36. The glycosidic linkages in cellulose and amylose are , respectively.
(A) α 1-4 and β 1-4
(B) β 1-4 and α 1-4
(C) β 1-4 and α 1-6
(D) α 1-4 and α 1-2
Cellulose vs Amylose Glycosidic Linkages Explained | Solution on α(1→4) and β(1→4) Bonds
Correct Answer
(B) β(1→4) and α(1→4)
Introduction
Carbohydrates are among the most abundant biomolecules found in living organisms and serve as major sources of energy as well as structural components of cells. Complex carbohydrates known as polysaccharides are formed by linking multiple glucose molecules through glycosidic bonds. Although cellulose and amylose are both polymers composed entirely of glucose units, they differ dramatically in their biological properties because of the type of glycosidic linkage connecting the glucose molecules.
The orientation of the glycosidic bond determines the three-dimensional structure, digestibility, mechanical strength, and biological function of the polysaccharide. Cellulose, the primary structural component of plant cell walls, contains β(1→4) glycosidic linkages, whereas amylose, a major component of starch, contains α(1→4) glycosidic linkages. This seemingly small structural difference is responsible for the remarkable contrast between a rigid structural polymer and an energy-storage molecule.
Understanding the Concept Behind the Question
Both cellulose and amylose are homopolysaccharides because they are composed entirely of glucose molecules. However, the glucose units are connected differently.
In cellulose, glucose molecules are joined through β(1→4) glycosidic bonds, producing long, straight chains that align parallel to one another and form extensive hydrogen bonding. This arrangement gives cellulose exceptional tensile strength and makes it the major structural polysaccharide of plants.
In amylose, glucose molecules are connected by α(1→4) glycosidic bonds, causing the polymer to coil into a helical structure. This compact conformation makes amylose suitable for storing glucose as an energy reserve in plants.
Therefore, the correct combination is:
- Cellulose → β(1→4)
- Amylose → α(1→4)
Hence, Option (B) is correct.
Why Option (A) Is Incorrect
α(1→4) and β(1→4)
This option reverses the glycosidic linkages of cellulose and amylose.
Cellulose does not contain α(1→4) linkages. Instead, it contains β(1→4) bonds that produce rigid, linear fibers.
Amylose likewise does not contain β(1→4) linkages. It contains α(1→4) linkages that allow the molecule to adopt a helical configuration.
Since both linkages are interchanged, this option is incorrect.
Therefore,
Option (A) is incorrect.
Why Option (B) Is Correct
β(1→4) and α(1→4)
This option correctly identifies the glycosidic bonds present in both polysaccharides.
Cellulose
- Polymer of β-D-glucose
- β(1→4) glycosidic linkages
- Linear, unbranched chains
- Structural component of plant cell walls
Amylose
- Polymer of α-D-glucose
- α(1→4) glycosidic linkages
- Helical, unbranched chains
- Major storage polysaccharide of plants
These are the characteristic glycosidic linkages of the two polymers.
Therefore,
Option (B) is correct.
Why Option (C) Is Incorrect
β(1→4) and α(1→6)
The cellulose linkage is correctly identified as β(1→4).
However, amylose does not contain α(1→6) glycosidic bonds.
The α(1→6) linkage is characteristic of amylopectin, the branched component of starch, where it forms branch points approximately every 24–30 glucose residues.
Amylose itself is an unbranched polymer containing only α(1→4) linkages.
Therefore,
Option (C) is incorrect.
Why Option (D) Is Incorrect
α(1→4) and α(1→2)
Neither linkage corresponds to cellulose or amylose.
Cellulose contains β(1→4) glycosidic bonds.
Amylose contains α(1→4) glycosidic bonds.
An α(1→2) linkage is not characteristic of either polysaccharide.
Therefore,
Option (D) is incorrect.
Structure of Cellulose
Cellulose consists entirely of β-D-glucose molecules connected by β(1→4) glycosidic bonds.
Because each glucose residue is rotated by 180° relative to the next, cellulose forms long, straight chains capable of extensive hydrogen bonding. Multiple cellulose chains associate to produce microfibrils, which provide remarkable mechanical strength to plant cell walls.
Humans cannot digest cellulose because they lack the enzyme cellulase, which is required to hydrolyze β(1→4) glycosidic bonds.
Structure of Amylose
Amylose is a linear polymer of α-D-glucose connected exclusively through α(1→4) glycosidic bonds.
Unlike cellulose, the α-linkage causes the polymer chain to coil into a helical structure. This conformation allows amylose to store glucose efficiently within starch granules.
Humans readily digest amylose because digestive enzymes such as α-amylase specifically hydrolyze α(1→4) glycosidic bonds.
Cellulose vs Amylose
| Feature | Cellulose | Amylose |
|---|---|---|
| Monomer | β-D-Glucose | α-D-Glucose |
| Glycosidic Linkage | β(1→4) | α(1→4) |
| Structure | Linear | Helical |
| Function | Structural | Energy Storage |
| Digestibility in Humans | No | Yes |
| Major Location | Plant Cell Wall | Starch Granules |
Biological Importance
The difference between α(1→4) and β(1→4) glycosidic linkages illustrates how a minor change in stereochemistry can produce molecules with entirely different biological functions. Cellulose forms rigid fibers that strengthen plant cell walls and resist enzymatic degradation, while amylose serves as a compact, digestible energy reserve.
This distinction is also nutritionally significant. Humans possess α-amylase, which hydrolyzes α(1→4) linkages in starch, but lack cellulase, preventing digestion of cellulose. Consequently, cellulose functions as dietary fiber, whereas amylose serves as an important source of glucose during digestion.
High-Yield Points
- Cellulose → β(1→4) glycosidic bonds.
- Amylose → α(1→4) glycosidic bonds.
- Amylopectin contains both α(1→4) and α(1→6) linkages.
- Glycogen contains α(1→4) and frequent α(1→6) branch points.
- Humans digest α-linkages but not β(1→4) cellulose.
- Cellulose forms plant cell walls.
- Amylose is a storage polysaccharide.
Frequently Asked Questions
Why can’t humans digest cellulose?
Humans lack the enzyme cellulase, which is required to hydrolyze β(1→4) glycosidic bonds present in cellulose.
Does amylose contain α(1→6) linkages?
No. Amylose is an unbranched polymer composed exclusively of α(1→4) glycosidic bonds. The α(1→6) linkage occurs in amylopectin and glycogen.
Why do cellulose and amylose have different properties?
Although both are polymers of glucose, the orientation of the glycosidic bond determines whether the polymer forms rigid linear fibers (cellulose) or flexible helical chains (amylose), leading to completely different biological functions.
Key Takeaways
Cellulose and amylose are both glucose polymers, but they differ fundamentally in the type of glycosidic linkage connecting their glucose units. Cellulose contains β(1→4) glycosidic bonds, producing strong, linear fibers that form the structural framework of plant cell walls. Amylose contains α(1→4) glycosidic bonds, resulting in a helical storage polymer that can be readily digested by humans. This single stereochemical difference illustrates how bond orientation determines the structure, function, and biological role of polysaccharides.
Final Answer
Correct Option: (B) β(1→4) and α(1→4)
Explanation
Cellulose is composed of β-D-glucose molecules linked by β(1→4) glycosidic bonds, producing long, straight chains that form strong hydrogen-bonded fibers in plant cell walls. Amylose, the unbranched component of starch, consists of α-D-glucose molecules connected by α(1→4) glycosidic bonds, giving rise to a helical structure suitable for energy storage. Because cellulose contains β(1→4) linkages and amylose contains α(1→4) linkages, the correct answer is Option (B).
