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Carbohydrates Fundamentals

Carbohydrates, also known as sugars, are found in all living organisms. They are essential to the very source of life (ex. Ribose sugars in DNA and RNA) or sustaining life itself (ex. Metabolic conversion of carbohydrates into usable biochemical energy, ATP). Another important role of carbohydrates is structural (ex. Cellulose in plants). General names for carbohydrates include sugars, starches, saccharides, and polysaccharides. The term saccharide is derived from the Latin word "sacchararum" from the sweet taste of sugars. The name "carbohydrate" means a "hydrate of carbon." The name derives from the general formula of carbohydrate is Cx(H2O)y - x and y may or may not be equal and range in value from 3 to 12 or more. For example glucose is: C6(H2O)6 or is more commonly written, C6H12O6.

Introduction

The chemistry of carbohydrates most closely resembles that of alcohol, aldehyde, and ketone functional groups. As a result, the modern definition of a CARBOHYDRATE is that the compounds are polyhydroxy aldehydes or ketones. The chemistry of carbohydrates is complicated by the fact that there is a functional group (alcohol) on almost every carbon. In addition, the carbohydrate may exist in either a straight chain or a ring structure. Ring structures incorporate two additional functional groups: the hemiacetal and acetal.

A major part of the carbon cycle occurs as carbon dioxide is converted to carbohydrates through photosynthesis. Carbohydrates are utilized by animals and humans in metabolism to produce energy and other compounds.

Carbohydrate Functions

Carbohydrates are initially synthesized in plants from a complex series of reactions involving photosynthesis.

  • Store energy in the form of starch (photosynthesis in plants) or glycogen (in animals and humans).
  • Provide energy through metabolism pathways and cycles.
  • Supply carbon for synthesis of other compounds.
  • Form structural components in cells and tissues.

Photosynthesis is a complex series of reactions carried out by algae, phytoplankton, and the leaves in plants, which utilize the energy from the sun. The simplified version of this chemical reaction is to utilize carbon dioxide molecules from the air and water molecules and the energy from the sun to produce a simple sugar such as glucose and oxygen molecules as a by product. The simple sugars are then converted into other molecules such as starch, fats, proteins, enzymes, and DNA/RNA i.e. all of the other molecules in living plants. All of the "matter/stuff" of a plant ultimately is produced as a result of this photosynthesis reaction.

Di- and Poly-Carbohydrates

  • Monosaccharides contain one sugar unit such as glucose, galactose, fructose, etc.
  • Disaccharides contain two sugar units. In almost all cases one of the sugars is glucose, with the other sugar being galactose, fructose, or another glucose. Common disaccharides are maltose, lactose, and sucrose.
  • Polysaccharides contain many sugar units in long polymer chains of many repeating units. The most common sugar unit is glucose. Common poly saccharides are starch, glycogen, and cellulose.
Table 1: Common Carbohydrates
 Name
 Derivation of name and Source
Monosaccharides
 Glucose
From Greek word for sweet wine; grape sugar, blood sugar, dextrose.
 Galactose
Greek word for milk--"galact", found as a component of lactose in milk.
 Fructose
Latin word for fruit--"fructus", also known as levulose,found in fruits and honey; sweetest sugar.
 Ribose
Ribose and Deoxyribose are found in the backbone structure of RNA and DNA, respectively.
Disaccharides - contain two monosaccharides
  Sucrose
French word for sugar--"sucre", a disaccharide containing glucose and fructose; table sugar, cane sugar, beet sugar.
 Lactose
Latin word for milk--"lact"; a disaccharide found in milk containing glucose and galactose.
 Maltose

French word for "malt"; a disaccharide containing two units of glucose; found in germinating grains, used to make beer.

Common Polysaccharides
Starch Plants store glucose as the polysaccharide starch. The cereal grains (wheat, rice, corn, oats, barley) as well as tubers such as potatoes are rich in starch.
Cellulose The major component in the rigid cell walls in plants is cellulose and is a linear polysaccharide polymer with many glucose monosaccharide units.
 Glycogen This is the storage form of glucose in animals and humans which is analogous to the starch in plants. Glycogen is synthesized and stored mainly in the liver and the muscles.

Metabolism

Metabolism occurs in animals and humans after the ingestion of organic plant or animal foods. In the cells a series of complex reactions occurs with oxygen to convert for example glucose sugar into the products of carbon dioxide and water and ENERGY. This reaction is also carried out by bacteria in the decomposition/decay of waste materials on land and in the water.

Combustion occurs when any organic material is reacted (burned) in the presence of oxygen to give off the products of carbon dioxide and water and ENERGY. The organic material can be any fossil fuel such as natural gas (methane), oil, or coal. Other organic materials that combust are wood, paper, plastics, and cloth.

The whole purpose of both processes is to convert chemical energy into other forms of energy such as heat.

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