B io Factsheet
January 1999 Number 39
Carbohydrates: Revision Summary
Carbohydrates contain 3 elements: Carbon (C), Hydrogen (H) and Oxygen (O). Thus, if we remove water from carbohydrates, all that
remains is carbon. Carbohydrates can be divided into 3 categories; monosaccharides, disaccharides and polysaccharides (Table 1).
Table 1. Structure of monosaccharides, disaccharides and polysaccharides
Category Example Site Structure
Monosaccharide glucose fruit H OH
(made of 1 sugar fructose fruit, nectar
molecule) galactose milk
OH H
α-glucose β-glucose fructose
Disaccharide maltose = α-glucose + α-glucose germinating seeds
(made of 2 sucrose = glucose + fructose phloem tissue, fruit
O
monosaccharides lactose = glucose + galactose milk
joined together) maltose
Polysaccharide starch = polymer of glucose chloroplast stroma
(made of many glycogen = polymer of α-glucose muscle cells O O
monosaccharides cellulose = polymer of β-glucose plant cell wall O O
joined together) chitin = polymer of glucosamine exoskelteton of
arthropods cellulose
(glucose with an amino
acid attached)
Monosaccharides and Disaccharides Formation of disaccharides - typical exam questions
Monosaccharides and disaccharides are sugars. They all have the basic Common exam questions include:
formula (CH2O)n and can be classified according to how many carbon 1. Name the reaction involved when a disaccharide is formed
atoms they contain. 2. Name the type of bond formed
3. Show, by drawing a diagram, how a disaccharide is formed
3C = triose sugars e.g. glyceraldehyde C3H5O2
5C = pentose sugars e.g. ribose C5H10O5 Questions 1 and 2 are very simple - Disaccharides form in a condensation
6C = hexose sugars e.g. glucose C6H12O6 reaction which forms a glycosidic bond. The only way to get Question 3
correct is to practice! Fig 1 shows how maltose and sucrose are formed
from their monosaccharides.
Fig 1. Formation of maltose and sucrose from their monosaccharides
CH2OH CH2OH CH2OH
O O O O
H H H H H H HOH2C H
α-glucose + α-glucose glucose + fructose
OH OH OH H
HO H OH HO H OH HO H OH HO HO CH2OH
H H H OH
OH OH OH H
condensation condensation
reaction reaction
CH2OH CH2OH CH2OH
O O O O
H H H H H H HOH2C H
maltose + H2O sucrose + H2O
OH O OH OH O H
HO H H OH HO H HO CH 2OH
α1-4 glycosidic
H bond H
H OH
OH OH OH H
1
January 1999 Number 39
Carbohydrates: Revision Summary
Carbohydrates contain 3 elements: Carbon (C), Hydrogen (H) and Oxygen (O). Thus, if we remove water from carbohydrates, all that
remains is carbon. Carbohydrates can be divided into 3 categories; monosaccharides, disaccharides and polysaccharides (Table 1).
Table 1. Structure of monosaccharides, disaccharides and polysaccharides
Category Example Site Structure
Monosaccharide glucose fruit H OH
(made of 1 sugar fructose fruit, nectar
molecule) galactose milk
OH H
α-glucose β-glucose fructose
Disaccharide maltose = α-glucose + α-glucose germinating seeds
(made of 2 sucrose = glucose + fructose phloem tissue, fruit
O
monosaccharides lactose = glucose + galactose milk
joined together) maltose
Polysaccharide starch = polymer of glucose chloroplast stroma
(made of many glycogen = polymer of α-glucose muscle cells O O
monosaccharides cellulose = polymer of β-glucose plant cell wall O O
joined together) chitin = polymer of glucosamine exoskelteton of
arthropods cellulose
(glucose with an amino
acid attached)
Monosaccharides and Disaccharides Formation of disaccharides - typical exam questions
Monosaccharides and disaccharides are sugars. They all have the basic Common exam questions include:
formula (CH2O)n and can be classified according to how many carbon 1. Name the reaction involved when a disaccharide is formed
atoms they contain. 2. Name the type of bond formed
3. Show, by drawing a diagram, how a disaccharide is formed
3C = triose sugars e.g. glyceraldehyde C3H5O2
5C = pentose sugars e.g. ribose C5H10O5 Questions 1 and 2 are very simple - Disaccharides form in a condensation
6C = hexose sugars e.g. glucose C6H12O6 reaction which forms a glycosidic bond. The only way to get Question 3
correct is to practice! Fig 1 shows how maltose and sucrose are formed
from their monosaccharides.
Fig 1. Formation of maltose and sucrose from their monosaccharides
CH2OH CH2OH CH2OH
O O O O
H H H H H H HOH2C H
α-glucose + α-glucose glucose + fructose
OH OH OH H
HO H OH HO H OH HO H OH HO HO CH2OH
H H H OH
OH OH OH H
condensation condensation
reaction reaction
CH2OH CH2OH CH2OH
O O O O
H H H H H H HOH2C H
maltose + H2O sucrose + H2O
OH O OH OH O H
HO H H OH HO H HO CH 2OH
α1-4 glycosidic
H bond H
H OH
OH OH OH H
1
