Question

The carbohydrate which cannot be hydrolyzed by the human digestive system is (a) starch (b) cellulose (c) glycogen (d) all of these

Short answer

Cellulose is the carbohydrate that cannot be hydrolyzed by humans (Option b).

Step-by-step solution

Understanding the Problem

We need to identify which carbohydrate among the options (a) starch, (b) cellulose, (c) glycogen, and (d) all of these cannot be broken down by the human digestive system.

Identifying Digestible Carbohydrates

Starch and glycogen are polysaccharides that can be broken down by enzymes in the human digestive system into glucose units. Amylase breaks down starch, and enzymes in the intestines help process glycogen.

Recognizing the Indigestible Carbohydrate

Cellulose, a polymer of glucose units, cannot be broken down by human digestive enzymes. Humans lack the enzyme cellulase, which is required to hydrolyze the β-1,4-glycosidic bonds in cellulose.

Selecting the Correct Answer

Based on the digestion capabilities, cellulose cannot be hydrolyzed by human digestive enzymes, while starch and glycogen can be. Therefore, option (b) cellulose is the correct answer.

Key concepts

Carbohydrate Digestion

Carbohydrates are an essential part of our diet, serving as a major energy source. When we consume carbohydrates, our digestive system begins the process of breaking them down into simpler forms. Carbohydrate digestion occurs in several stages, each involving specific enzymes that act on different carbohydrate types.

Here's a simple breakdown:

• It starts in the mouth, where the enzyme amylase begins to break down starches into maltose.
• Then, in the small intestine, enzymes like maltase, sucrase, and lactase further split disaccharides into monosaccharides like glucose.
• These monosaccharides can be readily absorbed by the intestine into the bloodstream, providing energy to our cells.

However, not all carbohydrates undergo complete digestion. For example, cellulose remains largely undigested in the human body due to the absence of specific enzymes. This highlights the variability in how different carbohydrates are processed.

Polysaccharides

Polysaccharides are complex carbohydrates, composed of long chains of monosaccharide units bound together.

They play vital roles in both energy storage and structural functions, and include a variety of different polysaccharides such as:

• Starch, widely found in plants and serving as a primary energy source.
• Glycogen, primarily stored in animal liver and muscle cells as a quick energy reserve.
• Cellulose, a structural component in plant cell walls, known for its rigidity.

Polysaccharides can differ significantly due to the type of glycosidic linkages (bonds between sugar units) they contain. For humans, only starch and glycogen are digestible, providing energy upon breakdown. The unique structure of cellulose, however, makes it resistant to human digestive enzymes.

Cellulose

Cellulose is a fascinating polysaccharide that distinguishes itself by its role and properties. It is primarily found in the cell walls of plants and is notable for providing structural support.

The key to cellulose's indigestibility in humans lies in its molecular structure. It is made up of long chains of glucose units linked by β-1,4-glycosidic bonds.

These bonds are distinct from the α-glycosidic linkages found in starch and glycogen. Humans lack cellulase, the enzyme required to break these β-bonds. As a result, cellulose passes through the human digestive system without being broken down.

While cellulose may not provide energy, it plays an important role in digestion by adding bulk to the stool, aiding in bowel regularity, and supporting healthy gut function.