Levels of Organisation

Levels of Organisation

  • Living structures are arranged in a hierarchy:
    organelles→cells→tissues→organs→organ  systems→organismorganelles \rightarrow cells \rightarrow tissues \rightarrow organs \rightarrow organ\;systems \rightarrow organism

Cells

  • Basic unit of life; contain organelles (e.g. nucleus, mitochondria, ribosomes).
  • Unicellular organisms: one cell performs all functions.
  • Multicellular organisms: many specialised cells.

Specialisation in Multicellular Organisms

  • Differentiation produces cells adapted for specific roles (e.g. muscle, nerve, xylem).
  • Similar specialised cells group to form tissues.
  • Tissues combine to form organs; organs integrate into organ systems.

Major Organ Systems

Animals
  • Digestive, Circulatory, Respiratory, Nervous, Musculoskeletal, Excretory, Reproductive.
Plants
  • Shoot system (leaves, stems, flowers).
  • Root system (roots, root hairs).
  • Vascular tissue (xylem & phloem) links both systems.

Example (Leaf)

  • Organ: leaf.
  • Tissue layer: spongy mesophyll.
  • Cell: palisade mesophyll cell.

Key Takeaways

  • Organisation improves efficiency by division of labour.
  • Function emerges at each higher level (e.g. gas exchange in leaf, movement in muscle).