1. What are the organic chemical groups that compose carbohydrates? How are carbohydrates classified according to the presence of those groups?
Carbohydrates are also known as sugars (starches, cellulose and other substances are also carbohydrates).
Carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones (polyalcohol aldehydes or polyalcohol ketones).
Polyhydroxylated aldehydes are called aldoses and polyhydroxylated ketones are called ketoses.
2. What is the molecular formula for glucose? How can its structural formula be described?
The molecular formula for glucose is C₆H₁₂O₆.
Structurally, glucose is a hexagonal ring formed by one atom of oxygen and five atoms of carbon. A hydroxyl radical and a hydrogen atom are bound to each carbon atom of the ring, except for one of the carbon atoms bound to the oxygen atom of the ring, which binds to a CH₂OH radical. Concerning spatial position, hydroxyl bonds alternate sides.
Mono, Oligo and Polysaccharides
3. What are monosaccharides, oligosaccharides and polysaccharides?
Monosaccharides are simple carbohydrates molecules that cannot be broken down into smaller molecules of other carbohydrates. Oligosaccharides are carbohydrates made by bond of between a maximum of 10 monosaccharides. Polysaccharides are polymers of monosaccharides made of more than 10 units of such monomers. The most important polysaccharides are cellulose, starch, glycogen and chitin.
4. What is the difference between monosaccharides and disaccharides? What are some examples of them?
Monosaccharides are simple carbohydrates molecules that cannot be broken down into other carbohydrates. Glucose and fructose are examples of monosaccharides. Disaccharides are carbohydrates made up of two monosaccharides and which are missing one molecule of water (dehydration). The chemical bond between two monosaccharides is known as a glycosidic bond.
Sucrose (table sugar) is a disaccharide that consists of a bond between one molecule of glucose and one molecule of fructose. Maltose is a disaccharide that consists of two glucose molecules. Lactose (milk sugar) is another disaccharide and it is created by a bond between one molecule of galactose and one molecule of glucose.
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Hexoses and Pentoses
5. What are hexoses? What are some examples of hexoses with important biological functions?
Hexoses are carbohydrates made up of six carbon atoms. Glucose, fructose and galactose are all examples of hexoses. Hexoses have an important biological role, as they are energy sources for the metabolism.
6. What are pentoses? What are the roles of pentoses in DNA and RNA molecules?
Pentoses are carbohydrates made up of five carbon atoms.
The DNA molecule is made up of a sequence of molecules called nucleotides. Each nucleotide is formed by the bonding of a pentose called deoxyribose with phosphoric acid and a nitrogenous base (A, T, C or G). RNA is also formed by a sequence of nucleotides. RNA nucleotides are made through the bonding of one ribose (a pentose) molecule with one phosphoric acid molecule and one nitrogenous base (A, U, C or G).
Therefore, pentoses are fundamental components of DNA and RNA.
- Properties of Carbohydrates Review - Image Diversity: pentoses
Starch, Glycogen and Chitin
7. What are the main biological functions of polysaccharides?
Polysaccharides have an energy storage function and a structural function. Polysaccharides ingested by living organisms in the food chain are important sources of carbohydrates for the energetic metabolism of organisms of the next trophic levels.
Starch is the polysaccharide that plants use to store energy. Glycogen is a macromolecule responsible for the storage of glucose in the liver and muscles. Chitin is a polysaccharide with structural functions and which composes up the exoskeleton of arthropods and the cell wall in fungi.