AS and A Level Biology 17 Views 1 Answers
outline key structural features of a prokaryotic cell as found in a typical bacterium, including: • unicellular • generally 1–5 µm diameter • peptidoglycan cell walls • circular DNA • 70S ribosomes • absence of organelles surrounded by double membranes
outline key structural features of a prokaryotic cell as found in a typical bacterium, including: • unicellular • generally 1–5 µm diameter • peptidoglycan cell walls • circular DNA • 70S ribosomes • absence of organelles surrounded by double membranes
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Key Structural Features of a Prokaryotic Cell (Typical Bacterium)
I. Overall Cell Characteristics
- Unicellular: Exists as a single cell, not part of a multicellular organism.
- Size: Generally ranges from 1–5 µm in diameter (micrometers), though some bacteria can be larger or smaller.
- Shape: Varies among species; common shapes include:
- Spherical (Cocci)
- Rod-shaped (Bacilli)
- Spiral (Spirilla)
- Comma-shaped (Vibrio)
II. Cell Envelope
- Peptidoglycan Cell Wall (Murein):
- Provides structural support and maintains cell shape.
- Composed of a polymer of sugars cross-linked by short peptides.
- Thickness and composition can vary among different bacterial species, influencing their response to antibiotics (e.g., Gram staining differentiation).
III. Genetic Material
- Circular DNA (Nucleoid):
- Single, circular chromosome lacking a nuclear membrane.
- Located in a region of the cell called the nucleoid.
- No histone proteins to compact DNA; instead, other proteins help manage DNA structure.
IV. Protein Synthesis Machinery
- 70S Ribosomes:
- Smaller than eukaryotic 80S ribosomes, reflecting differences in protein synthesis processes.
- Composed of two subunits (50S and 30S).
- Responsible for translating mRNA into proteins.
V. Absence of Membrane-Bound Organelles
- No Mitochondria, Chloroplasts, Endoplasmic Reticulum, Golgi Apparatus, etc.:
- Metabolic processes, including energy production and protein modification, occur in the cytoplasm or are associated with the cell membrane.
- Simpler cellular organization compared to eukaryotic cells.
Additional Features (Not Always Present but Common in Many Bacteria)
- Flagella: For motility, composed of flagellin protein.
- Pili/Fimbriae: Short, hair-like appendages for adhesion to surfaces or other cells.
- Capsule/Slime Layer: A protective, gelatinous layer outside the cell wall, aiding in evasion of the host’s immune system.
- Plasmids: Small, circular, self-replicating DNA molecules separate from the chromosomal DNA, often carrying genes for antibiotic resistance or other beneficial traits.
Visual Representation:
Imagine a small, simple cell with:
- A peptidoglycan wall providing structure
- A circular DNA coil (nucleoid) without a membrane
- 70S ribosomes scattered throughout the cytoplasm
- No membrane-bound organelles
- Possibly equipped with flagella, pili, a capsule, or plasmids, depending on the bacterial species.
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