Biomolecules - Carbohydrates

Carbohydrates are the most abundant biological compounds. It is estimated that more than 50% of the total carbon content of the Earth is present in the form of carbohydrate compounds. The term carbonhydrate was first used back in the 1800s, when it was found that the ratio of the carbon, hydrogen, and oxygen atoms in these molecules was 1 carbon atom to 2 hydrogen atoms to 1 oxygen atom or put into a chemical formula CH₂O. The name carbohydrate is derived from carbon (carbo-) and water H₂O (-hydrate). More commonly in society carbohydrates are called sugars. This reflects the fact that carbohydrate are naturally occur sweetners

Carbohydrates are often used by living things for two main purposes. They are used as an energy source or storage form and they are used as a structural component. Carbohydrates can be used as an energy source because most cells can convert simple carbohydrates into energy that can be harnessed by the cell. Living things are also use carbohydrates as a means of storage for future use. Larger carbohydrate molecules can also be used by living organisms to construct physical structures. In plants and bacteria carbohydrates are the main component of the cell wall.

Monosaccharides

The simplest carbohydrates are the monosaccharides. The two most common monosaccharides are glucose (also called dextrose) and fructose. Monosaccharides are the fundamental units, or building blocks, that make up are other carbohydrates.

Monosaccharides typically have a chain of carbon atoms linked together called a "carbon backbone". The number of carbon atoms in chain is the way biochemists classify monosaccharides. There are 3 groups of monosaccharides. Trioses are monosaccharides that have a 3-carbon backbone. They are also called 3-carbon sugars. Sugars with a 5-carbon backbone are known as pentoses (5-carbon sugars). 6-carbon sugars are known as hexoses. differences are based on the number of carbon atom. All 3 carbon sugars have same chemical formula C₃H₆O₃. 5-carbon sugars all have C₅H₁₀O₅ for their formula. 6-carbon sugars all C₆H₁₂O₆.

Examples of 6-carbon sugars (hexoses)

Glucose ("blood sugar") is the most abundant monosaccharide in the human body. Glucose is a 6-carbon sugar. It is the chemical "fuel" that is carried in the bloodstream to tissues as an energy source for metabolism. Other carbohydrates that are absorbed by the human body must be converted to glucose before the body can break it down for energy. Fructose is the most abundant carbohydrate in fruits. Honey is a 1:1 mixture of glucose and fructose. Although glucose and fructose share the same molecular formula, their structures are different and they have different properties. Fructose is the sweest sugar - about 30% sweeter per gram than sucrose (table sugar) and is widely used as a lower calorie and lower cost than table sugar. Many soft drink and fruit drink are sweetened with "high fructose corn syrup".

Ribose and deoxyribose are both pentose sugars. These sugar sugars are an important part of nucleic acids. They are the sugars found in the nucleotides that form nucleic acids. Ribose is found in RNA and deoxyribose is found in DNA.

Disaccharides

When two monosaccharides are bonded together a disaccharide is formed. The process of linking the two monosaccharides together removes a hydrogen atom and a oxygen bonded to a hydrogen (-OH). This process is called dehydration synthesis because the atoms removed from the sugars bond to form water. There are 3 common disaccharides , lactose, sucrose, and maltose. Disaccharides, like monosaccharides, are soluble in water, but they are too big to pass through the cell membrane by diffusion.

Lactose is a disaccharide composed of a glucose and galactose molecule bonded together. Lactose (called "milk sugar") constitutes about 4-5%of cow's milk. A specific enzyme called lactase is required for the digestion of lactose. The enzyme breaks the bonds connecting the two monosaccharides. People who lack this enzyme are said to be "lactose intolerant" they cannot digest milk or milk products like ice cream, cheese, or yogurt.

Sucrose, or table sugar, is a disaccharide composed of glucose and a fructose monosaccharide joined together. It is harvested from sugar can or sugar beets and is also called can sugar. It is a widely used refined sugar in a typically Western diet. broken down in the small intestine during digestion to give the smaller monosaccharides that are able to pass into the to blood and through cell membranes into cells.

Maltose, or malt sugar, is a disaccharide that is composed of two glucose molecules. Maltose is sugar used is malted milk shakes and malted milk candies. Maltose is found primarily in plants, particularly in seeds that are sprouting. These sprouting seeds are taking larger carbohydrates and breaking them into maltose. Brewers and distillers use sprouted barley seeds and extract the maltose to brew beers and make spirits whiskey, bourbon, or scotch.

Polysaccharides

Polysaccharides are the most abundant carbohydrates in nature. They serve as reserve food or energy storage. Polysaccharides are composed of hundreds or thousands of monosaccharide unit bonded together. The monosaccharide building blocks are bonded together just like disaccharides are except instead of just linking two monosaccharides are continuously link creating a long chain of the monosaccharide units.

Three polysaccharides are the most significant to living organisms. They are starch, glycogen, and cellulose. Starch is the most important source of carbohydrates in the human diet. Starch is formed in plants. Starch is commonly found in seeds where it serves as a storage form of carbohydrate. As a seed sprouts the cells in the young plant convert starch molecules into disaccharides and then glucose that can be used as energy for the growing plant. Amylose is a component of starch that consists of many glucose molecules that are linked together.

Glycogen, often called animal starch, is the storage form of carbohydrate in animals. Almost all animal cells contain some glycogen to provide energy for the cell's functions. However, glycogen is most abundant in liver cells and muscle cells. When exercising or fasting cells will draw upon these reserves to obtain glucose needed to continue functioning.

Cellulose is a carbohydrate found only in plants. Cellulose is the structural component of plant cell walls; it is the most abundant of all carbohydrates in plants. Cotton and paper are close to 95% cellulose. Wood is about 50% cellulose. Humans cannot digest cellulose found in plants. Herbivores like cows, sheep, and deer have an enzyme called cellulase that allows their digestive system to break the chemical bonds in cellulose release the smaller monosaccharides. Despite the fact that humans cannot digest cellulose, it still is important to our diet. Cellulose in the human diet acts as roughage which keeps digested food moving regularly through the intestines.