A Level Biology Practice Exam

Which of the following best describes gills?

1. Organs responsible for nutrient absorption in fish

2. Gaseous exchange organs of fish

3. Structures that provide buoyancy in water

4. Hard structures for protection against predators

The correct answer is: Gaseous exchange organs of fish

The best description of gills is that they are gaseous exchange organs of fish. Gills are specialized structures that facilitate the extraction of oxygen from water and the release of carbon dioxide. When water flows over the gill membranes, oxygen diffuses into the blood while carbon dioxide diffuses out, allowing fish to efficiently breathe underwater. This function is crucial for the survival of aquatic organisms, making gills an essential component of their respiratory system. Other options refer to different functions: nutrient absorption is primarily the role of the digestive system; buoyancy is typically provided by swim bladders or other adaptations like fat deposits; and hard structures associated with protection would generally refer to scales or spines, not gills. Therefore, describing gills as gaseous exchange organs accurately captures their primary role in the physiology of fish.