Question

When a water beetle is placed in a laboratory situation where the atmosphere is pure O2 and the water that the beetle is in is equilibrated with the atmosphere, after the beetle obtains a new bubble from the atmosphere, it cannot stay underwater for nearly as long as it can when the atmosphere is ordinary air. This is true because the bubble does not operate effectively as a gill. Explain. Assume (almost accurately) that CO2 added to the bubble diffuses quickly out of the bubble into the surrounding water. (Hint: What is the partial pressure of O2 in the water and in the bubble?)

Answer 1

Assuming the experiment is done at sea level, the partial pressure of O2 in the laboratory atmosphere is 1 atm. Thus, when a bubble is grabbed from the atmosphere, the partial pressure in the bubble gas is 1 atm. Moreover, if the atmosphere and the water are at equilibrium, the O2 partial pressure in the water is also 1 atm. As the beetle removes O2 from the bubble, the CO2 added to the bubble exits rapidly. Thus, the O2 partial pressure in the bubble gas remains 1 atm. With no difference in partial pressure between the water and the bubble gas, O2 cannot diffuse into the bubble from the water (i.e., the bubble can't act as a gill). The only O2 available from the bubble is the O2 taken from the atmosphere. When that runs out, the bubble must be replaced. In the ordinary world with ordinary air, a new bubble consists of only about one-fifth O2. About 78% of the bubble is N2. N2 tends to stay in the bubble since it does not readily dissolve into the surrounding water in the time available. Because of this property of N2 in the bubble gas, the partial pressure of O2 in the bubble gas declines as the beetle removes O2, and it becomes lower than the O2 partial pressure in the atmosphere and water. Therefore, O2 diffuses into the bubble from the water. This means that the beetle can obtain more O2 from the bubble: not only O2 grabbed from the atmosphere but also O2 steadily entering the bubble by diffusion.