Self-reflection carbohydrate (christie)
Reflection on Carbohydrates
On 25 November 2019, I had my lesson about carbohydrates which taught by Dr Dayang. After the lesson, I was able to get to know the carbohydrates more specifically and in detail. Carbohydrates are organic compounds containing carbon, hydrogen and oxygen, usually in the proportion of 1:2:1 respectively. The general formula for carbohydrates is Cn (H2O)n . They have several key functions such as energy storage, structural components, cellular recognition and carbohydrate derivatives. Not only that, I also learnt that the carbohydrates are divided into four groups which are monosaccharides, disaccharides, oligosaccharides and polysaccharides.
Monosaccharides are the simple sugar units which make up all other carbohydrates. They cannot be broken into smaller molecules by hydrolysis. Mostly, monosaccharides are characterised by the number of carbon atoms in the molecules. For examples, Trioses (3C sugars) are glyceraldehyde and dihydroxyacetone; pentose (5C sugars) are ribose and ribulose; hexose (6C sugars) are glucose, fructose and galactose. Monosaccharides are polyhydroxyl aldehydes (aldoses) or ketones (ketoses) with two or more hydroxyl groups. Furthermore, monosaccharides also exhibits enantiomers (stereoisomers that are mirror images), diastereomers (stereoisomers that are not mirror images) and epimers (diastereomers that differ around 1 carbon) characteristics.
According to the conventions proposed by Fischer, D-monosaccharides that has –OH on it penultimate carbon on the right whereas L-monosaccharides that has –OH on it penultimate carbon on the left. From the lesson, I also knew that the aldehyde or ketone carbonyl group carbon (C=O) and hydroxyl group (-OH) react forming a hemiacetal with a new C-O-C bridge. When in the uncyclized form, monosaccharides acts as reducing agents. Free carbonyl group from aldoses or ketoses can reduce Cu2+ and Ag+ ions to insoluble products. Because of the –OH, aldehyde and ketone groups, sugars are easily derivatized. Several derivatizations of sugars include reduction and esterification.
Apart from monosaccharides, I also being taught about disaccharides which is formed by the condensation reaction between two monosaccharides. The bond linking the monosaccharides is known as a glycosidic bond. They are water-soluble, sweet-tasting and can be crystallised. In disaccharides, maltose and lactose are reducing sugars and sucrose is a non-reducing sugar. During the class, I also acknowledged that monosaccharides may be linked together to form small chains called oligosaccharides. Each oligosaccharides may contain 3 to14 monosaccharides. Oligosaccharides chains can be found attached to proteins and lipids forming glycoproteins and glycolipids of the plasma membrane. These can act as receptors involved in cell recognition.
Besides that, I also learnt about polysaccharides. Polysaccharides are polymers that made up of the condensation of hundreds of monosaccharides monomers. The process is called polymerisation. There are two types of polysaccharides which are homopolysaccharides (1 type of monomer) and heteropolysaccharides (different types of monomers). The functions of polysaccharides are storage, structure and information. Starch is a polysaccharides formed from the condensation of α-glucose units. It is made up of two components which are amylose and amylopectin. Amylose is a linear unbranched polymer of 200 to 1500α-glucose residues in a repeating sequence of α-1, 4-glycosidic linkages while amylopectin is a branched polymer of 2000 to 200000 α-glucose residues per starch molecule.
Glycogen is the storage polysaccharides in animals. It is like amylopectin but just even more highly compact and branched. In amylopectin, branches are formed every 24 to 30 glucose whereas in glycogen, branches are formed every 8 to 12 glucose. For cellulose, it is a homopolymer , a linear chain of several hundred to over ten thousand β linked D-glucose units. It is also the primary component of plant cell walls and green algae. Lastly, I also learnt that chitin is the main component of the cell walls of fungi and the exoskeletons of arthropods such as crustaceans. It is a homopolymer of β (1 to 4) linked N-acetylglucosamine residues. This is all what I learnt for today. I felt grateful and excited as I was able to gain extra knowledge about carbohydrates.
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