Carbohydrates Lecture Notes

Study Notes

Carbohydrates are widely distributed in plants and animals, fulfilling both structural and metabolic roles.
Plants synthesize glucose from CO2 and H2O through photosynthesis, storing it as starch or cellulose.
Animals synthesize some carbohydrates from fat and protein, but most carbohydrates come from plants.

Carbohydrates are defined as polyhydroxy aldehydes or ketones, or substances that yield them upon hydrolysis.
They are the major source of energy in most animal tissues, including the brain and RBCs.
Glucose is the most important sugar in the blood and the universal fuel for fetuses.
Carbohydrates have highly specific functions, such as glycogen for storage, ribose in nucleic acids, galactose in lactose, and combination with protein in glycoproteins and proteoglycans.

Carbohydrates are classified into four categories:
- Monosaccharides (simple carbohydrates): cannot be hydrolyzed into simpler carbohydrates.
- Disaccharides: yield two molecules of monosaccharide upon hydrolysis (e.g., maltose, sucrose, and lactose).
- Oligosaccharides: yield three to ten monosaccharide units upon hydrolysis (e.g., maltotriose).
- Polysaccharides: yield more than ten molecules of monosaccharides upon hydrolysis (e.g., starch and dextrins).

Simple carbohydrates found naturally in foods, such as fruits, vegetables, milk, and milk products, and added during food processing and refining.
May be subdivided based on the number of carbon atoms (e.g., trioses, tetroses, pentoses, hexoses, heptoses, or octoses).
Classified as aldoses or ketoses depending on the presence of an aldehyde or ketone group.
Glucose is the most important monosaccharide, with three ways to represent its structure: straight chain, Haworth projection (cyclic structure), and chair form.

Stereoisomers: compounds with the same structural formula but differing in spatial configuration.
Asymmetric carbon atoms allow the formation of isomers, with the number of possible isomers depending on the number of asymmetric carbon atoms.
Glucose, with 4 asymmetric carbon atoms, has 16 isomers.
Optical activity: the presence of asymmetric carbon atoms confers optical activity, causing rotation of plane-polarized light.
Types of isomerism:
- D and L isomerism: depends on the orientation of —H and —OH groups around the carbon atom adjacent to the terminal primary alcohol carbon.
- Pyranose and furanose ring structures: similar to the ring structures of pyran or furan.
- Alpha and beta anomers: isomerism about position 1 (the anomeric carbon atom).
- Epimers: isomers differing in configuration of —OH and —H on carbon atoms 2, 3, and 4 of glucose.
- Aldose-ketose isomerism: differs in the structural formula due to the presence of an aldehyde or ketone group.

Deoxy sugars: lack an oxygen atom, replacing it with hydrogen (e.g., deoxyribose in DNA).
Amino sugars: contain amino groups, found in glycoproteins, gangliosides, and glycosaminoglycans (e.g., D-glucosamine, D-galactosamine, and D-mannosamine).

Formed by condensation between the hydroxyl group of the anomeric carbon of a monosaccharide and a second compound.
O-glycosidic bond: formed with a hydroxyl group.
N-glycosidic bond: formed with an amine group (e.g., between adenine and ribose in nucleotides like ATP).