Growth of Cell Wall

The growth of the cell wall is a crucial process in plant cells that allows them to expand, divide, and differentiate while maintaining structural integrity. Cell wall growth occurs through controlled loosening, synthesis, and deposition of new materials.

Types of Cell Wall Growth

  1. Intussusception (Incorporation Growth)
    • New cell wall materials (cellulose, hemicellulose, pectins) are inserted within the existing wall.
    • The wall expands uniformly.
    • Common in primary cell walls of growing plant cells.
    • Example: Parenchyma cells in young tissues.
  2. Apposition (Layered Growth)
    • New layers of cell wall material are deposited on the inner surface of the existing wall.
    • Leads to thickening of the cell wall.
    • Common in secondary cell walls of mature cells.
    • Example: Xylem cells in woody plants.

Phases of Cell Wall Growth

1. Initiation Phase (Cell Wall Biosynthesis)

  • Begins during cytokinesis with the formation of the cell plate.
  • The phragmoplast helps in assembling new cell wall components.
  • Golgi apparatus produces vesicles containing pectins and hemicellulose.

2. Expansion Phase (Cell Wall Loosening and Extension)

  • Occurs in the primary cell wall during cell elongation.
  • Expansins and xyloglucan-modifying enzymes loosen the cell wall structure.
  • Turgor pressure helps push the wall outward.

3. Maturation Phase (Cell Wall Reinforcement)

  • New cellulose microfibrils are cross-linked with hemicellulose.
  • Lignin deposition occurs in secondary cell walls to provide rigidity.
  • The cell wall stops expanding when growth ceases.

Mechanisms of Cell Wall Expansion

  1. Turgor Pressure-driven Growth
    • Internal cell pressure pushes against the cell wall, forcing expansion.
    • Requires plasticity of the wall (regulated by expansins).
  2. Enzymatic Wall Loosening
    • Expansins weaken hydrogen bonds between cellulose and hemicellulose.
    • Xyloglucan Endotransglucosylase/Hydrolase (XTH) modifies cross-links.
  3. Microtubule-guided Deposition
    • Cortical microtubules direct the movement of cellulose synthase enzymes.
    • This ensures uniform cellulose microfibril arrangement.

Regulation of Cell Wall Growth

1. Hormonal Regulation

  • Auxin: Activates expansins, promoting cell elongation.
  • Gibberellins (GA): Stimulates synthesis of enzymes for wall loosening.
  • Brassinosteroids: Strengthens the cell wall by affecting cellulose production.

2. Genetic Control

  • Genes encoding cellulose synthase (CesA) regulate microfibril formation.
  • XTH genes control hemicellulose modifications.

3. Environmental Factors

  • Light: Phototropism influences localized cell wall growth.
  • Water availability: Affects turgor pressure and wall expansion.

MCQs on Cell Wall Growth

1-10: Basics of Cell Wall Growth

  1. Which of the following is the primary component responsible for cell wall growth? (CSIR NET 2018)
    a) Hemicellulose
    b) Lignin
    c) Cellulose
    d) Pectin
    ✔ Answer: c) Cellulose
  2. Which type of cell wall growth occurs by inserting new materials within the existing wall? (GATE 2019)
    a) Apposition
    b) Intussusception
    c) Lamellar growth
    d) Plasmodesmatal growth
    ✔ Answer: b) Intussusception
  3. Which organelle is responsible for synthesizing and transporting cell wall polysaccharides? (CSIR NET 2017)
    a) Ribosome
    b) Mitochondria
    c) Golgi apparatus
    d) Chloroplast
    ✔ Answer: c) Golgi apparatus
  4. Which phase of cell wall growth involves the deposition of new materials in layers? (ICAR JRF 2021)
    a) Intussusception
    b) Apposition
    c) Loosening
    d) Reinforcement
    ✔ Answer: b) Apposition
  5. Which enzyme plays a key role in loosening the plant cell wall for expansion? (CSIR NET 2019)
    a) Cellulase
    b) Expansin
    c) Amylase
    d) Peroxidase
    ✔ Answer: b) Expansin
  6. During cell elongation, which molecule helps in breaking bonds between cellulose and hemicellulose? (DBT JRF 2020)
    a) Xyloglucan
    b) Pectin
    c) Lignin
    d) Cutin
    ✔ Answer: a) Xyloglucan
  7. Turgor pressure-driven expansion of the cell wall requires which factor? (ICMR JRF 2019)
    a) Enzymatic softening of the wall
    b) Reduced vacuole size
    c) Increased lignin deposition
    d) ATP hydrolysis
    ✔ Answer: a) Enzymatic softening of the wall
  8. Which phase of cell wall growth involves secondary thickening? (GATE 2020)
    a) Expansion phase
    b) Initiation phase
    c) Maturation phase
    d) Cytokinesis phase
    ✔ Answer: c) Maturation phase
  9. Which microtubule structure directs cellulose deposition? (CSIR NET 2016)
    a) Spindle fibers
    b) Phragmoplast
    c) Cortical microtubules
    d) Centrosomes
    ✔ Answer: c) Cortical microtubules
  10. Which of the following is NOT a component of the plant cell wall? (DBT BET 2021)
    a) Cellulose
    b) Peptidoglycan
    c) Hemicellulose
    d) Pectin
    ✔ Answer: b) Peptidoglycan

11-20: Enzymes & Hormonal Regulation

  1. Which plant hormone is primarily responsible for cell wall loosening during elongation? (CSIR NET 2020)
    a) Auxin
    b) Cytokinin
    c) ABA
    d) Ethylene
    ✔ Answer: a) Auxin
  2. Which enzyme modifies hemicellulose linkages to allow cell wall expansion? (GATE 2017)
    a) Xyloglucan Endotransglycosylase (XTH)
    b) Laccase
    c) Peroxidase
    d) Amylase
    ✔ Answer: a) Xyloglucan Endotransglycosylase (XTH)
  3. Which plant hormone promotes secondary cell wall formation? (ICMR JRF 2018)
    a) Gibberellins
    b) Auxin
    c) Cytokinin
    d) Brassinosteroids
    ✔ Answer: d) Brassinosteroids
  4. Which structure is formed during cytokinesis and leads to cell wall synthesis? (CSIR NET 2015)
    a) Middle lamella
    b) Phragmoplast
    c) Plasmodesmata
    d) Xylem vessel
    ✔ Answer: b) Phragmoplast
  5. Which polysaccharide in the primary cell wall is responsible for hydration and flexibility? (DBT BET 2020)
    a) Pectin
    b) Lignin
    c) Xylan
    d) Chitin
    ✔ Answer: a) Pectin

21-30: Secondary Cell Wall & Mechanical Support

  1. Which polymer is mainly responsible for the rigidity of secondary cell walls? (ICAR NET 2019)
    a) Pectin
    b) Lignin
    c) Cutin
    d) Chitin
    ✔ Answer: b) Lignin
  2. In secondary cell wall growth, which layer is the thickest? (CSIR NET 2021)
    a) S1
    b) S2
    c) S3
    d) Middle lamella
    ✔ Answer: b) S2
  3. Lignin is primarily composed of which chemical units? (GATE 2022)
    a) Aromatic phenols
    b) Fatty acids
    c) Amino acids
    d) Simple sugars
    ✔ Answer: a) Aromatic phenols
  4. Cellulose synthase complex is located in which part of the cell? (CSIR NET 2014)
    a) Nucleus
    b) Golgi apparatus
    c) Plasma membrane
    d) Mitochondria
    ✔ Answer: c) Plasma membrane
  5. Which molecule forms the main structural network in the cell wall? (DBT JRF 2016)
    a) Chitin
    b) Cellulose
    c) Lignin
    d) Cutin
    ✔ Answer: b) Cellulose

21-32: Applied Concepts & Experimental Biology

  1. Which enzyme degrades cell wall pectins during fruit ripening? (ICAR JRF 2018)
    a) Pectinase
    b) Expansin
    c) Cellulase
    d) Laccase
    ✔ Answer: a) Pectinase
  2. Which factor influences cell wall expansion in response to gravity? (CSIR NET 2017)
    a) Gravitropism
    b) Hydrotropism
    c) Phototropism
    d) Electrotropism
    ✔ Answer: a)
  3. GravitropismWhich component is primarily responsible for cross-linking cellulose microfibrils in the plant cell wall?
    a) Lignin
    b) Pectin
    c) Hemicellulose
    d) Chitin
    ✔ Answer: c) Hemicellulose
    Reference: NEET 2019
  4. During cell wall synthesis, which organelle is responsible for the synthesis of matrix polysaccharides like pectins and hemicelluloses?
    a) Endoplasmic Reticulum
    b) Golgi Apparatus
    c) Mitochondria
    d) Chloroplast
    ✔ Answer: b) Golgi Apparatus
    Reference: CSIR NET 2017
  5. Which enzyme is crucial for the biosynthesis of cellulose in plant cell walls?
    a) Cellulase
    b) Cellulose Synthase
    c) Pectinase
    d) Lignin Peroxidase
    ✔ Answer: b) Cellulose Synthase
    Reference: GATE 2018
  6. The rigidity and impermeability of the secondary cell wall are primarily due to the deposition of which substance?
    a) Suberin
    b) Cutin
    c) Lignin
    d) Chitin
    ✔ Answer: c) Lignin
    Reference: ICAR JRF 2020
  7. Which process describes the incorporation of new cell wall materials into existing walls during cell expansion?
    a) Apposition
    b) Intussusception
    c) Aggregation
    d) Fusion
    ✔ Answer: b) Intussusception
    Reference: CSIR NET 2019

  8. Which plant hormone is primarily associated with the loosening of the cell wall to facilitate cell expansion?
    a) Cytokinin
    b) Gibberellin
    c) Auxin
    d) Abscisic Acid
    ✔ Answer: c) Auxin
    Reference: NEET 2018
  9. The protein expansin plays a critical role in which aspect of cell wall dynamics?
    a) Synthesis of cellulose
    b) Cross-linking of lignin
    c) Loosening of cell wall structure
    d) Degradation of pectin
    ✔ Answer: c) Loosening of cell wall structure
    Reference: CSIR NET 2020
  10. Which structural feature allows plant cells to communicate and transport substances between adjacent cells?
    a) Plasmodesmata
    b) Middle Lamella
    c) Tight Junctions
    d) Desmosomes
    ✔ Answer: a) Plasmodesmata
    Reference: GATE 2019
  11. In the context of cell wall growth, what is the primary role of the middle lamella?
    a) Providing rigidity to individual cells
    b) Acting as a barrier to pathogens
    c) Cementing adjacent cells together
    d) Facilitating cell elongation
    ✔ Answer: c) Cementing adjacent cells together
    Reference: ICAR JRF 2019
  12. Which phase of cell growth involves the synthesis of new cell wall materials to accommodate cell enlargement?
    a) Lag Phase
    b) Log Phase
    c) Stationary Phase
    d) Maturation Phase
    ✔ Answer: b) Log Phase
    Reference: CSIR NET 2018

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