Cell Structure and Function

Introduction

This study guide covers the fundamental differences between prokaryotic and eukaryotic cells, their representative microorganisms, cellular organization, and specialized structures. Understanding these differences is essential for microbiology students, as it forms the basis for studying microbial physiology, genetics, and classification.

Review Questions

• Halophile: A microorganism that grows in high concentrations of salt.

• Ignaz Semmelweis: Credited with inventing hand washing in medical practice.

• Mycology: The study of fungi.

• Anton von Leeuwenhoek: The first to observe and describe microorganisms using a microscope.

Prokaryotes vs. Eukaryotes

Definitions and Key Differences

• Prokaryotes: Organisms without a true nucleus or membrane-bound organelles. Includes Bacteria and Archaea.

• Eukaryotes: Organisms with a true nucleus and membrane-bound organelles. Includes Fungi, Algae, Protozoa, and all multicellular organisms.

Key Differences:

• Prokaryotes: Disorganized, non-compartmentalized cytoplasm.

• Eukaryotes: Organized, compartmentalized cytoplasm with specialized organelles.

Representative Microorganisms

• Prokaryotes: Streptococcus, Staphylococcus, E. coli, Cyanobacteria

• Eukaryotes: Diatoms (algae), Mold (fungi), Saccharomyces (yeast), Tetrahymena (protozoa)

Cell Size Comparison

• Viruses: ~0.03–0.3 μm

• Bacteria: ~0.5–5 μm

• Eukaryotic cells: ~10–100 μm

• Chicken egg: ~43 mm diameter (largest single cell)

Cellular Organization

Chromosomes

• Prokaryotes: Single, circular DNA molecule (nucleoid region).

• Eukaryotes: Multiple, linear chromosomes associated with histone proteins, contained within a nucleus.

Cell Membrane Structure

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

• Functions as a selective barrier and site for metabolic activities.

Ribosomes and Protein Synthesis

• Prokaryotes: 70S ribosomes (composed of 30S and 50S subunits), free in cytoplasm.

• Eukaryotes: 80S ribosomes (composed of 40S and 60S subunits), mostly bound to endoplasmic reticulum (ER).

• Clinical Relevance: Some antibiotics (e.g., tetracycline, aminoglycosides) target 70S ribosomes, inhibiting bacterial protein synthesis without affecting eukaryotic cells.

Photosynthetic Pigments

• Prokaryotes: Chlorophyll and other pigments (e.g., bacteriochlorophyll, bacteriorhodopsin) dissolved in membranes.

• Eukaryotes: Chlorophyll contained within chloroplasts (membrane-bound organelles).

Cell Wall Composition

• Bacteria: Thick, rigid cell wall containing peptidoglycan.

• Algae: Cell wall made of cellulose.

• Fungi: Cell wall made of chitin.

• Animals: No cell wall.

Eukaryotic Organelles

Nucleus

• Contains genetic information (DNA) and acts as the control center of the cell.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; site of protein synthesis.

• Smooth ER: Involved in lipid synthesis and detoxification.

Golgi Apparatus

• Packages and sorts proteins and lipids for transport within or outside the cell.

Lysosomes and Vacuoles

• Contain digestive enzymes (e.g., hydrolases) for breaking down waste and cellular debris.

Mitochondria

• Site of cellular respiration; converts chemical energy from food into ATP (the cell's energy currency).

• Known as the "powerhouse of the cell."

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• Mitochondria originated from a proto-eukaryote engulfing a gram-negative bacterium.

• Chloroplasts originated from a proto-eukaryote engulfing a cyanobacterium.

• Evidence:

  • Both have double membranes (like gram-negative bacteria).

  • Both contain prokaryotic 70S ribosomes.

  • Both have circular DNA.

Note: Some eukaryotes have both 80S and 70S ribosomes (e.g., in mitochondria and chloroplasts).

Bacterial Shapes and Configurations

Main Types

• Bacilli: Rod-shaped bacteria (e.g., Bacillus species). Can form chains called streptobacilli.

• Cocci: Spherical bacteria. Arrangements include:

  • Diplococci: Pairs (e.g., Neisseria gonorrhoeae)

  • Streptococci: Chains (e.g., Streptococcus pyogenes)

  • Staphylococci: Irregular clusters (e.g., Staphylococcus aureus)

• Spirals: Spiral-shaped bacteria, including:

  • Spirilla: Rigid, spiral-shaped

  • Spirochetes: Flexible, move via axial filaments (e.g., Treponema pallidum)

  • Vibrios: Comma-shaped (e.g., Vibrio cholerae)

Unusual Bacterial Anatomy

• Some bacteria have branching filaments (e.g., Actinomyces).

• Others have appendages or unique shapes (e.g., Caulobacter).

• Archaea can have unusual cell wall compositions and shapes (e.g., Thermoplasma).

Internal Structures of Bacteria

Cytoplasm

• Gel-like matrix composed of water, proteins, amino acids, carbohydrates, nucleotides, salts, vitamins, and ions.

Genetic Material

• Chromosome: Single, circular DNA molecule.

• Nucleoid: Region where the chromosome is located.

• Plasmids: Small, circular, extrachromosomal DNA molecules carrying additional genes (e.g., antibiotic resistance).

Ribosomes

• Composed of rRNA and protein; 70S type (30S and 50S subunits).

• Targeted by antibiotics such as tetracycline.

Inclusion Bodies

• Storage granules for nutrients (e.g., glycogen, sulfur, lipids).

• Chromatic granules contain polyphosphate and stain well.

Magnetosomes

• Contain magnetite (an iron compound) that aligns with magnetic fields.

• Help bacteria orient and move toward preferred environments.

Summary Table: Prokaryotes vs. Eukaryotes

Key Equations and Terms

• Svedberg Unit (S): A measure of sedimentation rate during centrifugation, used to describe ribosome size.

• Peptidoglycan: A polymer that forms a mesh-like layer outside the plasma membrane of most bacteria, providing structural strength.

Additional info:

• Some content was inferred and expanded for clarity and completeness, such as the full definitions of terms and the clinical relevance of ribosome differences.