Cytology and Cell Biology Principles

Cell Definition: Smallest units of life capable of metabolism, responding to the environment, and reproduction.
Cell Theory: * All living organisms consist of one or more cells. * The cell is the basic structural unit of life. * New cells arise only from pre-existing cells.
Microscopy: * Light Microscopes: Simple or compound instruments for viewing small specimens. * Electron Microscopes: Includes Transmission EM (TEM) for internal structures and Scanning EM (SEM) for surface details.

Surface-to-Volume Ratio ( $S/V$ ): As a cell increases in size, its volume grows faster than its surface area. Beyond a certain limit, the membrane cannot transport materials fast enough to sustain the volume.
Prokaryotic Cells: Smaller, simple structure; DNA is located in a non-membrane-bound nucleoid region; lack most organelles.
Eukaryotic Cells: Larger, complex structure; possess a membrane-enclosed nucleus and various specialized organelles.
Endosymbiont Theory: Proposed by Lynn Margulis; suggests mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotes.

Nucleus: Contains genetic material (chromatin) and the nucleolus; bounded by a double-membrane nuclear envelope with pore complexes.
Ribosomes: Composed of large and small subunits for protein synthesis; found free in the cytosol or bound to the ER.
Endoplasmic Reticulum (ER): * Smooth ER: Involved in fatty acid synthesis, carbohydrate metabolism, $Ca$ storage, and detoxification. * Rough ER: Involved in protein synthesis and glycosylation; studded with ribosomes.
Golgi Apparatus: Features a receiving cis face and a shipping trans face; refines, stores, and distributes chemical products.
Lysosomes: Contain digestive enzymes to break down food, macromolecules, and damaged organelles (autophagy).
Vacuoles: In plants, the central vacuole manages digestion, waste disposal, and water balance.

Mitochondria: Convert sugar to $ATP$ via cellular respiration; contain cristae, matrix, and their own DNA.
Chloroplasts: Perform photosynthesis to convert sunlight into $ATP$ and glucose; contain thylakoids, grana, and stroma.
Centrioles: Located in the centrosome of animal cells; organize microtubules and form cilia and flagella.
Cytoskeleton Components: * Microtubules: $25\,nm$ ; tubulin dimers; maintain shape and move chromosomes. * Microfilaments: $7\,nm$ ; actin subunits; muscle contraction and cell shape. * Intermediate Filaments: $8-12\,nm$ ; keratin proteins; anchor organelles like the nucleus.

Plant Junctions: Plasmodesmata allow passage of molecules between walls.
Animal Junctions: * Tight Junctions: Prevent fluid movement across cell layers. * Desmosomes: Fasten cells together into strong sheets. * Gap Junctions: Provide cytoplasmic channels for communication.
Cell Membrane: A phospholipid bilayer containing integral and peripheral proteins; cholesterol regulates fluidity.
Transport Mechanism: * Passive Transport: Movement down a concentration gradient; no energy required (Diffusion/Osmosis). * Active Transport: Movement against a gradient; requires $ATP$ . * Bulk Transport: Includes Phagocytosis, Pinocytosis, and Receptor-mediated endocytosis.
Tonicity: * Hypotonic: Animal cells may lyse; plant cells become turgid. * Hypertonic: Animal cells shrivel; plant cells undergo plasmolysis.