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Osmosis and tonicity

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Osmosis and tonicity. Hypertonic, isotonic, and hypotonic solutions and their effect on cells.

Introduction

Have you ever forgotten to water a plant for a few days, then come back to find your once-perky arugula a wilted mess? If so, you already know that water balance is very important for plants. When a plant wilts, it does so because water moves out of its cells, causing them to lose the internal pressure—called turgor pressure—that normally supports the plant.
Why does water leave the cells? The amount of water outside the cells drops as the plant loses water, but the same quantity of ions and other particles remains in the space outside the cells. This increase in solute, or dissolved particle, concentration pulls the water out of the cells and into the extracellular spaces in a process known as osmosis.
Formally, osmosis is the net movement of water across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. This may sound odd at first, since we usually talk about the diffusion of solutes that are dissolved in water, not about the movement of water itself. However, osmosis is important in many biological processes, and it often takes place at the same time that solutes diffuse or are transported. Here, we’ll look in more detail at how osmosis works, as well as the role it plays in the water balance of cells.

How it works

Why does water move from areas where solutes are less concentrated to areas where they are more concentrated?
This is actually a complicated question. To answer it, let’s take a step back and refresh our memory on why diffusion happens. In diffusion, molecules move from a region of higher concentration to one of lower concentration—not because they’re aware of their surroundings, but simply as a result of probabilities. When a substance is in gas or liquid form, its molecules will be in constant, random motion, bouncing or sliding around one another. If there are lots of molecules of a substance in compartment A and no molecules of that substance in compartment B, it’s very unlikely—impossible, actually—that a molecule will randomly move from B to A. On the other hand, it’s extremely likely that a molecule will move from A to B. You can picture all of those molecules bouncing around in compartment A and some of them making the leap over to compartment B. So, the net movement of molecules will be from A to B, and this will be the case until the concentrations become equal.
In the case of osmosis, you can once again think of molecules—this time, water molecules—in two compartments separated by a membrane. If neither compartment contains any solute, the water molecules will be equally likely to move in either direction between the compartments. But if we add solute to one compartment, it will affect the likelihood of water molecules moving out of that compartment and into the other—specifically, it will reduce this likelihood.
Why should that be? There are some different explanations out there. The one that seems to have the best scientific support involves the solute molecules actually bouncing off the membrane and physically knocking the water molecules backwards and away from it, making them less likely to crossstart superscript, 1, comma, 2, end superscript.
Regardless of the exact mechanisms involved, the key point is that the more solute water contains, the less apt it will be to move across a membrane into an adjacent compartment. This results in the net flow of water from regions of lower solute concentration to regions of higher solute concentration.
Illustration of osmosis. A beaker is divided in half by a semi-permeable membrane. In the left—initial—image, the water level is equal on both sides, but there are fewer particles of solute on the left than on the right. In the right—final—image, there has been a net movement of water from the area of lower to the area of higher solute concentration. The water level on the left is now lower than the water level on the right, and the solute concentrations in the two compartments are more equal.
Image credit: OpenStax Biology
This process is illustrated in the beaker example above, where there will be a net flow of water from the compartment on the left to the compartment on the right until the solute concentrations are nearly balanced. Note that they will not become perfectly equal in this case because the hydrostatic pressure exerted by the rising water column on the right will oppose the osmotic driving force, creating an equilibrium that stops short of equal concentrations.

Osmolarity

Osmolarity describes the total concentration of solutes in a solution. A solution with a low osmolarity has fewer solute particles per liter of solution, while a solution with a high osmolarity has more solute particles per liter of solution. When solutions of different osmolarities are separated by a membrane permeable to water, but not to solute, water will move from the side with lower osmolarity to the side with higher osmolarity.
Three terms—hyperosmotic, hypoosmotic, and isoosmotic—are used to describe relative osmolarities between solutions. For example, when comparing two solution that have different osmolarities, the solution with the higher osmolarity is said to be hyperosmotic to the other, and the solution with lower osmolarity is said to be hypoosmotic. If two solutions have the same osmolarity, they are said to be isoosmotic.

Tonicity

In healthcare settings and biology labs, it’s often helpful to think about how solutions will affect water movement into and out of cells. The ability of an extracellular solution to make water move into or out of a cell by osmosis is known as its tonicity. Tonicity is a bit different from osmolarity because it takes into account both relative solute concentrations and the cell membrane’s permeability to those solutes.
Three terms—hypertonic, hypotonic, and isotonic—are used to describe whether a solution will cause water to move into or out of a cell:
If a cell is placed in a hypertonic solution, there will be a net flow of water out of the cell, and the cell will lose volume. A solution will be hypertonic to a cell if its solute concentration is higher than that inside the cell, and the solutes cannot cross the membrane.
If a cell is placed in a hypotonic solution, there will be a net flow of water into the cell, and the cell will gain volume. If the solute concentration outside the cell is lower than inside the cell, and the solutes cannot cross the membrane, then that solution is hypotonic to the cell.
If a cell is placed in an isotonic solution, there will be no net flow of water into or out of the cell, and the cell’s volume will remain stable. If the solute concentration outside the cell is the same as inside the cell, and the solutes cannot cross the membrane, then that solution is isotonic to the cell.

Tonicity in living systems

If a cell is placed in a hypertonic solution, water will leave the cell, and the cell will shrink. In an isotonic environment, there is no net water movement, so there is no change in the size of the cell. When a cell is placed in a hypotonic environment, water will enter the cell, and the cell will swell.
Diagram of red blood cells in hypertonic solution (shriveled), isotonic solution (normal), and hypotonic solution (puffed up and bursting).
Image credit: Mariana Ruiz Villareal
In the case of a red blood cell, isotonic conditions are ideal, and your body has homeostatic (stability-maintaining) systems to ensure these conditions stay constant. If placed in a hypotonic solution, a red blood cell will bloat up and may explode, while in a hypertonic solution, it will shrivel—making the cytoplasm dense and its contents concentrated—and may die.
In the case of a plant cell, however, a hypotonic extracellular solution is actually ideal. The plasma membrane can only expand to the limit of the rigid cell wall, so the cell won't burst, or lyse. In fact, the cytoplasm in plants is generally a bit hypertonic to the cellular environment, and water will enter a cell until its internal pressure—turgor pressure—prevents further influx.
Maintaining this balance of water and solutes is very important to the health of the plant. If a plant is not watered, the extracellular fluid will become isotonic or hypertonic, causing water to leave the plant's cells. This results in a loss of turgor pressure, which you have likely seen as wilting. Under hypertonic conditions, the cell membrane may actually detach from the wall and constrict the cytoplasm, a state called plasmolysis (left panel below).
Image of a plant cell under hypertonic conditions (plasmolyzed/shriveled), isotonic conditions (slightly deflated, not fully pressed up against the cell wall), and hypotonic conditions (pressed firmly against the cell wall, normal state).
Image credit: OpenStax Biology, modification of work by Mariana Ruiz Villareal
Tonicity is a concern for all living things, particularly those that lack rigid cell walls and live in hyper- or hypotonic environments. For example, paramecia—pictured below—and amoebas, which are protists that lack cell walls, may have specialized structures called contractile vacuoles. A contractile vacuole collects excess water from the cell and pumps it out, keeping the cell from lysing as it takes on water from its hypotonic environment.
Microscope image of a paramecium, showing its contractile vacuoles.
Image credit: OpenStax Biology, modification of work by the National Institutes of Health (NIH), scale-bar data from Matt Russell

Want to join the conversation?

  • male robot hal style avatar for user Dovid Shaw
    Why doesn't the pressure of the cell (even a red blood cell that isn't rigid), balance out the net inflow in a hypotonic solution? The net inflow doesn't work with energy, but because their is room to slide around!?
    (16 votes)
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    • blobby green style avatar for user Joshua Schwimer
      I think this is the case with a plant cell that has a rigid cell wall thus in a fixed volume hydrostatic pressure will increase until osmotic pressure is opposed. But with an RBC the volume is not fixed (due to lack of cell wall) so osmotic pressure increases unopposed until the cell lyses.
      (32 votes)
  • piceratops ultimate style avatar for user Paul Norris
    It seems odd to me that the sole factor driving osmosis is the relative concentration of the solute (osmolarity), and that other characteristics of the solute (size of molecules, polarity, etc..) don't play a role as well. Is this really true and, if so, can someone explain why?
    (13 votes)
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    • leaf green style avatar for user Anika Sharma
      when addressing something like osmosis, it is really another form of diffusion for water but flipped. in diffusion, we don't see the polarity, size of molecules, or charge playing a role in how the molecules go from high concentration to low concentration. in the cell, constantly we see that it is trying to maintain and achieve equilibrium. from using channel proteins to diffusion, the cell constantly looks for ways to be in an equal environment. the way i like to look at it, water molecules flowing to an area with more solute rather than staying in the one with less, in other words, flowing from low water concentration to high, helps the cell reach equilibrium.
      (1 vote)
  • leaf orange style avatar for user Valeria Ventosa
    What could be an example of solute in a plant cell?
    (9 votes)
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  • piceratops seedling style avatar for user Nomunaa
    what is ion and molecule? and how do elements become positive / negative charged?
    (4 votes)
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    • aqualine ultimate style avatar for user Prajjwal Rathore
      An Ion is basically a charged atom. The atom can be either positively charged (by losing one electron) or negatively charged ( by gaining one electron).
      Molecules are groups of electrically neutral atom/s which are chemically bonded.
      Charge is due to loss or gain of an electron in an atom.
      (14 votes)
  • blobby green style avatar for user timar.pink
    My group and I are making lab project by estimating the osmolarity in tissues by bathing the blood samples from the 3 members of my group with hypotonic and hypertonic solutions and observing it by using our microscope. Since we are done with observations, we are assigned to do a group lab report, and my individual task is to basically do the data analysis. However, I do not know which type of graph should I create regarding the observation and its results of the osmolarity of the blood samples in all three solutions. Should it be line graph, bar graph, pie graph, or, etc.?
    (3 votes)
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  • male robot hal style avatar for user shreypatel0101
    Why does the cells of stomata becomes flaccid instead of shrinking when they loss water from them?
    (2 votes)
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  • blobby green style avatar for user Natalie
    what effect does concentration have on osmosis? does a higher concentration create faster or slower rates of osmosis?
    (4 votes)
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    • piceratops tree style avatar for user Jen
      If osmosis depends on the presence of a concentration gradient (in other words, if there is no concentration gradient, no osmosis will occur), what do you think would happen if you had one solution with a much higher solute concentration than another solution?
      (1 vote)
  • leafers seed style avatar for user bgao20
    Why do plants die from over-watering if plant cells don't explode from an influx of water?
    (2 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      Because xylem keeps getting water (it is thanks to the forces not based on the real plant needs). If the plant has nowhere to let go of water in the flower pot (no hole to let go of excessive water) it keeps 'drowning'. If transpiration cannot help all the water got through the body to the leaves, then water will just stay in the plant tissues.
      (4 votes)
  • aqualine seed style avatar for user 63052
    What exactly does hyper-tonic mean?
    (2 votes)
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  • blobby green style avatar for user Stav Shmueli
    In the introduction passage, it says: "The amount of water outside the cells drops as the plant loses water, but the same quantity of ions and other particles remains in the space outside of the cells."
    I can't understand how it makes sense. If water leaves the cells shouldn't there be MORE water outside the cells, hence there will be less concentration of solute particles in the solution outside the cell?
    (2 votes)
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    • winston baby style avatar for user Ivana - Science trainee
      While I understand your logic, here's what really is happening:
      In order for the cell to Lose water, there must be Less water outside!

      Just rephrasing.

      Do you know that plants lose water by transpiration? So you really cannot say that after cell lost water there is more water outside (it would make no sense since that same water would reverse back, right?)

      On the other side, the rhizome of the plant which is in direct contact with soil is able to obtain water by osmosis. It has high root pressure which soaks up water from soil (higher concentration of minerals in root cell than in soil).
      (2 votes)