What Carbohydrates or Sugars are found in Living Organisms

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.

GlucoseC₆H₁₂O₆It is the main energy source for life, and all plant and animal contain glucose.
FructoseC₆H₁₂O₆Sweet fruit and honey
GalactoseC₆H₁₂O₆mammal milk, plant gum, nerve system
LactoseGlucose + Galactosemammal milk
SucroseGlucose + FructoseIt can store energy in the plant.
MaltoseGlucose + GlucosePlant tissue in which starch hydrolysis occurs


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 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.


Two monosaccharide molecules form a disaccharide through a glycosidic bond. Lactose, sucrose and maltose are all common disaccharides.


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 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 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.

Frequently Asked Questions

Why do some people get diarrhea after drinking milk?

Some people experience diarrhea after drinking milk due to a condition known as lactose intolerance. It is a common digestive disorder in which the body is deficient in lactase and unable to digest lactose, a type of sugar found in milk and other dairy products. When a lactose-intolerant person consumes milk, the lactose passes undigested into the intestine, where it is fermented by bacteria. This fermentation produces gas and can cause symptoms such as abdominal bloating, cramping, and diarrhea.

What are the risks of eating a lot of sucrose?

Fruits are rich in fructose and one sucrose is made up of one glucose and one fructose. Fructose cannot be used directly as cellular fuel like glucose. It has to be converted to glucose, glycogen and fat in the liver, a process that consumes a lot of ATP to produce uric acid, which increases the risk of gout. Fructose is more likely to be converted into fat which accumulate in the liver, causing fatty liver. It is importantto reduce the intake of snacks in your daily diet, especially carbonated drinks, because they contain a lot of sucrose.