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 μm)
  • No true nucleus (DNA in nucleoid)
  • Circular, naked DNA
  • 70S ribosomes
  • Cell wall (peptidoglycan in bacteria)
  • No membrane-bound organelles
• Eukaryotic:
  • Larger (10-100 μm)
  • 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.