Prokaryotic and Eukaryotic Cell Structure: Key Concepts for Microbiology

Prokaryotic Cell Structure

Overview of Prokaryotic Cells

Prokaryotic cells are unicellular organisms that lack membrane-bound organelles. Their internal structure is simpler than that of eukaryotic cells, but they possess specialized features essential for survival and adaptation.

• Cytoplasm: The semifluid material inside the cell, composed mainly of water, ions, solutes, free amino acids, carbohydrates, and enzymes. Most metabolic reactions occur here.

• Plasma Membrane: Surrounds the cytoplasm and regulates the movement of substances in and out of the cell.

Nucleoid

The nucleoid is the region within a prokaryotic cell where the genetic material (DNA) is located.

• Chromosome: Prokaryotic DNA is organized into a single, circular chromosome found in the nucleoid region.

• The nucleoid may also contain some RNA and proteins.

• Essential genes: Chromosomal genes are essential for the viability of the organism.

Plasmids

Plasmids are small, extrachromosomal DNA elements found in many bacteria.

• They are usually circular and much smaller than the main chromosome.

• Plasmids can replicate independently and be passed to daughter cells.

• They are not essential for bacterial viability but often carry genes that confer advantages, such as:

  • Virulence factors (enhancing pathogenicity)

  • Antibiotic resistance

  • Tolerance to toxic substances

  • Useful tools in genetic engineering

Prokaryotic Ribosomes

Ribosomes are the sites of protein synthesis in all cells. In prokaryotes, ribosomes are structurally distinct from those in eukaryotes.

• Composed of ribosomal RNA (rRNA) and proteins (ribonucleoprotein complex).

• Prokaryotic ribosomes are 70S, made up of:

  • Large subunit: 50S

  • Small subunit: 30S

• Function: Translate mRNA into proteins by linking amino acids.

• Clinical relevance: Many antibiotics target bacterial ribosomes, inhibiting protein synthesis without affecting eukaryotic ribosomes.

Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria to survive harsh environmental conditions.

• Formed when conditions are unfavorable (e.g., nutrient depletion, heat, desiccation, radiation).

• Endospores can remain dormant for extended periods and germinate into vegetative cells when conditions improve.

• Sporulation: The process of forming an endospore.

• Resistance: Endospores are resistant to extreme temperatures, drying, chemicals, and radiation.

Important Spore-Forming Bacteria

Clinically relevant spore-forming bacteria are mainly found in the Bacillus and Clostridium genera.

• Clostridium tetani (tetanus)

• Clostridium botulinum (botulism)

• Clostridium perfringens (gas gangrene)

• Clostridium difficile (severe diarrhea)

• Bacillus anthracis (anthrax)

• Endospores can survive for extended periods on surfaces, posing challenges in healthcare settings.

Eukaryotic Cell Structure

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic cells as a result of symbiotic relationships between ancestral prokaryotic cells.

• Mitochondria are thought to have evolved from engulfed non-photosynthetic prokaryotes.

• Chloroplasts are believed to have originated from engulfed photosynthetic prokaryotes (e.g., cyanobacteria).

• Supporting evidence includes:

  • Presence of circular DNA in mitochondria and chloroplasts

  • 70S ribosomes (similar to bacteria)

  • Double membrane structures

  • Replication by binary fission

  • Genetic similarities to certain bacteria

Comparison of Eukaryotes and Prokaryotes

Eukaryotic and prokaryotic cells differ in several structural and functional aspects.

Feature	Eukaryotes	Prokaryotes
Cellularity	Unicellular (protists, yeast) or multicellular (animals, plants, fungi)	Unicellular (bacteria, archaea)
Size	Usually much larger	Much smaller
Reproduction	Asexual (mitosis) and sexual (meiosis)	Asexual (binary fission)
Membrane-bound organelles	Present (nucleus, mitochondria, etc.)	Absent
Ribosomes	80S (cytoplasm, rough ER, mitochondria, chloroplasts)	70S (cytoplasm)
Genetic material	Multiple linear chromosomes in nucleus	Single circular chromosome in nucleoid
Cell wall	Present in plants, fungi, some protists (composition varies: cellulose, chitin)	Present in most (peptidoglycan, except Mycoplasma)
Sterols in membrane	Common (e.g., cholesterol)	Rare

Plasma Membrane and Cell Wall

The plasma membrane is a phospholipid bilayer that regulates cell entry and exit. Eukaryotic membranes often contain sterols (e.g., cholesterol) for fluidity regulation.

• Cell walls are present in plants, fungi, and some protists, but absent in animals.

• Plant cell walls are primarily composed of cellulose; fungal cell walls contain chitin.

• Some eukaryotic cells are covered by an extracellular matrix of glycoproteins and other molecules.

Nucleus

The nucleus is a membrane-bound organelle that houses the cell's genetic material.

• Contains DNA organized as chromatin (DNA + proteins).

• The nucleolus is a dense region where ribosomal RNA is synthesized and ribosome assembly begins.

• The nuclear envelope (double membrane) contains pores for RNA and protein transport between nucleus and cytoplasm.

Eukaryotic Ribosomes

Eukaryotic ribosomes are responsible for protein synthesis and are structurally larger than prokaryotic ribosomes.

• 80S ribosomes consist of:

  • Small subunit: 40S

  • Large subunit: 60S

• Located free in the cytoplasm or bound to the rough endoplasmic reticulum (ER).

Flagella and Cilia

Flagella and cilia are motile structures composed of microtubules (tubulin) in eukaryotic cells.

• Flagella are longer, fewer in number, and move with a wavelike, undulating motion.

• Cilia are shorter, more numerous, and move in a coordinated, wave-like fashion.

• Both are involved in cell movement and moving substances across cell surfaces.

Membrane-Bound Organelles

• Endoplasmic Reticulum (ER): Network of membranes involved in protein (rough ER, with ribosomes) and lipid (smooth ER, without ribosomes) synthesis.

• Golgi Apparatus: Series of flattened sacs that modify, sort, and package proteins and lipids for secretion or delivery to other organelles.

• Mitochondria: Double-membraned organelles with a smooth outer membrane and a highly folded inner membrane (cristae). Site of ATP (adenosine triphosphate) production via aerobic respiration.

• Chloroplasts: (in plants and algae) Organelles responsible for photosynthesis, containing their own DNA and ribosomes.

Additional info:

• Some eukaryotic organelles (mitochondria, chloroplasts) retain features reminiscent of their prokaryotic ancestors, supporting the endosymbiotic theory.

• Endospores are a major concern in sterilization and infection control due to their resistance to standard disinfection methods.