Cell Structure and Function: Prokaryotic and Eukaryotic Microorganisms

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Cell Structure and Function

Types of Cellular Microorganisms

Microorganisms are classified into two main cellular types: prokaryotic and eukaryotic. Understanding their structural differences is fundamental in microbiology.

Prokaryotic cells: Lack a true nucleus and membrane-bound organelles. Examples include bacteria and archaea.
Eukaryotic cells: Possess a nucleus and various membrane-bound organelles. Examples include fungi, protozoa, algae, and animal cells.

Comparison of prokaryotic and eukaryotic cell structure

Acellular Microorganisms: Viruses

Viruses are considered acellular and non-living. They are obligate intracellular parasites, meaning they require a host cell to replicate. Viruses exist in two states:

Extracellular state (virion): Composed of nucleic acid, a protein capsid, and sometimes an envelope.
Intracellular state: Capsid/envelope is removed, and the virus exists as nucleic acid within the host cell.

Structure of a virus showing capsid and nucleic acid

Bacterial Cell Morphology

Size, Shape, and Arrangement

Bacterial cells exhibit a variety of shapes and arrangements, which are important for identification and classification.

Basic shapes: Bacillus (rod), coccus (spherical), spirillum (spiral).
Variations: Coccobacillus (short rod), vibrio (curved rod), spirochete (long, flexible spiral).

Basic bacterial shapes: cocci, bacilli, spirilla Variations in bacterial cell shapes

Name	Description	Illustration	Image
Coccus	Round	Single sphere	Image of cocci
Bacillus	Rod	Single rod	Image of bacilli
Vibrio	Curved rod	Comma-shaped	Image of vibrio
Coccobacillus	Short rod	Oval	Image of coccobacillus
Spirillum	Spiral	Rigid spiral	Image of spirillum
Spirochete	Long, flexible spiral	Flexible spiral	Image of spirochete

Table of common prokaryotic cell shapes

Bacterial Cell Arrangements

Bacteria can be arranged in characteristic patterns depending on their division and grouping.

Cocci: Single, pairs (diplococci), chains (streptococci), clusters (staphylococci), tetrads, sarcinae.
Bacilli: Single, pairs (diplobacilli), chains (streptobacilli), palisades.

Name	Description	Illustration
Coccus	Single coccus	Single sphere
Diplococcus	Pair of two cocci	Two spheres
Tetrad	Grouping of four cells arranged in a square	Four spheres
Streptococcus	Chain of cocci	Chain of spheres
Staphylococcus	Cluster of cocci	Cluster of spheres
Bacillus	Single rod	Single rod
Streptobacillus	Chain of rods	Chain of rods

Table of common prokaryotic cell arrangements

Prokaryotic Cell Structure

Basic Prokaryotic Cell Structure

A typical prokaryotic cell contains a cell membrane, chromosomal DNA concentrated in a nucleoid, ribosomes, and a cell wall. Some prokaryotic cells may also possess flagella, pili, fimbriae, and capsules.

Diagram of a typical prokaryotic cell

Cell Wall

The cell wall is a critical structure in bacteria, providing shape and protection against osmotic lysis. The main component is peptidoglycan, a polymer of alternating N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) subunits, cross-linked by peptide bridges.

Function: Maintains cell shape, prevents hypotonic lysis.
Structure: Peptidoglycan provides tensile strength.

Cell wall function in different osmotic solutions Peptidoglycan structure in bacterial cell wall Chemical structure of NAG and NAM

Gram-Positive vs Gram-Negative Cell Walls

Bacteria are classified as Gram-positive or Gram-negative based on cell wall structure, which affects staining and antibiotic susceptibility.

Gram-positive: Thick peptidoglycan layer (20–80 nm), teichoic acids, stains purple.
Gram-negative: Thin peptidoglycan layer (1–3 nm), outer membrane with lipopolysaccharide (LPS), periplasmic space, stains pink.

Gram-positive cell wall structure Gram-negative cell wall structure Summary of Gram-positive cell wall Summary of Gram-negative cell wall

Cell Wall Variations: Mycobacteria and Mycoplasma

Some bacteria have unique cell wall structures or lack a cell wall entirely.

Mycobacteria: Possess a waxy mycolic acid layer, detected by acid-fast staining. Example: Mycobacterium tuberculosis.
Mycoplasma: Lack a cell wall, resulting in variable shape. Example: Mycoplasma pneumoniae.

Acid-fast cell wall structure Acid-fast staining of mycobacteria Mycoplasma cell structure

Cell Membrane Structure

The bacterial cell membrane is a phospholipid bilayer with embedded proteins, glycoproteins, and glycolipids. It regulates transport, energy production, and cell signaling.

Structure of the bacterial cell membrane

Internal Structures

Prokaryotic cells contain several internal structures essential for their function:

Cytoplasm: Gel-like substance where metabolic reactions occur.
Inclusions: Storage granules for nutrients and other substances.
Ribosomes: Sites of protein synthesis (70S, composed of 30S and 50S subunits).
Nucleoid: Region containing the bacterial chromosome (usually a single, circular DNA molecule).
Plasmids: Small, extrachromosomal DNA molecules carrying non-essential genes, often for antibiotic resistance or virulence.
Endospores: Dormant, highly resistant structures formed by some genera (e.g., Bacillus, Clostridium).

Diagram of prokaryotic cell internal structures Cytoplasm in prokaryotic cells Poly-beta-hydroxybutyrate inclusions Gas vesicles inclusions Prokaryotic ribosome structure Nucleoid region in prokaryotic cell Plasmid and chromosomal DNA in bacterial cell Conjugation pilus between two bacteria Endospore structure in Bacillus Process of sporulation and endospore formation

External Structures

Flagella

Flagella are whip-like appendages used for motility. Their structure consists of a basal body, hook, and filament. Flagella arrangements include monotrichous, amphitrichous, lophotrichous, and peritrichous.

Diagram of prokaryotic cell with flagella Structure of bacterial flagellum Flagella arrangements

Flagellar Movement: Runs vs Tumbles

Bacterial movement is characterized by alternating runs (straight movement) and tumbles (random changes in direction), controlled by flagellar rotation.

Runs vs tumbles in flagellar movement

Chemotaxis

Chemotaxis is the movement of bacteria toward or away from chemical stimuli. Positive chemotaxis is movement toward attractants, while negative chemotaxis is movement away from repellents.

Chemotaxis: directional movement in response to chemical gradient

Pili and Fimbriae

Pili are hair-like appendages used for attachment and conjugation (transfer of plasmid DNA). Fimbriae are shorter and more numerous, aiding in attachment to surfaces and other cells.

Pili connecting two bacteria Fimbriae on bacterial cell Fimbriae on bacterial cell (SEM) Fimbriae in pathogenesis Fimbriae in biofilm formation Biofilm formation with fimbriae

Glycocalyx: Capsules and Slime Layers

The glycocalyx is an external layer composed of polysaccharides. It can be organized as a capsule or unorganized as a slime layer. Functions include protection from drying out, prevention of phagocytosis, and aiding in attachment and biofilm formation.

Capsule: Organized, tightly bound, enhances virulence.
Slime layer: Loosely organized, aids in environmental survival.

Slime layer structure Capsule structure Capsule stain of Pseudomonas aeruginosa Glycocalyx function: protection from drying out Protection from phagocytosis

Eukaryotic Cell Structure

General Eukaryotic Cell Structure

Eukaryotic cells are more complex, containing a nucleus and various organelles. External structures include glycocalyces (in animal and protozoan cells) and cell walls (in fungi and some protozoa).

Generalized eukaryotic cell structure

External Eukaryotic Structures

Glycocalyces: Found in animal and most protozoan cells, absent in cells with cell walls.
Cell walls: Found in fungi and some protozoa, absent in animal cells.

Cell Membranes in Eukaryotes

Eukaryotic cell membranes are similar to those in bacteria but contain steroids (cholesterol in animal cells, ergosterol in fungi) for structural support.

Flagella and Cilia in Eukaryotes

Some eukaryotic cells possess flagella or cilia for movement. These structures are more complex than their prokaryotic counterparts.

Eukaryotic flagella Eukaryotic cilia Flagella and cilia in eukaryotes

Nonmembranous and Membranous Organelles

Eukaryotic cells contain both nonmembranous (e.g., ribosomes) and membranous organelles (e.g., mitochondria, endoplasmic reticulum). Eukaryotic ribosomes are larger (80S, composed of 60S and 40S subunits) and are not targeted by antibiotics that affect prokaryotic ribosomes.

Eukaryotic ribosome structure