OCR AS Biology

Revision Notes

3.1.1 Specialised Exchange Surfaces

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

11. Principles of surface area _ volume, downloadable AS & A Level Biology revision notes

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

The Relationship between Surface Area: Volume Ratio & Metabolic Rate

  • The metabolic rate of an organism is the amount of energy expended by that organism within a given period of time
  • The basal metabolic rate (BMR) is the metabolic rate of an organism when at rest. The BMR is significantly lower than when an organism is actively moving
  • During periods of rest, the body of an organism only requires energy for the functioning of vital organs such as the lungs, heart and brain
  • The metabolic rate of an organism can be measured/estimated using different methods and apparatus:
    • Oxygen consumption (respirometers)
    • Carbon dioxide production (carbon dioxide probe)
    • Heat production (calorimeter)

Body Mass and Metabolic Rate (1), downloadable AS & A Level Biology revision notes

Metabolic rate increases with body mass

Body Mass

  • Experiments conducted by scientists have shown that the greater the mass of an organism, the higher the metabolic rate
    • Therefore, a single rhino consumes more oxygen within a given period of time compared to a single mouse
  • Although metabolic rate increases with body mass the BMR per unit of body mass is higher in smaller animals than in larger animals
    • Smaller animals have a greater SA:V ratio so they lose more heat, meaning they have to use up more energy to maintain their body temperature

Exam Tip

Plants have much lower metabolic rates than animals as they do not move around their habitat and don’t have to maintain a high body temperature.


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