Cell Structure and Function in Microbiology: Prokaryotic and Eukaryotic Cells

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Cell Structure and Function

Introduction to Cellular Microorganisms

Microorganisms are classified based on their cellular organization into prokaryotic and eukaryotic types. Understanding the differences in their structure and function is fundamental to microbiology.

Prokaryotic cells lack a true nucleus and membrane-bound organelles.
Eukaryotic cells possess a nucleus and various membrane-bound organelles.

Comparison of prokaryotic and eukaryotic cells

Viruses: Acellular Microorganisms

Characteristics of Viruses

Viruses are acellular entities that are not considered living organisms. They are obligate intracellular parasites, meaning they require a host cell to replicate.

In the extracellular state (virion): composed of nucleic acid, a protein capsid, and sometimes an envelope.
In the intracellular state: the capsid/envelope is removed, and the virus exists as nucleic acid within the host cell.

Structure of a virus showing capsid and nucleic acid

Prokaryotic Cell Morphology

Size, Shape, and Arrangement

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

Basic shapes: Bacillus (rod), Coccus (spherical), Spirillum (spiral)
Variations: Coccobacillus, Vibrio, Spirochete

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

Common Prokaryotic Cell Shapes

Name	Description	Illustration	Image
Coccus	Round	●	Micrograph
Bacillus	Rod	▬	Micrograph
Vibrio	Curved rod	∽	Micrograph
Coccobacillus	Short rod	◦▬	Micrograph
Spirillum	Spiral	~	Micrograph
Spirochete	Long, helical spiral	~~	Micrograph

Table of common prokaryotic cell shapes

Common Prokaryotic Cell Arrangements

Name	Description	Illustration
Coccus	Single coccus	●
Diplococcus	Pair of two cocci	●●
Tetrad	Group of four cells in a square	●● ●●
Streptococcus	Chain of cocci	●●●●
Staphylococcus	Cluster of cocci	●●● ●●
Bacillus	Single rod	▬
Streptobacillus	Chain of rods	▬▬▬

Table of common prokaryotic cell arrangements

Basic Prokaryotic Cell Structure

Overview of Structures

A typical prokaryotic cell contains a cell membrane, chromosomal DNA (nucleoid), ribosomes, and a cell wall. Some may also have flagella, pili, fimbriae, and capsules.

Diagram of a typical prokaryotic cell

Cell Wall Structure and Function

Major Functions

The cell wall provides shape and protects the cell from osmotic lysis, especially in hypotonic environments.

Osmotic effects on cells with and without cell walls

Peptidoglycan Structure

Peptidoglycan is a polymer consisting of alternating subunits of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), cross-linked by peptide bridges. This structure provides tensile strength to the bacterial cell wall.

Structure of NAG and NAM subunits Peptidoglycan structure in Gram-positive and Gram-negative bacteria

Gram-Positive vs. Gram-Negative Cell Walls

Bacteria are classified as Gram-positive or Gram-negative based on differences in their cell wall structure, which affects their staining properties and susceptibility to antibiotics.

Gram-positive: Thick peptidoglycan layer, teichoic acids, stains purple.
Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS), stains pink.

Comparison of Gram-positive and Gram-negative cell wall structure Gram-positive cell wall structure

Cell Wall Variations

Mycobacteria

Some Gram-positive bacteria, such as Mycobacterium species, have a cell wall rich in mycolic acids, making them acid-fast and resistant to many chemicals and antibiotics. The acid-fast stain is used to identify these organisms.

Acid-fast cell wall structure Acid-fast bacteria and staining

Mycoplasma

Mycoplasma species lack a cell wall, resulting in variable shapes and resistance to antibiotics targeting cell wall synthesis.

Mycoplasma cell structure

Cell Membrane Structure

Phospholipid Bilayer

The cell membrane is a phospholipid bilayer with embedded proteins, glycoproteins, and glycolipids. It controls the movement of substances into and out of the cell and is involved in various cellular processes.

Structure of the bacterial cell membrane

Internal Structures of Prokaryotic Cells

Cytoplasm

The cytoplasm is the gel-like substance inside the cell membrane, containing water, enzymes, nutrients, wastes, and cell structures.

Cytoplasm of a prokaryotic cell

Inclusions

Inclusions are storage granules for nutrients, such as poly-β-hydroxybutyrate and gas vesicles, which help bacteria survive fluctuating environmental conditions.

Poly-β-hydroxybutyrate inclusions Gas vesicles in prokaryotic cells

Ribosomes

Prokaryotic ribosomes (70S) are composed of 30S and 50S subunits and are the site of protein synthesis.

Prokaryotic ribosome structure

Nucleoid Region

The nucleoid is the region where the bacterial chromosome (usually a single, circular DNA molecule) is located.

Nucleoid region in a prokaryotic cell

Plasmids

Plasmids are small, extrachromosomal DNA molecules that carry genes beneficial for survival, such as antibiotic resistance and virulence factors.

Plasmid and chromosomal DNA in a bacterial cell

F Plasmids and Conjugation

F plasmids enable the formation of a conjugation pilus for the transfer of plasmid DNA between bacteria, a process known as conjugation.

Bacterial conjugation via pilus

Endospores

Endospores are highly resistant, dormant structures formed by genera such as Bacillus and Clostridium under unfavorable conditions. They can withstand heat, drying, chemicals, and radiation.

Endospore structure Sporulation process and endospore formation

External Structures of Prokaryotic Cells

Flagella

Flagella are whip-like appendages used for motility. Their structure includes a basal body, hook, and filament, with arrangements varying among species (monotrichous, amphitrichous, lophotrichous, peritrichous).

Structure of bacterial flagellum Flagella arrangements in bacteria

Movement: Runs and Tumbles

Bacteria move by alternating between runs (straight movement) and tumbles (random changes in direction), controlled by the rotation of flagella.

Runs and tumbles in bacterial movement

Chemotaxis

Chemotaxis is the movement of bacteria toward or away from chemical stimuli, achieved by modulating the frequency of runs and tumbles.

Chemotaxis: movement in response to chemical gradients

Pili

Pili are longer, hair-like appendages involved in attachment and genetic exchange (conjugation).

Pili connecting two bacterial cells

Fimbriae

Fimbriae are short, bristle-like structures that enable bacteria to adhere to surfaces and each other, playing a key role in colonization and biofilm formation.

Fimbriae on bacterial cells Fimbriae and flagella on bacteria Fimbriae in pathogenesis Fimbriae in biofilm formation Biofilm structure with fimbriae

Glycocalyx (Capsule and Slime Layer)

The glycocalyx is an external layer composed of polysaccharides and/or proteins. It exists as a capsule (organized, firmly attached) or slime layer (loose, unorganized).

Slime layer structure Capsule structure Capsule stain of Pseudomonas aeruginosa

Functions: Protection from desiccation, phagocytosis, and antibiotics; aids in attachment and biofilm formation.

Protection from drying out Protection from phagocytosis

Eukaryotic Cell Structure

Overview

Eukaryotic cells are generally larger and more complex than prokaryotic cells, containing a nucleus and various organelles. Structures such as the glycocalyx, cell wall, and membrane-bound organelles differ among eukaryotic microorganisms.

Generalized eukaryotic cell structure

External Structures

Glycocalyces: Found in animal and most protozoan cells, but not in those with cell walls.
Cell walls: Present in fungi and some protozoa; absent in animals and protozoa with glycocalyces.

Cell Membranes in Eukaryotes

Eukaryotic cell membranes are similar to those of bacteria but contain steroids such as cholesterol (in animals) or ergosterol (in fungi), which affect membrane fluidity and function.

Flagella and Cilia in Eukaryotes

Some eukaryotic cells possess flagella or cilia for movement. These structures are more complex than their prokaryotic counterparts and are composed of microtubules arranged in a 9+2 pattern.

Eukaryotic flagella structure Eukaryotic cilia structure Flagella and cilia in eukaryotes

Nonmembranous and Membranous Organelles

Eukaryotic cells contain both nonmembranous (e.g., ribosomes, cytoskeleton) and membranous organelles (e.g., nucleus, mitochondria, endoplasmic reticulum).

Ribosomes

Eukaryotic ribosomes (80S) are larger than prokaryotic ribosomes (70S) and are composed of 60S and 40S subunits. They are not targeted by antibiotics such as erythromycin or tetracycline.