Unit 3 · Topic 3.5 end-of-topic test
Suggested time: about 56 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
A student claims that only animal cells carry out cellular respiration, because plants make their own food by photosynthesis.
Which of the following observations contradicts the student’s claim?
The figure shows a mitochondrion in cross-section, inside a cell. Four positions are numbered.
Which number marks the membrane that holds the electron transport chain and ATP synthase?
In one step of respiration, a three-carbon molecule hands two electrons (traveling with hydrogen) to NAD⁺.
Which molecule is reduced?
A researcher puts mitochondria from potato tubers in a tube with oxygen, ADP and phosphate. One batch also gets NADH and makes ATP steadily. A second batch gets the same amount of NAD⁺ instead and makes almost no ATP.
Why does only the NADH batch make ATP?
A researcher gives two dishes the same mixture of glucose, oxygen, ADP and phosphate. One dish holds intact liver cells. The other holds mitochondria isolated from the same cells. After 10 minutes the intact cells have made 20.0 μmol of ATP and the isolated mitochondria 0.3 μmol.
Why did the isolated mitochondria make so little ATP?
Glycolysis has just produced two pyruvate molecules in the cytosol of a cell that has plenty of oxygen.
What happens to each pyruvate next?
In the matrix of a potato cell’s mitochondrion, a two-carbon fragment from pyruvate oxidation enters the Krebs cycle.
Which of the following happens to the fragment and its energy in the Krebs cycle?
A researcher lets a mouse breathe, for ten minutes, air in which the oxygen atoms are the heavy form ¹⁸O, so that those atoms can be traced. As the mouse’s cells respire, they make water and carbon dioxide.
Which product do the cells make directly from the oxygen the mouse breathes in?
The table shows what one glucose molecule gives a yeast cell, stage by stage, by the end of the Krebs cycle.
By the end of the Krebs cycle, how many ATP has the cell made directly from the glucose, and where is most of the glucose’s energy?
Artificial vesicles hold only ATP synthase, ADP and phosphate: no fuel and no electron transport chain. A researcher keeps the inside of every vesicle at pH 8 and the outside at pH 7, and the vesicles make ATP steadily.
Which of the following changes would make the vesicles form ATP faster?
A gut bacterium has no mitochondria. Given glucose and oxygen, its live cells use up the oxygen and their ATP rises. When a researcher adds a chemical that lets protons pass freely across the plasma membrane, the cells keep using oxygen, but their ATP stops rising.
How do the live bacteria make their ATP?
Mitochondria in a hummingbird’s flight muscle have an inner membrane folded far more densely than the mitochondria in its fat cells. Both kinds of mitochondria have plenty of pyruvate and oxygen.
Which of the following explains why the flight-muscle mitochondria make ATP faster?
A bacterium from estuary mud respires with oxygen when oxygen is present, and with nitrate when there is no oxygen. Given glucose and oxygen, it makes about 30 ATP per glucose. Given glucose and nitrate with no oxygen, it makes 12 ATP per glucose.
Why does the bacterium make less ATP per glucose with nitrate?
A newborn lamb in a cold field warms itself with brown fat. In its brown-fat mitochondria a protein in the inner membrane lets protons leak back into the matrix. Compared with the lamb's liver mitochondria, they use oxygen twice as fast, make a third as much ATP, and give off far more heat.
Why does the leak have these three effects?
A student seals two jars, and both jars soon use up their oxygen. The apple-juice jar holds apple juice with yeast: over a week, bubbles of gas collect at the top and alcohol builds up. The cucumber jar holds cucumbers in salty water with bacteria: over a week, the water turns sour and no gas collects.
Which fermentation route is each jar using?
Two flasks of a bacterium from salt-marsh mud are grown with glucose and no oxygen. Flask 1 also contains sulfate; flask 2 contains none. After a day, the sulfate in flask 1 is used up, and lactate has built up in flask 2. The other results are in the table.
Which of the following best explains the role of sulfate in flask 1?
Which of the following animals warms itself from within, using brown fat?
A bee’s flight-muscle cells are packed with mitochondria.
Which of the following do those mitochondria do for the cell?
In the matrix, an enzyme removes two electrons (with hydrogen) from a molecule made from the fragments of glucose, and FAD picks them up, becoming FADH₂. The enzyme comes out of the reaction unchanged.
Which of the following describes FAD?
(a) Describe the graph the students should construct from the living-bean data: the kind of graph, what goes on each axis, the trend it shows, and how the plotted means should be joined. (2 points)
A full-credit answer: Temperature is a measured amount, not a set of separate categories.
So the means are plotted as points, not bars.
Temperature (°C), the quantity the students set, goes on the x-axis.
Oxygen used in 20 minutes (mL), the quantity measured, goes on the y-axis.
Oxygen use rises steeply from 10 °C to 20 °C and only a little from 20 °C to 30 °C: the trend levels off.
The means do not follow one straight trend.
So they are joined one to the next, not fitted with one straight best-fit line.
Check the box for each point your answer earns
Accept "line graph" for a point graph, and a smooth curve through the means for the joining. Do not award the graph point for a bar graph (temperature is a measured amount, not a set of separate categories) or for the axes swapped. Do not award the trend point for one straight best-fit line through the three means, or for a trend with no joining stated.
Common slip: Drawing bars, one per temperature, or fitting one straight best-fit line through the three means. Temperature is a measured amount, so each mean is a point on a temperature scale. The means rise steeply and then level off, so they are joined one to the next.
(b) Calculate the upper end of the ±2SE error bar on the 30 °C mean. (1 point)
Write down the values in the question:
30 °C: mean = 0.88 mL, SE = 0.08 mL
Write down the equation:
Substitute the values into the equation:
A full-credit answer: The upper end of the 30 °C bar is 1.04 mL: the mean of 0.88 mL plus two standard errors, 2 × 0.08 = 0.16 mL.
Accept 1.04 mL with the working shown as 0.88 + 0.16. Do not award the point for 0.96 mL (±1SE) or for the lower end, 0.72 mL.
(c) Determine, using the error bars, whether the data show a difference in oxygen use between 20 °C and 30 °C. Explain why the gas volume falls in the respirometers of living beans but hardly at all in the respirometers of boiled beans. (2 points)
A full-credit answer: The 20 °C bar runs from 0.64 to 0.84 mL.
The 30 °C bar runs from 0.72 to 1.04 mL.
The two bars overlap, between 0.72 and 0.84 mL.
Overlapping ±2SE bars mean the data do not show a difference.
So the data do not show a difference in oxygen use between 20 °C and 30 °C.
The living beans’ cells respire: they break food down to make ATP.
Oxygen takes the electrons off the end of the electron transport chain and becomes water.
So the living cells take oxygen out of the gas, and the volume falls.
The chemical absorbs the carbon dioxide released.
Boiling denatured the boiled beans’ enzymes, so those cells are dead and do not respire.
So they take up almost no oxygen, and their volume hardly changes.
Check the box for each point your answer earns
Determine needs the decision and the observation it rests on. Accept "the null hypothesis of no difference is not rejected for 20 versus 30 °C". Do not award the determination point for "20 and 30 °C are the same", or for claiming they differ because the means differ. For the explanation, accept "oxygen is used up by respiration" or "by the electron transport chain" for the living beans, with the boiled beans described as dead or as unable to respire. Do not award the explanation point for "the beans breathe" or "the beans use up air" with no role for oxygen, for oxygen turned into carbon dioxide, or for an answer that does not mention the boiled beans.
Common slip: Reading the overlapping bars as 'the same', or treating the gap between the means (0.74 and 0.88 mL) as a difference: overlap means no difference shown. For the explanation, saying the beans 'breathe' or 'use up air': the point needs oxygen's job, taking electrons at the end of the chain, and the reason the boiled beans use none, that their cells are dead.
(d) A student adds a poison that stops electron transfer at a protein near the end of the chain to a respirometer of living beans at 20 °C. Predict what happens to the gas volume in that respirometer, and justify your prediction. (1 point)
A full-credit answer: The gas volume stops falling, or falls far more slowly, like the boiled beans' respirometer.
Oxygen is taken up only at the end of the electron transport chain, where it accepts the electrons.
The poison blocks transfer near the end of the chain.
So electrons no longer reach oxygen.
So the beans stop using oxygen, and the volume stays constant.
Check the box for each point your answer earns
Accept a note that NADH builds up, or that ATP output falls, alongside the oxygen prediction. Do not award the point for a prediction with no link to oxygen's role as the terminal electron acceptor, or for "the beans use more oxygen to make up for it".
Common slip: Predicting what happens to ATP, or that the beans 'use more oxygen to compensate'. The reading measures oxygen. So the answer must follow the block to oxygen uptake: electrons no longer reach oxygen, so oxygen is no longer used.
(a) Describe how the untreated cells made ATP by oxidative phosphorylation, from the electrons delivered by NADH to the ATP. (1 point)
A full-credit answer: NADH delivers electrons to the chain, and they pass from one protein to the next, down toward oxygen.
Their energy pumps protons from the matrix into the intermembrane space, which is why that space (pH 6.8) is more acidic than the matrix (pH 7.8).
The inner membrane lets protons back into the matrix almost only through ATP synthase.
Protons flowing down their proton gradient through ATP synthase drive the formation of ATP from ADP and inorganic phosphate.
Check the box for each point your answer earns
Accept "a difference in concentration and charge across the membrane" for the proton gradient, and "ATP synthase uses the proton flow to join ADP and phosphate" for the last step. Do not award the point for ATP synthase making the proton gradient, for protons pumped into the matrix, or for the chain making the ATP directly.
Common slip: Saying that ATP synthase pumps the protons, or that the chain makes the ATP. The chain pumps, outward into the intermembrane space. ATP synthase is the return path, and that is where the ATP is made.
(b) Calculate the ATP made per glucose before and after the drug, and explain why the cells used more glucose after it. (2 points)
A full-credit answer: Before the drug the cells made 30 ATP per glucose.
After it, they made 2 ATP per glucose.
Each glucose now gives 2 ATP instead of 30.
So the cells must break down far more glucose for the ATP they make.
before the drug: glucose used = 12 mmol, ATP made = 360 mmol
after the drug: glucose used = 40 mmol, ATP made = 80 mmol
Check the box for each point your answer earns
Accept the two values with their working shown, and any reason that links the smaller yield per glucose to the greater glucose use. Do not award the calculation point for one value only or for dividing the glucose by the ATP. Do not award the explanation point for the values restated with no reason.
Common slip: Dividing the glucose by the ATP, or giving the two values with no reason for the extra glucose. The yield is the ATP made divided by the glucose used; a smaller yield per glucose means more glucose for the same ATP.
(c) Describe the effect of the drug on the proton gradient across the inner membrane. Use the pH readings to support your answer. (1 point)
A full-credit answer: The proton gradient grows.
The chain keeps pumping protons out of the matrix.
The channel through ATP synthase is blocked, so the protons cannot flow back.
So they pile up in the intermembrane space.
That is why its pH fell from pH 6.8 to pH 6.4: more hydrogen ions outside.
The matrix rose from pH 7.8 to pH 8.0: fewer hydrogen ions inside.
So the proton gradient is steeper than before.
Check the box for each point your answer earns
Accept "the difference in pH across the membrane widened" as the justification if the direction (more H⁺ outside) is clear. Do not award the point for the proton gradient collapsing, or for a prediction about ATP output with nothing said about the proton gradient.
Common slip: Answering about ATP ('ATP falls') when the question asks about the proton gradient, or predicting that the proton gradient collapses. The chain is still pumping. Only the return path is shut, so protons accumulate.
(d) Explain why lactate appeared in the dish after the drug. (1 point)
A full-credit answer: The drug shut the channel through ATP synthase, almost the only way back into the matrix for protons.
The chain kept pumping protons out until it could no longer pump against the grown proton gradient, and nearly stopped.
A stopped chain cannot take electrons from NADH, so NADH could not be unloaded and glycolysis, which needs NAD⁺, was about to stall.
The cells passed NADH’s electrons to pyruvate instead, making lactate: fermentation.
That regenerates NAD⁺, so glycolysis kept going.
Check the box for each point your answer earns
Accept "NADH can no longer be unloaded", "the concentration of NAD⁺ falls" or "the chain has stopped because the protons it pumps can no longer return" as the reason the cells ferment; no name for the chain's stalling is needed. Accept "the cells ferment, making 2 ATP per glucose" or "fermentation yields 2 ATP per glucose", provided NAD⁺ regeneration or the stopped chain is given as the reason for the lactate. Do not award the point for the lactate-forming step itself producing ATP, for fermentation making ATP instead of glycolysis, or for "the cells used up their oxygen" (oxygen was plentiful).
Common slip: Saying that the cells used up their oxygen, or that the lactate-forming step itself makes the ATP. Oxygen was plentiful; the chain stopped because the protons it pumps had no way back. Glycolysis makes the two ATP per glucose, and fermentation regenerates the NAD⁺ that lets it keep going.