Problem 15 - Biological Science

Question

True or false: Water moves by osmosis across a fish's gills to an area with a higher sodium ion concentration because water molecules are attracted to the sodium ions.

Step-by-Step Solution

Answer

False. Water does move across the fish's gills by osmosis, but it is not because water molecules are attracted to sodium ions. Instead, movement is primarily due to concentration gradients of solutes and the natural tendency for water to equalize concentrations across the semipermeable membrane (gills).

Step 1: Understand Osmosis

Osmosis is the process where water molecules move across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration until equilibrium is reached. This occurs due to the natural tendency for water to try to equalize the concentration of solutes in two different environments.

Step 2: Consider Fish Gills

Fish gills are involved in the process of respiration, where oxygen from the surrounding water is extracted and sent to the fish's bloodstream. Gills are made up of thin layers of tissue, which work as a semipermeable membrane that allows certain substances, like water and ions, to pass through.

Step 3: Water Movement in Fish

The movement of water across a fish's gills depends on the balance between the concentration of ions (like sodium) and water in the fish's blood and the surrounding water. In addition to gaseous exchange, fish gills also regulate the balance of ions and water in the fish's body, a process known as osmoregulation.

Step 4: Evaluate the Statement

The statement that water moves by osmosis across a fish's gills to an area with a higher sodium ion concentration because water molecules are attracted to the sodium ions is not entirely accurate. While water does move across the gills, it is primarily driven by the concentration gradient of solutes, not the attraction between water molecules and sodium ions. Osmosis aims to balance solute concentrations, not to create an attraction between water and solute particles.

Step 5: Conclusion

The given statement is false. Water does move across the fish's gills by osmosis, but it is not because water molecules are attracted to sodium ions. This movement is primarily due to concentration gradients of solutes and the natural tendency for water to equalize concentrations across the semipermeable membrane (gills).

Key Concepts

Semipermeable Membrane

A semipermeable membrane is a special barrier that selectively allows certain substances to pass through while blocking others. This characteristic plays a crucial role in osmosis, particularly in biological systems like fish gills.
Fish gills, with their thin layers of tissue, act as semipermeable membranes, permitting the movement of water and smaller molecules such as ions but keeping larger particles out.
Interestingly, these membranes do not require energy to perform this selective transfer. Instead, they rely on the physical and chemical properties of the materials on either side of the membrane.
This allows cells and organisms to maintain optimal conditions by regulating the movement of water and solutes through processes like osmosis.

Osmoregulation

Osmoregulation refers to the way in which organisms control the concentration of water and electrolytes in their bodies to maintain homeostasis. It is an essential function for aquatic animals like fish.
Fish gills are not only involved in breathing but also play a significant role in osmoregulation. They help in balancing the intake and outflow of water and salts such as sodium ions, maintaining internal fluid equilibrium.

• In freshwater fish, the concentration of ions inside their bodies is higher than in their surrounding environment. Thus, they actively expel water while absorbing required ions.
• Conversely, in saltwater fish, the external ion concentration is greater. Hence, they tend to lose water to their environment and must take up water and ions to remain hydrated.

This ability to regulate water and ion balance ensures that the fish survive and thrive in various aquatic environments.

Concentration Gradient

A concentration gradient is a crucial concept in understanding how osmosis works. It refers to the difference in the concentration of particles, such as ions or molecules, between two regions.
In the case of osmosis across fish gills, a concentration gradient exists between the water outside the fish and the fluids inside its body. Water naturally moves from areas of low solute concentration to areas of high solute concentration in an attempt to equalize the concentrations on both sides of the semipermeable membrane.

• This movement doesn't require energy and occurs until equilibrium is reached, where the concentrations become equal.
• Consider it like a slope where water 'flows' down from a higher concentration to a lower one, driven by the gradient.

The concentration gradient is a key player in directing the passive transport of water and solutes in biological systems, contributing to vital processes like osmoregulation.