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A plant cell differs from that of an animal cell due to the presence of an extra layer on top of the cell membrane called the cell wall.
- This cell wall of plants consists of the primary cell wall and the secondary cell wall.
- The primary cell wall is the cellulose-containing layer that is capable of growth and gradually diminishes with cell maturity.
- The secondary cell wall is formed on the inner side of the primary cell wall once the cell is completely grown.
- The primary cell wall majorly contributes to the structural and supportive role of cells.
| Table of Content |
Key Terms: Cell wall, Primary cell wall, Secondary cell wall, Polysaccharides, Glycoproteins, Cellulose, Cell membrane, Plant cell
What is Primary Cell Wall?
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The primary cell wall is a cellulose-containing layer composed of polysaccharides and smaller proportions of glycoproteins.
- It is a strong yet thin and flexible wall layer capable of plastic and elastic expansion.
- Primary cell walls are of great tensile strength because of the presence of cellulose fibers.
- They are present in between the middle lamella and secondary cell walls.
- The primary cell wall of the plant cuticle is usually impregnated with cutin and wax resulting in a permeability barrier for the plant cuticle.
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Primary Cell Wall Structure
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In plant cells, the primary wall is composed of cellulose, pectin, and hemicellulose and is primarily involved in the division and growth of cells.
- Apart from plant cells, primary cell walls are also present in fungi, algae, and most prokaryotes.
- In between primary cell walls, the middle lamella which is a pectin-rich intercellular material glues the adjacent cells together.
Primary Cell Wall Structure
Cell Wall Composition in Different Organisms
The composition of cell walls in some different organisms are
- The Gram-positive bacteria consist of many layers of peptidoglycan and teichoic acids which make the cell wall.
- The cell walls of Gram-negative bacteria consist of a few layers of peptidoglycan.
- In archaea, the cell wall is composed of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides.
- In fungi, the cell wall is made up of chitin and other polysaccharides.
- The cell wall in algae is made up of glycoproteins and polysaccharides.
Primary And Secondary Cell Wall
Difference Between Primary Cell Wall And Secondary Cell Wall
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All cell walls consist of three layers, the primary wall, the secondary wall, and the middle lamella. In due course, the primary walls disappear leading to the production of the secondary wall.
| Primary wall | Secondary wall |
|---|---|
| The primary wall is located inner to the middle lamella | The secondary wall is located inner to the primary wall |
| Young growing cells have primary wall | Secondary walls are produced when cells have stopped growing |
| Primary walls are open for expansion | Secondary walls are resistant to expansion |
| It is single-layered | It is usually three or multilayered |
| The primary wall is relatively thin | The secondary wall is thick-walled |
| Hydration is 60% | Hydration is 30-40% |
| Cellulose content is available in a relatively low amount | Cellulose content is available in a relatively high amount |
| Cellulose microfibrils are shorter, wavy, and slackly arranged | Cellulose microfibrils are longer, strongly arranged, straight, and parallel |
| Protein content is relatively high up to 5% | Protein content is relatively low 1% or less |
| Hemicellulose content is up to 50% | Hemicellulose content is up to 25% |
| Lipid content is up to 5-10% | Lipid content is negligible |
Functions of Primary Cell Wall
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The following are the functions of the primary cell wall
- It provides structural and mechanical support to plant cells.
- The primary cell walls surround and protect the inner cell.
- It provides wall structural integrity, signal transduction, and cell adhesion.
- It contributes to the increase of extensibility of the cell wall.
- It protects against pathogens, dehydration, and other environmental factors.
Features of Primary Cell Wall
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Some features of the primary cell wall are
- The primary cell wall is a thin and flexible layer of the cell wall capable of both plastic and elastic extension.
- It is composed of pectin, hemicellulose (especially, xyloglucan), and cellulose.
- The primary cell wall contains more pectin compared to the secondary cell wall.
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Things to Remember
- The primary cell wall is a cellulose-containing layer, capable of growth and gradually diminishes with cell maturity.
- It is located above the plasma membrane and below the middle lamella.
- The composition of the primary cell wall consists of polysaccharides, smaller proportions of glycoproteins along noncarbohydrate substances.
- The primary cell wall contributes to the rate and direction of cell expansion.
- Due to their thin and flexible nature, primary cell walls are capable of providing structural and supportive roles only.
Sample Questions
Ques. Which enzymes are present in the primary cell wall? (2 Marks)
Ans. Primary cell walls consist of a variety of proteins. The most notable ones are Acetylesterase, Endopeptidases, XTH(with XET activity), XTH (with XEH activity), Peroxidase, etc.
Ques. Which compound is the main constituent of the primary cell wall? (2 Marks)
Ans. The principal compound that constitutes the primary cell wall is cellulose, a complex carbohydrate made up of several thousand glucose molecules linked end to end.
Ques. What are the major carbohydrates present in the primary cell wall? (2 Marks)
Ans. The primary cell wall consists of the following major carbohydrates:
- Cellulose
- Hemicellulose
- Pectin
Ques. How are primary cell walls formed? (2 Marks)
Ans. Cell wall biosynthesis leads to the generation of a cell plate in the middle of the cell during the cytokinesis phase of cell division. This, in turn, generates primary cell walls by the deposition of polymers of cellulose, hemicelluloses, and pectin.
Ques. What is a Cell Wall? (1 Mark)
Ans. A cell wall is a rigid, exterior layer that has been designed to give structural support and rigidity. It also protects the cell's internal components from the external environment.
Ques. What are the 3 layers of the cell wall? (1 Mark)
Ans. A typical plant cell wall consists of three layers: the primary cell wall, the secondary cell wall, and the middle lamella.
Ques. Where are pits in the primary cell wall? (2 Marks)
Ans. Pits are thin portions of the cell wall that offer interruption in the primary cell wall which the plasmodesmata can cross. They aid in the communication of adjacent cells or the exchange of fluid between them.
Ques. What are the functions of microfibrils in the primary cell wall? (3 Marks)
Ans. Inside primary cell walls, microfibrils are a group of glucose polymer cellulose, which control cell expansion which in turn play a significant role in the growth and morphogenesis of plants. These microfibrils are a few nanometers in diameter, composed of cellulose which are arranged in orthogonal layers.
Ques. Describe the growth mechanism of the primary cell wall. (3 Marks)
Ans. The acid growth mechanism contributes to the expansion of the primary cell wall whereby plant cell wall elongation takes place at low pH. In this process, a protein named Expansin loosens the connections between the cellulose microfibrils within a cell wall which in turn increases the cell volume by turgor and osmosis.
Ques. What is the plant cuticle? (2 Marks)
Ans. The plant cuticle is the outer part of the primary cell wall mostly impregnated with cutin and wax offers protection to the epidermis of leaves and young shoots of plants. Thus the primary cell wall offers a permeability barrier for the plant cuticle.
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| NCERT Class 12 Biology Book | NCERT Solutions for Class 11 Biology | Biology MCQs |
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