IB Chemistry III B2 Presentation

1. Carbohydrates

2. Monosaccharides
• All monosaccharides have the empirical
formula CH2O.
• They contain a carbonyl group in addition to
this and at least two –OH groups.
• They have between 3 and 6 carbon atoms.

3. Types of Monosaccharides
• Monosaccharides with the formula C5H10O5
are generally known as pentoses (ribose is an
example of this).
• Monosaccharides with the formula C6H12O6
are known as hexoses (glucose is an example
of this).

4. Monosaccaharides Structures
• Many structural isomers are possible.
• In addition several carbon atoms are chiral
(asymmetric) and give rise to optical
isomerism.
• Open chain structures and ring structures are
also possible.

5. • The form of natural glucose
is known as D-glucose.
• The picture on the left is of
a straight chain formation
of D-glucose.

6. Ring Structure of D-glucose
The ring structure of D-glucose can exist in two separate
crystalline forms known as α-D-glucose and β-D-glucose.
The only difference is the the –OH group on the first
carbon atom is inverted.

7. Monosaccharides cont.
• Six-membered ring monosaccharides are
known as pyranoses.
• Hexoses can also have a furanose structure
where they have a five-membered ring
containing an oxygen atom.

8. Polysaccharides
• Monosaccharides can undergo condensation
reactions to form disaccharides and eventually
polysaccharides.

9. Sucrose
• Sucrose is a disaccharides formed from the
condensation of α-D-glucose in the pyranose
form and β-D-frutose in the furanose form.
• The bond formed is known as a glycosidic link.

10. Sucrose continued
• In the case of sucrose the link is between the
C-1 atom of glucose in the α-configuration and
the C-2 atom of frutose. The links is known as
a β-1,2 bond.

11. • Maltose, another disaccharide is formed from
two glucose molecules condensing to form an
α-1,4 bond.

12. • Lactose is a disaccharide in which the β-D-
galactose is linked at the C-1 atom to the C-4
atom of β-D-glucose. This is called a β-1,4
bond.

13. Starch
• Starch is one of the most important
polysaccharides.
• Starch exists in two forms: amylose, which is
water soluble and amylopectin, which is
insoluble in water.

14. Amylose
• Amylose is a straight chain polymer of α-D-
glucose units with α-1,4 bonds.

15. Amylopectin
• Amylopectin also contains α-D-glucose units
but it has a branched structure with both α-
1,4 and α-1,6 bonds.

16. • Most plants use starch as a store of carbohydrates
and their energy.
• Cellulose, a polymer of β-D-glucose contains β-1,4
links. Cellulose, together with lignin, provides the
structure to the cell wall of green plants.
• most animals, including all mammals, do not have
the enzyme cellulase so are unable to digest cellulose
or other dietary fiber polysaccharides.

17. Functions of Polysaccharides in the
Body
• To provide energy
– Food such as
bread, biscuits, cake, potatoes, pasta, and cereals
are all high in carbohydrates.
• To store energy
– Starch is stored in the livers of animals in the form
of glycogen. Glycogen has almost the same
chemical structure as amylopectin.

18. (cont.)
• As precursors
– For other important biological molecules they are
components of nucleic acids and thus play an
important role in the biosynthesis of proteins.
• As dietary fiber
– Dietary fiber is mainly plant material that is not
hydrolysed by enzymes secreted by the human
digestive tract by may be digested by microflora in the
gut.
– It may be helpful in preventing conditions such as
diverticulosis, irritable bowel
syndrome, obesity, Crohn’s disease, haemerrhoids and
diabetes mellitus.