02.06 Cellulose
Cellulose
Cellulose is the most abundant organic molecule on Earth, primarily due to its presence in plant cell walls and its resistance to natural breakdown. It serves a structural role, providing mechanical strength to plants.
Key Characteristics of Cellulose
- Polymer Type:
- Made from β-glucose monomers, unlike α-glucose polymers such as starch and glycogen.
- Glycosidic Bond Formation:
- In cellulose, each β-glucose molecule is rotated 180° relative to the next.
- This rotation aligns the –OH groups on carbon atoms 1 and 4, allowing the formation of 1,4 glycosidic bonds.
Structure and Strength of Cellulose
- Hydrogen Bonding:
- Hydrogen atoms on –OH groups are weakly attracted to oxygen atoms within the same cellulose molecule and between adjacent molecules.
- These hydrogen bonds are individually weak but collectively create substantial strength because of the large number of –OH groups in cellulose.
- Microfibrils and Fibres:
- Microfibrils: Bundles of 60-70 cellulose molecules cross-linked by hydrogen bonds.
- Fibres: Groups of microfibrils bonded together by additional hydrogen bonds.
- The cell wall contains layers of fibres arranged in multiple directions, enhancing strength and rigidity.
Functional Role of Cellulose in Plant Cells
- Tensile Strength:
- Cellulose fibres have high tensile strength, comparable to steel, which prevents stretching or breaking under tension.
- This strength allows plant cells to withstand internal pressures from osmosis without bursting.
- Structural Support:
- The rigid cell wall formed by cellulose fibres helps maintain cell shape and supports cell expansion during growth.
- The arrangement of fibres contributes to tissue rigidity, providing structural support to the entire plant.
- Permeability:
- Despite their strength, cellulose fibres are freely permeable, allowing water and solutes to move in and out of the cell.
Comparison: Molecular Structures of Amylose and Cellulose
| Feature | Amylose | Cellulose |
|---|---|---|
| Glucose Type | α-glucose | β-glucose |
| Glycosidic Bonding | 1,4 links without 180° rotation | 1,4 links with each glucose rotated 180° relative to the next |
| Molecular Shape | Helical, unbranched chains | Straight chains forming microfibrils with high tensile strength |
Practise Questions
Cambridge AS Biology Style Questions on Cellulose with Detailed Marking Schemes
Question 1
Describe the structure of cellulose and how it differs from amylose. (6 marks)
Mark Scheme:
- Cellulose is a polymer of β-glucose, where each β-glucose molecule is rotated 180° relative to the next. (1 mark)
- It forms straight, unbranched chains linked by 1,4 glycosidic bonds. (1 mark)
- Amylose is a polymer of α-glucose, forming helical, unbranched chains without 180° rotation. (1 mark)
- Cellulose chains are cross-linked by hydrogen bonds, forming microfibrils, which are further grouped into fibres. (1 mark)
- These fibres provide high tensile strength, unlike amylose, which is more flexible and used for energy storage. (1 mark)
- The structural differences make cellulose suited for rigidity and support, while amylose is adapted for compact energy storage. (1 mark)
Question 2
Explain the role of hydrogen bonding in the strength of cellulose. (5 marks)
Mark Scheme:
- Hydrogen bonds form between the hydrogen atoms of –OH groups and oxygen atoms within and between cellulose chains. (1 mark)
- Individually, these bonds are weak, but the large number of bonds collectively provides significant strength. (1 mark)
- Hydrogen bonds link cellulose molecules into microfibrils, which are further arranged into fibres. (1 mark)
- These fibres create a strong, rigid network in the plant cell wall. (1 mark)
- This strength prevents cell stretching or bursting under osmotic pressure, maintaining structural integrity. (1 mark)
Question 3
How does cellulose provide structural support in plants? (6 marks)
Mark Scheme:
- Cellulose forms microfibrils and fibres through hydrogen bonding between chains, creating a rigid structure. (1 mark)
- These fibres have high tensile strength, comparable to steel, preventing stretching under tension. (1 mark)
- The cell wall, composed of cellulose, maintains the shape of plant cells. (1 mark)
- It allows cells to withstand turgor pressure from osmosis without bursting. (1 mark)
- The layered arrangement of fibres in the cell wall provides rigidity to plant tissues. (1 mark)
- This support enables upright growth and structural integrity of the entire plant. (1 mark)
Question 4
Why is cellulose considered the most abundant organic molecule on Earth? (5 marks)
Mark Scheme:
- Cellulose is a major component of plant cell walls, present in all plant tissues. (1 mark)
- It is synthesized by plants in large quantities due to its structural role. (1 mark)
- Its high resistance to natural breakdown ensures its accumulation in the environment. (1 mark)
- Enzymes capable of digesting cellulose, like cellulase, are rare in nature, contributing to its abundance. (1 mark)
- The wide distribution of plants and the durability of cellulose fibres make it the most prevalent organic molecule. (1 mark)
Question 5
Compare the molecular structures of cellulose and amylose, highlighting their roles in plants. (6 marks)
Mark Scheme:
| Feature | Cellulose | Amylose |
|---|---|---|
| Glucose Type | β-glucose | α-glucose |
| Glycosidic Bonding | 1,4 bonds with 180° rotation of glucose | 1,4 bonds without 180° rotation |
| Shape | Straight, unbranched chains | Helical, unbranched chains |
| Role | Provides structural support in cell walls | Acts as an energy storage molecule in starch |
| Strength | High tensile strength due to hydrogen bonding | Flexible but compact for storage |
| Functionality | Resists tension, maintains cell shape | Stores energy for metabolic processes |
Question 6
How does the permeability of cellulose fibres benefit plant cells? (5 marks)
Mark Scheme:
- Cellulose fibres form a rigid yet porous network in the cell wall. (1 mark)
- The pores between fibres allow free movement of water and solutes. (1 mark)
- This permeability ensures that nutrients and waste products can pass in and out of cells. (1 mark)
- It facilitates processes like diffusion and osmosis, critical for cellular function. (1 mark)
- While permeable, cellulose maintains structural integrity, balancing strength with functional adaptability. (1 mark)
Question 7
Discuss how cellulose structure contributes to its resistance to enzymatic breakdown. (6 marks)
Mark Scheme:
- Cellulose is composed of β-glucose monomers linked by 1,4 glycosidic bonds with 180° rotation. (1 mark)
- This arrangement creates straight chains that form hydrogen-bonded microfibrils, making the structure highly stable. (1 mark)
- The dense packing of microfibrils limits enzyme access to individual cellulose chains. (1 mark)
- Only specific enzymes, like cellulases, can hydrolyze the β-1,4 glycosidic bonds. (1 mark)
- Most organisms lack cellulase, making cellulose resistant to digestion. (1 mark)
- This resistance contributes to its durability and widespread presence in nature. (1 mark)
Question 8
Why are cellulose fibres important for plant cell wall permeability and tensile strength? (6 marks)
Mark Scheme:
- Cellulose fibres provide tensile strength, preventing plant cells from bursting under osmotic pressure. (1 mark)
- The arrangement of fibres in multiple directions enhances cell wall rigidity. (1 mark)
- Despite their strength, cellulose fibres are freely permeable, allowing water and solutes to move in and out. (1 mark)
- This permeability supports nutrient uptake and waste removal. (1 mark)
- The balance of strength and permeability is crucial for maintaining plant cell shape and function. (1 mark)
- These properties enable plants to grow, expand, and remain upright. (1 mark)
Question 9
Explain how microfibrils and fibres contribute to the mechanical strength of cellulose. (6 marks)
Mark Scheme:
- Microfibrils are bundles of 60-70 cellulose molecules cross-linked by hydrogen bonds. (1 mark)
- These microfibrils aggregate into larger fibres, forming the structural framework of the cell wall. (1 mark)
- The multiple layers of fibres arranged in different directions enhance wall strength. (1 mark)
- Hydrogen bonding within and between chains provides significant tensile strength. (1 mark)
- This mechanical strength allows plant cells to resist external forces and internal osmotic pressure. (1 mark)
- The robust structure supports the plant and protects it from physical damage. (1 mark)
Question 10
How does cellulose enable plants to grow and maintain structural integrity? (5 marks)
Mark Scheme:
- Cellulose forms the primary component of the plant cell wall, providing rigidity and shape. (1 mark)
- The tensile strength of cellulose fibres allows cells to withstand internal turgor pressure. (1 mark)
- The permeability of cellulose facilitates nutrient and water movement, supporting cellular metabolism. (1 mark)
- The arrangement of cellulose fibres allows controlled cell expansion during growth. (1 mark)
- This balance of flexibility and strength supports plant development and structural integrity. (1 mark)