Chemistry of Carbohydrates Lecture 7

Carbohydrates

• Carbohydrates are the most abundant organic molecules in nature, formed of carbon, hydrogen, and oxygen atoms with a ratio of 1:2:1 and the general formula Cn(H2O)n.
• They are organic substances (polyhydroxy aldehyde or ketone) and most names for sugars end in -ose.

Biomedical Importance of Carbohydrates

• Carbohydrates are the chief source of energy for most organisms.
• They act as a storage form of energy in the body.
• They serve as cell membrane components that mediate some forms of intercellular communication.
• Ribose and deoxyribose sugars form part of the structure of RNA and DNA.
• Major antigens are carbohydrates in nature, e.g., blood group substances.

Classification of Carbohydrates

• Carbohydrates are classified according to the hydrolysis products (number of individual simple units) into:
  • Monosaccharides: contain 1 sugar unit (cannot be hydrolyzed), e.g., Glucose, galactose, mannose, and fructose.
  • Disaccharides: contain 2 sugar units, e.g., Maltose, lactose, and sucrose.
  • Oligosaccharides: contain 3-10 sugar units.
  • Polysaccharides: contain more than 10 sugar units, e.g., Starch, glycogen, and cellulose.

Monosaccharides

• Monosaccharides are the simplest units of carbohydrates and the building blocks.
• The general formula is (CH2O)n, so the name of carbohydrates (carbon + H2O).
• The ratio between carbon and H2O is 1.
• Most monosaccharides occurring in mammals are D sugars.
• Examples of monosaccharides include Glucose, mannose, galactose, and fructose.

Classification of Monosaccharides

• Classification based on the number of carbon atoms:
  • Trioses: contain 3 carbon atoms.
  • Tetroses: contain 4 carbon atoms.
  • Pentose: contain 5 carbon atoms.
  • Hexoses: contain 6 carbon atoms.
• Classification based on the functional group:
  • Aldoses: monosaccharides containing an aldehyde group (-CH=O), e.g., Glyceraldehyde, erythrose, ribose, glucose, galactose, and mannose.
  • Ketoses: monosaccharides containing a ketone group (-C=O), e.g., Dihydroxyacetone, erythrulose, ribulose, and fructose.

Disaccharides

• Disaccharides consist of two monosaccharides united together by glycosidic linkage.
• Disaccharides are classified into reducing disaccharides and non-reducing disaccharides.
• Examples of disaccharides include:
  • Sucrose (non-reducing): hydrolyzed by sucrase enzyme into Glucose + Fructose.
  • Lactose (reducing): hydrolyzed by lactase enzyme into Galactose + Glucose.
  • Maltose (reducing): hydrolyzed by maltase enzyme into 2 Glucose.

Oligosaccharides and Polysaccharides

• Oligosaccharides are polymers of 3-10 monosaccharide units and are not an important source of carbohydrates in the diet for humans.
• Polysaccharides are composed of more than 10 monosaccharide units linked by glycosidic bonds and are non-reducing.
• Examples of polysaccharides include Starch, glycogen, and cellulose.

This quiz covers the chemistry of carbohydrates, including their structure, composition, and properties. It's based on Lecture 7 by Dr. Wael Sobhy Darwish, a Biochemistry PhD.