• Single-celled organisms have a high SA:V ratio which allows for the exchange of substances to occur via simple diffusion
  • The large surface area allows for maximum absorption of?nutrients?and?gases?and secretion of?waste products
  • The small volume means the diffusion distance to all organelles is short

   

• As organisms?increase in size?their?SA:V ratio decreases
  • There is less surface area for the absorption of nutrients and gases and secretion of waste products
  • The greater volume results in a?longer diffusion distance?to the cells and tissues of the organism

   

• Large multicellular animals and plants have evolved adaptations to facilitate the exchange of substances between their environment
• They have a large variety of specialised cells, tissues, organs and systems
  • Eg. gas exchange system, circulatory system, lymphatic system, urinary system, xylem and phloem

   

As the size of an organism increases, it’s surface area : volume ratio decreases. Notice for this particular shape the distance between the surface and the centre increases with size.

The Need for a Specialised System for Gas Exchange

• Supply of Oxygen:
  • Organisms require?ATP?in order to carry out the biochemical processes required for survival. The majority of ATP is produced through aerobic respiration which requires oxygen

   

• Removal of Carbon Dioxide:
  • Carbon dioxide is a toxic waste product of aerobic respiration
  • If it accumulates in cells/tissues it alters the pH

   

Diffusion for Single-celled Organisms vs Multicellular Organisms

• Chlamydomonas?is a single-celled organism that is found in fresh-water ponds. It is spherical in shape and has a diameter of 20μm. Oxygen can diffuse across the cell wall and membrane of the?Chlamydomonas
• The maximum distance that oxygen molecules would have to diffuse to reach the centre of a?Chlamydomonas?is 10μm, which would only take 100 milliseconds
• If the cell was larger and the diffusion distance increased to 20μm the?diffusion time would increase substantially?to 400 milliseconds
• This demonstrates how diffusion is a viable transport mechanism for single-celled organisms but not for larger multicellular organisms
  • The time taken for oxygen to diffuse from the cell-surface membrane to the tissues would be?too long