Cellular Structures of Microbes & Microbial Metabolism

Cellular Structures of Microbes

Introduction

Microbes, including bacteria, archaea, fungi, protozoa, and algae, exhibit diverse cellular structures that are fundamental to their function and classification. Understanding these structures is essential for studying microbial physiology, taxonomy, and their roles in the environment and disease.

Prokaryotic vs. Eukaryotic Cells

Microbial cells are broadly classified into prokaryotic and eukaryotic types based on their structural organization.

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No nucleus (DNA in nucleoid region)	True nucleus (membrane-bound)
Internal Structures	Lack membrane-bound organelles	Contain membrane-bound organelles
Size	Generally small (~1.0 μm in diameter)	Generally larger (>10 μm in diameter)
Complexity	Simple structure	More complex structure
Examples	Bacteria, Archaea	Algae, Protozoa, Fungi, Animals, Plants

Prokaryotic Cell Structure

Prokaryotic cells, such as those of bacteria and archaea, have a relatively simple organization:

• Cell Wall: Provides structural support and shape; composed of peptidoglycan in bacteria.

• Plasma Membrane: Phospholipid bilayer that controls movement of substances in and out of the cell.

• Cytoplasm: Gel-like substance containing enzymes, nutrients, and ribosomes.

• Nucleoid: Region containing the cell's genetic material (DNA), not enclosed by a membrane.

• Ribosomes: Sites of protein synthesis; prokaryotes have 70S ribosomes.

• Other Structures: May include flagella (motility), pili (attachment), and capsules (protection).

Eukaryotic Cell Structure

Eukaryotic microbial cells (e.g., fungi, protozoa, algae) possess complex internal structures:

• Nucleus: Membrane-bound organelle containing genetic material; site of DNA replication and transcription.

• Cell Wall: Present in fungi (composed of chitin) and algae (composed of polysaccharides); absent in animal cells.

• Plasma Membrane: Phospholipid bilayer with embedded proteins and sterols (e.g., cholesterol) for fluidity.

• Cytoskeleton: Network of protein fibers (microtubules, microfilaments, intermediate filaments) providing structural support and facilitating intracellular transport.

• Ribosomes: Sites of protein synthesis; eukaryotes have 80S ribosomes (composed of 60S and 40S subunits).

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; involved in protein synthesis and modification.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

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

• Mitochondria: Double-membraned organelles; site of ATP production via cellular respiration. Contain their own DNA and 70S ribosomes.

• Other Organelles: May include lysosomes (digestion), peroxisomes (detoxification), and chloroplasts (photosynthesis in algae).

Specialized Processes in Eukaryotes

• Endocytosis: Process by which cells absorb particles by engulfing them.

  • Phagocytosis: Uptake of solid particles.

  • Pinocytosis: Uptake of liquid particles.

• Exocytosis: Release of substances from the cell via fusion of vesicles with the plasma membrane.

Summary Table: Key Differences

Structure	Prokaryotic	Eukaryotic
Cell Wall Composition	Peptidoglycan (bacteria)	Chitin (fungi), polysaccharides (algae)
Ribosome Size	70S	80S
DNA Location	Nucleoid	Nucleus
Organelles	Absent	Present

Microbial Metabolism

Introduction

Microbial metabolism encompasses all the biochemical reactions that occur within a microbe, enabling it to acquire energy, grow, and reproduce. These reactions are tightly regulated and essential for cellular function.

Types of Metabolic Pathways

• Catabolism:

  • Breaks down larger molecules into smaller products.

  • Releases energy, some of which is stored in ATP.

  • Example: Glycolysis, where glucose is broken down to pyruvate.

• Anabolism:

  • Synthesizes large molecules from smaller building blocks.

  • Requires energy, usually supplied by ATP.

  • Example: Synthesis of proteins from amino acids.

ATP: The Energy Currency

• Adenosine Triphosphate (ATP): Stores and transfers energy within cells.

• Energy is released from ATP by hydrolysis of its terminal phosphate group.

• Equation: $ATP ightarrow ADP + P_i + ext{energy}$

Oxidation and Reduction Reactions

Many metabolic reactions involve the transfer of electrons, known as oxidation-reduction (redox) reactions.

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Electron donor transfers electrons to an electron acceptor.

• Mnemonic: OIL RIG – Oxidation Is Loss, Reduction Is Gain.

Summary of Metabolism

• Metabolism is the sum of all controlled biochemical reactions in a microbe.

• Catabolic pathways break down molecules and release energy.

• Anabolic pathways build complex molecules and require energy.

• Redox reactions are central to energy transfer in metabolism.

Example: Synthesis of Membrane Lipids

• Anabolic pathway: Fatty acids and glycerol are joined to form membrane lipids, requiring ATP.

Additional info:

• Some eukaryotic cells have multiple nuclei (e.g., certain fungi).

• Mitochondria and chloroplasts are believed to have originated from prokaryotic cells via endosymbiosis.