Cell Structure and Function in Prokaryotes

Overview of Prokaryotic Cell Structure

Prokaryotic cells, which include all Bacteria and Archaea, are characterized by their simple structure and lack of membrane-bound organelles. Their main components include the cell envelope, cytoplasm, and nucleoid region.

• Cell Envelope: Composed of the cytoplasmic membrane and, in most cases, a cell wall.

• Cytoplasm: Contains ribosomes, storage granules, and the nucleoid.

• Nucleoid: Region containing the cell's genetic material (DNA).

Cell Envelope: Cytoplasmic Membrane

The cytoplasmic membrane is a selectively permeable barrier that separates the interior of the cell from the external environment. It is primarily composed of a phospholipid bilayer and proteins.

• Phospholipid Bilayer: Provides fluidity and flexibility to the membrane. In Bacteria, the bilayer contains fatty acid tails, while Archaea have different lipid structures.

• Transmembrane Proteins: Function as gates for nutrients and waste, and as sensors/receptors for environmental signals.

• Fluid Mosaic Model: Describes the dynamic nature of the membrane, with proteins moving within the lipid bilayer.

Transport Across the Cytoplasmic Membrane

Transport of substances across the membrane can occur via simple diffusion, facilitated diffusion, or active transport. The membrane is selectively permeable, allowing only certain molecules to pass through.

• Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2) down their concentration gradient.

• Facilitated Diffusion: Movement of molecules via specific transmembrane proteins.

• Active Transport: Movement of molecules against their concentration gradient, requiring energy (often from ATP or proton motive force).

Osmosis and Water Movement

Osmosis is the movement of water across the membrane, driven by differences in solute concentration. Water moves from areas of low solute concentration (hypotonic) to high solute concentration (hypertonic).

• Hypotonic Solution: Water enters the cell, causing swelling.

• Isotonic Solution: No net movement of water.

• Hypertonic Solution: Water leaves the cell, causing shrinkage.

Cell Wall Structure and Function

The cell wall provides structural support and shape to prokaryotic cells. In Bacteria, it is primarily composed of peptidoglycan, a polymer of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by peptides.

• Gram-Positive Bacteria: Thick peptidoglycan layer with peptide interbridges.

• Gram-Negative Bacteria: Thin peptidoglycan layer with direct peptide linkages and an outer membrane containing lipopolysaccharide (LPS).

Gram Stain and Bacterial Identification

The Gram stain differentiates bacteria based on cell wall structure. Gram-positive bacteria retain the crystal violet dye (purple), while Gram-negative bacteria do not (pink/red).

Lipopolysaccharide (LPS) in Gram-Negative Bacteria

LPS, also known as endotoxin, is a major component of the outer membrane of Gram-negative bacteria. It consists of three parts: O antigen (variable), core polysaccharide, and Lipid A (toxic, inflammatory).

• Lipid A: Anchors LPS in the membrane and triggers strong immune responses.

• O Antigen: Used for bacterial identification.

• PAMP: LPS acts as a Pathogen Associated Molecular Pattern, recognized by the immune system.

Porins and Periplasm

Porins are proteins in the outer membrane of Gram-negative bacteria that allow the passage of small molecules. The periplasm is the space between the cytoplasmic and outer membranes, containing enzymes and proteins for nutrient processing and defense.

• Porins: Contribute to antibiotic resistance by controlling entry of substances.

• Periplasm: Site of nutrient degradation, electron transport, and detoxification.

Specialized Cell Envelope Structures

• Mycoplasma: Bacteria lacking a cell wall, making them resistant to many antibiotics.

• Archaea: Diverse cell wall structures, but lack peptidoglycan.

• Slime Layers & Capsules (Glycocalyx): Polysaccharide layers that protect cells and aid in biofilm formation (e.g., dental plaque).

Flagella and Cell Motility

Flagella are long, whip-like structures used for motility. They are composed of flagellin and are powered by a proton gradient. Bacterial flagella are structurally distinct from those of Archaea and Eukaryotes.

• Immunodominant Antigen: Recognized by the immune system as a PAMP.

• Movement: Bacteria move by alternating "runs" (straight movement) and "tumbles" (random reorientation).

Pili and Fimbriae

Pili are hair-like appendages that facilitate attachment to surfaces, movement, and genetic exchange (sex pili for conjugation).

• Fimbriae: Specialized pili for adherence.

• Sex Pili: Involved in DNA transfer between cells (conjugation).

Internal Structures: Chromosome, Plasmids, and Ribosomes

Prokaryotic cells contain a single, circular chromosome located in the nucleoid. Plasmids are smaller, accessory DNA molecules that often carry antibiotic resistance genes. Ribosomes (70S) are the sites of protein synthesis and are distinct from eukaryotic ribosomes (80S).

Endospores

Endospores are highly resistant, dormant structures formed by certain bacteria (e.g., Bacillus, Clostridium) in response to harsh conditions. They can survive extreme heat, desiccation, chemicals, and radiation.

• Formation: Involves asymmetric cell division and development of a tough spore coat.

• Significance: Endospores are important in disease transmission and sterilization challenges.

Binary Fission

Prokaryotic cells reproduce by binary fission, a process in which the cell duplicates its chromosome, grows, and divides into two identical daughter cells.

Cell Structure and Function in Eukaryotes

Overview of Eukaryotic Cell Structure

Eukaryotic cells (animals, plants, fungi, protists) are larger and more complex than prokaryotic cells. They contain membrane-bound organelles and a true nucleus.

• Nucleus: Contains linear DNA wound around histones.

• Organelles: Specialized compartments for various cellular functions (e.g., mitochondria, chloroplasts, ER, Golgi apparatus).

• Cytoskeleton: Provides structural support and facilitates movement.

Plasma Membrane and Endocytosis

The plasma membrane of eukaryotes is similar in structure to that of prokaryotes but is involved in more complex processes such as endocytosis and exocytosis.

• Endocytosis: Uptake of materials via phagocytosis, pinocytosis, or receptor-mediated mechanisms.

• Exocytosis: Secretion of materials out of the cell.

Ribosomes

Eukaryotic ribosomes are 80S (60S + 40S subunits), distinct from the 70S ribosomes of prokaryotes. Mitochondria and chloroplasts contain 70S ribosomes, supporting the endosymbiotic theory.

Cytoskeleton

The cytoskeleton is composed of actin filaments, microtubules, and intermediate filaments, each with specialized functions.

• Actin: Involved in cell shape and movement; constantly remodeled.

• Microtubules: Serve as tracks for organelle movement, form spindle fibers during mitosis, and are major components of cilia and flagella.

• Intermediate Filaments: Provide structural support; protein composition varies by cell type.

Nucleus and Genetic Material

The nucleus stores DNA and is the site of RNA synthesis. Eukaryotic DNA is linear and organized with histones. Cell division occurs via mitosis, producing diploid cells.

Mitochondria and Chloroplasts

Mitochondria are the sites of ATP synthesis via degradation of organic compounds. Chloroplasts perform photosynthesis in plants and algae. Both organelles have internal folds to maximize surface area and contain 70S ribosomes, supporting their prokaryotic origin (endosymbiotic theory).

Endoplasmic Reticulum (ER) and Golgi Apparatus

• ER: Rough ER is studded with ribosomes for protein synthesis; smooth ER is involved in lipid synthesis and detoxification.

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

Lysosomes and Peroxisomes

• Lysosomes: Contain hydrolytic enzymes for intracellular digestion.

• Peroxisomes: Contain enzymes for breakdown of reactive oxygen species (e.g., H2O2).