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Ch.6 Carbohydrates–Life’s Sweet Molecules
Chapter 3, Problem 90

ALLIED Health Which of the components in starch is more likely to be broken down more quickly in plants, amylose or amylopectin? Why?

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1
Understand the structure of starch, which is composed of two polysaccharides: amylose and amylopectin.
Recognize that amylose is a linear polymer of glucose units connected by α(1→4) glycosidic bonds.
Identify that amylopectin is a branched polymer, with glucose units connected by α(1→4) glycosidic bonds and branches formed by α(1→6) glycosidic bonds.
Consider that the branched structure of amylopectin provides more terminal ends for enzymatic action, making it more accessible to enzymes.
Conclude that amylopectin is more likely to be broken down more quickly in plants due to its branched structure, which allows enzymes to work on multiple sites simultaneously.

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Key Concepts

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

Starch Structure

Starch is a polysaccharide composed of two main components: amylose and amylopectin. Amylose is a linear chain of glucose units, while amylopectin is a branched structure. The structural differences between these two components influence their digestibility and breakdown rates in plants.
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Digestibility of Amylose vs. Amylopectin

Amylopectin is generally more easily broken down than amylose due to its branched structure, which provides more ends for enzymes to act upon. This results in a faster rate of digestion and energy release in plants. In contrast, the linear structure of amylose makes it more resistant to enzymatic breakdown.
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Enzymatic Breakdown

The breakdown of starch components is facilitated by enzymes such as amylase. These enzymes hydrolyze the glycosidic bonds in starch, converting it into simpler sugars. The efficiency of this process varies between amylose and amylopectin, with amylopectin being more readily accessible to these enzymes, leading to quicker digestion.
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Related Practice
Textbook Question

On an exam, a student was asked to draw the Fischer projection of l-glucose, but he had only memorized the structure of d-glucose. He wrote the structure of d-glucose and switched the hydroxyl group on C5 from the right to the left. Was his answer correct? If not, what is the name of the aldose that he drew?

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Textbook Question

Carbohydrates are abbreviated using a three-letter abbreviation followed by their glycosidic bond type. For example, maltose and sucrose can be written respectively as

Glcα (1→4) Glc Glcα (1→2) ßFru

Maltose Sucrose


Provide the structure for the O-type blood carbohydrate set given the following abbreviation:


L-Fucα (1→2) Galß (1→4) GlcNAc

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Textbook Question

ALLIED Health The structure of sucralose, found in the artificial sweetener Splenda, is shown in the figure. It consists of a chlorinated disaccharide made up of galactose and fructose. In its structure shown,

(a) identify the galactose unit and the fructose unit.

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Textbook Question

How much energy is produced if a person eats 50 g of digestible carbohydrate (not fiber) in a day? In this case, what percent of a 2200 Calorie diet would be digestible carbohydrate? Recall that carbohydrates provide four Calories of energy per gram consumed.

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Textbook Question

Trehalose is a naturally occurring disaccharide used in cosmetics because of its ability to retain moisture. The formal name of trehalose is glucose α, α (1→1) glucose. Draw the structure of trehalose. Is it a reducing or nonreducing sugar?

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