Passage
Within the Actinopterygii (bony fish) lineage, marine teleosts (bony fish with mobile jaws) live in an aquatic environment where there is a higher external concentration of salt and a lower concentration of water relative to the internal concentrations. Owing to these conditions, salt ions tend to diffuse into the teleost through its skin, whereas water molecules within the organism osmotically traverse the opposite path.Irrespective of lineage, many fish species actively assess their environment and regulate their internal concentration of fluids and electrolytes via a homeostatic process known as osmoregulation. Teleosts with either glomerular kidneys (eg, eels, sculpin) or aglomerular kidneys that are purely tubular (eg, goosefish, toadfish) have served as experimental subjects for the study of marine teleost osmoregulation. Analyzing the ionic (ie, Na + , Cl − , Mg 2+ , SO 4 2− ) concentration of intestinal fluids, urine, and plasma in each species led to the discovery that the renal tubule of the goosefish has both excretory and reabsorptive functions.Scientists have concluded that marine teleost osmoregulation involves the ingestion of seawater, retrieval of NaCl and water from the intestine, and primary excretion of divalent ions via urine and monovalent ions through the gills (thin barriers between the organism's blood and the aquatic environment).In an experiment to further study osmoregulatory mechanisms, researchers emptied the gut of an eel and sealed the anus to prevent fluid loss. The eel was transported into seawater that had been treated with the volume marker phenol red. After 20 hours, the eel's gut contained 2.3 mL of fluid but showed a phenol red concentration equivalent to 12.3 mL of ingested seawater. Researchers found that the eel lost 2.3 g of weight due to urine output. They ultimately reported that the eel had lost all the seawater it absorbed through the gut via various mechanisms.
-Scientists also study freshwater teleosts, which live in an environment where there is more water and less salt outside their bodies than inside. Osmoregulation in freshwater teleosts would most likely serve to combat their natural tendency to:

Question

Passage
Within the Actinopterygii (bony fish) lineage, marine teleosts (bony fish with mobile jaws) live in an aquatic environment where there is a higher external concentration of salt and a lower concentration of water relative to the internal concentrations.  Owing to these conditions, salt ions tend to diffuse into the teleost through its skin, whereas water molecules within the organism osmotically traverse the opposite path.Irrespective of lineage, many fish species actively assess their environment and regulate their internal concentration of fluids and electrolytes via a homeostatic process known as osmoregulation.  Teleosts with either glomerular kidneys (eg, eels, sculpin) or aglomerular kidneys that are purely tubular (eg, goosefish, toadfish) have served as experimental subjects for the study of marine teleost osmoregulation.  Analyzing the ionic (ie, Na + , Cl − , Mg 2+ , SO 4 2− ) concentration of intestinal fluids, urine, and plasma in each species led to the discovery that the renal tubule of the goosefish has both excretory and reabsorptive functions.Scientists have concluded that marine teleost osmoregulation involves the ingestion of seawater, retrieval of NaCl and water from the intestine, and primary excretion of divalent ions via urine and monovalent ions through the gills (thin barriers between the organism's blood and the aquatic environment).In an experiment to further study osmoregulatory mechanisms, researchers emptied the gut of an eel and sealed the anus to prevent fluid loss.  The eel was transported into seawater that had been treated with the volume marker phenol red.  After 20 hours, the eel's gut contained 2.3 mL of fluid but showed a phenol red concentration equivalent to 12.3 mL of ingested seawater.  Researchers found that the eel lost 2.3 g of weight due to urine output.  They ultimately reported that the eel had lost all the seawater it absorbed through the gut via various mechanisms.
-Scientists also study freshwater teleosts, which live in an environment where there is more water and less salt outside their bodies than inside.  Osmoregulation in freshwater teleosts would most likely serve to combat their natural tendency to:

Quizplus · Accepted Answer

11ecba2d_1026_de2a_a3bf_23ef15c5a38b_MD0008_00 In the absence of external energy input, molecules in solution will diffuse from areas of high concentration to low concentration. Such flow down a concentration gradient is thermodynamically favorable. Molecules that are separated from an area of low concentration by a membrane will still flow across the membrane down their concentration gradient as long as the membrane is permeable to those molecules. The passage introduces osmoregulation, the homeostatic process by which organisms actively assess their environment and regulate their internal concentration of fluid and electrolytes. In addition, the passage states that marine (saltwater) teleosts inhabit an environment that has a higher concentration of salt and a lower concentration of water molecules than their internal environment. Consequently, it is thermodynamically favorable for salt ions at high external concentration to diffuse through the skin into the low-salt internal environment of the fish. Water will traverse the opposite path via osmosis, moving from the high-concentration environment inside the fish to the lower concentration environment of the surrounding salt water. Osmoregulation uses external energy to reverse these processes and maintain constant salt and water concentrations within the fish. Unlike marine teleosts, freshwater teleosts live in an environment where there is more water and less salt outside their bodies than inside. As a result, salt ions are at relatively high concentration within the teleost and will tend to move to the lower concentration environment of the freshwater. Water molecules will follow the opposite route and move from the higher concentration of the external environment to the lower concentration inside the fish. As a result, the freshwater teleosts must use osmoregulation to combat their natural tendency to lose salt and absorb water. (Choices A and B) The passage states that water and salt move in opposite directions. Because the natural tendency of freshwater teleosts is to lose salt and absorb water, both compounds cannot be absorbed, nor can they both be lost. (Choice C) Saltwater, not freshwater, fish have a natural tendency to absorb salt and lose water. Educational objective: In the absence of external energy input, molecules in solution diffuse from areas of high concentration to low concentration. Osmoregulation provides the energy needed to reverse this process and maintain constant salt and water concentrations within an organism.

Passage Within the Actinopterygii (Bony Fish) Lineage, Marine Teleosts (Bony Fish