Monosaccharides
Monosaccharides are carbohydrates that cannot be further hydrolysed to simpler units having the basic formula Cn(H2O)n , where n = 3 or more. The smallest ones, for which n = 3, are glyceraldehyde and dihydroxyacetone.
Monosaccharides are often given more detailed names that describe both the total number of carbon atoms and the functional groups (aldehyde and ketone groups).
Depending on the number of carbon atoms, monosaccharides are subdivided into trioses, tetroses, pentoses , hexoses and so on. Mono saccarides are further subdivided into aldoses or ketoses depending on whether they contain aldehyde or ketone group for example hexoses whitch are monosaccharides containing 6 carbon atoms are further classified into:
• Aldohexoses containing 6 carbon atoms and aldehyde group e.g. glucose.
• Ketohexoses containing 6 carbon atoms and ketone group e.g. fructose
Classification of monosaccharides
Number of Category Examples Carbons Name Aldose Ketose
3 Triose Glyceraldehyde Dihydroxyacetone
4 Tetrose Erythrose Erythrulose
Ribose Ribulose 5 Pentose Xylose Xylulose
Glucose 6 Hexose Fructose Galactose
2 Structural formulae of monosaccharides
The structure of glucose can be represented in one of the following ways:
1. The straight chain formula.
H C O H C OH HO C H H C OH H C OH
CH2 OH glucose (straight chain formula) 2. Fischer projection formula where the aldehyde (or ketone) group reacts with an alcohol group on the same sugar to form a hemiacetal or hemiketal ring .
OH H C H C OH HO C H H C OH H C O
CH2 OH
glucose (hemiacetal, Fischer projection formula)
3. Haworth formula where the cyclic structure is represented in six membered pyranose ring or five membered furanose ring.
CH2OH H O H H OH H OH OH H OH
glucose (Haworth formula) Monosaccharides of Biological Importance
3 1- Glucose (grape sugar)
Glucose is an aldohexose
It is the major source of energy in humans and animals.
It is the main sugar in the blood.
It is converted into other carbohydrates in the liver and other tissues e.g. galactose, fructose, ribose and glycogen.
2- Galactose
Galactose is an aldohexose
It is synthesized in mammary gland
It binds to glucose to form the disaccharide lactose (sugar of milk).
It can be converted into glucose in the liver.
3- Fructose (fruit sugar)
Fructose is a ketohexose
It is present in semen acting as the main energy source for the spermatozoa
It is a constituent of disaccharide sucrose (cane sugar).
It can be converted into glucose in the liver.
5- Ribose and deoxyribose
Ribose and deoxyribose are aldopentoses
They enter in nucleic acids structure.
Ribose enters in the structure of high energy phosphate compounds as ATP, GTP and CTP
Also, ribose enters in the structure of coenzymes such as NAD and FAD.