Question

Select the option that correctly fills the blanks (A –D).

For freshwater fish, the osmolarity of the surrounding water is ___A___ than the osmolarity of its body fluids, hence fish is said to be ___B____ to the surrounding freshwater. The reason for this being that the fish absorbs ions via ____C___ to maintain its body function.

This leads to endosmosis and to tackle the problem of excess incoming water, the fish excretes ____D_____urine.

• A. A – less, B – hypertonic, C – lungs, D – dilute
• B. A – less, B – hypertonic, C – gills, D – dilute
• C. A – less, B – hypotonic, C - lungs, D – concentrated
• D. A – less, B – hypotonic, C – gills, D – dilute

Answer 1

The correct option is B A – less, B – hypertonic, C – gills, D – dilute

Osmosis is the movement of water molecules from the region of their higher concentration to lower concentration across a semipermeable membrane.

Thus, if the surrounding environment has a higher osmotic concentration than an organism’s body, water moves out of the body via exosmosis.

However, if the surrounding environment has a lower osmotic concentration than an organism’s body, water enters into the body via endosmosis

Osmoregulation is the active regulation of the body fluid osmolarity. Osmoregulation is different for freshwater fishes and marine fishes as they live in different habitats.

Freshwater fishes have a higher osmotic concentration (high osmolarity) than the surrounding water because the gills actively absorb ions that are required to maintain its body function, from the surrounding water. This makes the fish hypertonic (having higher concentration of solute compared to the surrounding water) and hence water from the surrounding environment starts moving into the fish’s body through gills and skin (endosmosis). To compensate for the excess incoming water, the fishes excrete excessive dilute urine so that a constant osmotic concentration is maintained within their body.