Key Components of the Cell Envelope and Their Roles
The protective outermost cover of prokaryotes is called a cell envelope. A cell envelope involves a cell wall, cell membrane and in some cases, outer membrane. In simple words, it is a union of an inner cell and a wall of bacteria cells that surrounds the cell.
This thick layer provides the structural rigidity of a cell. Prokaryotes usually inhabit a dilute atmosphere. Therefore the concentration of inner macromolecules is higher than the outer part. So, it acts as a single protective unit of a bacterium cell. The cytoplasmic membrane, which divides the inside of the cell from its external environment, regulates the flow of nutrients, maintains the right intracellular milieu, and prevents the loss of the cell's contents, is the only trait shared by all cells. Many essential cellular processes are performed by the cytoplasmic membrane, including energy synthesis, protein secretion, chromosomal segregation, and efficient active transport of nutrients. It's a typical unit membrane made up of proteins and lipids, and it's quite similar to the membrane that encircles all eukaryotic cells. It appears as a triple-layered structure of lipids and proteins that fully surrounds the cytoplasm in electron micrographs.
Classification of Cell Envelope
Based on gram staining, bacteria are classified into two parts.
Gram-Positive
This group of bacteria has a thick Peptidoglycan layer that prevents the crystal purple dyes from leaking out while the gram staining process takes place.
They also contain Lipoteichoic and Teichoic acids. Teichoic acid is responsible for giving a negative charge to gram-positive bacteria because this acid shares a Phospodeister bond with monomers.
Gram-Negative
Unlike gram-positive, these bacteria have a thin Peptidoglycan layer. Because of this structure, this layer cannot contain the crystal purple dye. It means they can be easily washed off when the gram staining process occurs.
Gram-negative bacterial cell envelope consists of phospholipids and lipopolysaccharides. The latter provides a negative charge to gram-negative bacteria. It also safeguards the bacteria from certain drugs.
Mycobacteria
Apart from these two types of bacteria, there is another one that is entirely different from the two. The cell envelope of these bacteria does not possess the outer membrane. Instead, it has an exclusive cell wall made of arabinogalactan, peptidoglycan and mycolic acid that acts as an external barrier.
Bacteria without a Cell Wall composed of Peptidoglycans
Bacteria without a cell wall are composed of peptidoglycans. The obligate intracellular bacteria in the Chlamydiaceae family have a peculiar morphology in that their infectious forms have no detectable levels of peptidoglycan in their cell walls. Instead, the extracellular forms of these gram-negative bacteria rely on a layer of disulfide bond cross-linked cysteine-rich proteins situated between the cytoplasmic and outer membranes, similar to the peptidoglycan layer in other gram-negative bacteria, to preserve structural integrity. The disulfide cross bond is absent in the bacterium's internal forms, making this form more mechanically brittle.
The bacterial class mollicutes have no cell walls in their cell envelopes. Mycoplasma and ureaplasma are the most common pathogenic bacteria in this group. L-form bacteria are strains of bacteria that lack cell walls yet are produced from bacteria that do.
Components of Cell Envelope
The bacterium cell wall consists of mainly three layers, the glycocalyx, cell wall and lastly plasma membrane. The composition of a glycocalyx varies for different types of bacteria. The loose sheath-like one is called a slime layer, and the tough and thick one is called a capsule. Also, the Plasma membrane is a thin and semi-permeable layer that contracts with the outer part of the cell. The composition of plasma membranes remains the same even for eukaryotes.
It consists of another membrane-like structure, the mesosome. Mesosome is the extended part of this plasma membrane. They help in DNA replication, the formation of cell walls and daughter cell distribution.
Another component of the bacterial cell wall is a pill, a long structure made of pilin protein. Also, there is another small bristle-like fibre, fimbriae. It helps the bacteria to attach to different surfaces.
Bacterium cells also comprise a varied arrangement of flagella. More specifically, the bacterial flagellum is divided into three sections, filament, hook and the basal body.
The most gram-positive bacteria cell wall consists of a cell wall, a cytoplasmic membrane and a capsule.
The most gram-negative bacteria cell wall consists of a cell wall, a cytoplasmic-membrane, outer membrane, periplasmic space and capsule.
Purpose of Cell Envelope
The bacterial cell wall conducts a series of functions.
An essential function of it is to withstand the hydrostatic pressure of cells.
Periplasmic space between the plasma membrane and cell wall helps the gram-positive bacteria to digest things.
This cell wall is porous, thus helping the large molecules to move in and out of cells.
It helps in protein secretion, transportation of nutrients, etc.
Multiple Choice Questions
1. Which one lacks a cell wall?
Bacterial cells
Plant cells
Protozoa
Algae
2. What is S-layer?
Surface layer
Secondary layer
Soluble layer
Solid layer
Answers: 1-c), 2-a)
Fill in the Blanks
1. Cord factor is a _______________ derivation.
2. Glycerol residues present in ______________ acids.
Answers: 1-mycolic acid, 2-Teichoic.
Conclusion
A cell envelope is a union of an inner cell and a wall of bacteria cells. It contains the cytoplasmic membrane, which divides the inside of the cell from its external environment. It regulates the flow of nutrients, maintains the right intracellular milieu, and prevents the loss of contents.
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FAQs on Cell Envelope: Definition, Structure, Functions
1. What exactly is the cell envelope in a prokaryotic cell?
The cell envelope is the complex, multi-layered outer covering of a prokaryotic cell, such as a bacterium. It acts as a protective boundary between the cell's interior and the external environment. This structure is not a single layer but a combination of several distinct layers working together to maintain the cell's integrity and function.
2. What are the main components that make up the bacterial cell envelope?
The bacterial cell envelope typically consists of three main components, arranged from the outside in:
Glycocalyx: The outermost layer, which can be a loose slime layer or a thick, tough capsule. It aids in protection and adhesion.
Cell Wall: A rigid layer located beneath the glycocalyx that provides structural support and determines the cell's shape. Its composition is the basis for Gram staining.
Plasma Membrane: The innermost layer, a selectively permeable membrane that controls the passage of substances into and out of the cell.
3. What are the primary functions of the cell envelope in bacteria?
The cell envelope performs several critical functions for bacterial survival, including:
Protection: It shields the cell from physical damage, chemical toxins, and osmotic lysis (bursting due to water intake).
Structural Support: The rigid cell wall component maintains the characteristic shape of the bacterium (e.g., coccus, bacillus).
Regulated Transport: The plasma membrane controls which molecules enter and leave the cytoplasm.
Cell-to-Cell Interaction: It facilitates attachment to surfaces and communication with other cells, often leading to biofilm formation.
4. What is the purpose of Gram staining in relation to the cell envelope?
Gram staining is a differential staining technique used to classify bacteria into two main groups: Gram-positive and Gram-negative. The outcome of the stain depends entirely on the structural differences in their cell walls, a key part of the cell envelope. It helps microbiologists identify bacteria and predict their susceptibility to certain antibiotics.
5. How does the cell envelope of a Gram-positive bacterium differ from a Gram-negative one?
The primary difference lies in the composition of the cell wall. A Gram-positive bacterium has a very thick layer of peptidoglycan in its cell wall, which retains the crystal violet stain, making it appear purple. In contrast, a Gram-negative bacterium has a very thin layer of peptidoglycan and an additional outer membrane. This outer membrane prevents the stain from being retained, so the cell is counterstained pink or red.
6. Why is the complex structure of the cell envelope so crucial for the survival of a bacterium?
The cell envelope's complexity is a survival advantage. The rigid cell wall prevents the cell from bursting in dilute solutions due to osmotic pressure. The outer layers, like the capsule in some bacteria, protect against being engulfed by host immune cells (phagocytosis). Furthermore, the selective nature of the membranes allows the cell to thrive in diverse and often harsh environments by controlling its internal chemistry.
7. What are some specialised modifications of the cell envelope and their functions?
Some bacteria have specialised structures extending from their cell envelope that serve specific purposes:
Flagella: Long, whip-like appendages responsible for motility, allowing the bacterium to move towards nutrients or away from toxins.
Pili and Fimbriae: These are shorter, hair-like structures. Fimbriae are used for attachment to surfaces, including host tissues. Pili are typically longer, and specialised pili are used in bacterial conjugation (transfer of genetic material).
8. Is the cell membrane considered part of the cell envelope?
Yes, the plasma membrane (or cell membrane) is the innermost layer and a fundamental component of the cell envelope in prokaryotes. The term 'envelope' refers to the entire layered structure, including the plasma membrane, the cell wall, and the glycocalyx (if present), all functioning together as an integrated unit.
9. How does the glycocalyx layer contribute to the function of the cell envelope?
The glycocalyx is the outermost layer and varies in composition. If it's a loose, gelatinous sheath, it's called a slime layer, which helps prevent dehydration and nutrient loss. If it's a thick, well-organised layer, it's called a capsule. A capsule is a major virulence factor as it protects the bacterium from being destroyed by the host's immune system and helps it adhere firmly to surfaces to form biofilms.
10. Do eukaryotic cells like those in plants and animals have a cell envelope?
The term cell envelope is primarily and most accurately used to describe the prokaryotic structure. While eukaryotic cells have a plasma membrane, and some (like plants and fungi) also have a cell wall, this combination is not typically referred to as a 'cell envelope' in the same context. Animal cells lack a cell wall entirely, possessing only a plasma membrane and an extracellular matrix.
