3.1 Cell: The Unit of Life

Cell Theory

  • Historical Figures: Hooke, Leeuwenhoek, Schleiden, Schwann, Virchow.
  • Modern Postulates:
    1. All living things are made of cells.
    2. All cells come from pre-existing cells (Omnis cellula-e cellula).
    3. The cell is the basic structural and functional unit of life.

Prokaryotic vs. Eukaryotic Cells

Feature Prokaryotic Eukaryotic
Nucleus Nucleoid (no true nucleus) True nucleus
Organelles No membrane-bound Membrane-bound
Ribosomes 70S 80S (cytoplasm), 70S (organelles)
DNA Circular Linear with histones

Plant vs. Animal Cells

Feature Plant Cell Animal Cell
Cell Wall Present (cellulose) Absent
Plastids Present (chloroplasts) Absent
Vacuole Large central vacuole Small, numerous vacuoles
Centrioles Absent (higher plants) Present

The Cell Envelope

  • Cell Wall (Plants, Fungi): Provides shape and protection.
  • Plasma Membrane:
    • Fluid Mosaic Model: Lipid bilayer with embedded proteins.
    • Functions: Selective permeability, transport (passive and active).

Endomembrane System

  • Endoplasmic Reticulum (ER):
    • Rough ER: Protein synthesis.
    • Smooth ER: Lipid synthesis.
  • Golgi Apparatus: Processing, packaging, transport.
  • Lysosomes: "Suicidal bags" with digestive enzymes.
  • Vacuoles: Storage (water, sap, waste).

Mitochondria & Plastids

  • Mitochondria:
    • "Powerhouse" of the cell.
    • Site of aerobic respiration and ATP synthesis.
  • Plastids (Plants):
    • Chloroplasts: Photosynthesis.
    • Chromoplasts: Color (fruits, flowers).
    • Leucoplasts: Storage (starch, oil, protein).

Other Key Organelles

  • Ribosomes: Protein synthesis (70S and 80S).
  • Cytoskeleton: Mechanical support, motility (microtubules, microfilaments).
  • Centrosome (Animals): Forms spindle fibers for cell division.
  • Cilia & Flagella: Motility (9+2 microtubule arrangement).

The Nucleus

  • Nuclear Envelope: Double membrane with pores.
  • Nucleolus: Site of rRNA synthesis.
  • Chromatin: DNA + histone proteins.
  • Function: Controls cell activities and contains genetic material.

Chromosomes

  • Structure: Condensed chromatin, consisting of two sister chromatids joined by a centromere.
  • Types based on Centromere Position:
    • Metacentric: Middle.
    • Sub-metacentric: Slightly off-center.
    • Acrocentric: Near one end.
    • Telocentric: At the tip.