Introduction
Polysaccharides are the large biological molecules that belong to the category of carbohydrates. They are the biological polymers made up of hundreds or thousands of monosaccharide subunits attached via glycosidic bonds.
According to the definition, polysaccharides are the carbohydrates that contain more than ten monosaccharide subunits. Upon degradation, they yield individual monosaccharides.
Commonly occurring polysaccharides are starch, glycogen, cellulose, and hemicellulose. In this section of notes, we will try to understand the structure, properties, and functions of polysaccharides using these examples.
Starch
It is a common storage polysaccharide present in plants. It is also known as amylum.
Structure
Starch is a polysaccharide made up of glucose subunits that are linked via glycosidic bonds. The glucose subunits combine to form two polymer components that together form starch. These polymer components of starch differ in their structure and properties. These include;
- Amylose
- Amylopectin
Amylose
Amylose make up of around 20-30% of the total starch. The structure and properties of amylose are mentioned below.
Structure
Amylose is made up of long chains of glucose molecules bound together via alpha 1-4 glycosidic linkages. This means that all the glucose molecules in long chains are in alpha orientation with the first carbon of one molecule attached to the fourth carbon of the next molecule via glycosidic bonds. The number of the glucose subunits range from 300 to 3000 or even more.
The long chain of amylose can fold onto itself or can bind with other hydrophobic compounds.
When the chain of amylose folds onto itself, a spiral-shaped structure is formed known as a helix. The helix is kept in shape by the hydrogen bonds between the hydrogen atoms and the hydroxyl groups of glucose molecules.
The amylose chain can also bind hydrophobic molecules using hydrogen bonds. This is the way amylose is bound to amylopectin in starch.
Properties
Amylose component of starch has the following properties;
- It is insoluble in cold water
- It is more resistant to digestion due to packed helical structure
- It limits the infiltration of water in starch
- It shows very limited branching or no branching at all
- It gives dark blue/black color with iodine solution
Amylopectin
Amylopectin makes around 70-80% of total starch. Its structure and properties are discussed below.
Structure
Amylopectin is also a polymer of glucose molecules linked together via alpha 1-4 glycosidic bonds just like amylose.
The main difference between the structure of amylose and amylopectin is of branching. The amylopectin component of starch shows extensive branching.
A branch breaks away from the original chain of glucose molecules after every 24 to 30 subunits. The glucose molecule at each branch point is attached to the original chain via an alpha 1-6 glycosidic bond. the rest of the glucose subunits in the branch are attached via the same alpha 1-4 glycosidic bonds as in the original chain. Each polysaccharide branch in amylopectin also contains 24 to 30 glucose subunits.
Properties
The properties of amylopectin are also different from amylose.
- It is soluble in both hot and cold water
- It does not resist digestion
- It shows extensive branching
- It gives reddish-brown color with iodine solution
Both these components are joined together to form starch.
Sources
Starch is the main storage carbohydrate in plants. They store glucose in the form of starch granules. It is present in grains, roots, tubers, fruits, etc.
Synthesis
Plants synthesize starch from glucose molecules that are made by the process of photosynthesis. The alpha 1-4 glycosidic bond is formed between the glucose molecules by starch synthase enzyme. Another enzyme called starch branching enzyme adds the alpha 1-6 glycosidic bond at branch points. All this process uses energy in the form of ATP.
Degradation
Starch undergoes hydrolysis into glucose molecules. In the human digestive system, starch is digested by amylase enzyme into smaller molecules. It yields maltose and glucose molecules. Maltose is further digested by maltase to form glucose.
Uses
Following are the uses of starch;
- It is the main source of carbohydrates for humans and other animals. It is present in our daily diet in the form of bread, rice, potatoes, corn, etc.
- It is used in confectionery and bakery to make pancakes, cereals, noodles, pasta, tortilla, etc.
- It is used in the beverage industry to make malt.
Glycogen
Glycogen is the storage polysaccharide in animals. It is also known as animal starch.
Structure
Glycogen is a branched polymer of glucose molecules. Its structure is similar to that of amylopectin. The individual glucose molecules in the chain are linked via alpha 1-4 glycosidic bonds. Branching takes place after every 8 to 12 subunits with each new branch containing 8-12 glucose molecules. Each branch is linked to the main chain via the alpha 1-6 glycosidic bond. All the glucose molecules have alpha orientation. Each branch has a free glucose molecule at its end known as the reducing end.
The glycogen molecules are arranged in the form of granules. These granules have a protein at their core known as glycogenin protein.
Properties
- It is an amorphous solid
- It is soluble in water
- It can be easily digested in the human digestive tract
- It gives red color with iodine solution
Sources
Glycogen is animal in origin. Humans and other animals store glucose in the form of glycogen. It is present abundantly in the muscles and liver of animals. However, in small amounts, it is present in all animal cells.
Synthesis
Glycogen is synthesized in our bodies in a process known as glycogenesis. It is an energy-consuming process in which extra glucose molecules in cells are stored in the form of glycogen.
An enzyme called the glycogen synthase makes alpha 1-4 glycosidic bonds between the glucose molecules to make straight chains of glucose. Another enzyme called glycogen branching enzyme removes a chain of six to seven glucose molecules from the reducing end of the original chain and adds this chain to a non-reducing glucose molecule by alpha 1-6 glycosidic bond. New glucose molecules are added to the reducing ends of both ends and the process repeats.
Remember that the end of the chain containing the last added glucose molecule is called the reducing end.
Degradation
The breakdown of glycogen occurs by a process known as glycogenolysis. The process begins at the non-reducing end of the chain. This means that the glucose molecule added at the beginning of the process is cleaved first. The enzyme involved in this process is known as glycogen phosphorylase. This enzyme cleaves the alpha 1-4 glycosidic bonds and releases glucose molecules in the form of glucose-1-phosphate.
A special enzyme known as glycogen debranching enzyme is required for breaking the alpha 1-6 glycosidic bonds.
Importance
- Glycogen is the most abundant polysaccharides present in animals.
- Extra glucose is stored in animal cells in the form of glycogen.
- It provides energy to the body in times of glucose depletion.
- Severe hypoglycemia and other metabolic disorders can occur in humans if glycogen is not efficiently made or degraded in our body.
Cellulose
It is the most abundant polysaccharide present in nature. Cellulose is a structural polysaccharide that is only present in plants. It is not present in animal cells.
Structure
Like the other two polysaccharides discussed above, cellulose is also made up of glucose subunits. However, it differs with respect to the glycosidic bonds present among the glucose subunits.
In cellulose, the glucose subunits are linked together via beta 1-4 glycosidic bonds. All the glucose molecules in cellulose are in beta orientation meaning that the OH groups of the anomeric carbons (carbon number 1) are oriented upwards.
In order to make the beta 1-4 glycosidic bonds, the successive glucose molecules should be arranged opposite to each other because the OH group on carbon 1 is above the ring while that on the carbon 4 is below the ring. Thus, every second glucose molecule in the structure of glucose is inverted at 180 degrees.
Unlike starch and glycogen, the chains of glucose subunits do not show branching in cellulose. The glucose chains also do not form a ring or any helical structure. Rather, these chains are arranged parallel to each other in cellulose.
The glucose chains in cellulose are held side by side through hydrogen bonds that are formed between the hydrogen atoms and the abundant hydroxyl groups attached to each carbon of glucose. In this way, these chains from strong microfibrils within cellulose. In living structure, these microfibrils of cellulose are arranged in the form of a polysaccharide matrix.
Properties
Following are the properties of cellulose;
- It is a tasteless crystalline solid
- It is not soluble in water
- It cannot be digested in the human body
- It does not give any color with iodine solution
- Cellulose fibers have a very high tensile strength. The tensile strength of cellulose fibers is almost equal to that of steel fibers.
Sources
Cellulose is a polysaccharide that is present in cell walls of plants. Humans consume cellulose in the form of green leafy vegetables that are abundant in fiber. It is also present in cell walls of algae and some fungi. Cellulose is also a component of biofilms secreted by some bacteria.
Degradation
As mentioned in the properties, humans cannot digest cellulose. We do not have the enzyme required to break the beta 1-4 glycosidic bond between two glucose molecules. Therefore, the cellulose we consume in the form of green leafy plants is excreted in feces without digestion.
However, cellulose can be digested in herbivores like cows, horses, sheep, goats, etc. because the symbiotic bacteria in these animals release the enzyme that can break the beta 1-4 glycosidic bonds.
Importance
- It provides strength to the cell walls of plants.
- It makes the leaves, stems, and branches of plants strong.
- It allows cells to withstand the fluid pressure in cytoplasm. This is the reason why plant cells do not burst when placed in a hypertonic solution.
- It is used in industry to make paper products such as paper, paper board, cardboard, etc.
- It is used as a stationary phase for thin layer chromatography in science labs.
- Cellulose derivates are used in the pharmaceutical industry as stabilizers.
- It is also used as a laxative.
Hemicellulose
Hemicellulose is another important polysaccharide present in cell walls of plants. It is a heteropolymer meaning that it is made up of more than one type of monosaccharide subunits.
In addition to glucose, it also contains arabinose, xylose, mannose, galactose, and some other monosaccharide.
Hemicellulose is an amorphous solid having shorter chain lengths as compared to that of cellulose. It also shows branching of chains.
It is not resistant to hydrolysis. It can yield monosaccharide subunits even in dilute acidic or basic solutions.
Its main function is to cross-link the microfibrils made up of cellulose and glycoproteins.
Summary
Polysaccharides are the biopolymers made up of repetitive monosaccharide subunits.
They differ greatly in their structure, properties, and functions.
Polysaccharide that are important include starch, glycogen, cellulose, and hemicellulose.
All of polysaccharides are non-reducing sugars.
Starch is storage polysaccharide in plants made up of two components, amylose and amylopectin.
- Amylose shows limited branching and is more resistant to digestion.
- Amylopectin shows abundant branching and can be hydrolyzed easily.
Starch is the main carbohydrate source of animals as it is present in every staple food.
Glycogen is storage polysaccharide in animals, made of glucose chains that show branching after every 8 to 12 subunits.
- It is mainly made in the liver and muscles by a process called glycogenesis.
- It is found only in animals.
- It prevent hypoglycemia and provides energy to the body when it is needed the most.
Cellulose is a structural polysaccharide present in the plant cell walls.
- The long chains of glucose molecules in cellulose make microfibrils that provide strength to plant cells.
- It cannot be digested in humans.
- Cellulose is also used in paper and pharmaceutical industry.
- It is also used in science labs.
Hemicellulose is another polysaccharide present in cell walls of plants.
- It is a heteropolymer made up of different monosaccharides.
- It is made up of small chains that also show branching.
- Its function is to cross-link the microfibrils in plant cell walls.
Frequently Asked Questions
What are polysaccharides?
Polysaccharides are the polymers of carbohydrates. They are large molecules that are made by combining hundreds or thousands of carbohydrate molecules.
What are some examples of polysaccharides?
Some examples of polysaccharides include starch, glycogen and cellulose. Starch is a storage polysaccharide found in plant cells while glycogen is a storage polysaccharide present in animal cells. Cellulose is present in the cell wall of plant cells.
What are the functions of polysaccharides?
Polysaccharides act as storage molecules in both animal and plant cells. They also provide structural support such as cellulose and have a role in cell signalling.
Can humans digest polysaccharides?
Humans can digest and break down starch. However, cellulose cannot be digested in humans because of lack of cellulase enzyme.
References
- Matthews, C. E.; K. E. Van Holde; K. G. Ahern (1999) Biochemistry. 3rd edition. Benjamin Cummings. ISBN 0-8053-3066-6
- N.A.Campbell (1996) Biology (4th edition). Benjamin Cummings NY. p.23 ISBN 0-8053-1957-3
- Anatomy and Physiology. Saladin, Kenneth S. McGraw-Hill, 2007.
- “Animal starch”. Merriam Webster. Retrieved May 11, 2014.