Chapter 4: Chemical Composition of the Cell
Chemical Composition of Carbohydrate:
A carbohydrate is any member of a widespread class of natural organic substances that includes sugars,starch and cellulose. The chemical composition of carbohydrates – as analysed in the nineteenth century – is a combination of carbon (C), hydrogen (H) and water (H2O). Many carbs have the general chemical formula Cx(H2O)x, but the class is too large to fit into a simple chemical formula. Carbohydrates are often isomers – meaning, they have the same atomic composition but different structures. Fructose, galactose and glucose are isomers with the chemical formula C6H12O6. There are many classification schemes for carbohydrates. The most common one separates them into 4 major groups: monosaccharides, disaccharides, oligosaccharidesand polysaccharides.
Types of Carbohydrates:
Molecules of monosaccharides (simple sugars) usually contain 5-6 carbon atoms. Three most common monosaccharide carbohydrates include (a) glucose (also called dextrose, grape sugar or corn sugar; (b) fructose (fruit sugar); and galactose. Glucose is a constituent of the two most widespread disaccharides, sucrose and lactose, and is the sole structural constituent of the polysaccharides cellulose, starch and glycogen. Galactose is a common constiuent of oligosaccharides and polysaccharides (eg. agar, carrageenan), and also is found in carbohydrate-containing lipids called glycolipids located in brain and nerve tissue.
Disaccharides are composed of 2 simple sugar molecules, hence they are sometimes referred to as “double sugars”. For example, the disaccharide sucrose contains one molecule of glucose and one molecule of fructose. Other disaccharide carbs include lactose (milk sugar), mannose, and maltose.
Polysaccharides (eg. cellulose, starch, glycogen) are much larger molecules which comprise up to 10,000 monosaccharides. Most of the stored carbohydrates in nature occur in the form of polysaccharides. For example, glycogen – the stored carbohydrate found in the muscles and liver of humans and many animals – consists of a complex chain of glucose molecules. The two most well known polysaccharides are cellulose and starch. Cellulose – the basic structural material in plants – contains over 3,000 glucose molecules. We encounter cellulose in the form of insoluble dietary fiber. Starch refers to a class of plant-based polysaccharides made up of units of glucose. Starches typically comprise a combination of two substances: amylose and amylopectin. We metabolize starch in our digestive system in stages. First, digestive enzymes called amylases convert the starch into maltose. As the maltose is absorbed through the walls of the intestine it is hydrolyzed to glucose and distributed to cells and muscles for energy or stored as glycogen or fat.