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In this article, we will discuss the molecular structure of starch and cellulose in detail. So, let us get started.
Introduction to Starch
Starch is the primary glucose storage compound found in plants. It is a polysaccharide in nature and is a biopolymer that is composed of several (thousands of) repeating units of glucose. Polysaccharides refer to the macromolecules that are polymers formed by several monosaccharides connected by glycosidic bonds in a condensation reaction to create chains. These chains can be either branched or unbranched. They can be straight or folded. Starch is considered polysaccharides because they are compact and insoluble. Insolubility means that they have no osmotic effect. Because starch is composed of several monomers, therefore, it takes a lot of time to digest as compared to glucose.
The following diagram shows different types of polysaccharides.
Animals cannot synthesize starch; however, it is the primary source of diet for animals. Plants rich in starch are the main source of nutrition not only for animals but also for humans. Let us now discuss the molecular structure of starch
Components of Starch
Starch is found in plants in the form of starch granules. It is composed of the following two components which have distinct structures:
- Amylose
- Amylopectin
Amylose
It is sometimes also referred to as amylose starch. This polysaccharide is composed of alpha-D-glucose monomers, and it makes up 10% to 30% of the starch. Amylose is the unbranched helix-shaped chain that has 1,4 glycosidic bonds between α-glucose molecules. The helix shape of the amylose starch allows it to be more compact and hence it is hard to digest.
Amylopectin (70 – 90% of starch)
Amylopectin makes up 70 to 90% of starch and has 1,4 glycosidic bonds between α-glucose molecules, but it also creates 1,6 glycosidic bonds between glucose molecules to form a branched molecule.
Amylopectin exhibits branching after every 20 to 30 subunits of glucose. Each branch of the amylopectin also has the same number of glucose subunits. All the subunits of glucose in one branch are connected through alpha 1,4 glycosidic bonds except the first one. The first subunit of each branch is connected to the parent chain through a 1,6 glycosidic bond.
The branches result in several terminal glucose molecules that are easy to hydrolyze for use during the process of cellular respiration. These branches also help amylopectin to store easily.
Properties of Amylose and Amylopectin
As both the components possess different molecular structures, therefore they have different properties. The properties of starch are the mixture of the properties of its two components, i.e. amylose and amylopectin.
Properties of Amylose
The properties of amylose are listed below:
- It is characterized by its crystalline nature
- Amylose is insoluble in cold water and has greater resistance to digestion because of the lack of branching
- Due to the amylose, the gel strength of the starch is reduced
Properties of Amylopectin
The properties of amylopectin are listed below:
- It not only dissolves in cold water but also the hot water
- Because it has extensive branching, hence it can be digested easily
- It enhances the get strength of the starch because of its branching
Properties of Starch
So far, we have discussed the individual properties of the starch components. Let us now elaborate on the collective properties of starch.
- Starch is hydrolyzed to produce constituent sugars
- It forms a gel and becomes insoluble in water
- Upon heating, it decomposes and forms dextrin
- With iodin solution, it yields a blue color
Biosynthesis of Starch
Starch is biosynthesized only in plant cells. Enzymes that synthesize starch are absent in animals. The process of biosynthesis of starch occurs in the chloroplasts of the plant cells.
Synthesis of ADP glucose is the first step of the starch’s biosynthesis. This ADP glucose plays the role of precursor for all the subunits of glucose present in starch. ADP glucose synthesis is connected to the Calvin cycle of photosynthesis.
Importance of Starch
Starch is a vital polysaccharide not only for plants but also for animals and humans.
In plants:
- Starch in plants is the primary storage compound. Plants store additional glucose in the form of starch during the daytime and utilize it as energy during night time when the process of photosynthesis stops.
- Stored starch provides energy to the plant’s embryo
In animals:
- Starch is the major source of carbohydrates for animals
- It acts as the source of energy for animals
- Animals get their carbon skeleton from the starch. This carbon skeleton can be employed in the synthesis of different other compounds in their bodies.
In humans:
- Like animals, starch is the main source of carbohydrates for humans.
- Humans consume starch in the form of cereals, grains, potatoes, and rice.
So far, we have discussed the molecular structure and properties of starch. Now, we will discuss the molecular structure and functions of another important polysaccharide – cellulose.
Cellulose
Cellulose is a polysaccharide like starch and glycogen. It is a polymer that consists of long β-glucose chains that are connected by 1,4 glycosidic bonds. Because β-glucose is an isomer of α-glucose to create 1,4 glycosidic bonds, so consecutive β-glucose should be rotated at 180 degrees to each other. Several hydrogen bonds create between the long chains because of the inversion of β-glucose molecules. This gives strength to cellulose.
Functions of Cellulose
- Cellulose is the primary structural component of the cell walls owing to its strength which is a consequence of several hydrogen bonds present between parallel chains of microfibrils
- The cellulose is characterized by its high tensile strength which enables it to be more stretched without breaking. Due to this, cell walls can bear turgor pressure.
- The fibers of cellulose and other molecules, for instance, lignin present in the cell wall create a matrix that enhances the strength of the cell walls in plants
- Because the cell wall of the plants is strong, hence they offer structural support to the plants.
- The fibers of cellulose are freely permeable which enables solutes and water to leave or reach the cell membrane of the cell.
- Some organisms contain an enzyme known as cellulase which can hydrolyze cellulose; therefore it is the main source of fiber in those organisms.
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