IB Biology HL - Cell Structure (A2.2)

Cell Theory

  • The cell theory states:
    • All living organisms are composed of one or more cells.
    • The cell is the basic unit of structure and function in all living organisms.
    • All cells arise from pre-existing cells.
  • Inductive reasoning contributed to the formation of the cell theory through observations by scientists like Hooke, Leeuwenhoek, Schleiden, Schwann, and Virchow.
  • In science, a "theory" is a well-substantiated explanation backed by repeated observations and experiments.

Common Cell Structures

  • All cells have these structures:
    • Plasma Membrane: Controls movement of substances in and out of the cell.
    • Cytoplasm: Site of biochemical reactions; contains organelles.
    • Ribosomes: Synthesize proteins.
    • DNA: Stores genetic information.

Prokaryotic Cell Structure

  • Prokaryotes (bacteria and archaea) are unicellular with no membrane-bound organelles.
  • DNA is "naked" (lacks histone proteins) and is not enclosed in a nucleus.
  • Key structures include:
    • Cell Wall (peptidoglycan)
    • Plasma Membrane
    • Cytoplasm
    • Ribosomes (70S)
    • Nucleoid (contains genophore)
    • Plasmid
    • Pili
    • Flagella
    • Slime Capsule (glycocalyx)

Eukaryotic Cell Structure

  • Eukaryotes (plants, animals, fungi, and protists) have complex, compartmentalized cells.
  • Compartmentalization increases efficiency in cellular processes.
  • Key structures and functions:
    • Plasma Membrane: Regulates substance movement, maintains homeostasis; phospholipid bilayer with proteins.
    • Cytoplasm: Contains cytosol and organelles; allows compartmentalization.
    • 80S Ribosomes: Protein synthesis; free-floating (internal use) or attached to rough ER (secretion/membrane).
    • Nucleus: Stores DNA, controls cell activities; double membrane with pores; contains chromosomes and nucleolus (rRNA).
    • Cytoskeleton: Structural support, intracellular transport; microtubules (tubulin), microfilaments (actin).
    • Mitochondria: Aerobic respiration, ATP production; double membrane, own DNA/ribosomes.
    • Endoplasmic Reticulum (ER):
      • Rough ER (RER): Protein synthesis and modification; has 80S ribosomes.
      • Smooth ER (SER): Lipid synthesis, detoxification; lacks ribosomes.
    • Golgi Apparatus: Modifies, sorts, and packages proteins/lipids; produces lysosomes.
    • Vesicles & Vacuoles: Transport and storage; transport vesicles, secretory vesicles, vacuoles.
    • Lysosomes: Breakdown waste, debris, pathogens; contain digestive enzymes.

Prokaryotic vs. Eukaryotic Cells

  • Prokaryotic:
    • Small (1-5 μmμm)
    • No true nucleus (DNA in nucleoid)
    • Circular, naked DNA
    • 70S ribosomes
    • Cell wall (peptidoglycan in bacteria)
    • No membrane-bound organelles
  • Eukaryotic:
    • Larger (10-100 μmμm)
    • DNA in nuclear membrane
    • Linear DNA with histone proteins
    • 80S ribosomes
    • Cell wall (cellulose in plants, chitin in fungi; absent in animals)
    • Contains membrane-bound organelles (mitochondria, ER, Golgi, lysosomes, etc.)

Atypical Eukaryotic Cells

  • Aseptate Fungal Hyphae: Continuous cytoplasm with many nuclei (no septa).
  • Skeletal Muscle: Multinucleated, long cells.
  • Red Blood Cells: No nucleus (increases oxygen-carrying capacity).
  • Phloem Sieve Tube Elements: Lack nuclei, rely on companion cells.

Processes of Life

  • Essential life processes:
    • Metabolism
    • Homeostasis
    • Excretion
    • Growth
    • Nutrition
    • Movement
    • Reproduction
    • Response to Stimuli
  • Unicellular organisms must perform all life processes within a single cell.
  • Paramecia: Use cilia for movement, engulf food via phagocytosis, and excrete waste through an anal pore.
    • Maintain homeostasis using contractile vacuoles.
    • Reproduce asexually (mitosis) or sexually (fusion).
    • Contain enzymes for metabolic reactions.