Cell Structure and Function

Overview

This section explores the fundamental differences and similarities between prokaryotic and eukaryotic cells, focusing on how cell structure relates to function and microbial pathogenicity. Understanding these concepts is essential for recognizing how microbes cause disease and how their unique features can be targeted in antimicrobial strategies.

Processes of Life

Major Processes of Living Cells

• Growth: Increase in size and number of cells.

• Reproduction: Production of new cells or organisms, either asexually (binary fission in bacteria) or sexually (in eukaryotes).

• Responsiveness: Ability to respond to environmental stimuli (e.g., chemotaxis in bacteria).

• Metabolism: Sum of all chemical reactions in the cell, including catabolism (breaking down molecules for energy) and anabolism (building cellular components).

Example: Bacteria metabolize nutrients to grow and divide, responding to changes in their environment by moving toward nutrients or away from toxins.

Prokaryotic vs. Eukaryotic Cells

Comparison and Contrast

• Prokaryotes: Lack a membrane-bound nucleus; DNA is in a nucleoid region. Examples: Bacteria and Archaea.

• Eukaryotes: Have a true nucleus enclosed by a nuclear membrane and possess membrane-bound organelles. Examples: Fungi, Protozoa, Algae, plants, and animals.

Additional info: Prokaryotes reproduce mainly by binary fission, while eukaryotes can reproduce by mitosis or meiosis.

External Structures of Bacteria

Glycocalyces

• Definition: Gelatinous, sticky substance surrounding the outside of some bacterial cells.

• Composition: Polysaccharides, polypeptides, or both.

• Function: Protects cells from desiccation, helps in adherence to surfaces, and can prevent phagocytosis by host immune cells.

• Relevance to Health: Contributes to bacterial virulence (e.g., Streptococcus pneumoniae capsule).

Capsules vs. Slime Layers

• Capsule: Organized, firmly attached to cell wall; often associated with pathogenic bacteria.

• Slime Layer: Loosely attached, unorganized; aids in biofilm formation.

Bacterial Flagella

• Structure: Long, whip-like appendages made of flagellin protein.

• Function: Motility; enables bacteria to move toward or away from stimuli (chemotaxis).

Flagellar Arrangements

• Monotrichous: Single flagellum at one end.

• Lophotrichous: Tuft of flagella at one end.

• Amphitrichous: Flagella at both ends.

• Peritrichous: Flagella all over the cell surface.

Fimbriae, Pili, and Flagella

• Fimbriae: Short, bristle-like structures for attachment to surfaces and other cells.

• Pili: Longer than fimbriae, usually one or a few per cell; involved in conjugation (DNA transfer).

• Flagella: Primarily for motility.

Bacterial Cell Walls

Shapes and Arrangements

• Coccus: Spherical

• Bacillus: Rod-shaped

• Spirillum: Spiral-shaped

• Arrangements: Chains (strepto-), clusters (staphylo-), pairs (diplo-), etc.

Peptidoglycan Structure

• Sugar Portion: Alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

• Peptide Portion: Short chains of amino acids cross-linking the sugar chains.

Gram-Positive vs. Gram-Negative Cell Walls

Clinical Implications: Gram-negative bacteria are often more resistant to antibiotics due to the outer membrane, which can act as a barrier to drugs and contains endotoxin (LPS) that can trigger strong immune responses.

Acid-Fast Bacteria

• Cell Wall: Similar to Gram-positive but with waxy mycolic acids (e.g., Mycobacterium species).

• Staining: Retain carbol fuchsin dye after acid-alcohol wash (acid-fast stain).

Endospores

Formation and Function

• Definition: Dormant, tough, non-reproductive structures formed by some Gram-positive bacteria (e.g., Bacillus, Clostridium).

• Function: Allow survival in harsh conditions (heat, desiccation, chemicals).

• Formation: Sporulation is triggered by nutrient depletion; involves DNA replication, membrane invagination, and spore coat formation.

Nonmembranous Organelles: Ribosomes

Structure and Function

• Function: Protein synthesis (translation of mRNA into polypeptides).

• Structure: Composed of rRNA and proteins; two subunits.

Prokaryotic vs. Eukaryotic Ribosomes

Additional info: Some antibiotics (e.g., tetracycline) target 70S ribosomes, affecting bacteria but not eukaryotic cells.

Membranous Organelles (Eukaryotes)

Functions and Structures

• Nucleus: Contains genetic material (DNA); site of transcription.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Body: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosome: Contains digestive enzymes for breakdown of macromolecules and cellular debris.

• Peroxisome: Breaks down fatty acids and detoxifies hydrogen peroxide.

• Vesicle: Small membrane-bound sacs for transport within cells.

• Vacuole: Storage organelle, especially prominent in plant and fungal cells.

• Mitochondrion: Site of aerobic respiration and ATP production.

• Chloroplast: Site of photosynthesis in plants and algae.

Labeling: Students should be able to identify these organelles in diagrams and describe their functions.

Endosymbiotic Theory

Origin of Mitochondria, Chloroplasts, and Eukaryotic Cells

• Theory: Eukaryotic organelles such as mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Evidence:

  • Mitochondria and chloroplasts have their own circular DNA, similar to bacteria.

  • They have 70S ribosomes, like prokaryotes.

  • They replicate independently of the host cell by binary fission.

  • Double membranes suggest engulfment.

Example: The endosymbiotic origin of mitochondria explains why these organelles are susceptible to some antibiotics that target bacteria.

Additional info: This guide is based on the BIOL 31600 Basic Microbiology course at Purdue University Northwest and covers key learning outcomes for cell structure and function, as well as related exam preparation topics.