Concept explainers
Introduction:
The epithelial cells in the gills of freshwater have a higher concentration of solutes than the surrounding water where fishes live, that is they are hyper-osmotic to the surrounding water.
Answer to Problem 2TYK
Correct answer:
In freshwater fishes, water enters epithelial cells in their gills via osmosis. Whereas, the electrolytes leave the same cells via diffusion.
Explanation of Solution
Explanation/justification for the correct answer:
Option (c) is given that water enters epithelial cells in the gills of freshwater fishes via osmosis and electrolytes leaves the same cells via diffusion. The freshwater fishes maintain the electrolyte balance (osmoregulation) with their external environment. This osmoregulation is maintained by the entry of freshwater into the epithelial cells of the gills through the process of osmosis.
In this, the transfer of solvent (water) from the region of less solute concentration (water body, for example, a freshwater river) to the region of higher solute concentration (epithelial cells in the gills) takes place. In order to maintain homeostasis, the excess water intake is excreted in the urine.
The excess solute (electrolytes) present in the epithelial cells leaves the cells via the process of diffusion. The level of lost electrolytes is maintained by food intake or through their active transport from the freshwater present outside. Hence, option (c) is correct.
Explanation for incorrect answer:
Option (a) is given that water surrounding the fish is iso-osmotic. As a result, no special organ is required to maintain water and electrolyte balance. So, it is a wrong answer.
Option (b) is given that freshwater fishes lose water to their environment primarily through the gills, and replace this water by drinking. The epithelial cells of freshwater fishes have less amount of water and they possess a higher solute concentration; therefore, they take in water by osmosis instead of losing it via gills. So, it is a wrong answer.
Option (d) is given that freshwater fishes have specialized epithelia that actively pump Na+ (sodium ions) and Cl- (chloride ions) from the blood into their environment. The specialized epithelia in the gills help to actively pump in the electrolytes (sodium ions and chloride ions) from the outside environment to maintain the levels of electrolyte, which were lost due to diffusion. So, it is a wrong answer.
Hence, options (a), (b), and (d) are incorrect.
Therefore, it can be concluded that the concentration of solute in the epithelial cells of the gills of freshwater fishes is higher than in the water outside. The osmoregulation is maintained by osmosis (water enters the epithelial cells) and diffusion (water leaves the cells). Thus, the water enters epithelial cells in the gills of freshwater fishes via osmosis and the electrolytes leave those cells through diffusion.
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Chapter 43 Solutions
Biological Science
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