Biology gas exchange in single-celled organisms and insects

• C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O
• Diffusion


  The net movement of molecules, or ions, from a region where they are more highly concentrated to one where their concentration is lower, until evenly distributed.
• Osmosis


  The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane.
• Single-celled organisms


  • Their surface area:volume is large enough to allow all of their needs to be met via simple diffusion or osmosis
  • They are constantly respiring so the concentration gradients for oxygen and carbon dioxide are maintained
• Insects


  • SA:vol too small for simple diffusion. Need an exchange surface
  • Compromise between increasing gas exchange and reducing water loss
  • Insects have evolved several adaptations to balance this
• Insect exoskeleton


  • Made of chitin and covered with a cuticle
  • Impermeable to water - reduces water loss
  • Impermeable to gases - reduces gas exchange
• Spiracles


  1. Gases enter and leave through tiny pores called spiracles

  2. Spiracles are found all over the thorax and abdomen

  3. Can be opened and closed by a valve

  4. Open = water vapour can evaporate out but gases can be drawn in - increased water loss and gas exchange

  5. So spiracles are often closed to reduce water loss
• Chlamydomonas


  • Unicellular
  • Contains a chloroplast
  • Has 2 flagella
  • Maximum distance for diffusion is 10µm (micrometres)
  • Takes 100ms (milliseconds)
  • 1mm would take 100seconds
• Insect gas exchange system

  • The atmospheric air (containing oxygen) is therefore brought to the respiring tissues
  • The system of spiracles, trachea and tracheoles has reduced the diffusion distance
• How does the insect gas exchange system work?

  1. Along a diffusion gradient - oxygen is used in aerobic respiration, reducing concentration in cells, creating a concentration gradient. Carbon dioxide is produced causing its own gradient.

  2. Mass transport - contraction of muscles can squeeze the trachea enabling mass movements of air into and out of the tracheoles.
• How does the insect gas exchange system work?

  1. The ends of the tracheoles are filled with water - during times of high activity, some anaerobic respiration may occur. Produces lactate.

  2. Lactate is soluble

  3. Lowers water potential of the muscle cells

  4. Water moves into cells from tracheoles by osmosis

  5. Decreasing the pressure in the tracheoles and so draws air further in.
• Further adaptations


  • Very active insects, especially those that fly, have muscles that pump air into and out of the tracheal system - ventilation
  • e.g. grasshoppers
  • Spiracles are closed and muscles then pull the skeletal plates of the abdominal segments together
  • This squeezes the tracheal system and pumps air deeper in the tracheoles. The recoil also lowers the pressure inside the system so it is lower than atmospheric.