Structure of Bacterial Cell — Revision Notes

⚡ 30-Second Revision

Prokaryotic: — No true nucleus, no membrane-bound organelles.
Cell Wall: — Peptidoglycan (NAG-NAM), rigid, shape, protection. Gram-positive (thick PG, teichoic acids), Gram-negative (thin PG, outer membrane with LPS).
Plasma Membrane: — Phospholipid bilayer, selective permeability, respiration site, lacks sterols (except Mycoplasma).
Cytoplasm: — Semifluid matrix, site of metabolism.
Ribosomes: — $70S$ type ($50S + 30S$), protein synthesis.
Nucleoid: — Region with single, circular chromosome (no membrane).
Plasmids: — Extrachromosomal DNA, independent replication, carry resistance/virulence genes.
Glycocalyx (Capsule/Slime Layer): — Polysaccharides/polypeptides, adhesion, protection (phagocytosis, desiccation).
Flagella: — Flagellin protein, motility.
Pili/Fimbriae: — Pilin protein, adhesion (fimbriae), conjugation (sex pili).
Inclusion Bodies: — Storage granules (e.g., PHB, volutin).
Endospores: — Highly resistant, dormant survival structures (e.g., *Bacillus*, *Clostridium*).

2-Minute Revision

Bacterial cells are prokaryotic, meaning they lack a true nucleus and membrane-bound organelles. Their defining outer layer is the cell wall, primarily composed of peptidoglycan, which dictates cell shape and protects against osmotic lysis.

This wall differs significantly between Gram-positive (thick peptidoglycan) and Gram-negative bacteria (thin peptidoglycan with an outer membrane containing LPS), forming the basis of Gram staining. Inside the cell wall is the plasma membrane, a selectively permeable barrier where vital functions like respiration occur.

The internal cytoplasm houses 70S ribosomes for protein synthesis and the nucleoid, an area containing the single, circular bacterial chromosome, not enclosed by a membrane. Many bacteria also carry plasmids, small circular DNA molecules conferring advantageous traits like antibiotic resistance.

External to the cell wall, some bacteria have a glycocalyx (capsule or slime layer) for adhesion and protection, flagella for motility, and pili/fimbriae for adhesion and genetic transfer.

Certain bacteria can form highly resistant endospores to survive harsh conditions. Understanding these structures is crucial for NEET, especially their unique features compared to eukaryotes and their roles in bacterial survival and pathogenicity.

5-Minute Revision

To thoroughly revise the structure of a bacterial cell for NEET, focus on its prokaryotic nature and the specific components. Start with the cell wall, a rigid layer external to the plasma membrane.

Remember its unique composition: peptidoglycan (murein), a polymer of NAG and NAM units cross-linked by peptides. This peptidoglycan is the target of many antibiotics like penicillin. Crucially, differentiate between Gram-positive (thick peptidoglycan, teichoic acids) and Gram-negative (thin peptidoglycan, outer membrane with LPS and porins) cell walls, as this distinction is fundamental for Gram staining and antibiotic susceptibility.

The plasma membrane lies beneath the cell wall, a typical phospholipid bilayer that lacks sterols (except Mycoplasma). It's the site for respiration and transport.

The cytoplasm fills the cell, containing 70S ribosomes (composed of 50S and 30S subunits) for protein synthesis – a key difference from eukaryotic 80S ribosomes, exploited by antibiotics. The genetic material, a single, circular chromosome, is located in the nucleoid region, which is not membrane-bound.

Additionally, many bacteria possess plasmids, small, circular, extrachromosomal DNA that replicate independently and often carry genes for antibiotic resistance or virulence, making them vital in genetic engineering.

Accessory structures are equally important: Glycocalyx (capsule or slime layer) provides protection against phagocytosis and desiccation, and aids in adhesion. Flagella, made of flagellin, are long, whip-like structures for motility.

Pili (fimbriae), made of pilin, are shorter, hair-like structures for adhesion (fimbriae) and genetic transfer (sex pili). Finally, some Gram-positive bacteria form highly resistant, dormant endospores (e.

g., *Bacillus*, *Clostridium*) to survive extreme conditions. Remember to link each structure to its specific function and its relevance in bacterial biology and disease.

Prelims Revision Notes

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Prokaryotic Nature: — Bacterial cells are prokaryotic; they lack a true nucleus and all membrane-bound organelles (e.g., mitochondria, ER, Golgi, lysosomes).
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Cell Wall:

* Composition: Primarily peptidoglycan (murein), a polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by peptides. * Function: Provides shape, structural support, protects against osmotic lysis.

* Gram-Positive: Thick peptidoglycan layer, presence of teichoic acids. Stains purple. * Gram-Negative: Thin peptidoglycan layer, presence of an outer membrane (with LPS, phospholipids, porins) external to peptidoglycan.

Stains pink/red. * NEET Relevance: Peptidoglycan synthesis is target for penicillin. Gram staining is a key diagnostic tool.

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Plasma Membrane:

* Structure: Phospholipid bilayer, fluid mosaic model. Lacks sterols (except Mycoplasma). * Function: Selective permeability, site of respiration (electron transport chain), photosynthesis, cell wall synthesis, secretion.

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Cytoplasm: — Semifluid matrix, site of metabolic reactions.
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Ribosomes:

* Type: $70S$ (composed of $50S$ and $30S$ subunits). * Function: Protein synthesis (translation). * NEET Relevance: Target for antibiotics like streptomycin, tetracycline.

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Nucleoid:

* Location: Irregular region in cytoplasm. * Contents: Single, circular, double-stranded bacterial chromosome. Not membrane-bound.

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Plasmids:

* Structure: Small, circular, extrachromosomal DNA. Replicate independently. * Function: Carry non-essential genes (e.g., antibiotic resistance, virulence factors). * NEET Relevance: Crucial for horizontal gene transfer, antibiotic resistance spread, genetic engineering.

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Glycocalyx (Capsule/Slime Layer):

* Composition: Polysaccharides, polypeptides, or both. * Capsule: Well-organized, tightly attached. * Slime Layer: Loose, diffuse. * Function: Protection (phagocytosis, desiccation), adhesion (biofilm formation).

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Flagella:

* Structure: Long, filamentous, made of flagellin. * Function: Motility (chemotaxis). * Types: Monotrichous, Lophotrichous, Amphitrichous, Peritrichous.

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Pili (Fimbriae):

* Structure: Short, hair-like, made of pilin. * Fimbriae: Numerous, for adhesion to surfaces/host cells. * Sex Pili: Longer, fewer, for conjugation (genetic transfer).

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Inclusion Bodies: — Non-membrane-bound storage granules (e.g., PHB for carbon, volutin for phosphate).
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Endospores:

* Formation: By some Gram-positive bacteria (*Bacillus*, *Clostridium*) in harsh conditions. * Structure: Highly resistant, dormant. * Function: Survival against heat, radiation, chemicals, desiccation. * NEET Relevance: Extremely difficult to sterilize.

Vyyuha Quick Recall

To remember the key components of a bacterial cell, think of 'C.P.N.R.F.P.I.E.S.':

Cell Wall
Plasma Membrane
Nucleoid
Ribosomes
Flagella
Pili (and Plasmids)
Inclusion Bodies
Endospores (for some)
Slime Layer/Capsule (Glycocalyx)