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Sugars and other carbohydrates are highly variable in structure.
Monosaccharides are monomers that polymerize to form polymers called polysaccharides, and are joined by different types of glycosidic linkages.
Carbohydrates perform a wide variety of functions in cells: serving as raw material for synthesizing other molecules, providing structural support, indicating cell identity, and storing chemical energy.
Many distinct monosaccharides exist because so many aspects of their structure are variable: aldose or ketose placement of the carbonyl group, variation in carbon number, different arrangements of hydroxyl groups in space, and alternative ring forms. Each monosaccharide has a unique structure and function.
• Cellulose, chitin, and peptidoglycan form long strands with bonds between adjacent strands.
• These strands may then be organized into fibers or layered in sheets to give cells and organisms great strength and elasticity.
• Unlike the α-glycosidic linkages in the storage polysaccharides, the β-1,4-glycosidic linkages of structural carbohydrates are very difficult to hydrolyze – very few enzymes have active sites that accommodate their geometry or have the reactive groups necessary.
• Most monosaccharides are readily synthesized under conditions that mimic early conditions; thus, it is likely that the prebiotic soup contained a wide diversity of monosaccharides.
• Polysaccharides, however, despite their current relative abundance on Earth, probably played little to no role in the origin of life.
Carbohydrates have diverse functions in cells: In addition to serving as precursors to larger molecules, they provide fibrous structural materials, indicate cell identity, and store chemical energy.
• Although polysaccharides are unable to store information, they do display information on the outer surface of cells in the form of glycoproteins – proteins joined to carbohydrates by covalent bonds.
• Glycoproteins are key molecules in cell-cell recognition and cell-cell signaling.
• Each cell in your body has glycoproteins on its surface that identify it as part of your body.
• Carbohydrates store and provide chemical energy in cells.
• In chemical evolution, the kinetic energy of sunlight and heat were converted into chemical energy stored in the bonds of H2CO and HCN.
– Today, most sugars are produced via photosynthesis, a key process that transforms the energy of sunlight into the chemical energy of C–H bonds in carbohydrates.
• Carbohydrates have more free energy than CO2 because the electrons in C–H bonds and C–C bonds are shared more equally and held less tightly than they are in C–O bonds.
• Starch and glycogen are efficient energy-storage molecules because the α-linkages are readily hydrolyzed, whereas the β-linkages of structural carbohydrates resist enzymatic degradation.
• The enzymes amylase and phosphorylase catalyze the hydrolysis of α-glycosidic linkages in glycogen and starch, respectively. The released glucose subunits can then be used in the production of ATP.