Bacterial Cell Structure and Function

Characteristics of Life

Living organisms share several fundamental characteristics that distinguish them from non-living matter.

• Reproduction: The ability to produce offspring, either sexually or asexually.

• Metabolism: The sum of all chemical reactions that occur within a cell, including catabolism and anabolism.

• Responsiveness: The ability to sense and respond to environmental changes.

• Growth: Increase in size and/or number of cells.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as either prokaryotic or eukaryotic based on their structural features.

• Prokaryotic cells lack a true nucleus and membrane-bound organelles. Examples: Bacteria and Archaea.

• Eukaryotic cells have a true nucleus and various membrane-bound organelles. Examples: Fungi, Protozoa, Plants, and Animals.

External Structures of Bacterial Cells

Bacteria possess various external structures that contribute to their survival and pathogenicity.

• Glycocalyx: A viscous, gelatinous polymer external to the cell wall, composed of polysaccharide, polypeptide, or both.

• Capsule: A well-organized, firmly attached glycocalyx. Functions include protection from phagocytosis and desiccation.

• Slime layer: A loosely attached, unorganized glycocalyx. Aids in adherence to surfaces.

• Biofilms: Communities of microorganisms attached to a surface, embedded in a self-produced matrix. Example: dental plaque.

Flagella and Motility

Flagella are long, whip-like appendages that provide motility to many bacteria.

• Flagellar arrangement: Monotrichous (single flagellum), lophotrichous (tuft at one end), amphitrichous (flagella at both ends), peritrichous (flagella all over).

• Movement: Bacteria move by runs (straight movement) and tumbles (random changes in direction).

• Taxic responses: Chemotaxis (movement toward/away from chemicals), phototaxis (light), magnetotaxis (magnetic fields).

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

• Pili: Longer than fimbriae, involved in conjugation (DNA transfer).

Bacterial Cell Wall

The cell wall provides structural support and shape to bacterial cells.

• Peptidoglycan: A polymer consisting of sugars and amino acids forming a mesh-like layer outside the plasma membrane.

• Gram-positive bacteria: Thick peptidoglycan layer, teichoic acids present.

• Gram-negative bacteria: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS).

• Mycobacteria: Contain mycolic acid in their cell wall, making them acid-fast.

Peptidoglycan Structure:

• Composed of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by peptide bridges.

Acid-fast stain is used to identify Mycobacteria due to their waxy cell wall.

Bacterial Internal Structures

Bacteria contain several internal structures essential for their survival and function.

• Nucleoid: Region containing the bacterial chromosome (DNA), not membrane-bound.

• Plasmids: Small, circular DNA molecules that replicate independently of the chromosome.

• Ribosomes: Sites of protein synthesis. Bacterial ribosomes are 70S (composed of 50S and 30S subunits).

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

• Endospores: Highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium) for survival under adverse conditions.

Endospores

Endospores are specialized structures that allow bacteria to withstand extreme conditions.

• Formation: Sporulation is triggered by nutrient deprivation.

• Structure: Contains a core, cortex, spore coat, and exosporium.

• Function: Resistance to heat, desiccation, chemicals, and radiation.

• Spore positions: Central, subterminal, terminal (location within the cell).

Archaea vs. Bacteria

Archaea are prokaryotes but differ significantly from bacteria in several aspects.

• Cell wall composition: Archaea lack peptidoglycan; may have pseudopeptidoglycan or proteinaceous walls.

• Membrane lipids: Ether-linked in Archaea, ester-linked in Bacteria.

• Ribosomes: Archaea have 70S ribosomes but with different protein and rRNA composition.

Eukaryotic Cell Structures

Eukaryotic cells contain membrane-bound organelles and a true nucleus.

• Nucleus: Contains genetic material (DNA) enclosed by a nuclear envelope.

• Ribosomes: 80S (60S and 40S subunits), free or bound to endoplasmic reticulum.

• Cytoskeleton: Network of protein filaments for cell shape and movement.

Staining Techniques

Staining is essential for visualizing and differentiating microorganisms under the microscope.

• Gram stain: Differentiates bacteria into Gram-positive (purple) and Gram-negative (pink/red).

• Acid-fast stain: Identifies acid-fast organisms like Mycobacterium.

Summary Table: Comparison of Bacterial Cell Wall Types

Key Equations and Terms

• Peptidoglycan repeating unit:

• Ribosome sedimentation coefficients: ,

Examples and Applications

• Example of capsule function: Streptococcus pneumoniae uses its capsule to evade phagocytosis.

• Example of endospore-forming bacteria: Bacillus anthracis (anthrax), Clostridium botulinum (botulism).

• Example of acid-fast bacteria: Mycobacterium tuberculosis.

Additional info:

• Some questions in the original file refer to specific textbook figures and tables (e.g., Table 3.1, Fig. 3.21). For detailed diagrams, refer to your course textbook.

• Concept mapping is a useful study tool for visualizing relationships between cell structures and their functions.