[Biology Class Notes] on Cell Envelope Pdf for Exam

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.

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.

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?

1. Bacterial cells

2. Plant cells

3. Protozoa

4. Algae

2. What is S-layer?

1. Surface layer

2. Secondary layer

3. Soluble layer

4. 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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