Chapter 40, Problem 1TYK

Which of the following statements is true of fishes that live infreshwater?

a. Water moves across the gills via osmosis until equilibrium is established, at which time the water molecules stop moving.

b. They lose water to their environment primarily through the gills. They replace this water by drinking.

c. Water enters epithelial cells in their gills via osmosis. Electrolytes leave the same cells via diffusion.

d. They have specialized epithelia that actively pump Na⁺ and CT from the blood into their environment.

Summary Introduction

Introduction:

The epithelial cells in the gills of freshwater have a higher concentration of solutes than the freshwater of the water body where fishes lived.

Answer to Problem 1TYK

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 moves across gills via osmosis until equilibrium is established, at which time the water molecules stop moving in freshwater fishes. The water enters the epithelial cells of the gills via osmosis, which results in osmotic shock. The equilibrium is maintained by excretion of the excess amount of water, in the form of urine. 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.

Conclusion

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.