We tackle Passage 3 on Next Step’s full-length 10 covering a passage about proteins in milk and what happens when you heat them up. Take a listen! As always, we’re joined by Bryan Schnedeker from Next Step Test Prep.
[01: 55] Passage 3: Whey Protein
Passage 3: Whey proteins are globular proteins found in milk that are an important component in the American diet, particularly among athletes. Whey is noted for its ability to enrich foods through enhancing their texture, and water and flavor holding capacity. As a whey containing solution is heated, the proteins denature, unfold, and aggregate. This process is called gelation and leads to an increase in the viscosity of the solution. It has also been observed that introducing ions into a protein solution causes thickening. Food scientists have become increasingly interested in thicker protein gels because subsequent aeration of these mixtures can induce gas bubbles to be trapped in the solution. This leads to a higher volume of product that can be sold without increasing the amount of ingredients used.
A group of scientists was contracted to determine optimal protein concentration, salt concentration, and temperature to obtain gels with the best capacity to hold air bubbles. It’s observed that no gelation occurs after 30 minutes of heating at 80 degree Celsius. Protein solutions were then mixed with calcium chloride, magnesium chloride, potassium chloride, or iron chloride. Solutions were foamed immediately after adding the salts for 30 seconds at 2,000 revolutions per minute. (The results shown on Table 1)
[04:00] Protein Structures
Question 14: Denaturation of whey proteins involves the loss of which levels of protein structure?
- Primary structure
- Tertiary structure
III. Quaternary structure
- (A) II only
- (B) I and II only
- (C) II and III only
- (D) I, II, and III
A well-prepared MCAT student should be walking into the test knowing that to denature proteins means to unfold it all the way down to its primary structure. But that you don’t actually break apart the primary structure.
It’s like a ball of yarn where the primary structure is the string of yarn itself. The secondary structure would be little coils of yarn. The tertiary structure is the whole folded up yarn ball. And then the quaternary structure is two yarn balls that get tangled into each other slightly.
So to denature is like ripping two yarn balls apart and unraveling them all the way down to just the string and then you leave the strong alone.
[05:40] Strategy for Looking at Passages
There is this strategy where you read the passage first before looking at the questions or vice versa. But which one should you actually do?
Bryan explains that typically, you do want to read the passage first. Although the one place you could maybe get away with going to the questions first is the Chem/Phys section.
And for the purposes of this podcast, Bryan and I have actually selected out questions that are a bit easier to answer by just listening.
But even in the first passage we did, we had to go back to two of the questions to directly reference the passage. For the most part though, we’ve selected out shorter, more readable questions. All this being said, you should always be reading the passage first as a mainline strategy. And if it’s not working for you or you’re running out of time, maybe you can try a questions first approach. But this is not the optimal first choice.
[06:55] Proteins and Ions
Question 16: What is a likely explanation for why whey protein solutions become thicker after the introduction of ions?
- (A) Ions form new compounds with atoms found within the whey proteins and these new compounds aggregate.
- (B) Ions disrupt bonding within in between protein subunits leading to protein unfolding and aggregation.
- (C) Ions will thicken any solution due to their large size relative to most other compounds found in solutions.
- (D) The introduction of ions causes novel bonding relationships between subunits to form ultimately leading to larger polymers of proteins that can aggregate.
Looking back at the passage, where it says “as a whey containing solution is heated, the proteins denature, unfold, and then aggregate.
So it’s like using the same ball of yarn analogy we mentioned above. First, unravel the ball of yarn. Now you have this loose string and it’s easy for that loose string to get tangled up with all the other strings and make a big, squishy mess, which is what gel is.
The heating itself makes the gel form but ions make the gel form more. You get more gel because it forms faster. But notice that the core process is you take the ball of yarn, unravel it, and then let that loose yarn get all mushed up with all the other strings in the solution. This is the process of gel forming. And what ions do is make that happen faster.
The correct answer here is B in that one of the things that holds quaternary into some tertiary structures together is called salt bridges. So the ball of yarn stays as a nice, neat compact ball due to a salt bridge. And the introduction of exogenous salts can actually disrupt the bridges. It helps untie the ball of yarn if you sprinkle a little salt on top.
[12:54] Heating Proteins
Question 17: After a solution-containing whey protein is heated to 70 degrees Celsius for 30 minutes, what is most likely happening to the proteins?
- (A) Hydrogen bonds within and in between the protein subunits are being broken.
- (B) Disulfide bonds within and in between the protein subunits are being broken.
- (C) Covalent bonds within and in between the protein subunits are being broken.
- (D) No significant breaking of bonds is occurring.
When you go back to the passage, it says nothing happens when the protein was heated at 80 degrees Celsius for 30 minutes. So nothing should also be happening at 70 degrees Celsius for 30 minutes. Hence, D is the correct answer here.
As a side note, this is an example of a thought process where there are multiples of the same so they’re all wrong.
[14:33] About Next Step Test Prep
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