Module 16: Unit 2. Carbohydrates Types and Naming
There are three types of carbohydrates: Monosaccharides, disaccharides and polysaccharides.
Monosaccharides: examples: galactose, glucose and fructose.
A video below illustrates the structure and the types of monosaccharides:
Galactose [OH group at carbon 4 is up]. A 6 member cyclic ring structure
Galactose [OH group at carbon 4 is up]. An Open and not Cyclic galactose structure.
Glucose [OH group at carbon 4 is down].
The carbon atoms in glucose are numbered as follows:
Six member Cyclic Ring Glucose (Haworth Structure)
Open and not Cyclic Glucose Structure (Fischer Structure)
Glucose and galactose have a six cyclic ring structures (most stable structure called Haworth structure).
Glucose and galactose have none cyclic ring structures (least stable structure called Fischer structure or projection)
Glucose can have two isomers called anomers: alpha glucose (α) anomer and beta glucose (β) anomer.
Fructose
Fructose has a five member cyclic ring structure.
Fructose has a Fischer structure as well.
Disaccharides: examples: maltose, lactose and sucrose.
A video below illustrates the disaccharides structure in some details.
Glucose is a monosaccharide that contributes in building several disaccharides and polysaccharides.
Maltose is made of two units of glucose molecules connected via α (1à4) glycosidic bond.
Maltose
Lactose
Lactose is made of one unit of galactose and one unit of glucose connected via β (1…> 4) glycosidic bond.
Sucrose
Sucrose is made of one unit of β–D-fructose and one unit of α–D-glucose connected via α, β (1…>2) glycosidic bond. Sucrose is called sugar cane or table sugar.
Polysaccharides: examples starch, glycogen and cellulose.
The videos below illustrate the structure and types of polysaccharides.
Starch is a form of glucose storage in plants. Starch is made of amylopectin and amylose.
Amylopectin is made of branched glucose molecule units every 25 glucose units via α (1à6) glycosidic bond [carbon number 1 connected to carbon 6 branched] and linear glucose molecule units connected via α (1à4) glycosidic bond [carbon number 1 connected to carbon 4 linearly]. Amylopectin is contributes 80% of the starch.
.Amylose is made of 250 to 4000 α – D glucose molecules units connected linearly to each other via α (1à4) glycosidic bond. Amylose contributes 20% of the starch.
Glycogen is an animal starch stored in the liver and muscles of animals. It is polymer of glucose molecules connected via α (1à4) glycosidic bond similar to amylopectin structure with many branching units connected via α (1à6) glycosidic bond every 10 to 15 glucose molecules unit.
Cellulose is the main component in plants especially in wood. Cellulose is made of polymer of glucose molecules units similar to amylose. The glucose molecules units are unbranched and connected linearly via β (1à4) glycosidic bond.
Cellulose
Cellulose cannot be digested by humans because human have no enzyme that can break the β (1à4) glycosidic bonds. Animals are using some bacteria and microorganisms in their stomach and intestines that can break this β (1à4) glycosidic bonds and hence they can digest the cellulose.
Carbohydrates Naming:
A video below illustrate the naming of carbohydrates:
Naming Monosaccharides:
The open and none cyclic monosaccharides according to the IUPAC rules will depend on number of carbon atoms and on the carbonyl group (aldehydes or ketones).
The number of carbons will follow the Greek numbering:
1 = mono
2= di
3 = tri
4 = tetra
5 = penta
6 = hexa
7 = hepta
8 = octa
9 = nona
10 = deca
Etc…
Note that the vowel a will be omitted in many naming of the carbohydrates.
The carbonyl group will end with Keto for a ketones and Aldo for an aldehyde.
The ending of the naming is ose for both ketones and aldehydes
The begin of the naming is given D or L abbreviation. D is given for the last OH being at the right of the none cyclic Fischer structure and L is given for the last OH being at the left of the Fischer structure.
Examples:
L – Aldopentose (5 carbons, aldehyde, last OH at left position)
D – Ketohexose (6 carbons, ketone, last OH at right position)
D – Aldotriose (IUPAC name)
L – Glyceraldehyde (common name)
L – Aldotriose (IUPAC name)
D – Therose (common name) L – Therose (common name)
D – Aldotetrose (IUPAC name) L – Aldotetrose (IUPAC name)
D – Erythrose (common name as well as IUPAC name not to mix with D – Therose which is also D- Aldotetrose)
L – Erythrose (common name as well as IUPAC name not to mix with L – Therose which is also L- Aldotetrose)
D – Ribose (common name) L – Ribose (common name)
D – Aldopentose (IUPAC name) L – Aldopentose (IUPAC name)
D – Fructose (common name) L – Fructose (common name)
D – Ketohexose (IUPAC name) L – Ketohexose (IUPAC name)
D – Glucose (common name) L – Glucose (common name)
D – Aldohexose (IUPAC name) L – Aldohexose (IUPAC name)
Naming Disaccharides and Polysaccharides:
In this chapter will follow the common names used above for disaccharides and polysaccharides. IUPAC naming of disaccharides and polysaccharides is very complex and will not discuss here.
For example the IUPAC of maltose is 4-O-(α-d-glucopyranosyl)- β-d-glucopyranose.
Important Disaccharides mentioned above:
Maltose
Lactose
Sucrose
Naming Polysaccharides:
Here also the common names will be used because IUPAC names of polysaccharides are very complex.
Important Polysaccharides mentioned are Starch, Glycogen and Cellulose.
The structures of these polysaccharides are given above.