AP Biology 2.8 - Facilitated Diffusion

This section of the AP Biology curriculum focuses on tonicity, water potential, and how different organisms regulate their cells to deal with these complex forces. We’ll start by taking a look at hypertonic, hypotonic, and isotonic substances. Then, we’ll see exactly what water potential is and how we can calculate the water potential of a given system. In the final section, we’ll take a look at how plants and animals deal with variations in osmolarity. We’ll see how plants manipulate the water potential of their cells to carry water to the top of the plant. Then, we’ll see how animals deal with water balance in various environments.

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Cells have membranes that allow them to establish and maintain internal environments that are different from their external environments.

Explain how concentration gradient affects the movement of molecules across membranes.
Explain how osmoregulatory mechanisms contribute to the health and survival of organisms.

External environments can be hypotonic, hypertonic, or isotonic to internal environments of cells-

  1. Water moves by osmosis from areas of high water potential/low osmolarity/low solute concentration to areas of low water potential/high osmolarity/high solute concentration.

Growth and homeostasis are maintained by the constant movement of molecules across membranes.


Water Potential:

𝚿 = 𝚿P + 𝚿S
𝚿P = pressure potential
𝚿S = solute potential

2.8 Tonicity and Osmoregulation Overview

Redwood trees are the tallest plants in the world. At up to 380 feet tall, these giant organisms need to transport water from their roots all the way to their leaves. In order for humans to pump water to the same height (equivalent to a 30 story building!), we would need massive water pumps that use a ton of energy. So, how do redwood trees accomplish this herculean task?

While humans need advanced technology to move water, cells in plants and animals have evolved incredible mechanisms for dealing with different environments, transporting water, and maintaining homeostasis. This overview will dive into the complexities of tonicity, osmoregulation, and water potential. There will definitely be questions about this material on the AP Biology test! So, follow along with us as we look at Tonicity and Osmoregulation in organisms and cover everything you need to know!

This section of the AP Biology curriculum starts with a discussion of tonicity and how we can compare different solutions based on their solute concentrations using words like hypertonic, hypotonic, and isotonic. Then, we’ll look at water potential and the water potential equation we can use to predict the movement of water. Once we understand these things, we’ll look at how plants manipulate their cells to decrease their water potential and drive water up to their leaves. Finally, we’ll look at how animals deal with different tonicities in different environments!

A solution is formed when a solute – such as ions, polar molecules, or other substances – dissolves into a solvent – a liquid capable of dissolving a substance.

In living systems, the solvent is always water. The solute dissolved in water can be anything from small, charged ions to large, polar glucose molecules. The phenomenon of hydrogen bonding is constantly separating these molecules and trying to distribute them evenly as each water molecule tries to maximize its number of bonds. Osmolarity is a term used to describe the concentration of solutes within a given volume of water. A high osmolarity means there are many solutes dissolved in a solution, whereas a low osmolarity describes a solution where few solutes dissolved in a solution.

The term “tonicity” describes a set of terms we can use to compare two solutions and predict how the water molecules and solute particles will react if the two solutions meet.

If we are trying to determine the tonicity of two solutions, there are three terms we can use: hypertonic, hypotonic, and isotonic. The important thing to remember is that these are comparative terms. You cannot describe a single solution using the terms of tonicity. For example, a certain sports drink may have an osmolarity of 300 mmol per kg. Without another substance to compare to, you cannot determine whether this sports drink is isotonic, hypertonic, or hypotonic. It depends entirely on the substance you are comparing it to.

A solution is described as isotonic to another solution if they have the same concentration of solutes diluted by the same amount of solvents. In other words, two solutions with the same osmolarity are each considered isotonic to the other. For example, if you pour the sports drink into two glasses, each has an osmolarity of 330 mmol/kg so the two solutions are isotonic to each other.

By contrast, a hypertonic solution has more dissolved solutes than the solution it is being compared to. For instance, if we were to compare the sports drink to pure water, we would say that the sports drink is hypertonic compared to pure water.

Finally, a hypotonic solution is one that has fewer dissolved solutes than the solution it is being compared to. Let’s say that we are trying to compare water to the sports drink. Now, we would say that the water is hypotonic compared to the sports drink. Notice how this is the same comparison, though now the water is the object we are focusing on instead of the sports drink.

We can use these terms to describe the environment a cell is in. Since water can flow through the cell membrane, cells that experience different tonicities are subject to different effects.

If a cell is in an isotonic environment, the solution surrounding the cell has the same osmolarity as the solution within the cell. This means that the cell will exchange water molecules into and out of the cell at the same rate.

If a cell is placed in a hypotonic environment, the solution outside the cell has fewer dissolved solutes than the solution inside the cell. Since water can flow through the cell membrane, water will pack into the cell in an effort to correct this imbalance. This can pressurize the water in the cell and cause lysis – or ruptures in the cell membrane – if the imbalance is too much.

By contrast, if we place a cell in a hypertonic environment it will become shriveled. This is because there is a higher concentration of solutes outside of the cell. Therefore, the water inside of the cell will begin to move out of the cell to try to bring the two solutions into equilibrium.

Water potential is a measure of the potential energy water that a solution carries when compared to pure water. You can think about it this way – because of water’s ability to form hydrogen bonds, water can physically move polar substances and ions. Moving anything takes energy.

So, pure water has a lot of potential energy stored up. It has the potential to move large amounts of solutes. When you add solutes to the water, some of the hydrogen bonds are used to surround and distribute the solutes, so the entire solution has a lower amount of potential energy. Or, you could also say that the entire solution has a lower water potential.

There are many things that can affect water potential.

Scientists use the letter “psi” (𝚿) from the Greek alphabet to denote water potential. The total water potential of a given solution is equal to all the individual aspects of water potential added together. We can add up the pressure potential, the solute potential, the gravity potential, the matrix potential, and many other aspects that contribute to the potential energy stored in a solution to get the overall water potential. In this course, we will focus on only the main two of these forces – the pressure potential and the solute potential.

Consider this experiment. If we were to fill up a small tub with water and divide it in half with a cell membrane we can replicate how tonicity affects the internal and external environment of a cell. We can measure the water potential of each half of this system. If we add salt to one side of the membrane, we are decreasing the solute potential of this side of the membrane.

Since all other things remain the same, the saltwater side now has a lower water potential than the freshwater side. This means that water will move by osmosis across the membrane to the saltwater side. By the same logic, if we increased the pressure potential on the saltwater side by pushing downward on the water, the water would move back through the membrane as the decrease in solute potential was countered.

Plants are different from animals in that they prefer a hypotonic solution to surround their cells.  You may remember that a hypotonic solution can cause red blood cells to lyse. This is because animal cells do not have a cell wall. Plants can pack their cells with water in order for the cells to be rigid – also known as turgidity. This turgidity is what allows plants to stand tall and have a defined structure. If you pour salt on a plant’s roots or otherwise bathe it in a hypertonic solution, the cells will be drained of water, become plasmolyzed, and the plant will collapse.

When it comes to water potential, plants live by a simple rule. The water potential of the soil must be greater than the roots, which themselves must have a higher water potential than the stems, which must have a higher water potential than the leaves, which must have a higher water potential than the atmosphere. Plants can actively control the water potential of their tissues by adding solutes like ions and glucose to their cells. This lowers the water potential of cells higher in the plant, allowing water to easily travel up the stem. That is essentially how massive redwoods get water all the way up to their leaves!

Animals deal with different tonicities in different ways, depending on both their evolution and the environment they are in. Let’s consider a freshwater fish vs a saltwater fish.

The freshwater fish has a problem. It lives in a hypotonic environment. That means that the cells in the freshwater fish are much more concentrated than the external environment, and they want to stay that way. Because of the dilute environment it lives in, water is constantly entering the fish through its mouth, gills, and skin. The kidneys of this freshwater fish are constantly removing excess water from its body, creating very dilute urine. Let’s compare that to the saltwater fish.

The saltwater fish lives in a hypertonic environment, meaning the solution outside of the fish contains more dissolved solutes than within the fish’s cells. This means that the fish is constantly losing water via osmosis through the skin and gills to the outside environment. To compensate, a saltwater fish must not only drink water but it also must have a much more efficient kidney to create highly concentrated urine to conserve water!