Carbohydrates
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Carbohydrates are a group of organic molecules encompassing sugars (monosaccharides and disaccharides) and their polymers. Most carbohydrates have a general formula of Cn(H2O)m.
The nutritional sources of carbohydrates include table sugar (a common name for sucrose) and starchy products such as potatoes and cereals.
Monosaccharides are simple sugars typically having a general formula of (CH2O)n. The number of carbon atoms in monosaccharides ranges from 3 to 7. The common example is a six-carbon sugar – glucose, with a general formula of C6H12O6. Like most monosaccharides, glucose adopts a ring structure in aqueous solutions.
Glucose is the major source of energy for the cell. The release of energy from monosaccharides occurs through cellular respiration, a process that involves breaking them down into smaller molecules.
Another function of monosaccharides is to serve as building material for other kinds of molecules, such as amino acids.
Two monosaccharide units can be joined together in a dehydration reaction.
The most common disaccharides formed by the dehydration reaction are sucrose ("table sugar"), maltose and lactose. The addition of more sugar units to the chain results in the synthesis of polysaccharides.
Polysaccharides are large polymers consisting of hundreds to thousands of monosaccharide units. Polysaccharides serve either energy storage or structural functions.
Both plant and animal cells use polymers made of glucose monomers to store energy. Glucose can be quickly retrieved from these polymers by a reaction called hydrolysis. Unlike glucose, these polysaccharides are insoluble, resulting in compact storage.
The plant storage polysaccharide is starch. Starch consists of two types of molecules: amylose, a linear molecule, and amylopectin, a more complex, branched molecule. Amylose allows a slower release of glucose, while amylopectin can be broken down faster due to more free ends.
The animal storage polysaccharide glycogen is similar to amylopectin but is more branched. Glycogen is predominantly stored in liver and muscle cells, though other cells also contain a limited amount of glycogen.
Cellulose, another polymer of glucose, is the most abundant structural polysaccharide in the world. Numerous linear and unbranched cellulose molecules align and are connected by weak electrostatic attractions known as hydrogen bonds. Aggregated cellulose molecules, forming structures called microfibrils, constitute a major component of plant cell walls.
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