Gas exchange

• What are the features of specialised exchange systems?

  1. Large surface area to volume ratio for exchange.

  2. High difference in concentration (i.e. the concentration gradient is steep)

  This can be done in two ways:

  • Blood flow - constant movement of blood maintains a steep concentration gradient.

  • Ventilation - in gas exchange in the lungs air is breathed in and out which maintains steep diffusion gradients

  3. A short diffusion path (i.e. a thin exchange surface) either a membrane or very thin cells, usually in a single layer.
• What is gaseous exchange?

  The process of getting oxygen from the external environment into respiring tissues, and removal of carbon dioxide from the respiring cells, into the environment again.
• Explain gas exchange in single-celled organisms:

  Large surface area - Organism is small, therefore has a large surface area to volume ratio

  Concentration gradient - Respiration uses oxygen, therefore there's always more oxygen outside the cell than inside, creating a concentration gradient which oxygen can diffuse down

  Short diffusion pathway - Cell membrane/cell wall are very thin, therefore there is a short diffusion pathway
•  Explain why single-celled organisms do not need specialised exchange surfaces. (4 marks)

  Metabolic activity relatively low (1);

  So relatively little oxygen needed or carbon dioxide produced (1).

  SA : V is large (1);

  So diffusion distances small (1).