The cell wall has been an area of study since Robert Hooke first observed it in cork cells. It surrounds each plant cell, providing a rigid and protective layer.
The presence of a cell wall is one of the distinguishing features between plant and animal cells. Additionally, cell walls are present in bacteria and fungi, although their chemical composition differs.
Structure
The structure of the cell wall includes microfibrils, which are composed of elementary fibrils or micelles, consisting of about 20 to 22 microfibrils.
Numerous thread-like fibrils make up the wall, with each fibril consisting of approximately 250 microfibrils, serving as the main structural elements.
In most cases, It is primarily composed of cellulose. However, in fungi, it is made up of fungal cellulose or chitin, while in bacteria and blue-green algae, it is composed of peptidoglycan.
Within the cell wall, there is a gel-like substance called the matrix, where substances such as pectin and lignin are found.
It is composed of four layers-
- Middle lamella
- Primary wall
- Secondary wall
- Tertiary wall
Middle lamella
The middle lamella is formed by calcium and magnesium pectate, acting as a cementing material between adjacent cells.
During cell division, the equatorial plate aligns and forms the cell plate, leading to the formation of the middle lamella.
Primary wall
The primary wall is a soft, delicate, and elastic layer that is formed by the deposition of substances by the protoplasm onto the middle lamella.
It contains pectic substances, cellulose, and hemicellulose. Parenchymatous cells and meristematic cells are composed solely of the primary wall.
Secondary wall
After the primary wall is fully developed, the deposition of materials begins, resulting in the formation of the secondary wall. The secondary wall, which is thicker and non-elastic, is made up of cellulose, hemicellulose, and pectin.
Lignin, cutin, and suberin are commonly deposited in the secondary wall.
Tertiary wall
In some cells, such as tracheids in gymnosperms, there is an innermost layer called the tertiary wall, mainly composed of hemicellulose and xylan. The tertiary wall and secondary wall are collectively referred to as secondary thickenings.
Chemical Composition
It has a diverse chemical composition that includes various components. The approximate percentages of these components in the cell wall are as follows-
- Water (30-60%): It provides hydration and flexibility to the structure.
- Hemicellulose (5-15%): It is a complex carbohydrate that contributes to strength and integrity.
- Pectin substances (2-8%): They play a role in cell adhesion and help maintain the structure of plant tissues.
- Lipids (0.5-3%): They including waxes and cutin, are present in small amounts and contribute to the hydrophobic properties.
- Proteins (microfibrils: 20-40%, deposition: 0.0-25%): They are essential components of the cell wall, with microfibrils providing structural support and deposition proteins contributing to the deposition of materials during cell wall growth and repair.
Together, these components create a complex and dynamic chemical composition that gives the cell wall its unique properties.
Read about – Shape and Size of Cells
Plasmodesmata
Plasmodesmata are cytoplasmic bridges between adjacent cells that form a continuous system called the symplast. These plasmodesmata contain endoplasmic reticulum (ER) tubules known as desmotubules.
Strassberger discoverd plasmodesmata.
Growth
Its growth involves the action of a protein called expansin, which loosens the cellulose microfibrils and facilitates the addition of new cell wall material. This process is known as the cell wall loosening factor. Its growth occurs through two methods-
- Intussusception
- Apposition
Intussusception Growth
Intussusception growth refers to the process in which particles or materials are deposited between existing substances in the cell wall.
This type of growth occurs in all three layers of the cell wall: primary, secondary, and tertiary.
It involves the addition of new materials within the structure of the cell wall, contributing to its expansion and strengthening.
Apposition Growth
Apposition growth, also known as accretion growth, occurs when new layers of materials are deposited onto the existing layers of the cell wall.
This type of growth specifically takes place in the secondary cell wall.
The deposition of additional layers contributes to the thickness and reinforcement of the cell wall, enhancing its structural integrity and providing support to the plant cell.
Read about – Cell Principle: Understanding the Building Blocks of Life
Functions
It serves various functions, including-
1. Protecting the protoplasm
- Role of it as a physical barrier
- Defense against pathogens and mechanical stress
2. Providing shape and size to the cell
- Contribution of it to cell shape
- The exoskeleton-like function of the cell wall
3. Offering mechanical support
- Rigidity provided by the cell wall
- Support for upright growth and structural integrity
4. Transport and Exchange of Substances
- Permeability of the cell wall
- Facilitation of water and mineral transport
5. Involvement in Cellular Processes
(A) Nutrient Absorption and Uptake
- Role of it in nutrient uptake
- Importance in the absorption of water and minerals
(B) Transpiration and Water Movement
- Contribution of it to water movement
- Regulation of transpiration processes
(C) Substances Transport within the Plant
- Facilitation of substance transport between cells
- Role in the movement of hormones and signaling molecules
(D) Secretion of Substances
- Involvement of it in secretion processes
- Release of cell wall components and metabolites
It also plays a crucial role in processes such as absorption, transpiration, transport, and secretion.
Read about – Foundations of Cytology and Cell Theory
Pits
The cell wall exhibits non-uniform thickness, with certain regions lacking the deposition of a secondary wall. These unthickened areas are referred to as pits.
Pits of adjacent cells typically align opposite each other, forming pit pairs. Sclerenchyma, vessels, and tracheids contain pits. Among these, gymnosperms’ tracheids exhibit the highest quantity of bordered pits.
There are two types of pits:
(i) Simple pits: In these pits, the pit cavity maintains a consistent diameter.
(ii) Bordered pits: These pits have a flask-shaped cavity, commonly observed in tracheids.
Pits serve as important features within the cellular wall structure, facilitating communication and transportation between neighboring cells.
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