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Chapter 2: Problem 6

What are similarities and differences between starch and cellulose?

Short Answer

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Question: List the similarities and differences between starch and cellulose in terms of their molecular structure, function, and properties. Answer: Both starch and cellulose are complex carbohydrates (polysaccharides) made up of glucose monomers, and they are both naturally occurring in plants. Starch consists of two glucose polymers, amylose and amylopectin, with alpha(α)-1,4 glycosidic linkages, while cellulose has a linear polymer with beta(β)-1,4 glycosidic linkages. Starch serves as an energy storage molecule in plants, whereas cellulose functions as a structural component in plant cell walls. Starch has a semi-crystalline structure and is water-soluble, allowing it to be broken down by amylase enzymes in the digestive system, whereas cellulose has a rigid crystalline structure, is insoluble in water, and is indigestible by humans, serving as a dietary fiber instead.

Step by step solution

01

Similarities

Both starch and cellulose are complex carbohydrates (polysaccharides) made up of glucose monomers. They are both composed of D-glucose units connected through glycosidic linkages, and they are both naturally occurring carbohydrates in plants.
02

Molecular Structure

Starch is composed of two types of glucose polymers: amylose (a linear polymer) and amylopectin (a branched polymer). Amylose has an alpha(α)-1,4 glycosidic linkage, while amylopectin has an alpha(α)-1,4 glycosidic linkage between the straight chain and an alpha(α)-1,6 glycosidic linkage at the branch points. Cellulose, on the other hand, is a linear polymer of glucose with beta(β)-1,4 glycosidic linkages.
03

Function

Starch serves as the primary energy storage molecule in plants, particularly in the form of granules found in seeds, roots, and other storage organs. When plants need energy, starch is broken down into glucose, which is then utilized for various functions within the cell. Cellulose functions as a structural component in the plant cell wall, providing rigidity and strength to the plant. It is the most abundant organic molecule on Earth and is an essential part of plant cell walls.
04

Structural Properties

Starch has a semi-crystalline structure, where the amylose polymers form a helical shape that can coil around each other. In addition, amylopectin molecules can form both crystalline and amorphous regions in starch granules. In contrast, cellulose has a more rigid and crystalline structure due to the hydrogen bonding between the cellulose chains. This allows cellulose fibers to form strong networks that resist deformation and breakage, giving plants their structural integrity.
05

Solubility and Digestibility

Starch is water-soluble and can be readily broken down by enzymes called amylases, which are present in the digestive systems of many animals, including humans. This allows starch to be an important source of glucose for animals. Cellulose is insoluble in water and cannot be broken down by human enzymes, making it indigestible by humans. However, certain microorganisms, such as those found in the digestive systems of ruminant animals like cows, can produce cellulase enzymes, which can break down cellulose. As a result, cellulose serves as a significant part of the dietary fiber in the human diet.

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