Carbohydrates is a biological molecule consisting of carbon, hydrogen and oxygen atom, with a hydrogen: oxygen atom ratio of 2:1 (as in water) and an empirical formula of Cm(H2O)n. Carbohydrates can be viewed as hydrates of carbon, hence their name. Structurally however, it is more accurate to view them as polyhydroxy aldehydes and ketones. Historically nutritionists have classified carbohydrates as either simple or complex; however, the exact delineation of these categories is ambiguous. Today, the term simple carbohydrate typically refers to monosaccharide and disaccharides, and complex carbohydrate means polysaccharides (and oligosaccharides).
Classification of carbohydrates
Monosaccharides (from Greek word mono; means single, sacchar; means sugar).Monosaccharide’s are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose (dextrose), fructose (levulose), galactose, xylose and ribose. Monosaccharides are the building blocks of disaccharides such as sucrose and polysaccharides (such as cellulose and starch).
A disaccharide is the carbohydrate formed when two monosaccharide’s undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharide, disaccharides also dissolve in water, taste sweet and are called sugars. The glycosidic bond can be formed between any hydroxyl groups on the component monosaccharide. So, even if both component sugars are the same (e.g., glucose), different bond combinations and stereochemistry (alpha- or beta-) result in disaccharides with different chemical and physical properties. Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-tasting and sticky-feeling.
Example of a disaccharide:
i. Sucrose (table sugar, cane sugar, beet sugar) sucrose is the combination of glucose and fructose joined by a gycosidic bond α(1→2),
ii. Lactose (milk sugar), a combination of galactose and glucose β(1→4)
iii. Maltose, having two glucose unit joined by a glycosidic bond α(1→4)
iv. Cellobiose, two glucose unit joined by a glycosidic linkeage β(1→4)
An oligosaccharide (from the Greek word oligos, means a few, and sacchar, means sugar) is a saccharide polymer containing typically three to ten component sugars, also known as simple sugars, or monosaccharide’s. Oligosaccharides can have many functions; for example, they are commonly found on the plasma membrane of animal cells where they can play a role in cell-cell recognition. In general, they are found either O- or N-linked to compatible amino acid side-chains in proteins or to lipid moieties. Example fructo-oligosaccharides (FOS), which are found in many vegetables, consist of short chains of fructose molecules. (Inulin has a much higher degree of polymerization than FOS and is a polysaccharide.) Galactooligosaccharides (GOS), which also occur naturally, consist of short chains of galactose molecules. These compounds can be only partially digested by humans.
Polysaccharides are polymeric carbohydrate structures, formed by repeating units of (either mono- or disaccharides) joined together by glycosidic bonds. These structures are often linear, but may contain various degrees of branching. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure, these macromolecules can have distinct properties from their monosaccharide building blocks. They may be amorphous or even insoluble in water.
Carbohydrate metabolism denotes the various biochemical processes responsible for the formation, breakdown and inter conversion of carbohydrates in living organisms.
The carbohydrate metabolism process is as follows:
- Food (a carbohydrate diet) is first consumed, the mouth being the first contact point.
- The Carbohydrates are broken down into glucose – by saliva and the gut.
- Glucose enters the bloodstream
- Pancreas responds to the presence of food by releasing stored insulin (phase 1 insulin response)
- Insulin allows glucose from the blood to enter into the body’s cells – where the glucose can be used for fuel.
- Insulin also allows glucose to be stored by muscles and the liver as glycogen
- If needed, the stored glycogen can later by returned to the blood as glucose
- If there is glucose remaining in the blood, insulin turns this glucose into saturated body fat.
- Proteins in the meal also get broken down into glucose to some degree; however, this is a much slower process than it is with carbohydrates.
- After the body’s initial release of insulin, the beta cells in the pancreas start to develop new insulin which can be released as well. This is known as the phase 2 insulin response.
- As mentioned above, if glucose is taken from the blood to the point where blood sugar levels start to approach a low level, the body releases glucagon.
- Glucagon works to change the stored glycogen into glucose which is released into the blood stream.
Note that the stored glycogen in the muscle cannot be broken down to glucose for the purpose of buffering blood glucose levels, only liver glycogen can because of the phosphatase in d hepatocytes.