Specialised Exchange Surfaces (OCR A Level Biology): Revision Note

The Need for Specialised Exchange Surfaces

• 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 diffusion distance increased to 15cm the diffusion time would increase substantially to 7 hours

• 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

SA:V ratio & metabolic rate

• The metabolic rate of an organism is the energy expended by that organism within a given period of time

  • The metabolic rate of an organism can be measured/estimated using different methods and apparatus, e.g.:

    • oxygen consumption (respirometers)

    • carbon dioxide production (carbon dioxide probe)

    • heat generation (calorimeter)

• The surface area to volume ratio of an organism is related to its metabolic rate; this relationship exists because of the relationship between SA:V and heat loss

  • Heat is lost to the environment at the body's surface, so having a large body surface in relation to volume will allow more heat to be lost

  • This means that:

    • small animals, with a higher SA:V ratio, will lose more heat to their surroundings, meaning that they need a relatively high metabolic rate to maintain body temperature

    • large animals, with a lower SA:V ratio, will lose less heat, meaning that they can maintain body temperature at a relatively low metabolic rate

• Smaller animals have a higher metabolic rate per unit of body mass

• Note that while smaller organisms have a higher metabolic rate per unit of body mass, larger organisms need to support the metabolism of more cells, so will consume more oxygen within a given period of time than smaller organisms