Organic Stereochemistry: Carbohydrates

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Organic Stereochemistry


Carbohydrates are used to provide energy to living cells in the body. They can also be used as storage in the body for example, in the form of glycogen. All carbohydrates contain the elements carbon, hydrogen and oxygen. The well known carbohydrates are monosaccharides, disaccharides and polysaccharides.

Monosaccharides are single sugar units which are building blocks that are used to make up larger carbohydrates. There is a large variety of monosaccharides that are different due to the number of carbon atoms that they possess and how the atoms are arranged in the molecule. An example of a monosaccharide is Glucose (C6H12O6) which is used to provide energy to heterotrophic cells.[1]

Disaccharides are made up of two monosaccharides that contain covalent bonds that are held together by a glycosidic bond. Disaccharides are present in the food that people take in; these are then broken down into two single sugar molecules so that they are able to be easily absorbed in the small intestine. An example of a disaccharide is sucrose which is formed from glucose and fructose.[2]

Polysaccharides are large molecules which are made from single sugars which can be in different forms such as branched, unbranched and coiled. Cellulose and Starch are examples of a polysaccharide. Cellulose provides support and rigidity in the cell wall in plant cells. It is made of thousands of glucose molecules that are joined together by beta (1-4) glycosidic bonds. Each second glucose unit is inverted, this means that the sixth carbon is on the opposite side which causes hydrogen bonding to occur. This is what provides the rigidity in the cell wall as there is cross linking involved which provides also flexibility to the cell wall.[3]

In monosaccharides, glucose can be structured as either a linear molecule or ring structure. It has the configurations of D-glucose and L-glucose.


D-Glucose and L-Glucose are enantiomers. Enantiomers are when isomers that are mirror images but are not super imposable. This means that the L-Glucose and D-Glucose will always have the same melting point, boiling point and solubility.[4] The naming of L-Glucose is shown on the fischer projection shown above in Figure 1. This is due to the fact that the OH group is indicated on the left of the chiral carbon. If the OH was indicated to the right of the chiral carbon then it will be called D-Glucose.[5]D-Glucose is the most common isomer found of glucose, this is because it occurs naturally as it is essential as part of the building blocks that is used to make disaccharides. It is the only sugar present in cellulose and starch. Also since it is made naturally in animals and plants, this makes D-Glucose a main source of energy.[6]However, L-Glucose does not occur naturally and so is synthesised from D-Glucose which is the more common isomer. This is because L-Glucose cannot be broken down by the body for the use of energy whereas D-Glucose can.[7]

Since D-Glucose is biologically active, this means that L-Glucose can’t be used in the same way as D-Glucose. Another name for D-Glucose is Dextrose which means that it can be used in the food industry. This is because D-Glucose can take on a 6-ring configuration or a linear shape, which is the same as glucose which is also a 6-ring configuration.[8]

Glyceraldehyde is also a three carbon monosaccharide which is essential for cellular respiration and photosynthesis.[9]Glyceraldehyde is a chiral molecule which means that there are four different substituent’s that are attached to the chiral carbon which consists of D-glyceraldehyde and L-glyceraldehyde. Also D-glyceraldehyde can be called R-glyceraldehyde and the same goes for L-glyceraldehyde as S-glyceraldehyde.[10] Also amino acids and sugars are synthesised from glyceraldehyde. The synthesised amino acid will be labelled after the D-isomer if it has been taken from D-glyceraldehyde when the order of the molecule...
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