Cellular Structures and Functions in Microbiology

Cellular Structures and Functions

Requirements of Life

All living organisms share several fundamental characteristics that define life. These requirements are essential for the survival and reproduction of cells.

• Growth: Increase in size of an organism. Growth is distinct from reproduction, as it refers to the enlargement of a single organism rather than an increase in number.

• Reproduction: The ability to produce new individuals, either sexually or asexually, leading to an increase in population.

• Responsiveness: The capacity to sense and respond to environmental changes, adjusting internal or external properties accordingly.

• Metabolism: The sum of all chemical reactions in a cell, including anabolic (building up) and catabolic (breaking down) processes. Nutrients are taken in and used for energy and biosynthesis.

The cell is the basic unit of life, and all living things are composed of one or more cells.

Types of Cells

Prokaryotes vs. Eukaryotes

Cells are classified into two main types based on their structural characteristics.

• Prokaryotes: Include the domains Archaea and Bacteria. They are generally small (0.5–10 μm), lack a membrane-bound nucleus, and do not have membrane-bound organelles.

• Eukaryotes: Include Protozoa, Fungi, Plants, and Animals. They have a membrane-bound nucleus, various membrane-bound organelles, and are typically larger (10–100 μm).

Prokaryotic Cell Structures

External Structures

• Cell Wall: Provides shape and protection. In bacteria, composed of peptidoglycan (alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)).

• Gram-Positive Bacteria: Thick peptidoglycan layer, contains teichoic acids (negatively charged, attract Ca2+ and Mg2+), and lipoteichoic acids. Stain purple in Gram stain.

• Gram-Negative Bacteria: Thin peptidoglycan layer between inner and outer membranes. Outer membrane contains lipopolysaccharide (LPS) with Lipid A (endotoxin), porins (control entry/exit), and a periplasmic space. Stain pink in Gram stain.

Feature	Gram-Positive	Gram-Negative
Peptidoglycan Thickness	Thick	Thin
Teichoic Acids	Present	Absent
Outer Membrane	Absent	Present
LPS	Absent	Present
Gram Stain Color	Purple	Pink

• Glycocalyx: Sticky carbohydrate layer outside the cell wall. Can be a capsule (firmly attached) or slime layer (loosely attached). Functions include protection from desiccation, immune evasion, and inhibition of phagocytosis.

• Flagella: Provide motility. Composed of a basal body, hook, and filament (flagellin). Movement is powered by hydrogen ions passing through Mot proteins, causing rotation. Arrangements include monotrichous, lophotrichous, amphitrichous, and peritrichous. Enable taxis (movement toward or away from stimuli).

• Fimbriae: Short protein projections for attachment to surfaces or host cells.

• Pili: Hollow protein tubes used for DNA exchange (conjugation) and sometimes attachment to host cells. DNA transferred is usually a plasmid.

Cell Membrane

The prokaryotic cell membrane is a selectively permeable barrier described by the fluid mosaic model. It consists of phospholipids and integral proteins (forming pores, channels, receptors, and carriers).

• Passive Transport: No energy required; moves substances down their concentration gradient. Includes simple diffusion, facilitated diffusion, and osmosis (isotonic, hypotonic, hypertonic conditions).

• Active Transport: Requires energy; moves substances against their concentration gradient. Types include uniport, antiport, symport, and group translocation.

Biofilms

Biofilms are complex communities of microorganisms encased in a self-produced matrix of organic molecules. They provide protection and are formed via quorum sensing (cell-to-cell signaling).

Internal Structures

• Cytoplasm: Semi-liquid matrix containing cytosol (water, ions, macromolecules), inclusions (storage granules), ribosomes, and cytoskeleton.

• Nucleoid: Region containing the cell's DNA (not membrane-bound).

• Ribosomes (70S): Sites of protein synthesis, composed of 50S (large) and 30S (small) subunits.

• Cytoskeleton: Provides shape; rod-shaped bacteria have cytoskeletons, cocci generally do not.

• Endospores: Highly resistant structures formed by some bacteria for survival under harsh conditions. Formation involves chromosome duplication, membrane formation, cortex and spore coat development. Endospores resist heat, UV, desiccation, alcohol, hydrogen peroxide, and bleach.

Archaea

Structures of Archaea

• Glycocalyces: Gelatinous, sticky polysaccharides, may aid in biofilm formation.

• Flagella: Structurally similar to bacterial flagella but thinner, grow at the base, contain different amino acids and sugars, and are powered by ATP (not hydrogen ions).

• Fimbriae and Hami: Fimbriae are similar to those in bacteria; hami (hook-like structures) aid in attachment.

• Cell Walls: Composed of specialized proteins, sometimes pseudopeptidoglycan, but never true peptidoglycan.

• Cell Membrane: Contains lipids with ether bonds (not ester bonds) and lacks phosphates.

• Cytoplasm: Contains 70S ribosomes (proteins more similar to eukaryotes), no membrane-bound organelles.

Eukaryotic Cell Structures

External Structures

• Glycocalyces: Sticky carbohydrates anchored to the membrane, involved in protection, cell recognition, and communication.

• Cell Walls: Present in plants, fungi, and algae (not animals). Provide protection, shape, and resistance to osmotic pressure. Composition varies: cellulose (plants), chitin (fungi), and various molecules (algae: cellulose, agar, carrageenan, silicates, algin, calcium carbonate).

Cell Membrane

The eukaryotic cell membrane follows the fluid mosaic model and contains sterols (e.g., cholesterol in animals, ergosterol in fungi) for stability.

• Transport Mechanisms: Diffusion, facilitated diffusion, active transport (uniport, antiport, symport), endocytosis (phagocytosis, pinocytosis), and exocytosis.

Internal Structures

• Cytoplasm: Contains cytosol and organelles (membrane-bound and non-membrane-bound).

• Ribosomes (80S): Made of 60S (large) and 40S (small) subunits. Two types: free (in cytoplasm) and bound (to rough endoplasmic reticulum or nuclear envelope).

• Cytoskeleton: Includes microtubules (tubulin, dynamic, involved in mitosis/meiosis, organelle movement, flagella/cilia), microfilaments (actin, dynamic, involved in organelle movement, cytokinesis, cytoplasmic streaming), and intermediate filaments (keratin, permanent, provide shape/support).

• Flagella and Cilia: Both have a 9+2 arrangement of microtubule pairs in the shaft and a 9+0 arrangement of triplets in the basal body. Cilia are shorter and more numerous than flagella.

Membrane-Bound Organelles

• Nucleus: Contains nucleoplasm, chromatin, nucleolus, and is surrounded by a double membrane (nuclear envelope) with nuclear pores. Site of DNA storage and RNA synthesis.

• Endoplasmic Reticulum (ER):

  • Rough ER (RER): Studded with ribosomes, site of protein synthesis.

  • Smooth ER (SER): Lacks ribosomes, involved in lipid synthesis and other functions.

• Golgi Body: Processes and packages proteins and lipids received from the ER into vesicles for transport.

• Lysosomes, Peroxisomes, Vesicles, and Vacuoles: Membrane sacs for digestion (lysosomes), breakdown of fatty acids (peroxisomes), storage (vacuoles), and transport (vesicles).

• Mitochondria: Site of aerobic respiration. Composed of two membranes (outer and inner), with inner membrane forming cristae. Contains matrix, 70S ribosomes, and its own DNA.

• Chloroplasts: Found in photosynthetic eukaryotes. Composed of two membranes, contain thylakoids (site of light reactions), grana (stacks of thylakoids), and stroma (site of Calvin cycle).

Endosymbiosis Theory

Proposed by Lynn Margulis, this theory suggests that mitochondria and chloroplasts originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Evidence includes the presence of 70S ribosomes and their own DNA in these organelles.

Key Equations and Concepts

• Osmosis: Movement of water across a semipermeable membrane from low to high solute concentration.

• Concentration Gradient: $\Delta C = C_{outside} - C_{inside}$

• Endosymbiosis Evidence: 70S ribosomes and circular DNA in mitochondria and chloroplasts.

Example: Escherichia coli is a Gram-negative bacterium with a thin peptidoglycan layer and an outer membrane containing LPS, making it resistant to certain antibiotics.

Additional info: Some details, such as the specific arrangement of cytoskeletal elements and the full process of endospore formation, have been expanded for academic completeness.