Prokaryotic Cells

So far we have focused on eukaryotic cell structure. OCR 2.1.1 (h) also requires a detailed understanding of prokaryotic cells and the ability to compare them with eukaryotes. Prokaryotes (bacteria and archaea) are the smallest, simplest, and most abundant cells on Earth, and their architecture differs from eukaryotes in important ways.

Key Definition: A prokaryotic cell is a cell that lacks a membrane-bound nucleus and membrane-bound organelles. Prokaryotes include the domains Bacteria and Archaea. Their name (from Greek pro = before, karyon = kernel) reflects the evolutionary view that they predate eukaryotic cells.

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General Features of Prokaryotic Cells

Typical prokaryotic features include:

• Small size — most are 0.2 to 10 µm across (often around 1–5 µm). Roughly ten to a hundred times smaller in linear dimension than a typical eukaryotic cell.
• No true nucleus — DNA is free in the cytoplasm, concentrated in an area called the nucleoid.
• Genetic material is a single, circular DNA molecule, generally described as "naked" because it is not bound to histones (though archaea have histone-like proteins).
• Additional small circular DNA molecules called plasmids are common.
• No membrane-bound organelles — no mitochondria, ER, Golgi, lysosomes, or chloroplasts.
• 70S ribosomes are scattered throughout the cytoplasm (smaller than the 80S ribosomes of eukaryotes).
• A cell wall of peptidoglycan (murein) lies outside the plasma membrane.
• May have additional surface structures: capsule, pili/fimbriae, and flagella.

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Structure in Detail

The Cell Wall

• Made of peptidoglycan (also known as murein) — a polymer of sugars (N-acetylglucosamine and N-acetylmuramic acid) cross-linked by short peptide chains.
• Provides structural support and prevents the cell from bursting under osmotic pressure.
• Chemically distinct from the cellulose wall of plants or the chitin wall of fungi.
• The presence and structure of peptidoglycan is the basis of the Gram stain:
  • Gram-positive bacteria have a thick peptidoglycan layer and stain purple.
  • Gram-negative bacteria have a thin peptidoglycan layer plus an outer lipopolysaccharide membrane; they stain pink.
• Many antibiotics (e.g., penicillin) work by inhibiting enzymes that cross-link peptidoglycan, weakening the wall so the cell lyses under osmotic pressure. Because human cells do not have cell walls, penicillin is highly selective.

Capsule (Slime Layer)

• Some (not all) bacteria have a polysaccharide capsule outside the cell wall.
• Functions:
  • Protection against phagocytosis by host immune cells.
  • Protection against desiccation.
  • Aids attachment to surfaces and to host tissues (important in biofilm formation and pathogenesis).
• Capsules are often major virulence factors; for example, Streptococcus pneumoniae without its capsule is harmless, but with its capsule it causes severe pneumonia.

Pili (Fimbriae)

• Short, hair-like projections of protein (pilin) on the cell surface.
• Attachment pili allow bacteria to stick to host cells or surfaces (e.g., uropathogenic E. coli attaching to the bladder wall).
• Sex pili (conjugation pili) are used during conjugation, where one bacterium transfers plasmid DNA to another. This is a major route of horizontal gene transfer and of antibiotic resistance spread.

Flagella

• Long, whip-like structures for locomotion, made of the protein flagellin.
• Structurally very different from eukaryotic flagella: they are not surrounded by a membrane, and they rotate like a propeller, driven by a proton gradient across the plasma membrane, rather than bending like eukaryotic cilia.
• A bacterium can reverse the direction of rotation to change between smooth swimming and tumbling, enabling chemotaxis (movement towards food or away from harmful substances).

Plasma Membrane

• A phospholipid bilayer, similar in basic structure to that of eukaryotes, but usually lacks cholesterol.
• Site of many processes that in eukaryotes occur on organelle membranes: respiration (electron transport chain and ATP synthesis occur on the prokaryotic plasma membrane rather than on mitochondrial inner membranes) and in photosynthetic prokaryotes, photosynthesis (on infoldings).

Mesosomes

• Mesosomes are infoldings of the plasma membrane seen in some bacterial electron micrographs.
• Historically, they were thought to increase the surface area for respiration and perhaps anchor DNA.
• Modern evidence suggests that many mesosomes seen in TEMs are actually fixation artefacts rather than real structures. However, OCR requires that you know the term and the suggested role.

Nucleoid and Circular DNA

• The nucleoid is the area of the cytoplasm containing the circular chromosome. It is not membrane-bound and so is not a true nucleus.
• The chromosome is typically a single circular molecule of DNA, supercoiled and anchored at several points to the plasma membrane.
• The DNA is considered "naked" because it is not wound around histones (although archaea have histone-like proteins, a reason why some biologists argue they should be studied separately).

Plasmids

• Small, circular, double-stranded DNA molecules independent of the main chromosome.
• Replicate separately from the chromosome.
• Carry genes that are often non-essential for basic metabolism but give a selective advantage — for example, antibiotic resistance, heavy metal tolerance, production of toxins, or the ability to metabolise unusual compounds.
• Can be transferred between bacteria via conjugation using a sex pilus, spreading adaptive traits rapidly.
• Are widely exploited in genetic engineering as vectors to introduce genes into bacteria.

70S Ribosomes

• Smaller than the 80S ribosomes of eukaryotic cytoplasm.
• Consist of a 50S large subunit and a 30S small subunit.
• Translate mRNA into protein — functionally equivalent to 80S ribosomes but different in composition.
• The differences between 70S and 80S ribosomes are exploited by antibiotics such as tetracycline, streptomycin, erythromycin, and chloramphenicol, which bind selectively to 70S ribosomes and inhibit bacterial protein synthesis without affecting human ribosomes.

Food Storage Granules

• Some prokaryotes store carbon and energy as glycogen granules, poly-β-hydroxybutyrate, or polyphosphate granules within the cytoplasm.

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Comparison: Prokaryotic vs Eukaryotic Cells