different bond types
The formation of a glycosidic link is shown in Fig 1. in essay answers.
Fig 1. Formation of a glycosidic link
carbon 1 carbon 4 CH2OH CH2OH
CH2OH CH2OH
H C O C O
H H H
C O C O hydrolysis
H H H H H H
C C C C + H2O
H H OH H OH H
C C C C
OH H OH H HO O OH
C C C C
HO C C OH + HO C C OH
condensation H OH H OH
H OH H OH X alpha-maltose Y
alpha-glucose alpha glucose 1,4 alpha-glycosidic link
Fig 2. Structure of alpha-amylose and anmylopectin
α - amylose
O O O O O
O O O O O O
1,4 glycosidic α - link
O O O
amylopectin
O O O O 1,6 glycosidic branch link
CH2
O O O O O
O O O O O O
1
