Cell Structure and Function in Microbiology: Prokaryotic and Eukaryotic Cells

Processes of Life

Characteristics of Living Cells

All living organisms share several fundamental processes that define life. These include:

• Growth: Increase in size or number of cells.

• Reproduction: Ability to produce new cells or organisms, either sexually or asexually.

• Responsiveness: Ability to respond to environmental stimuli.

• Metabolism: Collection of controlled chemical reactions that occur in organisms to maintain life.

Cell Types: Prokaryotic vs. Eukaryotic Cells

Overview of Cell Types

Cells are classified into two main types based on their structural characteristics: prokaryotic and eukaryotic cells.

• Prokaryotes:

  • Lack a nucleus and membrane-bound organelles.

  • Generally smaller (~1.0 µm in diameter) and simpler in structure.

  • Include Bacteria and Archaea.

• Eukaryotes:

  • Have a true nucleus and internal membrane-bound organelles.

  • Larger (10–100 µm in diameter) and more complex.

  • Include Algae, Protozoa, Fungi, Animals, and Plants.

Typical Prokaryotic Cell Structure

Prokaryotic cells have a simple organization, lacking compartmentalization found in eukaryotes. Key structures include:

• Cytoplasmic membrane

• Cell wall

• Glycocalyx

• Ribosomes

• Nucleoid (region containing DNA)

• Flagellum (for motility)

• Inclusions (storage granules)

Typical Eukaryotic Cell Structure

Eukaryotic cells are compartmentalized by internal membranes, allowing for specialized functions. Major components include:

• Nucleus (with nucleolus and nuclear envelope)

• Mitochondria (ATP production)

• Endoplasmic reticulum (smooth and rough)

• Golgi body (processing and packaging)

• Lysosomes, peroxisomes, vacuoles (storage and digestion)

• Cytoskeleton (structural support)

• Cilia and flagella (motility)

Relative Sizes of Cells and Microorganisms

Microorganisms vary greatly in size, from viruses (nanometers) to protozoa and eggs (micrometers to millimeters).

• Viruses: ~0.3 µm

• Bacteria: ~1 µm

• Protozoa: ~14 µm

• Chicken egg: ~47,000 µm

External Structures of Bacterial Cells

Glycocalyces

The glycocalyx is a gelatinous, sticky substance surrounding the outside of the cell, composed of polysaccharides, polypeptides, or both. It serves as protection and aids in attachment.

• Capsule: Organized, firmly attached, may prevent recognition by host immune system.

• Slime layer: Loosely attached, water-soluble, helps bacteria adhere to surfaces.

Flagella

Flagella are long, whip-like structures responsible for bacterial motility. They are composed of a filament, hook, and basal body. The basal body anchors the flagellum to the cell wall and membrane.

• Not all bacteria possess flagella.

• Flagellar rotation propels the bacterium and can be reversed (clockwise or counterclockwise).

• Bacteria move in response to stimuli (taxis), alternating between runs and tumbles.

Fimbriae and Pili

Fimbriae and pili are proteinaceous, hair-like appendages on the surface of many bacteria.

• Fimbriae: Short, bristle-like, used for attachment to surfaces, host cells, and in biofilm formation.

• Pili (conjugation pili): Longer than fimbriae but shorter than flagella, usually one or two per cell, mediate DNA transfer between cells (conjugation).

Bacterial Cell Walls

Functions and Composition

Bacterial cell walls provide structural support, shape, and protection from osmotic pressure. They are primarily composed of peptidoglycan, a polymer of sugars and amino acids.

• Cell wall structure is a target for antibiotics.

• Cell wall composition determines Gram staining characteristics.

Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present (contains LPS)
Gram Stain	Purple	Pink
Mycolic Acid	Up to 60% in acid-fast bacteria	Absent

Bacterial Cytoplasmic Membranes

Structure and Function

The cytoplasmic membrane is a phospholipid bilayer with embedded proteins, described by the fluid mosaic model. It serves as a selective barrier, energy storage site, and maintains gradients.

• Integral and peripheral proteins facilitate transport and communication.

• Maintains electrical and chemical gradients essential for cell function.

Transport Across Membranes

Substances cross the cytoplasmic membrane via passive and active processes:

• Passive processes: Do not require energy (diffusion, facilitated diffusion, osmosis).

• Active processes: Require energy (active transport, group translocation).

Cytoplasm of Bacteria

Internal Components

• Cytosol: Liquid portion containing enzymes, nutrients, and ions.

• Inclusions: Reserve deposits of chemicals (e.g., glycogen, polyphosphate).

• Endospores: Dormant, highly resistant structures formed by some bacteria for survival under adverse conditions.

• Ribosomes: Sites of protein synthesis (70S in prokaryotes).

• Cytoskeleton: Maintains cell shape and aids in division.

Archaeal Cell Structure

External and Internal Features

• Glycocalyces: Aid in biofilm formation and adherence.

• Flagella: Structurally distinct from bacterial flagella.

• Fimbriae and Hami: Attachment structures; hami are unique to archaea.

• Cell Walls: Lack peptidoglycan; composed of specialized polysaccharides and proteins.

• Cytoplasmic Membrane: Maintains gradients and controls transport.

• Cytoplasm: Contains 70S ribosomes, fibrous cytoskeleton, circular DNA, and unique metabolic enzymes.

Eukaryotic Cell Structure

External Structures

• Glycocalyces: Less organized than prokaryotic capsules; function in cell recognition, adhesion, and protection.

Cell Walls and Cytoplasmic Membranes

• Present in fungi, algae, plants, and some protozoa.

• Composed of cellulose (plants), chitin and/or glucomannan (fungi), and various polysaccharides (algae).

• All eukaryotes have a cytoplasmic membrane with steroid lipids and membrane rafts.

Membranous Organelles

• Nucleus: Contains DNA, nucleolus, and is surrounded by a nuclear envelope with pores.

• Endoplasmic Reticulum (ER): Smooth (SER) and rough (RER); involved in synthesis and transport.

• Golgi Body: Processes and packages molecules for export.

• Lysosomes, Peroxisomes, Vacuoles, Vesicles: Storage, digestion, and detoxification.

• Mitochondria: ATP production; contain their own DNA and 70S ribosomes.

• Chloroplasts: Photosynthesis in plants and algae; also contain DNA and 70S ribosomes.

Nonmembranous Organelles

• Ribosomes: Larger (80S) than prokaryotic ribosomes; composed of 60S and 40S subunits.

• Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments for structure and movement.

• Centrioles and Centrosome: Involved in cell division and organization of microtubules.

Flagella and Cilia

• Flagella: Structurally different from prokaryotic flagella; composed of microtubules, undulate rather than rotate.

• Cilia: Shorter, more numerous, used for movement and moving substances across cell surfaces.

Endosymbiotic Theory

This theory proposes that eukaryotic cells originated from a symbiotic relationship between small aerobic prokaryotes (which became mitochondria) and larger anaerobic prokaryotes. Chloroplasts are thought to have a similar origin.