Introduction

Glucose is the most important monosaccharide present in our body. It belongs to the hexose category of monosaccharides. Glucose provides energy to all the cells in our body except the cardiac myocytes. Excess glucose is stored in the body in the form of storage molecules. 

Glucose is present in human blood within a specific range. Different medical conditions can arise if the blood glucose levels fall or increase above this range. In this article, we will discuss the structure of glucose in its various forms, properties, isomers, metabolism in the human body, and medical conditions associated with it. So, keep reading. 

Structure

Glucose is a monosaccharide made up of six carbon atoms. It is the most important hexose present in our body. The molecular formula of glucose is represented as C₆H₁₂O₆. The structural formula of glucose can be represented in two ways;

• Linear chain
• Closed ring

The linear chain structure and the ring structure co-exist in equilibrium with each other in an aqueous solution. Both these forms are interconvertible. 

Linear chain

When the glucose structure is represented in the form of an open chain, it can be seen to have a linear chain of six carbon atom. One hydroxyl group is attached to each carbon atom, except the first carbon. The first carbon of glucose is a part of the aldehyde group. Because of the presence of the aldehydic functional group, glucose is also known as an aldohexose. 

Closed chain

Glucose makes a ring when it is dissolved in an aqueous solution. The ring formed by glucose is hexagonal in structure. Five carbon atoms and one oxygen atom, belonging to the aldehydic functional group, make the corners or angles of the hexagon. This ring structure of glucose is known as glucopyranose.   

Isomers

Isomers are the molecules that have the same molecular formula but differ with respect to the arrangement of atom in the three-dimensional space. The number of isomers of a molecule depends on the number of chiral carbons in it. A chiral carbon is the one that is attached to four different groups of atoms. The formula to find the  number of isomers based on chiral carbons is as follows;

Number of isomers = 2ⁿ (here, n=number of chiral carbons)

Except for the first and the last carbon atom, the other four carbon atoms in glucose are chiral. Thus, glucose has 2⁴=16 isomers. 

When in ring form, each of these 16 isomers can have one of the two possible orientations; alpha or beta. Thus, glucose actually has 32 isomers. 

The two different structural forms of glucose are as follows;

• D-Glucose
• L-Glucose

When dissolved in solution, each of them can have one of the following ring structure. 

• Alpha-Glucose
• Beta-Glucose

D and L isomers

These are the two isomeric forms of glucose that differ in the optical properties. 

The D-glucose is dextrorotatory. When a ray of light is passed through a solution of D-glucose, it bends the light in the right direction. It is the most abundant form of glucose present in nature. When the structure of D-glucose is drawn on a paper, the -OH groups are written on the right side of carbon atoms, except the 3^rd carbon atom that has the -OH group on its left side. 

The L-Glucose is levorotatory. It bends the light rays to the left when passed through its aqueous solution. It is normally present in living cells. Its structure is opposite to that of D-glucose i.e. the -OH groups are attached to the left side of carbon atoms except the third carbon. 

Alpha and beta isomers

When either D-glucose or L-glucose is dissolved in water, it forms a hexagonal ring structure known as glucopyranose. The ring formed by each of these molecules can have alpha or beta orientation. 

In alpha-D-glucose or alpha-L-glucose, the hydroxyl group attached to the carbonyl carbon or the first carbon is on the side of the ring opposite to that of the sixth carbon. 

On the other hand, in beta-D-glucose or beta-L-glucose, the hydroxyl group of the carbonyl carbon is on the same side of the ring as the sixth carbon. 

Epimers of Glucose

Epimers are the molecules that differ in structure around one carbon atom only. Glucose has eight epimers. 

• Glucose
• Allose
• Altrose
• Mannose
• Idose
• Galactose
• Talose
• Gulose

All these eight epimers have D-form and L-form making a total of 16 isomers. 

Each of the 16-isomers can have either an alpha ring or a beta ring when dissolved in aqueous solution. 

Properties

The following are the important properties of glucose that should be kept in mind. 

• It is sweet in taste
• It is a polar compound that readily dissolves in water 
• It is a reducing sugar that gives positive Benedict’s test
• Like other monosaccharides, it cannot undergo hydrolysis. 
• Upon reduction, it forms alcohol like sorbitol
• The oxidation of glucose yields carboxylic acids like pyruvic acid
• It can also undergo fermentation and ester formation

Occurrence

Glucose is abundantly present in nature. 

Plants are the major source of glucose as they make it by the process of photosynthesis. It is present in free form in all fruits being abundantly present in dates, figs, grapes, etc. In the form of large storage molecule starch, it is present in the roots and tubers of plants. 

In humans, glucose is present in free form in blood, cerebrospinal fluid, and other tissue fluids. It is also stored in cells in the form of large storage molecule glycogen. 

Synthesis

Glucose is mainly synthesized in plants by the process of photosynthesis. However, animals are also capable of synthesizing glucose to some extent from non-carbohydrate sources by a process known as gluconeogenesis. 

A brief detail of these processes is mentioned below. 

Photosynthesis

It is a process of food production that takes place in plants in the presence of sunlight. Photosynthesis is a chain of chemical reactions that uses energy from sunlight and converts water and carbon dioxide into glucose and oxygen. 

This process takes place in special organelles of plant cells called chloroplasts. Chloroplasts are the factories of glucose production. They are present in almost all plant cells, being abundantly present in leaves. They impart the green colors to plants. 

The process of photosynthesis is divided into two steps, light reactions and dark reactions. 

• Light reactions involve the capturing of energy present in photons of sunlight in the form of ATP and NADPH. 
• Dark reactions involve the carbon fixing reactions that convert carbon dioxide molecules into glucose using the energy provided by ATP and NADPH. They are not dependent on light and are also called collectively known as the Calvin cycle. 

Gluconeogenesis

This process involves the synthesis of glucose from non-carbohydrate sources in animals. Not all the animal cells are capable of carrying out gluconeogenesis. In humans it only takes place in two organs;

• Liver (majorly)
• Kidneys (to a small extent)

Cells present in other organs of our body lack enzymes for gluconeogenesis. 

Amino acids provide the carbon skeleton for the synthesis of glucose. The process is a reverse of glycolysis and takes place in two steps. 

• In the first step, the carbon skeleton of an amino acid is converted into pyruvic acid or pyruvate within the mitochondria. The pyruvate molecule is then transported outside the mitochondria into the cytoplasm. 
• The second step is the reverse of glycolysis in which a glucose molecule is generated from two pyruvate molecules within the cytoplasm of a cell. 

Both these steps use energy in the form of ATP and NADH. The glucose molecules thus formed are released into the blood so that they can be transported to the other cells of the body. 

Metabolism 

The first step of glucose metabolism in human body is the entry of glucose molecules into the cells. Once inside the cytoplasm, glucose molecules can undergo oxidation to yield energy or they can be stored in the form of glycogen. 

Entry into the cell

Glucose molecules are large and thus, cannot cross the plasma membrane freely. They require special protein channels for entry into the cell, called glucose transporter or GLUT. 

Different cells have different types of GLUT. The most common forms of GLUT along with the cells having them are listed below. 

• GLUT-1 is present in blood-brain barrier and heart
• GLUT-2 is present in liver, pancreas and small intestine
• GLUT-3 is present in brain
• GLUT-4 is present in skeletal muscles and adipose tissues

These glucose transporters have different affinities for the glucose molecules and allow their entry into the cell at different concentrations. It should also be noted that GLUT-4 is dependent on glucose. 

Oxidation

Once the glucose molecules have entered a cell, they can be oxidized to obtain energy. The energy thus obtained is used to carry out various functions performed by the cell. 

Oxidation involves two processes; an oxygen-independent glycolysis, and oxygen-dependent Krebs cycle.

Glycolysis

Glycolysis is the process of breaking down a molecule of glucose into two pyruvic acid molecules. 

It takes place within the cytoplasm of cells. Glycolysis can occur in the presence or absence of oxygen.

The net product of glycolysis is two molecules of glucose, two ATP molecules, two NADH molecules, and two water molecules. 

Krebs cycle

The pyruvate molecules are transported into the mitochondria for further processing. Here, the two pyruvate molecules are converted into two acetyl-CoA molecules. 

The acetyl-CoA molecules are then passed through a cyclic process for complete oxidation into carbon dioxide. This cyclic process is called the Krebs cycle or citric acid cycle. 

It is an oxygen-dependent process that takes place only in aerobic organisms.

This process yields two carbon dioxide molecules, three NADH molecules and one FADH₂ molecule from one acetyl-CoA.

Fermentation

The alternative to Krebs cycle in non-aerobic organisms as well as humans in the absence of oxygen is fermentation. In this process, the pyruvate molecule is converted into lactic acid molecule using NADH as a reducing agent. 

Clinical conditions

Glucose is present in human blood in free as well as dissolved form. The normal range of blood glucose levels are 60-100 mg/dL in fasting condition, and 170-200mg/dL in non-fasting condition. Any increase or decrease in this level results in some serious clinical conditions mentioned below. 

Hypoglycemia

It is a condition in which blood glucose levels fall below 60mg/dL in fasting conditions. 

It is characterized by pale, cold and clammy skin, very rapid pulse, tachycardia, with normal or shallow breathing. As the hypoglycemia progresses, the person becomes irritable, confused, aggressive and uncoordinated, finally resulting in unconsciousness. Other symptoms include headache, dizziness, tremors, and convulsions. 

Hypoglycemia is a medical emergency and should be treated immediately. 

It is usually associated with alcohol consumption, too much insulin administration, or unexpected exercise in diabetic patients. 

Hyperglycemia

It is characterized by blood glucose levels above 200mg/dL in fasting conditions. 

The signs and symptoms of hyperglycemia include flushed, hot and dry skin, weak and rapid pulse, low blood pressure, and low breathing.

The three classical signs of hyperglycemia include;

• Polyphagia (excessive food intake)
• Polydipsia (excessive thirst)
• Polyuria (excessive urine output)

It is a gradual condition that develops over time. It is associated with excessive food intake, insufficient insulin, insulin resistance, or an infectious disease. 

Summary

• Glucose is the most important monosaccharide that provides energy to cells present in our bodies. 
• It is an aldohexose having an aldehydic group and multiple hydroxyl groups attached to six carbon atoms. 
• Its structure can be represented by an open-chain structure or a closed ring. 
• Glucose has 16 isomers. Each of these isomers can have either alpha ring or beta ring when dissolved in aqueous solution.  
• D-glucose and L-glucose are the two optical isomers of glucose. 
• The properties of glucose resemble other monosaccharides. 
• It is majorly present in fruits. 
• Plants make glucose from carbon dioxide and water by using sunlight as an energy source in the process of photosynthesis. 
• Animals can also make glucose by the process of gluconeogenesis.
• Once the glucose has entered the cells, it can either be used as an energy source can be converted into glycogen for use in the future. 
• Oxidation of glucose in aerobic organisms involves glycolysis and the Krebs cycle. 
• The alternative of the Krebs cycle in non-aerobic organisms and bacteria is fermentation.  
• Increase or decrease in blood glucose levels can result in either of the two clinical conditions, hypoglycemia and hyperglycemia.

Frequently Asked Questions

Is glucose a sugar?

The term sugar is often used for glucose. Glucose is the simplest form of sugar that is made up of six carbon atoms. It is a monosaccharide belonging to the category of hexoses. 

Is glucose soluble in water?

Being the simplest sugar, glucose is soluble in water. All monosaccharides are soluble in water. Glucose is also a monosaccharide that has six carbon atoms in its structure. 

What are the functions of glucose?

Glucose is the main source of energy for us. All carbohydrates that we consume in our diet are broken down into glucose molecules that are carried to the cell. Cells majorly use glucose to obtain energy and use it in various cellular processes.

What are the normal blood glucose levels?

The normal blood glucose level is less than 200 mg/dl two hrs after a meal. If it is more than that, the person may be diabetic. 

References

1.  Matthews, C. E.; K. E. Van Holde; K. G. Ahern (1999) Biochemistry. 3rd edition. Benjamin Cummings. ISBN 0-8053-3066-6
2. N.A.Campbell (1996) Biology (4th edition). Benjamin Cummings NY. p.23 ISBN 0-8053-1957-3
3. Kwan, Lam Peng (2000). Biology- A course for O Level. p. 59. ISBN 9810190964.
4. “Carbohydrates”. Chemistry for Biologists. Royal Society of Chemistry. Retrieved 10 March 2017.
5. https://commons.wikimedia.org/wiki/File:Gluconeogenesis-es.svg