Carbohydrates Overview

Carbohydrates

• Carbohydrates are organic compounds with the empirical formula Cn(H2O)n.
• They are aldehyde or ketone derivatives of polyhydric alcohols.
• Importance of carbohydrates:
  • Energy storage (e.g., starch and glycogen).
  • Structural components (e.g., cellulose in plants and chitin in arthropods).
  • Ribose is a component of coenzymes (e.g., FAD, NAD) and the backbone of RNA.
  • Deoxyribose is a component of DNA.
  • Saccharides and their derivatives play roles in the immune system, fertilisation, preventing pathogenesis, blood clotting, lubrication, and development.
  • Hydroxyl groups allow carbohydrates to interact with the aqueous environment and participate in hydrogen bonding.
  • Carbohydrate derivatives can contain nitrogen, phosphate, and sulfur compounds.
  • Carbohydrates can combine with lipids to form glycolipids or with proteins to form glycoproteins.

Glucose

• Glucose is the most important carbohydrate.
• Synthesized in plants from CO2 and water through photosynthesis and stored as starch or used to synthesize cellulose for plant cell walls.
• Animals acquire most of their carbohydrates from plants, but can synthesize them from amino acids.
• Most dietary carbohydrate is absorbed into the bloodstream as glucose, formed by the hydrolysis of dietary polysaccharides and disaccharides.
• Glucose is the major metabolic fuel for mammals, except ruminants, and a universal fuel for the fetus.
• Glucose is the precursor for all other carbohydrates in the body, including glycogen, ribose, deoxyribose, galactose, glycolipids, glycoproteins, and proteoglycans.

Isomerism in Carbohydrates

• Carbohydrates contain at least one chiral carbon, making them optically active.
• A chiral carbon atom is attached to four different atoms or chemical groups.
• Mirror images of a chiral molecule are called enantiomers or optical isomers.
• Isomers have the same molecular formula but different structural formulas.
• The number of isomers depends on the number of chiral centers in the molecule, following the formula: Number of isomers = 2n (where n is the number of chiral centers). For example, glucose has 4 chiral carbons, resulting in 24 = 16 isomers.

Types of Isomerism

• D and L isomerism: Carbohydrates exist in D or L conformations determined by the orientation of the hydroxyl group on the chiral carbon farthest from the carbonyl carbon. Most physiologically significant carbohydrates are in the D-conformation.

• Pyranose and furanose ring structures: Rings resemble pyran (six-membered) or furan (five-membered) derivatives, leading to pyranoses and furanoses respectively. Glucose predominantly exists in the pyranose form in solution.

• Alpha and beta anomers: The rings can open and close, leading to rotation around the anomeric carbon, creating α and β configurations of hemiacetals and hemiketals.

• Epimers: Isomers with different configurations of –OH and –H groups at carbons 2, 3, or 4. Important glucose epimers include mannose (carbon 2) and galactose (carbon 4).

• Aldose and ketose isomerism: Fructose and glucose have the same molecular formula but different structures due to the presence of a keto group in position 2 in fructose (C2 is the anomeric carbon) as opposed to an aldehyde group in position 1 in glucose (C1 is the anomeric carbon).

                                    ### Classification of Carbohydrates
  

• Monosaccharides: Simple carbohydrates that cannot be hydrolyzed to smaller units.

  • Aldehydes or ketones with two or more hydroxyl groups.
  • General chemical formula: (CH2O)n.
  • Classified by the number of carbon atoms: trioses, tetroses, pentoses, hexoses, etc.

• Disaccharides: Formed by the condensation of two monosaccharides.

• Oligosaccharides: Short chains of monosaccharides.

• Polysaccharides: Long chains of monosaccharides.

                                  ### Monosaccharides
  

• Triose (3 carbons): Glyceraldehyde and Dihydroxyacetone

• Tetrose (4 carbons): Erythrose

• Pentose (5 carbons): Ribose, Ribulose, Xylulose

• Hexose (6 carbons): Glucose, Galactose, Mannose, Fructose

• Heptose (7 carbons): Sedoheptulose

• Nonose (9 carbons): Neuraminic acid (sialic acid)

• The aldehyde and ketone moieties of five and six-carbon carbohydrates spontaneously react with alcohol groups to form intramolecular semiacetals or hemiketals.

• This forms five- or six-membered rings.

• Five-membered rings are termed furanoses as they resemble furan, while six-membered rings are termed pyranoses as they resemble pyran.

• Fischer projection: A method of representing monosaccharides in a two-dimensional format.

• Haworth projection: A method of representing cyclic monosaccharides in a two-dimensional format.

• Chair and boat conformations: The most stable form of furanose and pyranose rings is the chair conformation.

  • Axial: Constituents of the ring that project above or below the plane of the ring.
  • Equatorial: Constituents of the ring that project parallel to the plane of the ring.

This quiz covers the essential aspects of carbohydrates, including their structure, importance, and roles in biological systems. Participants will learn about various carbohydrate types, such as glucose and their derivatives, and how they contribute to energy storage and structural components in organisms.