TRA - Osmosis [LESSON]

Osmosis

Osmosis is the movement of water across a membrane from an area of high water (low solute) concentration to an area of low water (high solute) concentration. While diffusion transports materials across membranes and within cells, osmosis transports only water across a membrane.  So, osmosis is a special case of diffusion.

Tonicity and Osmolarity

Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis. A solution's tonicity often directly correlates with the osmolarity of the solution. Osmolarity describes the total solute concentration of the solution. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles. In a situation in which solutions of two different osmolarities are separated by a membrane permeable to water, though not to the solute, water will move from the side of the membrane with lower osmolarity (and more water) to the side with higher osmolarity (and less water). This effect makes sense if you remember that the solute cannot move across the membrane, and thus the only component in the system that can move—the water—moves along its own concentration gradient. An important distinction that concerns living systems is that osmolarity measures the number of particles (which may be molecules) in a solution. Therefore, a solution that is cloudy with cells may have a lower osmolarity than a solution that is clear if the second solution contains more dissolved molecules than there are cells.

Watch the Osmosis video to learn about the three types of tonicities.Three terms—hypotonic, isotonic, and hypertonic—are used to relate the osmolarity of a cell to the osmolarity of the extracellular fluid that contains the cells. In a hypotonic situation, the extracellular fluid has lower osmolarity than the fluid inside the cell, and water enters the cell. In this situation, water will follow its concentration gradient and enter the cell, causing the cell to expand. As for a hypertonic solution, the prefix hyper- refers to the extracellular fluid having a higher osmolarity than the cell's cytoplasm; therefore, the fluid contains less water than the cell does. Because the cell has a relatively higher concentration of water, water will leave the cell, and the cell will shrink. In an isotonic solution, the extracellular fluid has the same osmolarity as the cell. If the osmolarity of the cell matches that of the extracellular fluid, there will be no net movement of water into or out of the cell, although water will still move in and out.

Check your knowledge below with the Osmosis Practice.

Look at the images of the red blood cells and plant cells in each of the three types of solutions. Which do you think animal cells prefer? Plant cells?

Which environment does the red blood cell prefer?

Animal cells prefer isotonic solutions because there is no excess loss or gain of water. Remember that animal cells do NOT have a cell wall to counteract water flow.

 

Which environment does the plant cell prefer?

Plant cells prefer hypotonic solutions because the pressure of the cell wall and the full vacuole pushing on each other allows the plant to maintain turgor pressure and stand upright.

Osmoregulation

Various living things have ways of controlling the effects of osmosis —a mechanism called osmoregulation. Some organisms, such as plants, fungi, bacteria, and some protists, have cell walls that surround the plasma membrane and prevent cell lysis in a hypotonic solution. The plasma membrane can only expand to the limit of the cell wall, so the cell will not lyse. Again, plants fare better with a constant inflow of water due to a hypotonic external environment to maintain turgor pressure.  Another example is found in Paramecia and Amoebas, which are protists that lack cell walls. They have contractile vacuoles which collect excess water from the cell and pump it out, keeping the cell from lysing as it takes on water from its environment.  Freshwater fish live in an environment that is hypotonic to their cells. These fish actively take in salt through their gills and excrete diluted urine to rid themselves of excess water. Saltwater fish live in the reverse environment, which is hypertonic to their cells, and they secrete salt through their gills and excrete highly concentrated urine.  In vertebrates, the kidneys regulate the amount of water in the body. If the levels of solutes increase beyond a certain range, a hormone is released that slows down water loss through the kidney and dilutes the blood to safer levels.

Answer the Conceptual Questions below to check your knowledge.

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