OXYGEN DIFFUSION IN SIMPLE ORGANISMS

Introduction: Many simple organisms do not have specialized respiratory structures and instead obtain oxygen by diffusion through their body surfaces.

Importance: We can use a simple equation to assess properties of an organism that can survive by diffusion alone.

Question: How is the oxygen need of an organism related to its metabolism and size?

Variable:

 S concentration of O2 required at surface of organism for survival (atm) C rate of oxygen consumption ((cm3 O2/cm3 tissue)/min) r radius of spherical organism (cm) K diffusion constant ((cm2/atm)/min)

Methods: E. Newton Harvey (1928) developed the following equation to describe the concentration of oxygen required to supply a spherical organism with oxygen by diffusion:

where S is the required concentration of oxygen at the surface of an organism, C is the rate of oxygen consumption (cm^3 of oxygen/cm^3 tissue/min), r is the radius of the organism (cm), and K is the diffusion constant (cm^2/atm/min).

We can plot S as a linear function of C and as a parabolic function of r. A diffusion constant of 11x10^-6 cm^2/atm/min is typical for many animal tissues

Interpretation: We can see that as organism metabolism (oxygen consumption) or organism size increases, a greater amount of oxygen is needed at the surface of the organism for survival. Well areated water typically contains 0.21 atm oxygen. We can compare the intercept S = 0.21 with the graphs of C and r. Even well-aerated water is well below the required oxygen level for organisms of large size or high metabolism.

Conclusions: By trying hypothetical organisms in the equation, one sees that for an organism to survive by diffusion alone, it must either be very small or have a very low metabolic rate. Larger organisms or those with high metabolic rates must develop respiratory structures in order to meet their oxygen needs.