A carbohydrate is a naturally occurring compound, or a derivative of such a compound, with the general chemical formula Cx(H2O)y, made up of molecules of carbon (C), hydrogen (H), and oxygen (O). Carbohydrates are the most widespread organic substances and play a vital role in all life. Carbohydrate is a group of organic compounds occurring in living tissues and foods in the form of starch, cellulose, and sugars. The ratio of oxygen and hydrogen in carbohydrates is the same as in water i.e. 2:1. It typically breaks down in the animal body to release energy.

1. LectureVII
Aqsa Akhtar
School of Food and Agricultural Sciences
University of Management andTechnology
FST-2101 3(3-0)
Food Chemistry
1

2. Lecture 7
Carbohydrates-2

3. Session 7: Learning Outcomes
Course Learning Objective
▪Differentiate between major and minor food components and their
functions and physiochemical properties
Session Learning Objective
▪Explain the properties and reactions of carbohydrates, lipids, proteins, and
enzymes during the storage and processing of food
Session Learning Outcomes
❖SLO 7.1: Understand the concepts of enantiomers, stereoisomers, and
epimers of simple sugars
❖SLO 7.2: Differentiate the ring structures of aldehyde and ketone sugars
are formed
❖SLO 7.3: Describe the role that mutarotation plays in interconversion
between the alpha and beta anomers

4. ▪ Most abundant dietary source of energy (4 Kcal/g)
▪ Precursor for many organic compounds (fats, amino acids)
▪ Carbohydrates (glycoprotein, glycolipids) participate in the
structure of cell membranes and cellular functions
▪ Structural components of many organisms, fibers (cellulose) of
plant, exoskeleton of some insects, and the cell wall of
microorganisms
▪ Serve as a storage form of energy (glycogen) to meet the
immediate energy demands of the body
Functions of Monosaccharides

5. ▪ Most monosaccharides have a sweet taste
❖ (fructose is the sweetest; 73% sweeter than sucrose)
▪ Solids at room temperature
▪ Extremely soluble in water
▪ Glucose can dissolve in minute amounts of water to make a syrup
(1 g / 1mL water)
Physical Properties

6. Stereoisomerism (Spatial Isomers)
▪ Have the same molecular formula and the same structural formula
but differ in the spatial arrangement of the atoms in the molecule
▪ The bond structure between atoms and functional groups is the
same in stereoisomerism, but the geometrical positioning can change
▪ This class of isomers includes enantiomers, which are non-
superimposable mirror images of each other, like left and right
hands
▪ Enantiomers always contain chiral centers
Isomerism in Sugars

7. ▪Monosaccharide exhibits various forms of isomerism due to
the presence of asymmetric carbon atoms
▪There are six types of isomerism found with
monosaccharides
✓Aldose-ketose isomerism
✓D & L isomerism
✓Optical isomerism
✓Pyranose and furanose isomerism
✓Alpha and Beta isomerism
✓Epimerism
Isomerism in Sugars

9. Carbohydrate Isomeric Forms
H

10. Asymmetric Carbon/Chiral Carbon
▪ An asymmetric carbon atom is a carbon atom that is attached to
four different types of atoms or groups of atoms
▪ Four groups of atoms attached to the carbon atom can be arranged
in space in two different ways that are mirror images of each other,
and which called left-handed and right-handed versions of the same
molecule
▪ Molecules cannot be superimposed on their own mirror image are
said to be chiral-like mirror image
▪ There could be more than one chiral centers in a molecule

12. ▪ Refers to molecules that interact with plane-polarized light
▪ Jean Baptiste Biot French Physicist - 1815
▪ He discovered that some natural substances (glucose, nicotine,
sucrose) rotate the plane of plane-polarized light and that others
did not
Optical Activity

13. ▪ A levorotatory (–) substance rotates polarized light to the left
[e.g., l-glucose;(-)-glucose]
▪ A dextrorotatory (+) substance rotates polarized light to the right
[e.g., d-glucose;(+)-glucose]
▪ Molecules which rotate the plane of polarized light are optically active
▪ Most biologically important molecules are chiral, hence optically active
Optical Activity

14. Enantiomers
▪ D and L sugars are referred to as
enantiomers
▪ Their structures are mirror images of
each other
▪ Only D-glucose or D-sugars are
utilized by humans
▪ D and L form depending on
arrangement of H and OH on the
penultimate (reference) carbon atom
▪ When the sugar has OH group on the
right, is D isomer
▪ If OH groupis on left side then it is
the L isomer

16. Epimers
➢Monosaccharides which differ in configuration around one specific C-
atom are called epimers of one another:
▪C-2 epimers
❑Glucose and mannose
▪C-4 epimers
❑Glucose and galactose

17. Chemical Properties of
Monosaccharides

18. Hemiacetal and Hemiketal
▪ When an alcohol adds to an aldehyde, the result is called a
hemiacetal; when an alcohol adds to a ketone the resulting product
is a hemiketal
▪ The conversion of an alcohol and aldehyde (or ketone) to a
hemiacetal (or hemiketal) is a reversible process

19. Furanose vs Pyranose
▪ The carbonyl carbon that reacts to form the hemiacetal is
referred to as the anomeric carbon
▪ Two ring arrangements can be produced
▪ These are termed anomers, and are referred to as the alpha (∂)
and beta (β) anomer
▪ The ring structure of D-fructose contains four carbons and an
oxygen to form a five-membered ring called a furanose
▪ In the six-member ring (five carbons and an oxygen) form of D-
isomers, the ring is called a pyranose

20. GlucoseAnomers
▪ In the pyranose form of glucose, carbon-1 is chiral, and thus two
stereoisomers are possible: one in which the OH group points
down (α-hydroxy group) and one in which the OH group points up
(β-hydroxy group)
▪ These forms are anomers of each other, and carbon-1 is called the
anomeric carbon

21. FructoseAnomers
• Fructose closes on itself to form a furanose ring:

23. Pyranose Formation by Intramolecular
Linkage

24. Furanose Formation by Intramolecular
Linkage

25. Oxidation of Monosaccharides
▪ Aldehydes and ketones that have an OH group on the carbon next to
the carbonyl group react with a basic solution of Cu2+ (Benedict’s reagent)
to form a red-orange precipitate of copper(I) oxide (Cu2O)
▪ Any carbohydrate (sugar) that can reduce the Benedict’s reagent is
called the reducing sugar
▪ All monosaccharides and disaccharides (except sucrose) are reducing
sugars

26. Benedict’sTest
▪ This test is used to test for simple carbohydrates
▪ The Benedict’s test identifies reducing sugars (monosaccharides
and some disaccharides), which have free ketone or aldehyde functional
groups

27. Formation of Phosphate Esters
▪ Phosphate esters can form at the 6-carbon of aldohexoses and
aldoketoses
▪ Phosphate esters of monosaccharides are found in the sugar-
phosphate backbone of DNA and RNA, in ATP
, and as
intermediates in the metabolism of carbohydrates in the body

29. Condensation and Hydrolysis
▪ Condensation reaction is a type of reaction that occurs when
two molecules are joined and a water molecule is produced. This
type of reaction is referred to as a dehydration reaction
▪ Hydrolysis reaction is the reverse of a condensation reaction
▪ A larger molecule forms two smaller molecules and water is
consumed as a reactant

30. Joining & Cleaving Sugar Molecules

31. Questions/Discussion
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