![]() ![]() We’re left with our correct answer, answer choice B. ![]() Not the case here because the minus end is anchored in the z-line. The only way answer choice D would work is if neither side was capped, and we had treadmilling. We expect both ends to be capped, and no growth. The – ends polymerize and the + ends depolymerize at the same rate.At a concentration above 4 micromolar, we would expect both the plus and minus end of the microfilament to add actin subunits. It’s not explaining why microfilament lengths don’t change. The actin subunit concentration is kept above 4 µM in muscle cells.Answer choice B is superior to answer choice A. We have both ends of the microfilaments capped, which prevents microfilament length from changing. This answer choice matches our breakdown, and what we just said when we went through answer choice A. The – ends of the microfilaments are capped by Z lines, and the + ends are capped by another protein.Second part of the answer choice isn’t a great explanation for why length doesn’t change. An actin subunit concentration above 1 micromolar would normally mean addition to the plus end, but we mentioned there are no actin subunits being added to that end. Both sides are capped to prevent addition or subtraction of actin subunits. The distance between microfilaments varies we said, not the length of the microfilaments. But the microfilament length isn’t actually varying on the plus side either. Microfilaments are capped by Z-lines on the minus end. First part of this answer choice is consistent with our breakdown. The – ends of the microfilaments are capped by Z lines, and the actin subunit concentration is kept above 1 µM in muscle cells.Even though we have different parts of the sarcomere shortening, the microfilament is anchored in the Z-line, and only the distance between microfilaments will decrease. The minus sign of the microfilament is anchored in that Z-line. When the sarcomere shortens, the H-zone, I-band, the distance between Z-lines all become smaller. I-band is the region with only thin filaments, and H-zone contains only thick filaments. We have the Z-line labeled which defines the boundaries of each sarcomere. The A band spans the length of the thick filament and also contains thin filaments. Thin filament is actin, and also troponin and tropomyosin. ![]() We have our image of the muscle sarcomere in the question stem, with labels. I want to explain why microfilament lengths do NOT change when the sarcomere shortens in a muscle contraction. We’re left with our correct answer: answer choice C.Ģ) I have to make sure to be careful with the wording. This contradicts our breakdown and Figure 1 from the passage. This is not true because at a concentration under 1 micromolar, there is a loss of actin subunits on both the plus and minus ends. This is superior to answer choices A and B. We said that at every concentration above 1 micromolar we have the plus end of the microfilament growing, and growing faster than the minus end. This answer choice is consistent with our breakdown of the question. At any concentration greater than 1 µM.Answer choice B is an incomplete answer, but it’s better than option A. But this answer choice says “Only.” We’re not limited to just these concentrations. We do have the plus end of the microfilament growing faster than the minus end between these concentrations. The rate of loss is equal to the rate of addition, so there’s no net change. This answer choice corresponds to a net change of zero to the plus end. That means at every concentration above 1 micromolar we have the plus end of the microfilament growing, and growing faster than the minus end. ![]() Dotted line represents the minus end and has the smaller slope, meaning it grows more slowly. That’s because the plus end of the microfilament grows faster. The solid line represents the plus end, and it has a much larger slope. Only after crossing the 1 micromolar threshold do we start getting addition of actin subunits on the plus end. At very low concentrations, meaning 1 micromolar or below, we have a loss of actin subunits on both the plus and minus end. Concentration of free actin is shown on the X-axis. We want to find the concentration of free actin where the plus end of the microfilament grows faster than the minus end. Biology Question Pack Volume 2: Passage 1ġ) First thing we want to do is look at Figure 1 from the passage and compare the rates of actin subunit addition of both ends.įigure 1 is above. ![]()
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