exchange and transport

• why is diffusion across the outer membrane too slow in multicellular organisms?


  three reasons

  1. some cells are deep within the body - there's a big distance between them and the outside environment
  2. larger animals have a low surface area to volume ratio
  3. multicellular organisms have a higher metabolic rate than single-celled organisms, so they use up oxygen and glucose faster
• special features of exchange surfaces

  1. a large surface area - to increase their efficiency e.g. root hair cells
  2. they are thin - decrease distance that substances are being exchanged over to improve efficiency e.g. alveoli
  3. a good blood blood supply and/or ventilation - to increase efficiency e.g. gills
• features of the gaseous exchange system

  goblet cells

  • lining the airways
  • secrete mucus
  • the mucus traps microorganisms and dust particles in the particles in the inhaled air, stopping them from reaching the alveoli
• features of the gaseous exchange system

  cilia

  • hair-like structures on the surface of epithelial cells lining the airways
  • they beat the mucus secreted by the goblet cells moving the mucus (and the trapped microorganisms and dust) upward away from the alveoli towards the throat
  • reduces the risk of lung infection
• features of the gaseous exchange system

  elastic fibres

  • in the walls of the trachea, bronchi, bronchioles and alveoli help the process of breathing out
  • on breathing in, the lungs inflate and the elastic fibres are stretched
  • then the fibres recoil to help push the air out when exhaling
• features of the gaseous exchange system 


  smooth muscle

  • in the walls of the trachea, bronchi and bronchioles (except the smallest bronchioles) allows their diameter to be controlled
  • during exercise the smooth muscle relaxes, making the tubes wider
  • this means their is more resistance to airflow and air can move in and out of the lungs more easily
• features of the gaseous exchange system

  cartilage

  • rings of cartilage in the trachea and bronchi provide support
  • it's strong but flexible
  • it stops the trachea and bronchi collapsing when you breathe in and pressure drops