When a salmon moves from a saltwater ocean to a freshwater river, it encounters a significant change in the osmotic environment. The ocean is a hypertonic environment, meaning it has a higher concentration of dissolved solutes compared to the freshwater river, which is a hypotonic environment. To survive, the salmon must maintain its internal osmotic balance by regulating the movement of water and solutes across its cell membranes. Here is an overview of the physiological changes that occur in the salmon's cells during this transition:
1. In the Ocean (Hypertonic Environment):
- The salmon's cells are exposed to a higher concentration of solutes in the ocean water compared to their internal environment.
- To prevent water loss and maintain cell volume, the salmon actively transports ions (such as sodium and chloride) into its cells, increasing the internal solute concentration.
- This process requires energy in the form of ATP and is carried out by ion pumps, such as the sodium-potassium ATPase pump, located on the cell membrane.
- The active transport of ions helps maintain osmotic balance and prevents cell shrinkage.
2. Transition to Freshwater (Hypotonic Environment):
- As the salmon moves into the freshwater river, the external osmotic pressure decreases, creating a hypotonic environment.
- The lower concentration of solutes in the freshwater causes water to move passively into the salmon's cells by osmosis.
- To prevent excessive swelling and potential cell rupture, the salmon adjusts its ion transport processes.
- It reduces the active transport of ions into its cells and may even reverse the process, actively transporting ions out of the cells.
- This shift in ion transport helps regulate water movement and maintains cell volume.
3. Ion Regulation:
- The change in environment triggers adjustments in the expression and activity of ion transport proteins in the salmon's cells.
- The increased activity of specific ion channels and pumps, such as the sodium-potassium ATPase pump, allows the salmon to adapt to the new osmotic conditions.
- The regulation of ion concentrations, particularly sodium and potassium, is crucial for maintaining the proper electrochemical gradients across cell membranes and ensuring essential cellular functions.
4. Kidney and Gill Function:
- The kidneys and gills play vital roles in osmoregulation in salmon.
- The kidneys are responsible for regulating water and ion excretion, helping the salmon balance its internal fluid volume.
- Modifications in urine production and ion reabsorption occur in response to the change in salinity.
- The gills are also involved in ion transport and gas exchange. They help in the uptake of essential ions, such as sodium and chloride, and the excretion of waste products.
Overall, as a salmon moves from the ocean to a freshwater river, its cells undergo physiological adaptations to maintain osmotic balance and survive in the changing environments. These adaptations involve adjustments in ion transport processes, regulation of water movement, and modifications in kidney and gill function.