Question

Read the following statements (I - II) and choose the correct option.

I : Freshwater fishes have higher osmotic concentration compared to the surrounding water.
II : To maintain a constant osmotic concentration in the body, freshwater fishes drink water.

• A. Both statement I and statement II are true
• B. Both statement I and statement II are false
• C. Statement I is true but statement II is false
• D. Statement I is false and statement II is true

Answer 1

The correct option is C Statement I is true but statement II is false

Osmosis is the movement of water molecules from its higher concentration to lower concentration through a semipermeable membrane (egg membrane).

Changes in the osmotic concentration of the environment affects the survival of organisms, especially organisms living in aquatic habitats. Thus if the surrounding environment has a higher osmotic concentration than an organism’s body, water moves out of the body cells via exosmosis making the organism dehydrated. Water enters inside the body cells (endosmosis), if the surrounding environment has a lower osmotic concentration than an organism’s body.

Depending on the changes in the osmotic concentration of their surroundings, fishes respond either by regulating their body to maintain a constant osmotic concentration (osmoregulators) or by adjusting their osmotic concentration according to the changes in the environment (osmoconformers).

Freshwater fishes have a higher osmotic concentration than the surrounding water body because the gills actively absorb ions (required to maintain its body function), from the surrounding water. This makes the fish hypertonic. Hence statement I is correct.

These fishes do not drink water but water from the surrounding starts moving into the fish’s body through gills and skin. To compensate for the excess incoming water, the fishes excrete excessive dilute urine so that a constant osmotic concentration is maintained within their body. Hence statement II is false.