Cell Theory and Structure

Cell Theory

• Formulated by Schleiden and Schwann, later modified by Rudolf Virchow.
• Virchow introduced the concept that new cells arise from pre-existing cells ("Omnis cellula-e cellula").
• Current cell theory includes:
  • All living organisms are composed of cells and their products.
  • All cells arise from pre-existing cells.

Overview of Cell Structure

• Onion and Cheek Cells:
  • Onion cells have a distinct cell wall and membrane; human cheek cells have an outer membrane.
  • Both cell types contain a nucleus housing chromosomes and DNA.

Cell Types

• Eukaryotic Cells:
  • Have membrane-bound nuclei and organelles (e.g., ER, Golgi complex, mitochondria).
• Prokaryotic Cells:
  • Lack membrane-bound nuclei and organelles; have a simpler structure.

Cytoplasm

• A semi-fluid matrix in both cell types where chemical reactions necessary for cellular activities occur.

Organelles

• Eukaryotic Organelles:
  • Include various structures like mitochondria, lysosomes, and vacuoles.
  • Ribosomes are present in both eukaryotic and prokaryotic cells, found in cytoplasm and organelles.
• Centrosome:
  • Contains centrioles that aid in cell division and are crucial for the formation of cilia and flagella.

Bacterial Classification

• Bacteria classified based on cell envelope differences; groups:
  • Gram-positive (stains) and Gram-negative (no stain).
• Glycocalyx:
  • Varies in composition, may form a slime layer or thick capsule, aiding in structure and protection.

Plasma Membrane and Mesosome

• The plasma membrane is selectively permeable, allowing interaction with the environment.
• Mesosomes, formed by plasma membrane extensions, play roles in respiration, secretion, and DNA distribution.

Motility Structures

• Flagella:
  • Present in motile bacteria, consisting of filament, hook and basal body.
  • Various arrangements and numbers affect bacterial motility.
• Cilia:
  • Shorter than flagella, facilitate fluid movement or cell movement.

Centrosomes and Centrioles

• Each centrosome comprises two perpendicular centrioles, organized like a cartwheel.
• Centrioles are essential for spindle formation during cell division.

Nucleus

• First described by Robert Brown; contains chromatin and nucleolus.
• Regulates organelle activities and plays a major role in heredity.

Endoplasmic Reticulum (ER)

• Composed of rough (with ribosomes) and smooth (without ribosomes) regions.
• Functions in protein synthesis and transport of substances.

Golgi Complex and Lysosomes

• Golgi Complex:
  • Membranous organelle for packaging and transporting cell secretions.
• Lysosomes:
  • Contain digestive enzymes for macromolecule breakdown.

Mitochondria and Plastids

• Mitochondria:
  • Double-membrane organelles involved in ATP generation through oxidative phosphorylation.
• Plastids:
  • Found in plant cells, with chloroplasts vital for photosynthesis (light reactions in thylakoids, dark reactions in stroma).

Cellular Unit

• The cell is the fundamental structural and functional unit of life, containing specialized organelles that perform specific functions.