Saccharide is a very important organic compound in cells. It contains C, H and O, and their ratio is generally 1:2:1. Its chemical formula is written as Cn(H₂O)n. sometimes, so it is also called carbohydrate. But there is an exception, deoxyribose lacks one oxygen atom and its chemical formula is C₅H₁₀O₄. Saccharide is the main energy substance needed for life activities. Some Saccharides are also raw materials for biological macromolecules (proteins and nucleic acids). Saccharides are divided into monosaccharides, oligosaccharides and polysaccharides according to their constituents.
| Monosaccharide | ||
|---|---|---|
| Types | Component | Distribution |
| Glucose | C₆H₁₂O₆ | It is the main energy source for life, and all plant and animal contain glucose. |
| Fructose | C₆H₁₂O₆ | Sweet fruit and honey |
| Galactose | C₆H₁₂O₆ | mammal milk, plant gum, nerve system |
| Disaccharide | ||
| Lactose | Glucose + Galactose | mammal milk |
| Sucrose | Glucose + Fructose | It can store energy in the plant. |
| Maltose | Glucose + Glucose | Plant tissue in which starch hydrolysis occurs |
Monosaccharides
Monosaccharides are the smallest units that make up sugars. The common monosaccharides are glucose, fructose, and galactose. They are all sugars consisting of 6 carbon atoms and are isomers of each other.
Glucose or grape sugar
Glucose was first isolated from raisins or grapes and it is also known as grape sugar. Starch, glycogen and sucrose are all digested into glucose before they can be used by the body. Glucose is the main source of energy for cells. Glucose has a simple structure of five carbon atoms and one oxygen atom arranged in a ring, and can exist in two forms: alpha-glucose and beta-glucose. The difference between them is the orientation of a hydroxyl group (-OH) attached to the first and fourth carbon atom. Glucose can be found in many foods, such as fruits, honey, corn syrup and starch.
Fructose or fruit sugar
It is also known as fruit sugar. Fructose is another simple carbohydrate that has the same molecular formula as glucose, but a different structure. Fructose has a ring with four carbon atoms and one oxygen atom. The other two carbon are attached to first and fourth carbon respectively. It is 1.8 times sweeter than sucrose and is commonly used as a sweetener in the food industry. Some particularly sweet natural foods contain large amounts of fructose, such as pears, lychees and honey. Fructose is not available to humans directly; it must be converted to glucose in the liver before it can be used to provide energy to cells. However, excessive consumption of fructose can have negative effects on health, such as increasing the risk of obesity, diabetes and fatty liver disease.
Galactose
Galactose has the same molecular formula as glucose and fructose, but a different structure. The ring of galactose is the same as glucose, but the hydroxyl groups attached to the fourth carbon in a different orientation. Galactose is not very sweet, and is not found in free form in nature. It is usually combined with glucose to form lactose, the main sugar in milk and dairy products. It is also found as a polysaccharide in plant gums or as glycolipids in nervous system. It needs to be converted to glucose in liver before being used by body.
Disaccharides
Two monosaccharide molecules form a disaccharide through a glycosidic bond. Lactose, sucrose and maltose are all common disaccharides.
Lactose
Lactose is found in mammalian milk and it is composed of glucose and galactose. Lactose is not digested and absorbed in the human stomach and goes directly to the intestine where it is broken down by lactase into glucose and galactose. Galactose is a component of glycolipids that make up brain and nerve tissue and is important for infant intellectual development.
Sucrose
Sucrose is the most common disaccharide, and it is composed of glucose and fructose. Much of glucose formed by photosynthesis is converted to sucrose and stored, making it one of the most abundant sugars in plant tissues. Edible sucrose is mainly obtained from sugar cane and beet.
Maltose
Maltose consists of two glucoses. It is found only in the tissues where starch is hydrolyzed. Maltose is present in large amounts in germinated grains; amylase in saliva hydrolyzes starch into maltose, which is why rice tastes a little sweet when it is chewed.