Unit 2 practice test: Cell Structure and Function

The enzyme breaks down 42 to 46 µg per hour at pH 4.5 to 5.0, less on either side, and just 3 µg per hour at pH 7.2.

• The enzyme breaks down protein fastest at pH 7.2, the pH of the cytosol.: At pH 7.2 the enzyme breaks down only 3 µg per hour, the lowest value in the table.
• Correct: The enzyme breaks down protein fastest near pH 4.5 to 5.0 and very slowly at pH 7.2.: This matches the table: a peak near pH 5, falling steeply toward neutral.
• The amount of protein broken down rises steadily as the pH rises from 3.5 to 7.2.: The values rise from 3.5 to 5.0 and then fall, so there is no steady rise.
• The enzyme breaks down about the same amount of protein at each pH that the students tested.: The values range from 3 to 46 µg per hour, more than a fifteenfold difference.

At the cytosol's pH (7.2) the enzyme breaks down only 3 µg per hour, against 46 at its best. A few leaked molecules work too slowly to do much damage.

• Correct: At the cytosol's pH of 7.2, the enzyme works at under a tenth of its fastest pace.: This uses the table's value at pH 7.2 to explain the low damage.
• The cytosol is more acidic than the lysosome, and the enzyme does not work in acid.: The cytosol (pH 7.2) is less acidic than the lysosome (pH 4.5 to 5), and the enzyme works best in acid, not worst.
• Proteins in the cytosol are made of different amino acids from those the enzyme attacks.: Cell proteins are built from the same 20 kinds of amino acids, so this does not explain the protection.
• The enzyme works faster in the cytosol, so it finishes its job before it reaches anything important.: The table shows the enzyme works much more slowly, not faster, at the cytosol's pH.

At a pH near 7 the enzymes work at a small fraction of their usual pace, so material delivered to lysosomes is broken down far more slowly and accumulates.

• Worn-out organelles break down faster, so the cell has fewer lysosomes.: Raising the pH slows the enzymes, so breakdown is slower, not faster.
• The enzymes leave the lysosomes and digest the cytosol, killing the cell within minutes.: The drug changes the pH inside the lysosome; it does not release the enzymes, and at pH 7 they work slowly anyway.
• Correct: Undigested material builds up inside the lysosomes, which swell.: Material keeps arriving but is digested far more slowly, so it builds up.
• The cell stops making the enzyme, because it is no longer needed.: Nothing in the data says the cell stops making the enzyme; the problem is that the enzyme works poorly at the new pH.

The host engulfed the bacterium the way a cell takes in a particle by endocytosis: its plasma membrane closed around it. In this model the inner membrane is the bacterium's own and the outer comes from the host.

• Correct: The host cell's plasma membrane, which wrapped around the bacterium as it was taken in: Correct: the engulfing membrane becomes the outer layer around the bacterium.
• The bacterium's cell wall, which turned into a membrane once it was inside: A cell wall is not a membrane, and it does not turn into one; the model shows the host's membrane wrapping around.
• A second bacterium that was taken in at the same time and fused with the first: The model shows a single bacterium being taken in.
• The host cell's nucleus, which sent a layer of its own envelope around the bacterium: The model's outer membrane forms where the host's surface wraps around the bacterium, not from the nucleus.

The model shows step 3 (mitochondria) before step 4 (chloroplast, in one line). Every descendant of the first event has mitochondria; only descendants of the later event have chloroplasts as well.

• Correct: Mitochondria came first; a photosynthetic bacterium was later taken in by one line of those cells.: Correct: the order of events explains why one organelle is nearly universal and the other limited to one group.
• Chloroplasts were gained first, and animal cells later lost theirs while keeping their mitochondria.: The model has mitochondria first; nothing in it shows animals losing chloroplasts.
• Both were gained at the same moment, and animal cells later turned their chloroplasts into mitochondria.: The model shows two separate events, and the organelles differ in membranes, DNA and job.
• Plant cells are larger, so they had room to take in two kinds of bacteria at once while animal cells did not.: Cell size does not explain the pattern; the order and branching of the events does.

The model says mitochondria descend from bacteria, not from the host. If that is right, their DNA should resemble the DNA of living bacteria more than the host's own DNA, and it does.

• Correct: Mitochondrial DNA matches DNA of certain living bacteria better than it matches the cell's nuclear DNA.: Correct: a close DNA match to bacteria tests the model's central claim about where mitochondria came from.
• Mitochondria are found in the cells of most eukaryotes, including animals, plants, fungi and many single-celled groups.: Wide distribution is consistent with the model, but organelles that arose in the host would also be widespread, so it does not test the origin.
• Mitochondria move around inside the cell along the cytoskeleton, toward regions that need more ATP.: Movement along the cytoskeleton tells you how mitochondria are moved now, not where they came from.
• Cells with more mitochondria use more oxygen than cells with fewer mitochondria in the same tissue.: This connects mitochondria to oxygen use, a fact about their job, not about their origin.

Mitochondria come only from other mitochondria, just as bacteria come only from other bacteria. They carry their own DNA, and part of what they need is coded there, so a cell with none left has no way to make new ones.

• Correct: The cell cannot build new mitochondria, because mitochondria form only by splitting of existing ones.: Correct: like bacteria, mitochondria arise only by division of existing ones.
• The cell builds new mitochondria from its own membranes, because it has the genes to make them.: The nucleus holds many genes for mitochondrial proteins, but not all of them; some are on the mitochondrial DNA, which was removed.
• The cell makes new mitochondria within hours by folding its plasma membrane inward.: Infolding is a hypothesis for the origin of the nucleus and ER, not a way cells make mitochondria today.
• The cell takes in free-living bacteria from its surroundings and turns them into mitochondria.: Endosymbiosis happened over a vast span of time; a modern cell does not convert bacteria into organelles on demand.