lunes, 17 de agosto de 2009
Carbohydrates are compounds that contain only C,H and O. These molecules are not only the fundamental energy stock of most of organims but they also play structural roles as in vegetal cell walls. They are formed by small molecules called sugars. There are three different kinds of carbohydrates depending on the number of sugars they are formed of:
Monosaccharides (Simple sugars).- They only have one sugar molecule. They have the general chemical formula (CH2O)n. This proportion gaves them the name of carbohydrates to these and all molecules derived from them. The main characteristic of monosaccharides is that they present hydroxyl groups and one ketone or aldehyde group. This, gives this sugars a high solubility and when molecules have more than 5 C atoms, they react with themselves to change their conformation dramatically. In a water solution, the aldehyde or the ketone group reacts with an hydroxyl group, then turning the molecule into a ring conformation. In glucose for example, the aldehyde group of the first C atom reacts with the hydroxyl group of the 5th C, thus producing a ring of six carbons. When this ring closes it can do it in two ways, over or under the level of the ring. When the hydroxyl group is situated under the level of the ring is called the alpha glucose while the form that is over the level of the ring is called the beta glucose. This small difference between alpha and beta forms have significant differences on the bigger molecules that are formed from glucose. Monosaccharides can be burned or oxidized. This is reaction releases a big amount of energy (673Kcal/glucose molecule). So glucose is the main energy source for many organisms. That is why glucose is the fuel for cells to live. Glucose is carried by the blood stream in superior vertebrates. Monosaccharides are ready to go energy for living systems.
Dissacharids (Transport forms of sugars).- Although glucose is the main transport sugar form in vertebrates, sugars are transported in the form of dissacharids, specially in plants. Sucrose (table sugar) is the common form in which carbohydrates are transported from the photosynthetic apparatus to the rest of plant. Sucrose is built up by one glucose molecule and one fructuose molecule. Trehalose is the common form in insects and it is composed of two binded glucose units. Lactose is formed of a glucose and a galactose.
In the synthesis reaction of dissacharids, two monosaccharids bind together releasing one molecule of water during the process. This chemical reaction is called condensation or dehydration reaction. In this way, only monosaccharides have the chemical ratio CH2O because two hydrogen atoms and one of oxygen are liberated in every bond formed.
When a dissacharid breaks up into their monosaccharides, when it is going to be used as a energy source, that water molecule is reincoporated. This reaction is called hydrolysis (separated by water) Hydrolysis liberates energy, for example in the hydrolysis of sucrose, 5.5 Kcal/mol are liberated. In the other hand, synthesis of sucrose requires an input of 5.5 Kcal/mol to bind fructose and glucose together.
Storage polysaccharids.-They are formed by long chains of monosaccharides. They are the storage forms of sugar. Starch for example, is the main energy storage in plants. Starch has two forms: Amylose and Amylopectine, both forms are a series of glucose units coupled together.
Glycogen is the main energy storage in superior animals. Glycogen has a very similar structure to that of amylopectine except for the fact that glycogen is more branched. In animals, glycogen is stored in the liver and muscular tissue. If there is an excess of glucose in the blood stream, the liver stores glycogen. When there is a need for energy (glucose) glucagon is secreted by the pancreas thus hydrolyzing glycogen into glucose, the ready to use form of energy.
Glycogen and Starch are formed only by Alpha units of Glucose.
Structural polysaccharids.- One of the main functions of molecules in the living organisms is to be constituent part of cells and tissues. The main structural molecule is cellulose. Cellulose forms the rigid part of cell walls in vegetable cells, forming fibers that wrap the cells up.
Cellulose is a polymer of glucose, such as glycogen and starch but not all organisms are capable of hydrolyze cellulose. This is a product of the kind of monomer that cellulose is composed of in comparison to glycogen. Although both polysaccharids are built up by glucose units, Cellulose has only beta glucose. This difference in the conformation of the molecules makes that in cellulose, glucose molecules stack together one over the other thus, creating fibers. This makes cellulose enzime proof so it cannot be degrated like the storage polysaccharides.
Chitin is another structural polysaccharid which is the main component of arthropod exoskeletons such as insects and crustaceans, and fungus cell walls. It is a very hard and resistant polysaccharid.
Cellulose and Chitin are formed only by Beta units of Glucose.
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