Introduction to Carbohydrates

Introduction to Carbohydrates

Introduction to Carbohydrates


       At the end of this lecture, student will be able to

      Explain carbohydrates

      Classify carbohydrates

      Discuss the major  pathways of carbohydrate metabolism

      Outline the functions of carbohydrate

Introduction to Carbohydrates

       Carbohydrates means hydrates of carbon

       Defined as polyhydroxy aldehydes or ketones or compounds which produce them on hydrolysis

       Soluble in water and sweet in taste 

       These are most abundant organic molecules in nature

       Primarily composed of C, H, & O


       Most abundant dietary source of energy (4 Cal/g)

       Precursors for many organic compounds (fats, amino acids)

       Participate in the structure of cell membrane and cellular functions such as cell growth, adhesion and fertilization

       Structural components of many organisms

                                Fiber (cellulose) of plants

                                Cell wall of microorganisms

       Serve as the storage form of energy (glycogen) to meet the immediate energy demands of the body

Classification of carbohydrates

Based on the number of sugar units – classified into 3 types

1.      Monosaccharides

       These are simplest group of carbohydrates and are often referred as simple sugars

       General formula Cn(H20)n, & cannot be further hydrolysed

       Based on the functional group & number of carbon atoms, they are furthers classified into two types

                a. Aldoses: here the functional group is aldehyde e.g. glyceraldehyde, glucose, erythrose etc

                b. Ketoses: here the functional group is keto e.g. fructose

Based on the number of carbon atoms;

       Trioses (3C)

       Tetroses (4C)

       Pentoses (5C)

       Hexoses (6C)

       Heptoses (7C)


2. Oligosaccharides

       Contain 2-10 monosaccharide molecules which are liberated on hydrolysis

       Based on the number of monosaccharide units, further subdivided to

                a. Disaccharides: sucrose, Lactose, maltose

                b. Trisaccharides: Raffinose

                c. Tetrasaccharides: Stachyose

3. Polysaccharides

       Polymers of monosaccharide units with high molecular weight

       Usually tasteless( non-sugars) and form colloids with water


       Polysaccharides are of two types

a. Homopolysaccharides: They contain single type of monosaccharide units e.g. Starch, Inulin, Glycogen, Dextrin.

b. Heteropolysaccharides: They possess two or more different type of monosaccharide units e.g. Heparin

       Carbohydrates are the major source of energy for the living cells

       They are the 1st cellular constituents synthesized by green plants during photosynthesis from CO2 &
H2O on absorption of light

       Glucose is the central molecule in carbohydrate metabolism since all the major pathways of carbohydrate metabolism are connected with it

       Fasting blood glucose level in normal individuals is 70-100 mg/dl

       Liver plays a key role in monitoring and stabilizing blood glucose levels


Dietary Importance of Carbohydrates

In human nutrition, carbohydrates are a major dietary energy source, providing approximately four calories per gram.

Common dietary sources of carbohydrates include grains, fruits, vegetables, legumes, and dairy products.

Dietary guidelines typically recommend consuming a balanced mix of carbohydrates, proteins, and fats to maintain optimal health, emphasizing whole grains, fruits, and vegetables for their fiber and nutrient content.

Major pathways of carbohydrate metabolism

1. Glycolysis: oxidation of glucose to pyruvate and lactate

2. Citric acid cycle: oxidation of acetyl CoA to CO2. it is the final common oxidative pathway for carbohydrates, fats or amino acids, through acetyl CoA

3. Gluconeogenesis: synthesis of glucose from non-carbohydrate precursors (e.g. amino acids, glycerol etc.)

4. Glycogenesis: formation of glycogen from glucose

5. Glycogenolysis: breakdown of glycogen to glucose

6. Hexose monophosphate shunt: This pathway is an alternative to glycolysis and TCA cycle for the oxidation of glucose (directly to carbon dioxide and water)

7. Uronic acid pathway: Glucose is converted to glucuronic acid, pentoses and in some animals to ascorbic acid (not in man) this pathway is also an alternative oxidative pathway for glucose

8. Galactose metabolism: Pathways concerned with the conversion of galactose to glucose and the synthesis of lactose

9. Fructose metabolism: Oxidation of fructose to pyruvate and the relation between fructose and glucose metabolism

10. Amino sugar and mucopolysaccharide metabolism: The synthesis of amino sugars and other sugars for the formation of mucopolysaccharides and glycoproteins

Amino sugar and mucopolysaccharide metabolism


       Carbohydrates means hydrates of carbon

       Primarily composed of C, H, & O

       Based on the number of sugar units, they are classified into 3 types; monosaccharide, oligosaccharide and polysaccharides

       Based on the number of carbon atoms, they are classified as trioses, tetroses, pentoses, hexoses & heptoses

       Fasting blood glucose level in normal individuals is 70-100 mg/dl

       Major pathways of carbohydrate metabolism are glycolysis, TCA, gluconeogenesis, glycogenesis, glycogenolysis, HMP shunt, uronic acid pathway, galactose metabolism, Fructose metabolism & amino sugar and mucopolysaccharide metabolism


  1. What are some examples of monosaccharides? Examples of monosaccharides include glucose, fructose, and galactose.
  2. How do carbohydrates contribute to energy production in cells? Carbohydrates are broken down into glucose, which is then metabolized through glycolysis and oxidative phosphorylation to produce ATP, the energy currency of cells.
  3. What is the recommended daily intake of carbohydrates? Dietary guidelines typically recommend that carbohydrates should comprise 45-65% of total daily caloric intake.
  4. Are all carbohydrates equally beneficial for health? Not all carbohydrates are equally beneficial. While complex carbohydrates from whole grains, fruits, and vegetables are generally health-promoting, excessive consumption of simple sugars and refined carbohydrates may have negative health effects.
  5. How does fiber in carbohydrates contribute to health? Dietary fiber, found in plant-based carbohydrates, promotes digestive health, helps regulate blood sugar levels, and may reduce the risk of cardiovascular disease and certain cancers.

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