Question

Which of the following is not an advantage of breathing air over breathing water? a. Air is less dense than water, so it takes less energy to move during ventilation. b. Air is less viscous than water, so it takes less energy to move during ventilation. c. The oxygen content of air is greater than that of an equal volume of water d. Air breathing leads to high evaporation rates from the respiratory surface.

Short answer

d. Air breathing leads to high evaporation rates from the respiratory surface.

Step-by-step solution

Evaluate Option a

Air is less dense than water, so it takes less energy to move during ventilation. This statement is true as air being less dense than water requires less energy to move while breathing. This is an advantage of breathing air over breathing water.

Evaluate Option b

Air is less viscous than water, so it takes less energy to move during ventilation. This statement is also true because air has a lower viscosity than water, which means that it provides less resistance and requires less energy to move during ventilation. This is another advantage of breathing air over breathing water.

Evaluate Option c

The oxygen content of air is greater than that of an equal volume of water. This statement is true because the amount of dissolved oxygen in water is considerably less than in the air (air has about 20% oxygen while water has around 1% or even less). This higher oxygen content present in the air makes it easier to extract oxygen for respiration, which is an advantage for air-breathing organisms.

Evaluate Option d

Air breathing leads to high evaporation rates from the respiratory surface. This statement is also true. However, unlike the other options, this is not an advantage of breathing air over water. High evaporation rates from the respiratory surface cause increased water loss, which leads to the need for more water intake to maintain hydration levels within the organism. Based on the analysis of all the given options, the option that is not an advantage of breathing air over water is: d. Air breathing leads to high evaporation rates from the respiratory surface.

Key concepts

Respiratory Surfaces

An essential aspect of respiration in organisms is the exchange of gases through respiratory surfaces. These surfaces enable the intake of oxygen and the release of carbon dioxide which are fundamental for metabolic processes.

For efficient gas exchange, these surfaces have common characteristics regardless of the organism: they are moist, thin, and have a large surface area relative to the size of the organism. Examples include lung alveoli in humans, gills in fish, and the tracheal system in insects.

In the context of the textbook exercise, the respiratory surface acts differently when facing air or water. Air, having lower density and viscosity, allows for easier expansion and contraction of the lungs, reducing the energy required to ventilate them. This is a significant advantage for terrestrial animals.

Oxygen Content in Air vs Water

Understanding the difference in oxygen content in air versus water is fundamental for realizing why certain respiratory adaptations have evolved in different species. Air is approximately 21% oxygen by volume, whereas water typically contains much less oxygen, especially in warmer or stagnant conditions.

The disparity is due to oxygen's physical properties and its solubility in water. Additionally, oxygen diffuses much faster in air than in water, making air-breathing a more efficient means of acquiring oxygen. Respiratory adaptations across species are influenced by this factor, and this was echoed in the exercise, where the higher oxygen content in air was highlighted as an advantage over water for respiration.

Ventilation Energy Cost

The ventilation energy cost refers to the amount of energy an organism must expend to move air or water through its respiratory system.

The cost is influenced by several factors, including the density and viscosity of the medium. Air is less dense and less viscous than water, meaning that moving air in and out of respiratory surfaces requires less energy compared to water. This was addressed in the exercise, highlighting the energy efficiency of air ventilation. Respiratory adaptations to reduce energy cost are evident in numerous biological systems, from the streamlined bodies of fish that reduce resistance underwater to the comparatively lighter structures of bird lungs.

Water Loss in Respiration

All respiratory processes come with the trade-off of water loss. For air-breathing organisms, the evaporation of water from respiratory surfaces is a significant concern.

As air passes over these surfaces, it picks up moisture and leads to water loss. This is particularly pronounced in arid environments or in organisms with high metabolic rates that require extensive ventilation. In the exercise, this disadvantage was marked as being the only non-advantageous aspect of air-breathing compared to water-breathing, underscoring the balance that organisms need to strike between efficient respiration and water retention.