Cells have unique features developed to maintain with cellular organization.
All prokaryotic cells predominantly have a rigid cell wall that provides mechanical and structural support to the cell. Except animal cells, bacteria, plant cells and fungi possess cell wall. Peptidoglycan subunits are the backbone of the cell wall.
Some key functions of cell wall
- Determines the shape, structure and maintains integrity of the cell
- Provides protection from external toxins
- Protects from mechanical stress
- Maintains internal turgor pressure
- Helps in cell-cell communication
- Acts as the interface between internal cellular space and external environment
Do prokaryotes have a cell wall and membrane?
Prokaryotic cells along with plant cells and fungi have cell wall and cell membrane.
Yes, certainly prokaryotes have cell membranes and cell walls. The cell membrane is also known as the plasma membrane which is particularly important as it performs multifaceted roles. Contrastingly the cell wall lies outside the cell membrane, usually a rigid structure encompassing the prokaryotic cells.
The cell membrane retains the cytoplasm and forms a selectively permeable layer that allows only specific ions and molecules to pass through it. Additionally, characteristic proteinaceous structures are embodied in the cell membrane which acts as receptors and responds to external stimuli.
Apart from cell-cell communication, the cell membrane is also the site for various metabolic activities including respiration, photosynthesis, and synthesis of various biomolecules.
The basic elements of a prokaryotic cell
The cell wall plays a crucial role in prokaryotic cells including it provides the shape of the cell, protecting cells from osmotic lysis, protecting cells from toxic substances and other pathogens, and lastly, it also contributes to pathogenicity.
What do the prokaryotic cell walls contain?
The most studied prokaryotic cell wall is of eubacteria, based on its response towards the Gram-stain, the whole bacterial domain is divided into two parts. Gram-positive bacteria contain characteristic thick-walled cell walls in comparison to gram-negative bacteria.
The cell wall of gram-positive bacteria contains a 20 to 80 nm thick peptidoglycan (murein) layer lying outside the cell membrane. Peptidoglycan is a polymer of two sugar derivatives – N-acetylglucosamine and N-acetylmuramic acid present alternatively cross-linked by peptide bonds.
D-glutamic acid, D-alanine, and meso-diaminopimelic acid (some bacteria replace with L-lysine) are usually not found in proteins and D-amino acids provide protection against peptidase degradation.
Gram-positive bacterial cell wall composition
To the enormous peptidoglycan layer, residues of teichoic acids are covalently attached. These are polymers of glycerol or ribitol linked by phosphate groups. Amino acids are attached to the glycerol and ribitol subunits.
The cell walls of gram-negative bacteria are uniquely different from gram-positive bacteria as they contain a thin peptidoglycan layer followed by periplasmic space which constitutes 20-40% of the total cell volume. Gram-negative bacterial peptidoglycan layer contains a unique protein – the Braun’s lipoprotein that is adhered covalently to the cell wall.
Additionally, to the cell wall and periplasmic space, gram-negative bacteria contain an unusual outer membrane composed of lipids and carbohydrates – the lipopolysaccharide (LPSs).
Gram-negative bacterial cell wall composition
Where cell wall found in prokaryotic cell?
Cell wall is the integral part of unicellular prokaryotes.
The cell wall is the outermost external layer of the prokaryotic cell, present next to the plasma membrane. However, there are some extra membranes such as capsules present next to the cell wall. This layer provides the cell to retain its shape and integrity under any environmental conditions.
How is a prokaryotic cell wall different from a eukaryotic cell?
The prokaryotic cell wall is greatly different from eukaryotic cell wall. The composition of both the cell walls are vastly different. The striking differences of both the cell walls are listed below.
| Prokaryotic cell wall | Eukaryotic cell wall | |
| Occurrence | Most prokaryotic cells have a cell wall. | Absent in vertebrates, present only in plant cells and fungal cells |
| Thickness | 20-80 nm for gram-positive and 10 nm for gram-negative | ~ 20nm in thickness |
| Chemical composition | Bacterial cell walls contain polymers of NAG (N-acetyl glucosamine) and NAM (N-acetyl muramic acid) crosslinked by peptide bonds that constitute the peptidoglycan layer. | The cell wall of plant cells is composed of cellulose while fungal cells contain chitin or fungal cellulose (a polymer of N-acetyl glucosamine) |
| Structure | Most bacterial cells contain a cell membrane followed by a layer of the peptidoglycan layer. In addition to the peptidoglycan layer, a few more layers are also present. | The plant cell contains a primary cell wall followed by middle lamellae. Sometimes there is a secondary cell wall also. |
Is the prokaryotic cell wall rigid?
The cell wall are fairly rigid in nature due to the presence of extensive cross linking (either peptide or glycosidic bond depends upon the kind of cell) across the wall.
Yes, the prokaryotic cell wall is a stiff and rigid protective layer. It is composed of an enormous mesh of polymers of peptidoglycan. The backbone of this polymer consists of alternating units of N-acetylglucosamine and N-acetylmuramic acid cross-linked by peptide bonds.
Do all prokaryotic cells have a cell wall?
Not all prokaryotes have a cell wall.
Prokaryotic cells predominately have cell walls. However there are some exceptions. Mycoplasma is a the smallest bacteria (about 150- 250 nm size) that lacks the peptidoglycan layer. Due to this they are present in various shapes.
Which prokaryotic cells have no cell wall?
There are some exceptions, few prokaryotic members lack this external rigid layer.
Mycoplasma is a group of fastidious bacteria, that usually remain as commensals or parasites of humans, animals, and plants. These are the smallest prokaryotic cells about 0,2 -0.3 µm in diameter. They lack a cell wall. They are only surrounded by the plasma membrane and present in various shapes – pleomorphic in nature.
Do archaea have a cell wall?
Archaeal cell wall structure is uniquely different from either gram-positive or gram-negative bacteria.
Yes, archaea do contain a cell wall. The archaeal cell wall lacks a peptidoglycan layer. The cell wall consists of heteropolysaccharides – pseudomurein in place of peptidoglycan, which is a polymer of L-amino acids, unlike D-amino acids that are present in the bacterial cell wall.
The cell wall contains N-acetyltalosaminuronic acid instead of N-acetylmuramic acid and N-acetylglucosamine, crosslinked with β (1→3) glycosidic bonds along with glycoproteins.
Membrane structures of archaeal and bacterial cell wall
1. archaeal phospholipid- isoprene chains; 2. ether linkages; 3. L-glycerol moiety; 4. phosphate group; 5. a bacterial or eukaryotic phospholipid – fatty acid chains; 6. ester linkages; 7. D-glycerol moiety; and 8. phosphate group
The archaeal cell membrane can be bilayer like bacterial and eukaryotic ones, however, sometimes it can be monolayer. The cell membrane contains branched-chain hydrocarbons (20 carbons in a chain – diether hydrocarbon while 40 carbons in a chain – tetraether) linked by an ether bond to the glycerol groups. The archaeal cell wall also contains nonpolar lipids – squalene and tetrahydro-squalene.
Do cyanobacteria have a cell wall?
Similar to other prokaryote members, cyanobacteria have complex membranous structure.
Yes, cyanobacteria also possess a distinct cell wall enclosed by a layer of mucilaginous sheath. The cyanobacterial cell wall contains five specific chemical substances – three of which are amino acids and the rest of the two are sugar residues viz., glucosamine, and muramic acid. The cyanobacterial cell wall is a complex structure with multiple layers.
There are four layers in the cyanobacterial cell wall that are designated as LI, LII, LIII, and LIV.
LI layer-
It is the innermost layer of 3-10 nm thickness. This transparent layer lies next to the plasma membrane.
LII layer
This layer is composed of mucopeptide and muramic acid, along with glucosamine, alanine, glutamic acid, and di-amino-pamelic acid. This layer provides mechanical support to the cell wall and maintains the cell shape. The thickness of the layer ranges from 10 to 1000 nm.
LIII layer
This layer is similar to layer LI.
LIV
This is the outermost layer that appears wrinkled, and undulating. It is a thin and electron-dense layer.
All four layers are interconnected by plasmodesmata. There are numerous pores present in the cell wall. These pores act as a passage for the secretion of mucilage by the cell.
A typical cyanobacterial cell
Do bacteria have cell walls?
Bacteria are an important class of prokaryotic micro organisms that certainly have cell wall.
Yes, bacteria have an extra layer outside the plasma membrane – the cell wall. Both gram-positive and gram-negative bacteria possess this additional layer. The layer consists of peptidoglycan that provides rigidity to the bacterial cell wall. It serves as a barrier between internal cellular space and the external environment. While there are some exceptions, Mycoplasma are cell wall- less bacteria.
Conclusion
Cell wall is an integral part of the cell that retains its shape and protects the cell from external injury. Since prokaryotic cells have simplest cellular organization, there cell wall becomes the indispensable part of it. Cell walls are composed of different polymers either peptidoglycan (prokaryotes) or cellulose (plants) or chitin (fungi) forms a rigid structure.
Also Read:
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- Nucleic acid
- Active transport examples
- Food vacuole in plant cell
- Types of rna splicing
- Do animal cells have cytoskeleton
- Abdomen anatomy
- Isotonic solution example
- Errors in dna replication
- Hydrophilic protein example
I am a doctoral student of CSIR- CIMAP, Lucknow. I am devoted to the field of plant metabolomics and environmental science. I have completed my post-graduation from the University of Calcutta with expertise in Molecular Plant Biology and Nanotechnology. I am an ardent reader and incessantly developing concepts in every niche of biological sciences. I have published research articles in peer-reviewed journals of Elsevier and Springer. Apart from academic interests, I am also passionate about creative things such as photography and learning new languages.
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