Fundamentals of Microbial Cell Structure, Function, and Metabolism

Overview of Microbial Cells

Types of Organisms

Microorganisms can be classified based on their cellular organization and structure. Understanding these differences is fundamental in microbiology.

• Unicellular: Consist of a single cell (e.g., most bacteria).

• Multicellular: Composed of multiple cells (e.g., some fungi).

Cell Theory

Cell theory is a foundational concept in biology, stating that all living organisms are composed of cells, and the cell is the smallest unit capable of life.

• All cells arise from previously existing cells (no spontaneous generation).

Cell Functions

Cells must be able to carry out essential life functions:

• Growth

• Maintenance

• Reproduction

• Waste removal

• Protection

Structural Components of All Cells

• Plasma membrane: Encloses the cell, controlling movement of substances.

• Cytoplasm: Gel-like substance inside the cell.

• DNA: Genetic material.

• Ribosomes: Sites of protein synthesis.

Cell Types

• Prokaryotic: Cells without a nucleus (e.g., bacteria, archaea).

• Eukaryotic: Cells with a nucleus (e.g., fungi, protozoa, plants, animals).

Prokaryotic Cells

Types of Prokaryotes

• Archaea: Distinct from bacteria, often found in extreme environments.

• Bacteria: Most common prokaryotes, diverse in form and function.

Cell Morphology

Prokaryotic cells exhibit various shapes and arrangements:

• Coccus: Spherical

• Bacillus: Rod-shaped

• Spirillum: Spiral-shaped

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

Cell Surface Structures

• Capsules and Slime Layers: Protective layers outside the cell wall. Capsules are well-organized; slime layers are more diffuse. Both can aid in evading host defenses and forming biofilms.

• Flagella: Structures for motility. Placement (polar, peritrichous, monotrichous, lophotrichous, amphitrichous) helps in identification.

• Pili and Fimbriae: Hair-like appendages for attachment and conjugation.

Cell Wall Structure

The cell wall provides shape and protection. Two major types are distinguished by the Gram stain:

• Gram-positive: Thick peptidoglycan layer, stains purple.

• Gram-negative: Thin peptidoglycan layer, outer membrane present, stains pink.

Peptidoglycan is a polymer of sugars and amino acids forming the cell wall's backbone.

Gram Stain Procedure

• Crystal violet (primary stain)

• Iodine (mordant)

• Alcohol (decolorizer)

• Safranin (counterstain)

Atypical Cell Walls

• Mycoplasma: Lack cell walls, have sterols in plasma membrane.

• Acid-fast bacteria: Waxy cell wall, resist Gram staining (e.g., Mycobacterium).

Cellular Components and Functions

Ribosomes

• Prokaryotic ribosomes: 70S (smaller)

• Eukaryotic ribosomes: 80S (larger)

Inclusions

• Storage granules for nutrients, gases, or other substances.

Endospores

• Highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium).

• Enable survival in harsh conditions.

Eukaryotic Cells

Key Organelles

• Nucleus: Contains DNA.

• Mitochondria: Site of ATP production via aerobic respiration.

• Chloroplasts: Site of photosynthesis in plants and algae.

• Endoplasmic Reticulum: Protein and lipid synthesis.

• Golgi Apparatus: Modification and sorting of proteins.

Origin of Mitochondria and Chloroplasts

• Endosymbiotic theory: These organelles originated from free-living bacteria engulfed by ancestral eukaryotic cells.

• Both have their own DNA and ribosomes.

Microbial Metabolism

Metabolism and Enzymes

Metabolism encompasses all chemical reactions in an organism. Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Characteristics of Enzymes: Specificity, activity, substrate, product, temperature, pH.

• Cofactors: Non-protein helpers (e.g., metal ions, vitamins).

• Inhibitors: Competitive and non-competitive inhibitors can regulate enzyme activity.

Nutritional Classification of Organisms

Organisms are classified based on their energy and carbon sources:

Energy Source	Carbon Source	Type
Light	CO2	Photoautotroph
Light	Organic	Photoheterotroph
Chemicals	CO2	Chemoautotroph
Chemicals	Organic	Chemoheterotroph

ATP Production Pathways

• Aerobic Cellular Respiration: Oxygen is the final electron acceptor.

• Anaerobic Respiration: Inorganic molecules other than oxygen serve as final electron acceptors.

• Fermentation: Organic molecules serve as electron acceptors; less ATP produced.

Glycolysis

Glycolysis is the breakdown of glucose to pyruvate, producing ATP and NADH.

• Occurs in both cellular respiration and fermentation.

Lipid and Protein Catabolism

• Microbes can break down lipids and proteins for energy.

Bacterial Growth

Phases of Growth

Bacterial populations change over time in a predictable pattern:

• Lag phase: Adaptation, little growth.

• Log (exponential) phase: Rapid cell division.

• Stationary phase: Nutrient depletion slows growth; death rate equals division rate.

• Death phase: Cells die off.

Binary Fission

Bacteria reproduce by binary fission, a process where one cell divides into two identical daughter cells.

• Steps: DNA replication, cell elongation, septum formation, cell separation.

Key Terms and Definitions

• Biofilm: A community of microorganisms attached to a surface, embedded in a self-produced matrix.

• Plasmid: Small, circular DNA molecule in bacteria, separate from chromosomal DNA.

• Endospore: Highly resistant, dormant structure formed by some bacteria.

• Peptidoglycan: Polymer forming the bacterial cell wall.

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

Important Equations

• Enzyme Reaction Rate: $\text{Rate} = k[\text{Enzyme}][\text{Substrate}]$

• ATP Cycle: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$

• General Respiration Equation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$

Comparisons: Prokaryotes vs. Eukaryotes

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Ribosome Size	70S	80S
Cell Wall	Peptidoglycan (bacteria)	Cellulose (plants), chitin (fungi), none (animals)
Organelles	Absent	Present
DNA Structure	Circular	Linear

Additional info:

• Some details about cell wall structure, Gram stain, and metabolism were expanded for clarity.

• Tables and equations were inferred and formatted for study purposes.