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Carbohydrates: general principle
-contain elements C,H,O
-Produced by photosynthesis by combining CO2 and H20 Monosaccharides
- have the general formula (CH2O)n - most common form are hexoses (n=6)glucose, galactose, fructose
-aldose (sugar contains an aldehyde) glucose, ketose (sugar contains a ketone group)- Fructose
-sugars have chiral carbon atoms so exist as stereoisomers - D glucose (dextrose) is naturally occurring form
-sugars are also cyclised, there is a second asymmetrical centre giving two forms
-the alpha form is when OH is below the ring- the OH is attached to C1 is on the opposite side of the ring as C6
-the beta from is when it is above the ring Reactions of monosaccharides
-glucose is a reducing sugar- the aldehyde is easily oxidised to carboxylic acid. The reaction is catalysed by glucose oxidase. This reaction is used for blood tests to monitor glucose levels. Modified glucose oxidase oxidises glucose from blood sample and releases H202. Electrons from H2o2 are passed into an electrode to form an electrical signal that displays a value.
-glucose reacts with amine groups of haemoglobin to form glycosylated haemoglobin- this remains in circulation and the amound of modified haemoglobin corresponds to long term regulation
-monosaccharides can be joined together by glycosidic bonds which are formed by dehydration reactions.
-the glycosidic bonds are named from the number of the carbon atoms in the sugar (1,4 Bond)
-type of glycosidic bond can have major effects on the final structure and function of molecule
-glycogen (animal energy storage)- glucose monomers- has alpha 1,4 glycosidic bonds with branches linked by alpha 1,6 glycosidic bonds.
-starch (plant energy storage) -mixture of two glucose polymers: amylase is linear alpha 1,4 glucose polymer and forms a helix, amylopectin is similar to glycogen but with less branching
-cellulose- (plant structural molecule)- has beta 1,4 glycosidic bonds between glucose monomers to form long straight chains. Chains line parallel and are stabilised by hydrogen bonds to form fibrils and then firbe. Fibres have high tensile strength required for structural role in plant tissue..
-mammals lack enzyme to digest cellulose (cellulose) and passes undigested through intestine
-sucrose (glucose 1-2 fructose)- table sugar
- lactose (Galactose-beta,1-4-alpha glucose)- milk and requires B galactosidase to digest
- maltose (glucose-alpha 1-4 glucose)- released by amylase from digestion of starch. Releases glucose through maltase enzymes
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