Introduction

Starch is a major glucose storage compound present in plants. It belongs to the category of biopolymers and is polysaccharide in nature. It is made up of thousands of repeating units of glucose. 

Starch is a compound that belongs to plants in origin. It cannot be synthesized in animals. However, it makes a major source of diet for animals. Crops that are rich in starch are the major source of nutrition for humans and other animals. In this article, we will discuss the structure, properties, synthesis and metabolism of starch. We will also discuss its importance in human lives. 

Structure

As mentioned earlier, it is a polymer of glucose molecules. It is present in plants in the form of starch granules. Starch is made up two components both of which have different structures;

• Amylose
• Amylopectin

Amylose

It is also sometimes called amylose starch. It is a polysaccharide made up of alpha-D-glucose monomers. Amylose makes around 20-30% of starch. It is an unbranched compound having linear chains of glucose molecules. these glucose molecules are linked via alpha 1-4 glycosidic bonds.

Amylose starch is a linear chain of alpha D-glucose subunits that usually contains around 300 to 3000 glucosyl residues or even more. 

This amylose chain can exist in one of the three forms;

• Disordered amorphous chain
• A or B forms
• V form

The A and B forms are helical or spiral in appearance. Both these forms have similar structures. These are the left-handed helices having around d six glucose molecules in each turn. The term left-handed means that when the spiral is rotated clockwise, it turns towards the observer. These spiral helices of amylose can also wind around each other forming a double helix. 

The V form of amylose is formed when it interacts with some hydrophobic or non-polar compounds like fatty acids, lipids, iodine or amylopectin. 

Amylose in starch is present in V-form after interacting with amylopectin. 

Amylopectin

It is also called as amylopectin starch. Amylopectin makes around 70-80% of starch. It is a branched polymer of alpha D-glucose subunits that are linked via the same 1-4 glycosidic bonds as in amylose. However, amylopectin also has alpha 1-6 glycosidic bonds among the glucose molecules at the branch points. 

Amylopectin shows branching after each 24 to 30 glucose subunits.  Each branch also contains around the same number of glucose subunits. All the glucose subunits in a single branch are attached via alpha 1-4 glycosidic bonds except the first one. The first glucose subunit of each branch is linked to the parent chain via alpha 1-6 glycosidic bond. 

Within the starch granules, amylopectin interacts with amylose in its V-form.

Starch molecules in plants are present in the form of semi-crystalline granules. The size of starch granules varies from one species to other. For example, the starch granules in rice are around 2 micrometers while those present in potatoes are up to 100 micrometers. 

Properties

Because of their different structures, the two components of starch also have different properties. The properties of starch are the amalgam of the properties of these two components, amylose and amylopectin. 

Amylose

The amylose starch have following properties; 

• Amylose is more crystalline in nature
• It is insoluble in cold water
• Amylose is more resistant to digestion due to lack of branching
• Amylose decreases the gel strength of starch

Amylopectin

The properties of amylopectin are as follows;

• It dissolves in cold as well as hot water
• Amylopectin is readily digestible due to the presence of extensive branching
• It increases the gel strength and solubility of starch as mole water molecules can infiltrate due to branching

The collective properties of starch are as follows;

• It undergoes hydrolysis to yield constituent sugars
• It becomes insoluble in water and forms gel 
• It undergoes decomposition upon heating and forms dextrin
• It gives blue color with iodine solution

Biosynthesis

The biosynthesis of starch occurs only in plant cells. Enzymes for starch synthesis are absent in animal cells. This process takes place in the chloroplast of plant cells. 

The first step in the biosynthesis of starch is the synthesis of ADP-glucose. This ADP-glucose acts as a precursor of all the glucose subunits found in starch. The synthesis of ADP-glucose is linked to the Calvin cycle of photosynthesis. 

The glucose-6-phosphate generated in the Calvin-cycle is first converted to glucose-1-phosphate by phosphoglucomutase enzyme. In the next step, glucose-1-phosphate and ATP are converted to ADP-glucose by glucose-1-phosphate adenyl transferase enzyme. One pyrophosphate enzyme is also released in this process. The pyrophosphate undergoes hydrolysis to phosphate groups making the process irreversible. 

This synthesis of ADP-glucose is regarded as the rate-limiting step in the starch biosynthesis. 

Once the ADP-glucose has been made, the starch synthase enzyme comes in action. This enzyme uses the ADP-glucose molecules as precursors and transfers their glucosyl residues to the amylose or amylopectin chain.

The starch synthase enzyme can only transfer glucose from ADP-glucose molecules to the non-reducing end of an already existing glucosyl chain. Its function is to create the alpha 1-4 glycosidic bonds and elongate the chain. 

The final step in the starch synthesis is the introduction of branches to the linear molecule in order to form amylopectin. This process is performed by a starch branching enzyme. It is much similar to the glycogen branching enzyme. 

Just like the glycogen branching enzyme in animal, the starch branching enzyme in plants have two activities;

• Alpha 1-4 glucosidase activity: It breaks the terminal alpha 1-4 glycosidic bond of the amylose chain and releases a free glucosyl residue
• Alpha 1-6 glucosidic activity: It attaches the released glucose molecule to a non-reducing glucosyl residue by making an alpha 1-6 glycosidic bond

The structure and synthesis of amylopectin starch and glycogen looks similar. However, there are two key differences in them;

• In glycogen, glucose residues come from UDP-glucose while in starch, they come from ADP-glucose. 
• In glycogen, there is one branch after every ten alpha 1-4 bonds, while in amylopectin, one branch occurs after around 30 alpha 1-4 bonds. 

Starch production in Industry

In the starch industry, the term starch production is used for the process by which starch is extracted and refined from the leaves, roots, and tubers, etc. 

Occurrence

Starch is produced by all plants to store excess glucose. It is majorly stored in the stroma of plant cells. In some plant cells, it is stored in specialized organelles called amyloplasts. Starch is present abundantly in the roots, tubers, and rhizomes. It is also present in large amounts in embryos and fruits of plants. 

Metabolism

The metabolism of starch can be discussed under two headings, degradation in plants and degradation in animals. 

Degradation in Plants

Starch is the main energy storage compound in plants, just like glycogen in animals. Plants make starch during daytime when the glucose production is more than the glucose required by the cells. The extra glucose is stored in the form of starch.

During the night, when the photosynthetic machinery of the plants is no more working, glucose production in plant cells is zero. They use the stored glucose and obtain it by degrading the starch. 

To make starch accessible for the degradation enzymes, it needs to be phosphorylated first. The phosphorylation of starch is carried out by two enzymes; 

• Glucan, water dikinase (GWD): It phosphorylates the C-6 of glucose residues
• Phosphoglucan, water dikinase (PWD): It phosphorylates the C-3 of glucose residues

After phosphorylation, the degrading of starch begins. It is carried out by three enzymes;

• Beta-Amylase: It acts on the non-reducing end of the amylose chain and releases maltose as a product. It can act only if the chain length is more than three subunits. 
• Disproportionating enzyme-1 (DPA-1): This enzyme joins the two maltotriose chains (chains having three glucose residues) and releases a water molecule. Beta-Amylase then acts on this chain and releases further maltose molecules. 
• Isoamylase: Beta-amylase does not act on the phosphorylated residues. These phosphorylated residues are degraded by isoamylase enzyme. 

Although some glucose molecules are also released, the major product of starch degradation is maltose. It is a disaccharide made up of two glucose subunits. Both these molecules are transported into the cytoplasm of plants cell where they can be used for sucrose synthesis. Sucrose is then used for obtaining energy. 

Degradation in Animals 

Starch is the major carbohydrate source for humans and other animals. It enters the human diet in the form of wheat, rice and potatoes, etc. It is present in every form of the staple diet. Other animals consume starch by eating leaves and other green parts of plants. 

Degradation of starch in animals means its digestion. Starch can be readily digested in the digestive tract of animals. 

In humans, the digestion of starch begins in the oral cavity. The amylase enzyme present in the saliva releases cleaves starch into maltose. 

The digestion of starch halts in the stomach due to the acidic pH. It continues again in the small intestine. 

The amylase enzyme present in the intestinal juice completely digests the entire starch present in diet into maltose. The maltose molecules are cleaved by maltase enzyme to release glucose molecules. 

The release of glucose molecules completes the starch digestion in humans. These glucose molecules are then absorbed into the blood and carried to the liver for further processing. 

Importance

The starch is an important polysaccharide for plants, animals, and humans. Its importance can be discussed under the following headings.

Plants

• In plants, starch acts as the main energy storage compound. They store excess glucose during daytime in the form of starch and use it as an energy source during the night.
• It provides energy to the embryo.  

Animals

• Starch is the primary source of carbohydrates for animals. 
• It provides energy to the animals.
• It provides carbon skeleton to the animals that can be used in the synthesis of various other compounds in their bodies. 

Humans

• Like other animals, it is the major carbohydrate source for humans.
• It is present in the human diet in the form of grains, cereals, rice, potatoes, etc.  

Industry

Starch found great uses in the food and paper industry. 

Food Industry

• Starch is used as a sweetening agent in beverages
• It is used as a bulking agent for fairy products
• It is used in ice creams as it lowers their freezing point
• It gives brown effect to caramels
• It is added as a preservative in jams

Paper Industry

• Starch is used to increase the strength of paper
• It is used as a bounding powder for boards
• It is used in glues and adhesives to increase their binding strength
• It is used in bioplastic production

In addition, starch also provides raw material for ethanol production.

Summary

Starch is a polysaccharide made up of repeating glucose subunits. 

It is made up of two components, amylose and amylopectin.

Amylose consists of a linear chain of glucose molecules attached via alpha 1-4 glycosidic bonds. It makes 20-30% of starch and can exist in one of the three forms;

• Amorphous chain
• A or B helix
• V form

Amylopectin consists of branched polymeric chains of glucose having both alpha 1-4 and 1-6 glycosidic bonds. It takes around 70-80% of the starch.  

Both these components wind around each other to form semi-crystalline granules of starch.

Both the components have different properties and together decide the overall properties of starch. 

Starch is present in the leaves, roots, tubers, fruits, and embryo of plants. 

It is synthesized in the chloroplasts of plants during sunlight. 

The biosynthesis of starch is coupled with the Krebs cycle and involves the following steps;

• Synthesis of ADP-glucose
• Elongation of chain by starch synthase
• Introduction of branches by starch branching enzymes

Starch is degraded in plants during the night to obtain glucose for energy. It involves the following steps;

• Phosphorylation
• Release of maltose by the action of Amylase enzyme
• Joining of maltotriose and degradation of the resulting compound
• Release of phosphorylated residues

Starch is readily digested in animals. In humans, it is digested into maltose by amylase in the oral cavity and small intestine. Maltose is then digested into glucose by maltase enzyme in the small intestine. 

Starch finds important uses in;

• Animal diet
• Human diet
• Food industry 
• Paper industry

Frequently Asked Questions

What is starch in food?

Starch is a carbohydrate found in a number of food products such as wheat, rice, potatoes, etc. It is one of the major sources that provide energy to human beings. It is a polymer that is made up of hundreds or thousands of glucose molecules. 

What is the structure of starch?

Starch is a polysaccharide that is made up of glucose molecules. It consists of a large number of glucose molecules that are combined together via glycosidic bonds in straight or branched chains to form starch. 

What enzyme breaks down the starch in your saliva?

Human saliva has an enzyme called amylase. This enzyme breaks down starch into maltose molecules. Maltose is a disaccharide consisting of two glucose molecules. 

Is starch a storage molecule?

Starch is a storage carbohydrate in plants. Glucose molecules in plants are stored in the form of starch. It is mainly stored in the stroma of plant cells. 

References

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2. Lindeboom, Nienke; Chang, Peter R.; Tyler, Robert T. (1 Apr 2004). “Analytical, biochemical and physicochemical aspoects of starch granule size, with emphasis on small granule starches: a review”. Starch-Stärke. 56 (3–4): 89–99. doi:10.1002/star.200300218.
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7. https://en.wikipedia.org/wiki/Calvin_cycle#/media/File:Calvin-cycle4.svg