Cell Structure and Function in Microbiology: Study Notes

Cell Structure and Function

Major Processes of Living Cells

Understanding the fundamental processes of living cells is essential in microbiology, as these processes underlie the survival, growth, and reproduction of microorganisms.

• Metabolism: The sum of all chemical reactions occurring within a cell, including catabolism (breakdown of molecules) and anabolism (synthesis of molecules).

• Growth: Increase in cell size and number, often measured by cell division rates.

• Reproduction: The process by which cells produce offspring, typically through binary fission in prokaryotes and mitosis in eukaryotes.

• Response to Environment: Cells detect and respond to changes in their surroundings, such as nutrient availability or toxins.

• Homeostasis: Maintenance of stable internal conditions despite external fluctuations.

Overview of Eukaryotic and Prokaryotic Cells

Microbiology distinguishes between two major cell types: prokaryotic and eukaryotic cells. Each type has unique structural and functional characteristics.

• Prokaryotic Cells: Lack a true nucleus and membrane-bound organelles. Examples include Bacteria and Archaea.

• Eukaryotic Cells: Possess a true nucleus and various membrane-bound organelles. Examples include Fungi, Protozoa, and Algae.

• Comparison: Eukaryotic cells are generally larger and more complex than prokaryotic cells.

External Structures of Bacterial Cells

Bacterial cells possess various external structures that contribute to their survival, motility, and pathogenicity.

• Glycocalyces: Gel-like layer outside the cell wall, including capsules and slime layers. Functions include protection from desiccation and immune evasion.

• Flagella: Long, whip-like appendages used for motility. Bacterial flagella rotate to propel the cell.

• Fimbriae and Pili: Short, hair-like structures involved in attachment to surfaces and in genetic exchange (pili).

• Comparison: Flagella are primarily for movement, while fimbriae and pili are for attachment and conjugation.

Bacterial Cell Walls

The cell wall provides structural support and determines the shape of bacterial cells. It is also critical in distinguishing between Gram-positive and Gram-negative bacteria.

• Composition: Mainly composed of peptidoglycan, a polymer of sugars and amino acids.

• Gram-Positive Cell Walls: Thick peptidoglycan layer, teichoic acids present.

• Gram-Negative Cell Walls: Thin peptidoglycan layer, outer membrane containing lipopolysaccharides.

• Function: Protects against osmotic pressure and environmental stress.

• Clinical Implications: Gram staining is used to classify bacteria and guide antibiotic therapy.

Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria to survive extreme conditions.

• Formation: Triggered by nutrient depletion; involves complex cellular differentiation.

• Function: Allows bacteria to withstand heat, desiccation, chemicals, and radiation.

• Clinical Importance: Endospore-forming bacteria (e.g., Bacillus, Clostridium) are significant in medicine due to their persistence and resistance.

Nomenclature and Classification of Ribosomes

Ribosomes are the molecular machines responsible for protein synthesis. Their structure and classification differ between prokaryotes and eukaryotes.

• Prokaryotic Ribosomes: 70S (composed of 50S and 30S subunits).

• Eukaryotic Ribosomes: 80S (composed of 60S and 40S subunits).

• Function: Translate mRNA into proteins.

• Antibiotic Target: Many antibiotics selectively inhibit prokaryotic ribosomes.

Membranous Organelles in Eukaryotes

Eukaryotic cells contain various membrane-bound organelles, each with specialized functions.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Endoplasmic Reticulum (ER): Synthesizes proteins (rough ER) and lipids (smooth ER).

• Golgi Body: Modifies, sorts, and packages proteins and lipids for secretion or use within the cell.

• Mitochondria: Site of cellular respiration and energy (ATP) production.

• Chloroplasts: Present in photosynthetic eukaryotes; site of photosynthesis.

• Other Organelles: Lysosomes (digestion), peroxisomes (detoxification), vacuoles (storage).

Endosymbiotic Theory

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

• Concept: Mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Evidence: These organelles have their own DNA, ribosomes similar to prokaryotes, and replicate independently of the cell.

• Significance: Explains the double-membrane structure and genetic similarities to bacteria.

Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound Organelles	Absent	Present
Ribosome Size	70S	80S
Cell Wall Composition	Peptidoglycan (in bacteria)	Cellulose (plants), chitin (fungi), or absent
Examples	Bacteria, Archaea	Fungi, Protozoa, Algae, Animals, Plants

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

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Layer	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
Lipopolysaccharide	Absent	Present
Staining Result	Purple	Pink/Red

Key Equations

• Binary Fission (Bacterial Growth):

• $N = N_0 \times 2^n$ Where $N$ is the final number of cells, $N_0$ is the initial number of cells, and $n$ is the number of generations.

Example

• Antibiotic Action: Penicillin targets the synthesis of peptidoglycan, making it effective against Gram-positive bacteria.

• Endosymbiotic Theory Application: The presence of circular DNA in mitochondria supports their prokaryotic origin.

Additional info: Academic context and explanations have been expanded for clarity and completeness.