Q. 13.49 - An Introduction to General, Organic, and Biological Chemistry

Question

Give the name of one or more polysaccharides that matches each of the following descriptions:

a. not digestible by humans

b. the storage form of carbohydrates in plants

c. contains only $\alpha (1\to 4)$-glycosidic bonds

d. the most highly branched polysaccharide

Step-by-Step Solution

Answer

Part a) Cellulose cannot be digested by humans 

Part b) Starch is a glucose storage 

Part c) Amylose is one of two types of starch polysaccharides with only one $\alpha (1\to 4)$ glycosidic bond.

Part d) Glycogen is a polysaccharide that is more branched than amylopectin.

Step 1: Introduction about Polysaccharides (Part a)

• Polysaccharides are carbohydrates that include $10$ or more monosaccharides.
• The glycosidic bond is the most important bond in polysaccharide structural stability. 
• These polysaccharides create more than $10$ monosaccharide units after complete hydrolysis.

Step 2: Explanation (Part a)

• In plants, cellulose is a primary structural carbohydrate. 
• The $beta(1 \to 4)$glycosidic bond connects lengthy unbranched glucose units in cellulose.
• Humans have an enzyme called $alpha$-amylase, which can hydrolyze $alpha(1 \to 4)$ glycosidic bonds but not $beta(1 \to 4)$ glycosidic bonds.
• That is why cellulose cannot be digested by humans.

Step 3: Explanation (Part b)

• Starch is a glucose storage which is form in plants made up of two types of polysaccharides: amylose and amylopectin. 
• As a result, two types of starch polysaccharides, amylose and amylopectin, operate as storage polysaccharides in plants.

Step 4: Explanation (Part c)

• Amylose is one of two types of starch polysaccharides with only one $\alpha (1 \to 4)$ glycosidic bond.
• Amylose is formed of of about $4000$ $\alpha$-D-glucose units that are connected by a $\alpha (1 \to 4)$glycosidic linkages.

Step 5: Explanation (Part d)

• Glycogen is a glucose storage type in mammals. 
• It's a glucose polymer with a structure which is comparable to amylopectin. 
• $a(1 \to 4)$-glycosidic linkages connect $\alpha$-D-glucose units in glycogen.
• Branches with $a(1 \to 6)$-glycosidic links occur every $10$ to $15$ glucose units, though.
• It is a polysaccharide that is more branched than amylopectin.