Three common sugars, glucose, galactose and fructose, share the same molecular formula: C6H12O6. Because of their six carbon atoms, each is a hexose. Glucose is the most common monosaccheride and acts as the immediate source of energy for cellular respiration. It has the structure:
The similar structure of both fructose and galactose is shown below:
Fructose Galactose
Although all three share the same molecular formula (C6H12O6), the arrangement of atoms differs in each case. Substances such as these three, which have identical molecular formulas but different structural formulas, are known as isomers.
Monosaccharides in the ring form can link together to form disaccharides or in greater numbers to form polysaccharides. In some polysaccharides, such as cellulose, the chain is unbranched; in others, such as glycogen, the chain is branched. Some polysaccharides, such as cellulose, contain only a single type of monosaccharide building block.
Linkage of two glucose molecules forms a disaccharide such as maltose, lactose and sucrose. The links are designated as alpha (a) or beta (b) depending on the orientation of the -OH group at the number one carbon forming the bond. The bonds formed are glycosidic bonds. Although the process of linking the two monomers is rather complex, the end result in each case is the loss of a hydrogen atom (H) from one of the monosaccharides and a hydroxyl group (OH) from the other. Thus the molecular formula of each of these disaccharides is: C12H22O11 2 C6H12O6 + H2O
The most common disaccharides are sucrose, lactose and maltose.
Sucrose Lactose
Maltose
Sucrose is commonly used as table sugar and is comprised of glucose and fructose. Maltose is a product of starch digestion and is comprised of two glucose molecules.
Lactose has a molecular structure similar to maltose, but consists of galactose and glucose. It is of interest because it is associated with lactose intolerance that is the intestinal distress caused by a deficiency of lactase, an intestinal enzyme needed to absorb and digest lactose in milk. Undigested lactose ferments in the colon and causes abdominal pain, bloating, gas, and diarrhoea. Yoghurt does not cause these problems because the bacteria that transform milk into yoghurt consume lactose.
All sugars are very soluble in water because of their many hydroxyl groups. Sugars are the most important source of energy for many cells.
Polysaccharides are polymers of simple sugars. They are formed from chains of monosaccharides and are insoluble in water. Starches are polymers of glucose. There are two types. Amylose consists of linear, unbranched chains of several hundred glucose molecules. The glucose molecules are linked between their number 1 and number 4 carbon atoms. Amylopectin differs from amylose in being highly branched. At approximately every thirtieth molecule along the chain, a short side chain is attached to the number 6 carbon atom. The total number of glucose molecules in a chain of amylopectin is several thousand.
As starches are insoluble in water, they can serve as storage depository of glucose. Plants convert excess glucose into starch for storage. Before starches can enter (or leave) cells, they must be digested. The hydrolysis of starch is done by amylases. With the aid of an amylase (such as pancreatic amylase), water molecules enter at the 1 to 4 linkages, breaking the chain and eventually producing a mixture of glucose and maltose. A different amylase is needed to break the 1 to 6 bonds of amylopectin.
Glycogen is another common polysaccharide. Animals store excess glucose by polymerising it to form glycogen. The structure of glycogen is similar to that of amylopectin, although the branches in glycogen tend to be shorter and more frequent. Glycogen is broken back down into glucose when energy is needed (a process called glycogenolysis).
In glycogenolysis, phosphate groups break the 1 to 4 linkages. The phosphate group must then be removed so that glucose can leave the cell.
There is some evidence that intense exercise and a high-carbohydrate diet can increase the reserves of glycogen in the muscles and thus may help marathoners work their muscles somewhat longer and harder than otherwise. But for most of us, a high-carbohydrate diet leads to increased deposits of fat.
Cellulose is the major structural material of which plants are made. Like starch, cellulose is a polysaccharide with glucose as its monomer. However, cellulose differs profoundly from starch in its properties. Because of the orientation of the bonds linking the glucose residues, the rings of glucose are arranged in a zigzag formation. This produces a long, rigid molecule. There are no side chains in cellulose as there are in starch. The absence of side chains allows these linear molecules to lie close together. Because of the many -OH groups, as well as the oxygen atom in the ring, there are many opportunities for hydrogen bonds to form between adjacent chains. The result is a series of stiff, elongated fibrils - the perfect material for building the cell walls of plants.
Cellular respiration is the process in which cells break down molecules of food, such as carbohydrates and fats, into carbon dioxide and water. They release a chemical energy stored in molecules. When oxygen is used, the process is called aerobic respiration. The energy released in aerobic respiration is used by cells to give energy to a huge variety of complex reactions to make diverse processes like growth, the contraction of the muscle cells, the transmission of nerve impulses, and the maintenance of body temperature in warm-blooded animals.
Some organisms can carry out cellular respiration without oxygen but this yields much less energy, this process is called anaerobic respiration. When plants respire, they take in oxygen through pores called stomata, and release carbon dioxide and water produced from the breakdown of carbohydrates. During the day, respiration still continues, but photosynthesis is also occurring at the same time. This process is the opposite of respiration, and involves the synthesis of new carbohydrates from carbon dioxide and water. Photosynthesis is powered by energy from the sun and becomes trapped in the new carbohydrate. During respiration, the trapped energy is released and transferred to the energy-carrying molecule ATP (adenosine triphosphate), which fuels reactions throughout the cell.
Glycolysis is the first stage of respiration. It takes place in the cell cytoplasm and does not require oxygen. During glycolysis, each molecule of glucose, which contains six carbon atoms, is split into two halves, each containing three carbon atoms. Each of these is converted into a compound called pyruvic acid, which is used in the second stage. Glycolysis produces a net gain of two molecules of ATP for each molecule of glucose.
Carbohydrates have several functions in cells. They are an excellent source of energy for the many different activities going on in our cells. Some carbohydrates may have a structural function. For example, the material that makes plants stand tall and gives wood its tough properties is a polymer form of glucose known as cellulose. Other types of sugar polymers make up the stored forms of energy known as starch and glycogen. Starch is found in plant products such as potatoes and glycogen is found in animals. A short molecule of glycogen is shown below.
Carbohydrates are essential for cells to communicate with each other. They also help cells adhere to each other and the material surrounding the cells in the body. The ability of the body to defend itself against invading microbes and the removal of foreign material from the body (such as the capture of dust and pollen by the mucus in our nose and throat) is also dependent on the properties of carbohydrates.