AQA A Level Biology复习笔记3.1.3 Adaptation to Facilitate Exchange

Adaptations In Large Organisms to Facilitate Exchange


  • 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


Exam Tip

Students often only focus on the transport of nutrients and gases when writing about the adaptations for facilitating exchange. Make sure you don’t forget about the importance of removing toxic waste products from tissues and cells. If toxic waste products build-up (e.g. urea and carbon dioxide) within cells or tissues they can cause damage/death.