Multicelled organisms and specialisation

• Multicelled organisms


  Cells and regions of the body become specialised
• Levels of organisation in multicelled organisms


  • Cell
  • Tissue
  • Organ
  • Organ system
  • Organism
• Cell


  • Basic building block of living organisms, e.g. animal cell
• Tissue


  • Groups of cells with similar structures and functions, e.g. skin
• Organ


  • Groups of different tissues working together to form a structure with a particular function, e.g. brain
• Organ system


  • Organs organised into organ systems, e.g. the nervous system
• Organism


  • Different organ systems make up the organism, e.g. human
• Increasing complexity in multicelled organisms


  cell → tissue → organ→ organ system → organism
• As organisms get larger in size their surface area/volume (SA/V) ratio decreases
• Surface area


  Area of body surface across which diffusion can take place (of respiratory gases, for example)
• Volume


  Volume of cells in the organism that need to be supplied with nutrients and oxygen
• The decrease in SA/V ratio can be represented by cubes of increasing size
• SA/V ratio


  Surface area to volume ratio
• The consequence is that large multicelled organisms need to increase the surface area across which molecules can diffuse
• How large multicelled organisms increase surface area

  • Presence of specialised exchange surfaces, e.g. lungs in mammals and gills in fish
  • Development of transport systems to transport the diffusing molecules and substances around the body