What is glucose | Definition, Structure, & Function

Glucose is the compound that serves as an energy source for living beings . Normally we know it as sugar or monosaccharide. The term “saccharide” is derived from the Greek sakcharonmeaning “sugar”.

Glucose is an organic molecule composed of carbon, hydrogen and oxygen whose formula is C 6 H 12 O 6 . As such, it is part of a much larger group of sugars or carbohydrates.

Chemical structure of glucose

Glucose is a monomer or monosaccharide with six carbons attached in a line. The first carbon is a carbonyl group HC = O; the other carbons have OH hydroxyl groups. The glucose in aqueous solution has a cyclic or ring structure, resulting from the reaction of carbon 1 with the oxygen of the OH of carbon 5.

Characteristics of glucose

  • It is a hexose, that is, it has six carbon atoms.
  • It is an aldose: it has an aldehyde or carbonyl CHO group in the first carbon.
  • It has OH hydroxyl groups, which is also found in alcohols.
  • It forms solid crystals.
  • It is soluble in water.
  • It has a sweet flavor.

Where do we find glucose?

Glucose is the most abundant carbohydrate on earth. The sugar that we use to sweeten the food or prepare desserts is called sucrose , composed of a molecule of glucose and another of fructose. It is usually obtained from sugar cane and beet.

The lactose , which is the sugar found in milk, is formed by a glucose and galactose, another monosaccharide. The maltose is a disaccharide (two glucose molecules attached) found in germinated seeds.

Many glucose units together form polymers called polysaccharides. The two best known polysaccharides are starch and glycogen . This is the way to store glucose, starch in vegetables, glycogen in animals.

The plant walls are formed by cellulose , which is another polysaccharide formed by glucose chains.

Glucose function

Glucose is an important molecule in the metabolism of living beings.

  • Energy: the processing of glucose within cells translates into ATP molecules, which is the energy molecule par excellence.
  • Reserve: plants, which use sunlight as energy source, synthesize sugars, mainly glucose and starches, and store it in fruits, tubers and roots. In animals, glucose is stored as glycogen in the muscles and liver.
  • Structure: glucose is also a component of cellulose, which is the main framework of cell walls of plants and algae.

In the digestive system occurs the digestion of carbohydrates where glucose is found. Various enzymes are in charge of this work. In saliva is amylase, the enzyme that degrades polysaccharides and releases glucose. Lactase breaks lactose, releasing galactose and glucose in the intestine.

The starches that are found in large quantities in potatoes, corn, wheat, rice and legumes are degraded by the amylase of saliva and pancreatic juice. In the intestine there is an enzyme that breaks the junction of fructose and glucose in sucrose.

Once in the small intestine, glucose is absorbed. This enters the intestinal cell by special transports, which are passages in the plasma membrane. Once inside, the glucose exits at the opposite end and falls into the blood vessels of the intestine.

Insulin promotes the passage of glucose from the blood to the cells.

Glucose reaches the body’s cells through the blood. The first station is the liver, where glucose arrives to be stored in the form of glycogen. When we spend a long period without eating food, the liver is responsible for supplying glucose constantly, through the degradation of glycogen.

Glucose enters the cells through passages or transporters in the plasma membranes. Some of those gates are opened by the action of insulin, a hormone produced in the pancreas.

Glucose in muscle cells is also stored as glycogen, which is a fast and safe way to get energy when we are exercising.

Glucose cellular metabolism

Once inside the cells, glucose in living beings is part of several biochemical reactions.

Synthesis of glucose in plants

Glucose is synthesized in plants from carbon dioxide and water. They do this through the chemical process known as photosynthesis . In this process, light is an important ingredient. The equation that represents the formation of glucose by photosynthesis is the following:

style size 14px bold 6 bold CO subscript bold 2 bold more H subscript bold 2 bold O bold arrow with bold light above bold C subscript bold 6 bold H subscript bold 12 bold subscript bold 6 bold plus bold 6 subscript bold 2 end style

Glucose catabolism

Glucose in cells is the main source of energy, which in living beings is in the form of ATP or adenosine triphosphate. There are two main ways of transforming glucose:

  1. Glycolysis: since glucose enters the cell, it undergoes a series of chemical reactions in the cytoplasm until it turns into pyruvic acid. In muscle cells under physical exertion and reduced oxygen levels, pyruvic acid is transformed into lactic acid.
  2. Cycle of citric acid or Krebs: the pyruvic acid produced in glycolysis enters the mitochondria and in a cycle of reactions known as the Krebs cycle, by Hans Adolf Krebs (1900-1981) who discovered them.

On the other hand, by alcoholic fermentation, glucose is transformed into ethanol and carbon dioxide, a process that is used to produce bread, beer, wine and pulque.

Glucose and health

The glycemia refers to the presence of glucose in the blood. Many people know it as blood sugar, although glucose is not the only type of sugar we know. It is also known as serum or plasma glucose , since glucose is dissolved in the liquid part of the blood.

In humans, blood glucose levels are regulated by two hormones: insulin and glucagon. Both hormones are produced in the pancreas.

Thanks to the liver and muscle stores, we do not need to be constantly eating so that our cells have glucose at their disposal. Blood glucose levels are maintained in a stable range of values. In the table those values ​​are shown when we are fasting:

Blood glucose levels
Reference Interval

Conventional units

mg / dL

Reference Interval

International system units

mmol / L

54-99 3.0-5.5

After a meal, sugar levels increase slightly, returning to the values ​​before two hours of eating. When there is an increase of glucose in the blood we speak of hyperglycemia. When the levels are decreased, it is called hypoglycemia.

How is blood glucose measured?

In clinical analysis laboratories, a trained professional takes a blood sample and processes it to measure glucose levels. The usual conditions in which this is done is to go to the laboratory on an empty stomach in the early hours of the morning, that is, without having eaten since the dinner of the previous day.

There have been devices called glucometers for several years, which are designed to measure the amount of glucose in a drop of blood. These devices can be bought in pharmacies or other establishments, and anyone can use them.


Diabetes is a disease that results from an increase in the amount of glucose in the blood. This can happen for two main reasons:

  1. There is no production of insulin: when there is a problem at the pancreatic level and insulin is not produced, the glucose levels are abnormally high in the blood, because they can not enter the cells.
  2. The cells do not respond to insulin: when the insulin reaches the cell but it does not let the glucose pass inside, there is also an increase in blood glucose.

To diagnose if someone has diabetes, the doctor must study the symptoms and signs that the person presents, and ask for blood glucose levels. If fasting glucose is greater than 126 mg / dL (7.0 mmol / L) or greater than 200 mg / dL (11.1 mmol / L) it is indicative of diabetes.


There is hypoglycemia when the glucose values ​​are less than 54 mg / dL or 3 mmol / L. Below 45 mg / dL (2.5 mmol / L) the person begins to experience fatigue, dizziness, headache and blurred vision, loss of consciousness and even death.

Contrary to what is expected, people with diabetes are more likely to suffer from hypoglycemia, either through poor control of the disease or by skipping meals.

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