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Ch. 5 - An Introduction to Carbohydrates
Chapter 5, Problem 6

Although cellulose and starch are identical in terms of stored chemical energy, our ability to harvest the energy from these two polysaccharides differs considerably. What is responsible for this difference?

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Understand the composition of cellulose and starch: Both cellulose and starch are polysaccharides made up of glucose units, but they differ in the type of glycosidic linkage connecting these glucose units.
Identify the type of linkage in each polysaccharide: Starch consists of alpha-1,4 and alpha-1,6 glycosidic linkages, while cellulose consists of beta-1,4 glycosidic linkages.
Recognize the human digestive enzyme capabilities: Humans have enzymes like amylase that can break down alpha linkages in starch, allowing us to digest starch and access its glucose for energy.
Note the absence of necessary enzymes for cellulose digestion: Humans lack the enzyme cellulase, which is required to break down the beta linkages in cellulose, making it indigestible and unable to be used as an energy source.
Conclude the impact on energy harvest: The presence of digestible alpha linkages in starch allows humans to harvest its energy, whereas the indigestible beta linkages in cellulose prevent us from using it as an energy source.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Polysaccharides

Polysaccharides are large carbohydrate molecules composed of long chains of monosaccharide units. They serve various functions in living organisms, including energy storage and structural support. Cellulose and starch are two common polysaccharides, with cellulose providing rigidity in plant cell walls and starch serving as an energy reserve in plants.
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Digestibility and Enzymatic Breakdown

The ability to digest polysaccharides depends on the presence of specific enzymes that can break down their glycosidic bonds. Humans and many animals can easily digest starch due to the presence of amylase, an enzyme that hydrolyzes starch into glucose. In contrast, cellulose has β-1,4-glycosidic bonds, which are not easily broken down by human enzymes, making it indigestible for us.
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Energy Harvesting

Energy harvesting from carbohydrates involves converting stored chemical energy into usable forms, such as ATP. While both cellulose and starch store energy, the efficiency of energy extraction varies. Starch can be readily converted into glucose and subsequently metabolized for energy, whereas cellulose requires specialized organisms or processes (like fermentation) to access its energy, limiting its utility as a direct energy source for humans.
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Related Practice
Textbook Question

What holds cellulose molecules together in bundles large enough to form fibers? a. the cell wall b. peptide bonds c. hydrogen bonds d. hydrophobic interactions

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Textbook Question

What are the primary functions of carbohydrates in cells? a. cell identity, energy storage, raw material source for synthesis, and structure b. catalysis, energy storage, metabolism, and structure c. catalysis, digestion, energy storage, and information storage d. energy storage, information storage, polymerization, and raw material source for synthesis

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Textbook Question

Which of the differences listed here could be found among molecules of the same monosaccharide? Select True or False for each statement. T/F There is a difference in the orientation of a hydroxyl group in the ring form. T/F There is a difference in the number of carbons. T/F There is a difference in the position of the carbonyl group in the linear form. T/F There is a difference in the overall shape of the molecule—one is a ring and the other is linear.

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Textbook Question

Contrast the structure of glycogen and chitin in terms of their monosaccharides, glycosidic linkages, and interactions between polysaccharide chains.

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Textbook Question

Lysozyme, an enzyme found in human saliva, tears, and other secretions, catalyzes the hydrolysis of the ββ-1,4-glycosidic linkages in peptidoglycan. Predict the effect of this enzyme on bacteria and how it may be involved in human health.

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Textbook Question

Galactosemia is a potentially fatal disease that occurs in humans who lack the enzyme that converts galactose to glucose. If you were a physician treating a person with this disease, which of the following would you have them exclude from their diet? a. maltose b. starch c. mannose d. lactose

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