Question

When a water beetle is placed in a laboratory situation where the atmosphere is pure \(\mathrm{O}_{2}\) 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 under water 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 (alancet accurately) that \(\mathrm{CO}_{2}\) added to the bubble diffurses quickly cut of the bubble into the surruunding water. (Hint: What is the partial pressure of \(\mathrm{O}_{2}\) in the water and in the bubble?)

Short answer

When a water beetle is in a pure \(\mathrm{O}_2\) atmosphere, the bubble it obtains is composed entirely of \(\mathrm{O}_2\). This high oxygen content maintains a high partial pressure in the bubble, causing \(\mathrm{O}_2\) to diffuse from the bubble to the surrounding water. However, in the absence of \(\mathrm{CO}_2\) in the bubble, there is no gradient in partial pressures to facilitate the diffusion of \(\mathrm{O}_2\) out of the bubble in exchange for \(\mathrm{CO}_2\). Therefore, even with the quick diffusion of \(\mathrm{CO}_2\) out of the bubble, the bubble does not operate effectively as a gill – reducing the beetle's underwater survival duration.

Step-by-step solution

Understanding Gaseous Exchange

Gaseous exchange in a beetle involves the beetle obtaining a new bubble of gas (air) which acts as a temporary gill underwater. Oxygen from the bubble is used for respiration and replaced by carbon dioxide, a waste product. The difference in the concentration of gases (oxygen and carbon dioxide) between the bubble and the aquatic environment causes a gradient in partial pressures, initiating exchange of gases.

Effect of a Pure Oxygen Atmosphere

In a pure oxygen atmosphere, the bubble the beetle obtains will be full of \(\mathrm{O}_2\). Once underwater, the \(\mathrm{O}_2\) in the bubble will start to diffuse out into surrounding water due to the high partial pressure of \(\mathrm{O}_2\) in the bubble and relatively low partial pressure of \(\mathrm{O}_2\) in the water.

Impact on the Beetle's Underwater Survival Duration

Since the bubble is filled with only \(\mathrm{O}_2\), the lack of \(\mathrm{CO}_2\) in the bubble reduces the rate of \(\mathrm{O}_2\) diffusion from the bubble into the water. Without a sufficient gradient in partial pressures, the rate of gaseous exchange is slowed meaning the bubble 'shrinks' slower allowing the water beetle to stay underwater for a shorter period.

Impact of Quick \(\mathrm{CO}_2\) Diffusion on the Bubble

The assumption that \(\mathrm{CO}_2\) added to the bubble quickly diffuses out means that the relative concentration, and so the partial pressure, of \(\mathrm{O}_2\) within the bubble remains high. This lack of \(\mathrm{CO}_2\) in the bubble prevents a sufficient partial pressure gradient forming which impacts the rate of \(\mathrm{O}_2\) diffusion from the bubble to the water – resulting in the bubble not operating effectively as a gill.

Key concepts

Partial Pressure in Respiration

Partial pressure is a fundamental concept in the study of respiration, both in humans and in insects like beetles. It refers to the pressure exerted by a single type of gas when it is part of a mixture of gases. This concept is central to understanding how gases are exchanged during respiration because it is the difference in partial pressures that drives the diffusion of gases.

For beetles, respiration involves obtaining oxygen, which is essential for cellular processes, and getting rid of carbon dioxide, the waste product. When a beetle takes a bubble of air underwater, it relies on the partial pressure differences between the gases in its bubble and the surrounding water to exchange these gases. A key point for students to understand is that the higher the partial pressure of oxygen in the bubble compared to the water, the more oxygen will diffuse into the water, and vice versa for carbon dioxide.

However, when the surrounding atmosphere is pure oxygen, the partial pressure of oxygen in the bubble is much higher than in the case with normal air. This leads to a rapid loss of oxygen from the bubble to the water, which is not balanced by the diffusion of carbon dioxide into the bubble. The result is that the bubble cannot function effectively as a 'gill', reducing the beetle's underwater survival time.

Respiration in Insects

Respiration in insects like beetles is fascinating and differs significantly from that in humans. Beetles do not have lungs; instead, they respire through a specialized system of air tubes called tracheae. These tracheae deliver oxygen directly to the tissues that require it and transport carbon dioxide back out.

In the query at hand, water beetles introduce a unique method of aquatic respiration by trapping an air bubble from the atmospheric air, which then acts as a physical gill. This bubble adheres to their body and is used to extract dissolved oxygen from the water. The rate at which oxygen is transferred from the bubble to the beetle's body depends largely on the partial pressure of oxygen within the bubble, the volume of the bubble, and the surface area through which diffusion can occur.

Importance of Carbon Dioxide in the Gas Exchange Process

Moreover, carbon dioxide plays an essential role in the respiration process of beetles. The diffusion of carbon dioxide out of the bubble helps maintain a gradient that favors oxygen absorption. If carbon dioxide does not accumulate in the bubble (as in the case with a pure oxygen atmosphere), the rate of oxygen absorption from the bubble becomes less efficient, impacting the beetle's ability to stay submerged for longer durations.

Diffusion of Gases

Diffusion of gases is a passive transport mechanism that is vital for the exchange of gases in respiration across all species, including beetles. It is driven by a gradient of concentration – gases move from an area of higher concentration to one of lower concentration until equilibrium is reached.

The process of diffusion is governed by several factors, including the differential in partial pressures, which acts as the driving force for the exchange of gases. The rate of diffusion is also influenced by temperature, the surface area available for diffusion, and the distance through which the gases must diffuse.

In the context of our water beetle, the diffusion of gases occurs between the bubble of air it carries and the surrounding water. When the beetle is submerged in water with a high partial pressure of oxygen, such as when the atmosphere is pure oxygen, oxygen will rapidly diffuse out of the bubble, which decreases the efficacy of the bubble as a gill. Understanding diffusion is crucial for students as it shows the necessity of balanced partial pressures to maintain a consistent gas exchange, vital for the beetle's survival underwater.