Cell membranes are studded with proteins. Some of these protei...

Question

Cell membranes are studded with proteins. Some of these proteins, involved in the transport of molecules across the membrane into the cell, span the entire membrane and are called transmembrane proteins. The interior of the cell membrane is hydrophobic and nonpolar, whereas both the extracellular and intracellular fluids are water-based.
a. List three amino acids you would expect to find in the part of a transmembrane protein that lies within the cell membrane.

a. List three amino acids you would expect to find in the part of a transmembrane protein that lies within the cell membrane.

Accepted Answer

All right. Hi, everyone. So this question says that cell membrane proteins arbitrate the processes that are fundamental for cells spanning the entire membrane are transmembrane protein which are involved in the transport of molecules across the membrane into the cell. The fluids inside and outside of the cell are water based. While the interior of the cell membrane is nonpolar, which set of amino acids below is expected to be found in the part of a transmembrane protein that lies within the cell membrane interior. And here we have four different answer choices labeled A through D that we will discuss as we proceed through this video. Because recall first and foremost that all amino acids have the same general structure, right? We have an alpha carbon in the center connecting to an amino group containing a nitrogen as well as a carboxyl group. But in addition to this connecting to the alpha carbon is an R group or a side chain. And that side chain is incredibly important to us for a number of reasons, right? Recall that each amino acid has a different side chain or R group. So by recognizing what each of the R groups look like we can distinguish between different amino acids and it is the identity of the side chain that can determine whether or not an amino acid is considered to be hydrophobic or hydrophilic. Recall that a substance is said to be hydrophobic if it is simply not attracted to water molecules or insoluble in water. So in the context of our discussion of amino acids, an amino acid is said to be hydrophobic if the side chain or the R group is nonpolar. So amino acids that contain nonpolar side chains are hydrophobic. On the other hand, a substance is considered to be hydrophilic if it is very much soluble in water, right, it's attracted to water molecules. Now, amino acids that are hydrophilic include ones that have neutro polar, acidic and basic side shakes. These three categories are considered to be hydrophilic because they all dissolve in water. So when we talk about the makeup of a transmembrane protein, the protein itself should be able to mesh well with its environment because notice what the question is discussing here. The question makes a point of referencing the fact that the fluids inside and outside of the cell are water based. And so what that means is that the fluids on either end of the cell membrane or of the cell inside or outside are therefore going to be polar if they are water based. This means that the amino acids in the protein that are on either end of the cell membrane should therefore be polar as well to make sure that the protein meshes well or mixes well with its environment. Now, on the other hand, the interior of the cell membrane is nonpolar. This means that on the inside of or the interior of the transmembrane protein should be nonpolar amino acids. So if we consider the structure of this protein, nonpolar amino acids should be in the interior and polar amino acids should be in the exterior. So if we examine the four answer choices provided, all four of them propose three amino acids each. Now, the correct answer is going to be the one that proposes three nonpolar amino acids because the question is specifically asking about which amino acid can lie within the cell membrane interior. In other words, in the non polar region of the membrane. So let's start with option eight which is aspartic acid, arginine and tyrosine. Now recall that all of these are actually hydrophilic because aspartic acid is acidic, arginine is basic and thyroxine is simply neutro polar. So option A I can go ahead and eliminate which leaves me with option B which is sine alanine and tryptophan. Now recalled alanine and tryptophan are in fact nonpolar but sine is not right. Sine is neutral polar because its side chain contains a hydroxy group or an oh group. So option B is pretty close. But because one of them is actually polar, I cannot say that option B would be correct. And so option C is vain Iso Len and three E and this one is also close, right? Because vine and isoleucine are in fact nonpolar but th anine is not because th three anine like Cine has a hydroxy group in its side chain which would make it polar as well. So I can cross out options C which leaves me with option D glycine, prolene and phenyl alanine. And so option D would be our correct answer because all three of these amino acids are in fact nonpolar, which means that option D is the only set of amino acids that all have nonpolar side chains. Therefore, all of these amino acids can be found in the part of the transmembrane protein that is within the cell membrane and there you have it. So with that being said, thank you so very much for watching and I hope you found this helpful.

Key Concepts

Transmembrane Proteins

Hydrophobic and Hydrophilic Interactions

Amino Acid Properties

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