Cell Structure and Function: Prokaryotic and Eukaryotic Cells

Cell Structure and Function

Introduction

This study guide covers the fundamental differences and similarities between prokaryotic and eukaryotic cells, focusing on their structural components, external features, and the functional significance of these structures in microbiology.

Features of Prokaryotic Cells

General Characteristics

• Prokaryotes are unicellular organisms lacking a membrane-bound nucleus.

• They do not have internal membrane-bound organelles.

• Typical size: 1.0 μm in diameter.

• Simple structure compared to eukaryotes.

• Include Bacteria and Archaea.

External Structures of Prokaryotes

• Glycocalyces

• Capsule

• Slime Layer

• Flagella

• Fimbriae and Pili

Glycocalyces

• Gelatinous, sticky substances surrounding the outside of the cell.

• Composed of polysaccharides, polypeptides, or both.

• Functions:

  • Protects cells from desiccation (drying out).

  • May prevent bacteria from being recognized by the immune system.

  • Helps bacteria adhere to surfaces and form biofilms.

• Capsule: Organized, firmly attached glycocalyx.

• Slime Layer: Loosely attached, water-soluble glycocalyx.

Flagella

• Responsible for motility in prokaryotes.

• Long, whip-like structures extending beyond the cell surface.

• Composed of filament, hook, and basal body.

• Flagella rotate to propel the cell; movement can be clockwise or counterclockwise.

• Enable taxis (movement in response to stimuli), such as chemotaxis (chemical), phototaxis (light), and magnetotaxis (magnetic fields).

Arrangements of Flagella

• Monotrichous: Single flagellum at one end.

• Lophotrichous: Tuft of flagella at one or both ends.

• Amphitrichous: Single flagellum at both ends.

• Peritrichous: Flagella covering the entire cell surface.

Nonmotile Extensions

• Fimbriae: Short, bristle-like proteinaceous structures; help bacteria adhere to surfaces, hosts, and each other; important in biofilm formation.

• Pili: Longer than fimbriae but shorter than flagella; usually only one or a few per cell; involved in DNA transfer between cells (conjugation).

Comparison Table: Fimbriae vs. Flagella

Feature	Fimbriae	Flagella
Length	Short	Long
Function	Attachment	Motility
Number per cell	Many	Few
Structure	Proteinaceous	Filament, hook, basal body

Prokaryotic Cell Walls

General Features

• Most prokaryotes have a cell wall composed of peptidoglycan.

• Functions:

  • Provides structure and shape.

  • Protects against osmotic pressure.

  • Assists in cell attachment and evasion of immune responses.

Bacterial Cell Walls

• Peptidoglycan is a polymer of sugars (N-acetylglucosamine and N-acetylmuramic acid) cross-linked by short peptides.

• Two major types based on Gram stain:

Gram-Positive Cell Walls

• Thick layer of peptidoglycan (up to 40 layers).

• Contains teichoic acids.

• Retains crystal violet dye during Gram staining, appearing purple.

Gram-Negative Cell Walls

• Thin layer of peptidoglycan (1-3 layers).

• Outer membrane contains lipopolysaccharide (LPS), which includes lipid A (endotoxin).

• Does not retain crystal violet; appears pink after Gram staining.

Comparison Table: Gram-Positive vs. Gram-Negative Cell Walls

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick	Thin
Teichoic acids	Present	Absent
Outer membrane	Absent	Present (with LPS)
Gram stain color	Purple	Pink
Sensitivity to antibiotics	More sensitive	Less sensitive

Note: Many important antibiotics inhibit peptidoglycan synthesis.

Prokaryotic Cell Membrane

Structure and Function

• Also called the plasma membrane or cytoplasmic membrane.

• Composed of a phospholipid bilayer with embedded proteins (fluid mosaic model).

• Functions:

  • Controls passage of substances into and out of the cell.

  • Site of energy production (electron transport chain in bacteria).

  • Maintains concentration gradients.

Transport Across the Cell Membrane

• Passive Transport: No energy (ATP) required; substances move from high to low concentration.

  • Simple diffusion

  • Facilitated diffusion (via channel or carrier proteins)

  • Osmosis (diffusion of water)

• Active Transport: Requires energy (usually ATP); substances move against their concentration gradient.

  • Uses carrier proteins (permeases or pumps)

  • Examples: uniport, antiport, symport systems

Cytoplasm of Prokaryotes

• Includes all cellular material inside the cell membrane.

• Cytosol: Liquid portion containing water, ions, proteins, carbohydrates, lipids, and nucleic acids.

• Contains ribosomes (site of protein synthesis), inclusions (storage granules), and sometimes endospores.

Features of Eukaryotic Cells

General Characteristics

• Larger (10–100 μm in diameter).

• More complex structure than prokaryotes.

• Have a membrane-bound nucleus and organelles.

• Include algae, protozoa, fungi, animals, and plants.

Features of Eukaryotic Cells

• Nucleus: Contains genetic material (DNA).

• Endoplasmic Reticulum (ER): Synthesis and transport of proteins and lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Mitochondria: Site of ATP (energy) production.

• Chloroplasts (in plants and algae): Site of photosynthesis.

• Ribosomes: Protein synthesis (larger than prokaryotic ribosomes).

• Cytoskeleton: Provides structural support and shape.

External Structures of Eukaryotes

• Flagella: Longer and more complex than prokaryotic flagella; covered by the cell membrane.

• Cilia: Short, hair-like structures for movement or moving substances along the cell surface.

• Extracellular Matrix (ECM): Provides structural support and mediates cell signaling.

Extracellular Matrices

• Similar to prokaryotic glycocalyces but more organized and complex.

• Anchor cells to each other and to substrates.

• Strengthen cell surfaces and protect against dehydration.

• Play roles in cell-cell recognition and communication.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Organelles	Absent	Present
Cell size	1–10 μm	10–100 μm
Cell wall	Usually present (peptidoglycan)	Varies (cellulose, chitin, or absent)
Examples	Bacteria, Archaea	Algae, protozoa, fungi, plants, animals

Key Terms

• Peptidoglycan: A polymer forming the cell wall of most bacteria.

• Glycocalyx: Gelatinous outer layer found in many prokaryotes and some eukaryotes.

• Fimbriae: Short, hair-like appendages for attachment.

• Pili: Longer appendages involved in DNA transfer.

• Flagella: Long, whip-like structures for motility.

• Gram stain: Differential staining technique to classify bacteria.

• Fluid mosaic model: Describes the structure of cell membranes.

Formulas and Equations

• Osmosis (water movement):

• $J = -P (C_2 - C_1)$ Where $J$ is the flux, $P$ is the permeability coefficient, and $C_2 - C_1$ is the concentration gradient.

Applications and Examples

• Antibiotics such as penicillin target peptidoglycan synthesis, making them effective against bacteria but not eukaryotic cells.

• Gram staining is used in clinical microbiology to rapidly identify bacterial pathogens.

• Fimbriae are important in the formation of biofilms, which can contribute to antibiotic resistance.