Cell Wall: Structure and Functions: Plants are unique organisms, they cannot walk, yet they can obtain everything essential for life at the point of their origin. Whether it’s a hot or cold or rainy season, plants can resist all of this out in the open. How does a plant survive extreme conditions? The answer is the cell wall. The cell wall is a complex, non-living structure designed to perform various functions, including providing strength and protection to plant cells. Read along to learn in detail about cell walls, their composition, structure, and function.

What is a Cell Wall?

The cell wall is a non-living, rigid, and permeable structure surrounding the plasma membrane. It is found in most plants, bacteria, fungi, and algae.  Animals and protozoans lack this rigid structure. The composition of cell walls varies greatly among bacteria, fungi, plants, and algae. Depending on the cell type,  a cell wall may contain different types of polysaccharides, proteins, pigments, enzymes, and aromatics, which contribute to its multi-layered structure.

Plant Cell Wall

Composition: Cellulose, hemicelluloses, and pectin. 

Cellulose is the main content of the plant cell wall, which is the most abundant macromolecule on Earth. Cellulose is a fibrous polysaccharide made up of long, linear polymers of glucose molecules. Precursors of cell wall components are synthesized inside the cell and then assembled by enzymes associated with the cell membrane. Another component of plant cell walls includes lignin, suberin, or cutin.

Fig: Structure of Plant Cellulose

Structure: Plant cell walls are made up of three layers, namely a middle lamella, primary cell wall, and secondary cell wall, essentially in the same order as we move the outermost layer to the plasma membrane. 

Fig: Layers of Cell Wall in Plant

Middle lamella: It is a thin, amorphous, cementing layer present between the primary cell wall of neighbouring cells. This is the first layer formed during the cytokinesis of cell division. The equatorial plate formed during cytokinesis is deposited with pectin and phragmosomes. The equatorial plate becomes thick at the corners and narrows around the rest of the places as cells mature. It is made up of calcium pectate and magnesium pectate.

Primary Cell Wall: The primary cell wall develops in a young cell between the middle lamella and plasma membrane. This layer is thin and flexible and made up of cellulose microfibrils. These microfibrils are synthesised and secreted by the plasma membrane. Other constituents include hemicelluloses, which binds microfibrils to cell wall matrix and pectin. This layer is capable of growth, but it starts to diminish once the cell matures. Cells found in leaves, flowers, fruit, and cortex possess only the primary cell wall.

Secondary Cell Wall: The secondary cell wall is found only in mature cells like tracheids, vessels, fibres, etc. It is present between the primary cell wall and the plasma membrane. It is much thicker and made up of three layers named S₁, S_2,, and S₃. Different depositions of these three layers cause differences between parenchyma, collenchyma, and sclerenchyma. The secondary cell wall is often derived from the primary cell wall. The composition of the secondary cell wall is similar to that of the primary cell wall, i.e., cellulose and hemicellulose.

Other substances like lignin, suberin, and cutin deposition may be found on cell walls depending upon cell type and function.

Fig: Layers of the Cell Wall

Lignin is a polymer of aromatic alcohol that provides additional support for the plant. It helps protect the plant from attacks by insects or pathogens. Lignin also helps with water transport in the cells. Suberin makes cell walls impervious to air and water. Cutin reduces water loss from the surface of the plant.

Difference between Primary and Secondary Cell Wall

Primary Cell Wall	Secondary Cell Wall
It is found in growing cells.	It is found in matured cells.
It is thin and flexible.	It is thick and rigid.
It remains associated with the plasma membrane.	It remains associated with the primary wall.
It has a single layer.	It has multiple layers.
It has low cellulose content.	It has high cellulose content.
Pits are usually absent in the primary wall.	Pits are found in the secondary walls.
Components like lignin and suberin are absent.	Components like lignin and suberin are present.

Plasmodesmata: These are small channels present through the cell wall that directly connect the cytoplasm of neighbouring cells. Plasmodesmata is surrounded by the plasma membrane. It has an endomembrane derivative called desmotubules. These channels may be simple or branched.

Pits: Pits are found on the secondary cell walls. Pits form due to the uneven thickness of the cell wall. They perform a major role in transportation.

Cell Wall Function

1. The cell wall provides mechanical strength and support.
2. The cell wall prevents mechanical injury.
3. The cell wall helps a cell withstand turgor pressure.
4. The cell wall helps to maintain the osmolarity of the cell.
5. The cell wall has a role in cell growth regulation.
6. Pits help in rapid translocation between two adjacent cells.
7. The cell wall provides a barrier against pathogens present in the soil.
8. Cutin and suberin help to prevent water loss.
9. Enzymes associated with the cell wall help in metabolism.
10. The cell wall stores carbohydrates in the seed.
11. Cell to cell communication: Plant cells communicate via plasmodesmata. Plants exchange material between each other through plasmodesmata. Symplast transportation occurs through the plasmodesmata.
12. The cell wall of the xylem and phloem components are specially modified for long-distance transportation.

Bacterial Cell Wall

Composition: Peptidoglycans.

Structure: Peptidoglycan or murein is a mesh of polysaccharide strands composed of a poly-[N-acetylglucosamine (GlcNAc)-N-acetylmuramic acid (MurNAc)] backbone crosslinked via short peptide bridges. Bacteria can be classified into gram-positive and gram-negative based on the colour of bacteria after the process of gram staining.  Gram stain reacts with the peptidoglycans of the cell wall to produce the colour. 

Gram-positive bacteria have thick cell walls made from layers of peptidoglycans. However, gram-negative bacteria have a thin layer of peptidoglycans along with an additional layer of lipopolysaccharides (LPS). LPS serve as an endotoxin which makes them tougher and harder to kill. Gram-negative bacteria have an inner and outer plasma membrane, and the thin cell walls are in between the membranes.

Fig: Composition of Cell Wall of Gram-Positive and Gram-Negative bacteria

Function:

1. It helps the cell maintain its shape and structure.
2. It provides antibiotic resistance to some bacteria.

Fungal Cell Wall

Composition: chitin, glucans, and proteins

Structure: In general, the cell walls of fungi have three layers: chitin, glucans, and proteins. The innermost layer is made up of chitin, a fibrous polysaccharide. It provides rigidity and strength to the cell wall. The second layer is made up of α- and β- linked glucans. Glucans are glucose polymers that are present in crosslinking with chitin. The glucans also provide rigidity. The third layer comprises glycoproteins called the mannoproteins or mannans that possess a high level of mannose sugar. The cell wall also has enzymes and structural proteins, and pigment molecules.

Fig: Structure of Fungal Cell Wall

Function:

1. The fungal cell wall provides strength and rigidity to fungus.
2. Enzymes present in the cell wall helps in the digestion of organic molecules.
3. Some protein helps in attachment with the substratum.

Algal Cell Wall

Composition: composition of the cell wall in algae varies greatly. Mostly cellulose, Galatians and mannans, and some other glycoproteins are found in algae.

Structure: Most of the algal cell walls possess sulfonated polysaccharides. The cell walls of algae have mannans, xylans, alginic acid, and sulfonated polysaccharides. Mannans are fibrous proteins found in some green and red algae. Xylans are complex polysaccharides. Alginic acid is mostly found in brown algae. Diatoms or golden algae have unique cell walls with silica. Diatoms are found mostly in the aquatic environment, both marine and freshwater.

Fig: Microscopic Image of Cell wall of Diatoms

Function:

1. Provide strength and rigidity to algal cells.
2. The dead remains of diatoms from diatomaceous Earth are a major source of silica and are used in many industries like medical and chemicals.
3. The algal cell wall of red algae is the source of agar, a solidifying agent.

Summary

The cell wall is a protective layer present around bacterial, fungal, algal, and plant cells. The cell wall of plant cells is made up of cellulose, hemicellulose, and pectin. The cell wall of a plant cell comprises three layers: middle lamella, which is made up of calcium and magnesium pectate. The primary cell wall is thin and flexible, made up of cellulose. The secondary cell wall is made up of cellulose and lignin etc. Cell wall performs various functions in a cell. It provides protection and strength to the cell.

Plasmodesmata present between two neighbouring cells helps in cell-to-cell communication. The cell wall helps to provide turgidity to the cell. Bacteria can be categorised into gram-positive and gram-negative based on the composition of the cell wall. Gram-positive bacteria have three layers of peptidoglycan, whereas gram-negative bacteria have a single layer of peptidoglycan and an additional layer of lipopolysaccharide. The fungal cell wall is made up of chitin mainly, but it may also contain enzymes and pigments. The algal cell wall is made up of cellulose, mannans, etc. The cell wall of diatoms contains silica as well.

Frequently Asked Questions (FAQs) on Cell Wall

Q.1. What is lignification?
Ans: Lignification is the polymerization of lignin on cell walls to make it harder and make the plant disease resistant.

Q.2. Why do plants need cell walls?
Ans: Plant cells need cell walls for protection, strength, turgidity, and osmolarity.

Q.3. Who discovered the cell wall?
Ans: Robert Hooke observed it for the first time and called it the “wall” surrounding the cell.

Q.4. Differentiate between gram-positive bacteria and gram-negative bacteria based on their cell wall characteristics?
Ans: Gram-positive bacteria have three layers of peptidoglycan, whereas gram-negative bacteria have a single layer of peptidoglycan with an additional layer of lipopolysaccharide.

Q.5. Name the organism that lacks a cell wall.
Ans: Animals and Protozoa lack a cell wall around the plasma membrane.

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