Chapter 8 Summary

Chapter 8.1

• Maintaining water and ion balance is important for normal cell function.
• Osmotic stress is dysfunction caused by a sudden change in the osmotic pressure outside a cell, which causes a rapid change in osmosis of water across the cell membrane.
• Hyperosmotic stress causes decreased cell volume, increased intracellular ion concentration, and macromolecule aggregation. Membrane fusion damage may occur,
• Hypoosmotic stress causes increased cell volume and decreased intracellular ion concentrations. Cell lysis due to breaks in the plasma membrane may occur.

Chapter 8.2

• To respond to osmotic shock, cells must first be able to detect that osmotic shock.
• Many marine organisms use calcium-sensing receptors to detect increases in extracellular Ca²⁺, which can accompany hyperosmotic stress/shock, stimulating signal transduction that involves the second messengers IP₃ and Ca²⁺.
• Some cells can use stretch receptors such as integrin to detect hypoosmotic stress/shock via membrane stretching, stimulating signal transduction that involves the Rho kinase phosphorylation cascade.

Chapter 8.3

• After detecting osmotic shock, cells respond differently depending on whether the shock is hypoosmotic or hyperosmotic.
• The main strategy to tolerate hyperosmotic conditions is to increase the rates of water entering the cell.
• One example of a mechanism to support cellular water gain is to accumulate compatible osmolytes like glycerol in the cytosol, so water will enter by osmosis, as seen in the alga D. salina and the nematode C. elegans.
• The main strategy to tolerate hypoosmotic environments is to increase rates of water leaving the cell.
• Two examples of mechanisms to support cellular water loss include the use of a contractile vacuole (e.g., by single-celled freshwater protists like Amoeba and Paramecium) or pumping ions out of the cell so that water will follow by osmosis (e.g., by gill cells of the turbot fish).