Prokaryotic Cell Structure and Function: Study Guide

Prokaryotic Cell Structures and Their Functions

Overview

Prokaryotic cells, such as bacteria, possess unique cellular structures that are essential for their survival, growth, and reproduction. Understanding the function of each component is fundamental in microbiology.

• Nucleoid (Chromosome): The region containing the bacterial DNA. It is not surrounded by a membrane. The nucleoid holds the genetic material necessary for cell function and replication.

• Cytoplasm: The fluid matrix inside the cell where metabolic reactions occur. It contains enzymes, nutrients, and the cell's internal structures.

• Ribosomes: Structures responsible for protein synthesis. Prokaryotic ribosomes are 70S, smaller than eukaryotic ribosomes, but functionally similar.

• Cell Membrane: A semipermeable phospholipid bilayer that encloses the cytoplasm. It regulates the movement of substances in and out of the cell and is involved in energy generation.

• Cell Wall: Composed mainly of peptidoglycan and other molecules, the cell wall provides structural support and prevents osmotic lysis in hypotonic environments.

• Flagella: Long, whip-like appendages that propel the cell in liquid environments. Bacterial flagella rotate like a propeller, unlike the wave-like motion in eukaryotes.

• Thylakoid: Membranous structures containing photosynthetic pigments, found in photosynthetic bacteria. They are the site of light-dependent reactions of photosynthesis.

• Endospore: A highly resistant, dormant structure formed by some bacteria to survive adverse conditions. Not a means of reproduction.

• Pilus (plural: pili): Hair-like appendages used for attachment, movement, or genetic exchange (conjugation).

• Magnetosome: Intracellular structures containing magnetic minerals, allowing bacteria to orient along magnetic fields.

• Plasmid: Small, circular DNA molecules separate from the chromosome. Often carry genes for antibiotic resistance and can be transferred between bacteria.

• Inclusion: Storage granules for nutrients or other substances within the cytoplasm.

• Capsule: A gelatinous outer layer composed of polysaccharides or polypeptides. Protects against phagocytosis and desiccation.

• Glycocalyx: An outer coating of polysaccharide that aids in attachment to surfaces or other cells. Can form a capsule if organized and dense.

Prokaryotic Cell Division

Binary Fission

Prokaryotic cells divide by a process called binary fission, a simple form of asexual reproduction where the cell duplicates its DNA and splits into two identical daughter cells.

• Meiosis: Prokaryotic cells do not undergo meiosis.

Comparing Bacterial and Archaeal Cells

Plasma Membrane Differences

• Bacteria: Plasma membrane contains fatty acids linked by ester bonds; typically a phospholipid bilayer.

• Archaea: Membrane lipids contain isoprenoid chains linked by ether bonds; can form monolayers or bilayers.

Bacterial Shapes and Arrangements

Common Shapes

• Rods (bacilli)

• Spheres (cocci)

• Spirals (spirilla/spirochetes)

Common Arrangements

• Chains: e.g., Streptococcus

• Clusters: e.g., Staphylococcus

• Pairs: e.g., Diplococcus

Gram-Positive vs. Gram-Negative Bacteria

Cell Wall Differences

• Gram-Positive: Thick peptidoglycan layer, retains crystal violet stain (purple).

• Gram-Negative: Thin peptidoglycan layer, outer membrane present, does not retain crystal violet (appears pink/red after counterstain).

Specialized Bacterial Structures

Acid-Fast Bacteria

• Mycolic Acid: A waxy lipid found in the cell wall of Mycobacterium and Nocardia species, conferring resistance to desiccation and chemical damage.

L-forms and Mycoplasma

• L-forms and Mycoplasma: Bacterial species that lack a cell wall, making them resistant to antibiotics targeting cell wall synthesis.

Transport Mechanisms Across the Membrane

• Diffusion: Movement of molecules from high to low concentration.

• Facilitated Diffusion: Movement of molecules across a membrane via transport proteins, down their concentration gradient.

• Osmosis: Diffusion of water across a semipermeable membrane.

• Active Transport: Movement of molecules against their concentration gradient using energy (often ATP).

Bacterial Flagella

Structure and Function

• Movement: Bacterial flagella rotate like a propeller, enabling motility.

• Arrangements:

  • Peritrichous: Flagella distributed over the entire cell surface.

  • Polar: Flagella at one or both ends.

  • Lophotrichous: Tuft of flagella at one end.

  • Amphitrichous: Flagella at both ends.

  • Monotrichous: Single flagellum at one end.

  • Bipolar: Flagella at both ends (similar to amphitrichous).

• Flagella in Spirochetes: Internal flagellar apparatus (axial filaments) allows corkscrew motion.

• Flagella Rings: Gram-positive bacteria have 2 rings; gram-negative bacteria have 4 rings anchoring the flagellum to the cell envelope.

Bacterial Endospores

• Function: Endospores are survival structures, not reproductive forms. They allow bacteria to withstand extreme conditions such as heat, desiccation, and chemicals.

Summary Table: Key Prokaryotic Cell Structures and Functions

Structure	Function
Nucleoid	Contains genetic material (DNA)
Cytoplasm	Site of metabolic reactions
Ribosome	Protein synthesis
Cell Membrane	Selective barrier, energy generation
Cell Wall	Structural support, prevents lysis
Flagellum	Motility
Capsule	Protection, attachment
Endospore	Survival under harsh conditions
Pilus	Attachment, genetic exchange
Plasmid	Extra genetic information

Key Equations and Concepts

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

• Active Transport (generalized): $\text{Energy} + \text{Solute}_{\text{outside}} \rightarrow \text{Solute}_{\text{inside}}$

Additional info: Some explanations and examples have been expanded for clarity and completeness, based on standard microbiology textbooks.