Specialised Exchange Surfaces (OCR A Level Biology)

• 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

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)

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