Which statement about oxygen exchange in fish gills is correct?

• A. Oxygen diffuses from water into the blood across the gill membranes.
• B. Oxygen exchange in fish gills occurs only when the fish is stationary.
• C. Oxygen is actively transported from water into the blood by gill cells.
• D. Oxygen is absorbed by fish gills through a process of osmosis.

Answer: (A)

Verified step by step guidance

1. Understand the basic function of fish gills: Fish gills are specialized organs that allow for the exchange of gases between the fish and the surrounding water. The primary function is to extract oxygen from water and expel carbon dioxide.
2. Recognize the process of diffusion: Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. In the context of fish gills, oxygen in the water is at a higher concentration than in the blood, allowing it to diffuse across the gill membranes into the bloodstream.
3. Clarify why active transport is not involved: Active transport requires energy to move substances against a concentration gradient. Since oxygen moves from a higher concentration in water to a lower concentration in blood, it does not require active transport.
4. Distinguish between diffusion and osmosis: Osmosis specifically refers to the movement of water across a semipermeable membrane, not gases like oxygen. Therefore, oxygen exchange in fish gills occurs through diffusion, not osmosis.
5. Consider the fish's movement: Oxygen exchange in fish gills can occur whether the fish is stationary or moving, as long as water flows over the gills. Fish often use a mechanism called buccal pumping to ensure water movement over the gills even when stationary.