Hydrophobic Effect - Online Tutor, Practice Problems & Exam Prep

The hydrophobic effect is crucial for life, influencing protein folding and membrane formation. Hydrophobic molecules, which are water-fearing, form hydration shells that decrease local entropy. When these molecules clump together, they reduce their surface area and the number of water molecules in hydration shells, allowing water to break free and increase universal entropy. This process is thermodynamically favorable, facilitating the proper folding of proteins and the formation of lipid bilayers essential for cellular structures.

Intro to the Hydrophobic Effect

In this video, we're going to begin our discussion on the hydrophobic effect. So the hydrophobic effect is this phenomenon of the exclusion of hydrophobic substances by water. And so we've mentioned the hydrophobic effect several times in our previous lesson videos, so we already know that hydrophobic molecules are water fearing molecules that are insoluble or not soluble because they do not dissolve well with water. And, actually, when you try to mix hydrophobic substances with water, they will actually form a completely separate phase. Now the hydrophobic effect is actually super important for life, and that's why your professors want you guys to know about it. Now the reason that the hydrophobic effect is so important for life is because it's critical for both protein folding and for the formation of membranes, which are two processes that we'll talk more about in a different video later in our course.

Now down below in our example, we have an experiment that you guys can actually try at your homes, and that's taking a cup of water and pouring in oil into it. And what you'll notice is that the oil does not dissolve with the water and that's because the oil is hydrophobic. And so no matter how much you try to stir that oil and dissolve the oil with the water, the oil will not dissolve with the water. And eventually, all of these oil bubbles that you create are going to clump together to create a completely separate oil phase that's separate from the water. And so you might think that these oil bubbles are clumping together because hydrophobic substances must have a strong attraction for each other. Right? Well, that's what it appears to be. So in water, hydrophobic nonpolar substances appear to have a strong net attraction or a strong net affinity for each other. But that is not the case. And so other than the super weak Van der Waals forces that exist between all molecules, hydrophobic substances do not have a strong attraction for one another.

And so if hydrophobic substances don't have a strong attraction for one another, then why is it that they're clumping up like what we're seeing over here? Well, we'll explain that in our next lesson video, so I'll see you guys there.

Explanation of the Hydrophobic Effect

So we already said in our previous video that the hydrophobic effect is not explained by the strong net affinity of hydrophobic substances for each other, but the hydrophobic effect can be explained by looking at the water molecules that surround a nonpolar substance. And so when we take a nonpolar substance and we add it into water, it becomes a hydrated nonpolar substance, and all that means is that there's a cage-like shell or layer of water molecules surrounding the nonpolar substance. And this layer of water molecules is referred to as the hydration shell. And we already talked about hydration shells a little bit when we talked about water solubility in our previous videos. And really there are 3 things to note about the water molecules in a hydration shell surrounding a nonpolar substance. And the first thing to note is that water molecules in the hydration shell cannot participate in normal hydrogen bonding. Now the second thing to note is that water molecules in the hydration shell, they move slower than normal, they form fewer but stronger hydrogen bonds, and because they form stronger hydrogen bonds, they have more energy, and more energy means that they are less stable. And then the third thing to note about water molecules in the hydration shell is that they have fewer options for orientations in 3D space, and this has to do with the stronger hydrogen bonds. So when they form these strong hydrogen bonds, they're harder to break, and because they're harder to break, it's harder for them to take on different orientations. And so fewer orientations means that they're going to be more ordered. And we know that more order is associated with less entropy from our thermodynamic videos. But recall that the universe is moving towards a state of increased entropy. And so because water molecules in the hydration shell around a nonpolar substance are decreasing the entropy, this means that the formation of a hydration shell around a nonpolar substance is not thermodynamically favorable. But it actually is thermodynamically favorable for hydration shells to merge together when nonpolar substances clump and reduce their surface area. And so even though the entropy is decreased when nonpolar substances clump, this decrease in local entropy is largely offset by an overall increase in the entropy of the surroundings or of the universe when water molecules that used to be part of the hydration shell break free from the hydration shell and increase the universal entropy. And so, all we're saying here is that when nonpolar substances clump in water, this is actually a thermodynamically favorable process and a spontaneous process. So let's take a look at our example below to help clear this up a little bit. So down here, what we have is water and we're taking a nonpolar molecule and adding it to the water, which means that it's going to become a hydrated nonpolar substance. And hydrated nonpolar substances have a hydration shell around them. And we know that water molecules in the hydration shell are more ordered, and more order, again, is associated with lowered entropy. And so here we have a hydration shell of water molecules surrounding the nonpolar molecule, and that means that there's going to be a decrease in the entropy of the system. Now over here, we're adding a second nonpolar molecule, which means that we're getting a formation of another hydration shell. And so if the first hydration shell decreased the entropy of the system, the formation of a second hydration shell is going to further decrease the entropy of the system. And so, when nonpolar substances clump together, there's actually a decrease in 3 different things. The first is that there's a decrease in the surface area of the nonpolar molecules. The second is that there's a decrease in the number of molecules that are part of the hydration shell. And so notice over here, we have a total of 7 water molecules, part of this hydration shell, and another 7 water molecules, part of this hydration shell. So there's a total of 14 water molecules, part of the hydration shells. Now, when the nonpolar molecules clump together, they reduce their surface area and they reduce the number of water molecules that are part of the hydration shell. So here, in this hydration shell, notice that there are only a total of 9 water molecules part of the hydration shell, not 14. And so the other water molecules that used to be part of the hydration shells, they broke free. And so they broke free from the hydration shell and they're circled in red in this image here. And so, the last thing that's decreased when nonpolar substances clump together is that there's a decrease in local entropy. So two things combining into 1 decrease local entropy is largely offset by an increase in the entropy of the surroundings or of the universe when water molecules that used to be part of the hydration shell break free. So these water molecules that break free, they end up increasing the overall entropy of the universe, which means that when nonpolar molecules clump together, ultimately, they end up increasing the entropy of the universe, and that means that the clumping of nonpolar molecules is thermodynamically favorable. And so this concludes our lesson on the hydrophobic effect explanation, and if you have any questions, leave them in the comments below. And in our next video, we're going to talk about the role of the hydrophobic effect in protein folding and membrane formation. So I'll see you guys in that video.

Protein Folding & Membrane Formation

So we already said in our previous videos that the hydrophobic effect is important to life because it's critical for protein folding as well as the formation of membranes. In our example below, we have protein folding on the left and membrane formation on the right. When we take a protein that's an unorganized protein that's unfolded, and we fold it very nicely into a nice organized structure, that decreases local entropy because we're going from unorganized and spread out all over the place to a nice neat organized formation of the protein. Whenever there's a decrease in entropy or local entropy, we know that it needs to be accompanied by an increase in the universal entropy or the entropy of the surroundings. That's where the hydrophobic effect comes into play. Here we have a protein represented by this green line here. This is an unfolded protein. Extending off of the protein, we have these yellow, R groups, which are nonpolar R groups. And around these nonpolar R groups, we have our hydration shells with these water molecules. You can see that the water molecules are surrounding the nonpolar R groups, creating a hydration shell. We know that the water molecules in the hydration shell decrease entropy. This is not thermodynamically favorable. Now, when these nonpolar substances clump, what will happen is the water molecules will break free from the hydration shell and increase the universal entropy. So even though there's a decrease in local entropy where the nonpolar R groups are clumping together to decrease entropy, it's offset by the water molecules that break free and increase the universal entropy. Here you can see we have our folded protein, and the water molecules here have broken free from the hydration shell and that allows our protein to fold. The hydrophobic effect is very important for the proper folding of a protein and we'll talk more about this later on in our course when we talk about proteins.

Now, over here we have membrane formation, and we've seen this image a few times in our previous videos. And so we've got a phospholipid here, which of course has a polar head and two nonpolar tails. The nonpolar tails are going to be hydrated nonpolar tails. So they're going to have water molecules in a hydration shell surrounding these nonpolar tails. These water molecules that surround the high, nonpolar tails, can begin to break free when these phospholipids clump together. You can see here we're clumping and we're reducing the surface area and we're reducing the amount of water molecules that are in the hydration shell surrounding these nonpolar substances. The phospholipids are going to continue to accumulate, continue to reduce the surface area and reduce the hydration shells, and continue to increase the universal entropy by releasing water molecules from the hydration shells. That allows for the formation of a phospholipid bilayer. This is incredibly important to the formation of life, just like we talked about in our abiogenesis videos. This concludes our lesson on the hydrophobic effect, and I'll see you guys in the practice videos.

Which of the following best explains the hydrophobic effect?

• A. Hydrophobic substances have a strong net affinity for each other.
• B. Hydrophilic substances increase local entropy upon clumping.
• C. Hydrophobic substances clump due to their strong intermolecular forces.
• D. Hydrophobic substances increase universal entropy when they clump.

Which of the following is false concerning H₂O molecules in the hydration shell around nonpolar substances?

• A. Cannot participate in normal hydrogen bonding.
• B. Form stronger hydrogen bonds than free H₂O
• C. Less ordered & higher entropy than free H₂O
• D. Less options for orientations in 3D space than free H₂O

Here’s what students ask on this topic:

What is the hydrophobic effect and why is it important for life?

The hydrophobic effect is the phenomenon where hydrophobic (water-fearing) substances are excluded by water, leading to the formation of separate phases. This effect is crucial for life because it plays a key role in protein folding and membrane formation. In protein folding, hydrophobic amino acid residues tend to cluster together inside the protein structure, away from water, which helps the protein achieve its functional three-dimensional shape. In membrane formation, hydrophobic lipid tails aggregate to form the interior of the lipid bilayer, while the hydrophilic heads face the aqueous environment, creating a stable barrier essential for cellular structures.

How does the hydrophobic effect influence protein folding?

The hydrophobic effect influences protein folding by driving hydrophobic amino acid residues to cluster together inside the protein, away from the aqueous environment. This clustering reduces the surface area exposed to water, leading to a decrease in the number of water molecules in hydration shells around the hydrophobic residues. As these water molecules are released, they increase the overall entropy of the system, making the folding process thermodynamically favorable. This effect helps the protein achieve its functional three-dimensional structure, which is essential for its biological activity.

What role does the hydrophobic effect play in membrane formation?

The hydrophobic effect is crucial for membrane formation. Phospholipids, which have hydrophilic heads and hydrophobic tails, spontaneously arrange themselves in water to form bilayers. The hydrophobic tails cluster together, reducing their exposure to water, while the hydrophilic heads face the aqueous environment. This arrangement minimizes the number of water molecules in hydration shells around the hydrophobic tails, allowing these water molecules to be released and increase the overall entropy. This process is thermodynamically favorable and leads to the formation of stable lipid bilayers, which are essential for cellular structures and functions.

Why do hydrophobic molecules clump together in water?

Hydrophobic molecules clump together in water not because they have a strong attraction for each other, but because this arrangement minimizes the disruption to the hydrogen-bonding network of water. When hydrophobic molecules are dispersed, they form hydration shells that decrease the entropy of the system. By clumping together, they reduce their surface area and the number of water molecules in hydration shells, allowing these water molecules to be released and increase the overall entropy. This increase in entropy makes the clumping of hydrophobic molecules thermodynamically favorable.

How does the hydrophobic effect relate to entropy?

The hydrophobic effect is closely related to entropy. When hydrophobic molecules are introduced into water, they form hydration shells that decrease the local entropy because the water molecules become more ordered. However, when these hydrophobic molecules clump together, they reduce their surface area and the number of water molecules in hydration shells. This allows the water molecules to be released, increasing the overall entropy of the system. The increase in entropy of the surroundings offsets the decrease in local entropy, making the process thermodynamically favorable.