Cellular Respiration

Unit 3 · Topic 3.5 end-of-topic test

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback tells you what a wrong choice assumed. For the two free-response questions, write your answer in full sentences, draw any graph on paper, and show any calculation; then open the scoring guide and mark your own work against it. Error bars in this test are ±2SE.

Question 1

A potted basil plant is sealed in a clear box and kept in the dark for two hours. The oxygen in the box falls from 20.9% to 20.4%, and the carbon dioxide rises from 0.04% to 0.31%.

Which process explains both changes?

A. Cellular respiration in the plant's cells, which runs in light and in dark B. Photosynthesis running in reverse, which happens whenever light is removed C. Fermentation, which the plant switches to as soon as photosynthesis stops D. Breathing through the leaves, which plants do only when it is dark

Question 2

A mitochondrion in cross-section. Four regions are numbered.

The figure shows a mitochondrion in cross-section with four regions numbered.

Which label marks the intermembrane space?

A. Label 1 B. Label 2 C. Label 3 D. Label 4

Question 3

In one step of respiration, a three-carbon molecule hands two electrons (traveling with hydrogen) to NAD⁺.

Which molecule is oxidized, and which is reduced?

A. Both are oxidized, since electrons moved B. Both are reduced, since electrons moved C. The three-carbon molecule is oxidized and NAD⁺ is reduced D. The three-carbon molecule is reduced and NAD⁺ is oxidized

Question 4

Isolated mitochondria are given oxygen, ADP and phosphate. One batch is also given NADH and makes ATP steadily. A second batch is given the same amount of NAD⁺ instead and makes almost no ATP.

Why does only the NADH batch make ATP?

A. NADH carries electrons taken from food; NAD⁺ is the empty form with none to deliver B. NADH is itself a fuel that the Krebs cycle breaks down to carbon dioxide and water C. NAD⁺ is a protein that must first be switched on by the electron transport chain D. NADH supplies the phosphate that joins ADP to make ATP; NAD⁺ has none to give

Question 5

Equal amounts of intact, isolated mitochondria are placed in six tubes with oxygen, ADP and phosphate. Three tubes receive glucose and three receive pyruvate. After 10 minutes the glucose tubes have each made 0.4 units of ATP and the pyruvate tubes 18.0 units.

Why did the pyruvate tubes make so much more ATP?

A. Glycolysis needs oxygen, and the glucose tubes used up their oxygen before making ATP B. Glucose is too large to enter a mitochondrion or release energy; pyruvate is small enough C. Pyruvate holds more energy per molecule than glucose does, so it yields more ATP D. Glycolysis runs in the cytosol, which the tubes lack; pyruvate is used inside the mitochondrion

Question 6

Glycolysis has just produced two pyruvate molecules in the cytosol of a cell that has plenty of oxygen.

What happens to each pyruvate next?

A. It stays in the cytosol, where oxygen breaks all three of its carbons to CO₂ at once B. It enters the matrix and is oxidized: one carbon leaves as CO₂ and NAD⁺ is loaded to NADH C. It is reduced to lactate in the cytosol, which frees NAD⁺ so glycolysis can continue D. It passes its electrons straight to the electron transport chain in the inner membrane

Question 7

The Krebs cycle runs in the mitochondrial matrix and finishes taking glucose's carbon apart, releasing it as carbon dioxide.

Where does most of the energy released in the cycle go?

A. Into NADH and FADH₂ B. Directly into ATP C. Into the carbon dioxide released D. Into the oxygen that enters the matrix

Question 8

Respiring cells are given ordinary glucose and a pulse of oxygen gas in which the oxygen atoms are the heavy form ¹⁸O. The carbon dioxide and water they make are tested at once.

Which newly made product carries ¹⁸O?

A. Both water and carbon dioxide B. Carbon dioxide only C. Water only D. Neither product

Question 9

Isolated mitochondria with pyruvate, ADP and phosphate have used up their oxygen, and ATP production has stopped. A pulse of oxygen is added and ATP production restarts at once.

What does the oxygen do?

A. It supplies the energy that is put into ATP as it is used up B. It takes the electrons off the end of the chain and becomes water C. It is turned into carbon dioxide, and that change releases energy D. It hands its electrons to NAD⁺ at the start of the chain

Question 10

Artificial membrane vesicles hold only ATP synthase, ADP and phosphate: no fuel and no electron transport chain. With pH 8 inside and pH 4 outside they form ATP fast; with pH 6 outside, slowly; with pH 8 outside, barely at all.

What do these results show about how ATP synthase works?

A. It pumps protons out of the vesicle using the energy released from ATP B. It needs electrons from an electron transport chain to make any ATP C. The acid outside supplies the phosphate that it joins onto ADP D. Protons flowing through it down their gradient drive ATP formation

Question 11

An aerobic soil bacterium has no mitochondria. Given glucose and oxygen, live cells use up the oxygen and their ATP rises; in heat-killed cells the oxygen and the ATP both stay level.

How do the live bacteria make their ATP?

A. By an electron transport chain that pumps protons across their plasma membrane B. By glycolysis alone; the oxygen use is a separate process unrelated to ATP C. By tiny mitochondria too small to be seen with a microscope D. By taking in ATP made by other soil organisms, using oxygen to power the uptake

Question 12

Liver cells with plenty of glucose and oxygen are given a poison that stops electron transfer at a protein near the end of the electron transport chain. Within minutes, oxygen use falls to 5% of normal, ATP production to 14%, and NADH builds up.

Why do all three change?

A. The poison stops glycolysis, so less NADH is made, less oxygen is needed and less ATP forms B. The poison uses up the oxygen itself, so the cells run out of terminal acceptor and switch to fermentation C. Electrons no longer reach oxygen, so NADH is not unloaded, protons are not pumped and the gradient collapses D. The poison blocks ATP synthase, so protons pile up outside, the gradient stalls the chain and NADH reloads

Question 13

Cultured cells are given an inhibitor. Afterward their mitochondrial ATP output is 30% of normal, while the proton gradient across the inner membrane is 180% of normal.

What did the inhibitor block?

A. The electron transport chain; ATP synthase kept working as before B. ATP synthase; the electron transport chain kept pumping protons C. Both the chain and ATP synthase together D. Glycolysis, so that less NADH reached the chain

Question 14

A bat waking from hibernation 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 liver mitochondria, they use oxygen faster, make less ATP, and give off far more heat.

Why does the leak have these three effects?

A. The leak jams the enzyme that makes ATP, so protons pile up outside and the trapped gradient is turned into heat B. The leak lets oxygen into the matrix, where it burns fuel directly, releasing heat instead of ATP C. The leak slows the chain, so electrons stay on NADH and their energy is released as heat D. Protons return past ATP synthase, so the gradient's energy leaves as heat and the chain runs faster to rebuild it

Question 15

Oxygen supply to a muscle is cut off. Within seconds almost all of the muscle cells' NAD⁺ has become NADH, and glycolysis is about to stall.

Why does the loss of oxygen tie up the NAD⁺?

A. With no oxygen to take electrons from the chain, NADH cannot be unloaded back to NAD⁺ B. Oxygen normally makes NAD⁺ in the matrix; with no oxygen, no new NAD⁺ is made C. Glycolysis needs oxygen to run, so it stops and the NADH it made piles up unused D. With no oxygen the cell breaks its NAD⁺ down to save energy for later

Question 16

A bacterium from lake sediment is grown with glucose and no oxygen. With nitrate added, it makes 12 ATP per glucose and the culture grows five times as much as with no nitrate, when it makes 2 ATP per glucose and lactate builds up.

What is the nitrate doing?

A. Releasing oxygen when it is broken down, allowing aerobic respiration B. Speeding up fermentation so that glycolysis yields more ATP per glucose C. Serving as the terminal electron acceptor for an electron transport chain D. Supplying nitrogen for growth; the ATP comes from fermentation either way

Question 17

Mean CO₂ produced in 20 minutes by yeast fed each sugar (five tubes per sugar). Error bars are ±2SE.

A class measured the carbon dioxide given off in 20 minutes by yeast fed glucose, sucrose or lactose. The graph shows the mean of five tubes for each sugar, with error bars of ±2SE.

Which conclusion do the error bars support?

A. Glucose gave more CO₂ than sucrose and sucrose more than lactose, because the three means are all different B. Glucose and sucrose gave the same amount of CO₂, because their error bars overlap C. The bars overlap for all three sugars, so the data show no differences at all D. Glucose and sucrose each gave more CO₂ than lactose; the data do not show a difference between glucose and sucrose

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Interpreting and Evaluating Experimental Results with Graphing · 4 points

Students measured the oxygen used by germinating pea seeds. Ten grams of peas were sealed in a respirometer, a tube in which a chemical absorbs all the carbon dioxide the peas give off, so the gas volume falls only as oxygen is used. Five respirometers were run for 20 minutes at each of three temperatures, 10, 20 and 30 °C, and five more at 20 °C held 10 g of peas that had been boiled and cooled. The table gives the mean oxygen used and the standard error (SE) of each set of five.

Oxygen used by 10 g of pea seeds in 20 minutes; five respirometers per condition.

(a) Explain why the gas volume in a respirometer of living peas falls. (1 point)

A full-credit answer: The peas' cells are respiring. Oxygen is taken up because it is the terminal electron acceptor: it takes the electrons off the end of the electron transport chain and becomes water. The carbon dioxide the cells release is absorbed by the chemical, so the fall in volume is the oxygen used.

Check the box for each point your answer earns

the peas' cells are respiring and take up oxygen, which accepts the electrons at the end of the electron transport chain (becoming water), while the carbon dioxide they release is absorbed by the chemical, so only the oxygen taken up changes the volume.

Accept "oxygen is the terminal electron acceptor" as the reason oxygen is used, with the carbon dioxide absorption mentioned. Do not award the point for "the peas breathe" or "the peas use up air" with no role for oxygen, or for an answer in which oxygen is turned into carbon dioxide.

Common slip: Saying the peas 'breathe' or 'use up air', or having the oxygen turned into carbon dioxide. The point needs oxygen's job, taking electrons at the end of the chain, and a word about where the carbon dioxide went.

(b) Construct a graph of the mean oxygen used by the living peas at each temperature, with an error bar of ±2SE on each mean. Leave the boiled peas off the graph. (1 point)

A full-credit answer: Temperature (°C) along the x-axis with 10, 20 and 30 spaced by value; oxygen used in 20 min (mL) up the y-axis from 0 to about 1.3. A point at 0.42 for 10 °C, 0.95 for 20 °C and 1.10 for 30 °C. On each point an error bar of ±2SE: 0.34 to 0.50, 0.83 to 1.07 and 0.92 to 1.28 mL.

Write down the values in the question:

10 °C: mean 0.42 mL, SE 0.04 mL
20 °C: mean 0.95 mL, SE 0.06 mL
30 °C: mean 1.10 mL, SE 0.09 mL

Write down the equation:

error bar runs from mean − 2SE to mean + 2SE

Substitute in the values, and calculate:

10 °C: 0.42 − (2 × 0.04) = 0.34 mL   to   0.42 + (2 × 0.04) = 0.50 mL
20 °C: 0.95 − (2 × 0.06) = 0.83 mL   to   0.95 + (2 × 0.06) = 1.07 mL
30 °C: 1.10 − (2 × 0.09) = 0.92 mL   to   1.10 + (2 × 0.09) = 1.28 mL

Check the box for each point your answer earns

a graph with all of: temperature (°C) on the x-axis and oxygen used in 20 min (mL) on the y-axis, both labeled with units; a scale that fits the data (0 to about 1.3 mL); each mean plotted at its temperature; and an error bar of ±2SE on each mean, running 0.34–0.50 mL at 10 °C, 0.83–1.07 mL at 20 °C and 0.92–1.28 mL at 30 °C.

Accept a scatter of means with the three temperatures spaced by value (the course convention for a measured quantity), with or without the means joined, or a bar graph with one bar per temperature; accept the boiled peas added as a separate, labeled fourth point, although the task leaves them off. Do not award the point for bars of ±1SE, for missing error bars, for axes without units, or for a straight trend line forced through three means that level off.

Common slip: Drawing the bar as ±1SE (half the right length), leaving the units off an axis, or spacing 10, 20 and 30 °C as three equal categories on a line graph. Each of these loses the graph point on the real exam.

(c) Using the error bars, determine which pairs of temperatures differ in oxygen use, and state what the comparison shows about the effect of temperature. (1 point)

A full-credit answer: The 10 °C bar (0.34–0.50) does not overlap the 20 °C bar (0.83–1.07) or the 30 °C bar (0.92–1.28), so oxygen use at 10 °C is lower than at either warmer temperature. The 20 °C and 30 °C bars overlap, so the data do not show a difference between them. Warming from 10 to 20 °C raised the rate of respiration; the data cannot say whether 30 °C raised it further.

Check the box for each point your answer earns

10 °C differs from 20 °C and from 30 °C (its bar, 0.34–0.50, does not overlap either of theirs), while the 20 °C and 30 °C bars overlap (0.83–1.07 and 0.92–1.28), so the data do not show a difference between them; warming from 10 to 20 °C raised the rate of oxygen use.

Accept "the null hypothesis of no difference is rejected for 10 versus 20 °C and for 10 versus 30 °C, and is not rejected for 20 versus 30 °C". Do not award the point for "20 and 30 °C are the same", or for claiming all three differ because the means differ.

Common slip: Reading the overlapping 20 and 30 °C bars as 'the same', or treating any gap between means as a difference. Overlap means no difference shown; only a clear gap between bars supports a claim.

(d) A student adds a poison that stops electron transfer at a protein near the end of the electron transport chain to a respirometer of living peas 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 peas' respirometer. Oxygen is taken up only at the end of the electron transport chain, where it accepts the electrons; with transfer blocked near the end, electrons no longer reach oxygen, so the peas stop using it and the volume holds.

Check the box for each point your answer earns

the volume stops falling (or falls far more slowly), because electrons can no longer be passed along the chain to oxygen, so oxygen is no longer taken up; oxygen is used nowhere else in respiration.

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 peas use more oxygen to make up for it".

Common slip: Predicting what happens to ATP, or that the peas '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.

Free-response score: 0 of 4

Free response 2 · Conceptual Analysis · 4 points

Human liver cells are grown in a dish with plenty of glucose and oxygen. A drug is added that blocks the channel through ATP synthase, so protons can no longer pass through it; the electron transport chain itself is untouched. In the hour before the drug, the cells used 12 mmol of glucose and made 360 mmol of ATP; the intermembrane space was at pH 7.2 and the matrix at pH 7.8. In the hour after the drug, the cells used 40 mmol of glucose and made 80 mmol of ATP, lactate built up in the dish, and the intermembrane space reached pH 6.8.

(a) Describe how the electron transport chain builds the proton gradient across the inner mitochondrial membrane. (1 point)

A full-credit answer: As electrons delivered by NADH pass from one protein of the chain to the next, down toward oxygen, the energy they release pumps protons from the matrix into the intermembrane space. Protons pile up outside the inner membrane and are scarce inside, which is why the intermembrane space (pH 7.2) is more acidic than the matrix (pH 7.8).

Check the box for each point your answer earns

as electrons delivered by NADH (and FADH₂) pass down the chain toward oxygen, the energy they release pumps protons (H⁺) out of the matrix into the intermembrane space, so protons are concentrated outside the inner membrane and scarce inside (the intermembrane space is more acidic than the matrix).

Accept "a difference in concentration and charge across the membrane" for the gradient. Do not award the point for ATP synthase making the gradient, or for protons pumped into the matrix.

Common slip: Saying ATP synthase pumps the protons, or sending them into the matrix. The chain pumps; it pumps outward, into the intermembrane space.

(b) Explain how the untreated cells used that gradient to make ATP. (1 point)

A full-credit answer: The inner membrane lets protons back into the matrix only through ATP synthase. Protons flowing down their gradient through ATP synthase drive the formation of ATP from ADP and inorganic phosphate. This flow is chemiosmosis, and the whole process from chain to ATP is oxidative phosphorylation.

Check the box for each point your answer earns

protons flow back down their gradient into the matrix only through ATP synthase, and their flow through it drives the formation of ATP from ADP and inorganic phosphate (chemiosmosis; oxidative phosphorylation).

Accept "ATP synthase uses the proton flow to join ADP and phosphate". Do not award the point for protons flowing back anywhere across the membrane, or for the chain making ATP directly.

Common slip: Having the chain make the ATP, or the protons flow back 'through the membrane'. The return path is ATP synthase alone, and that is where the ATP is made.

(c) Predict what the drug does to the proton gradient, and justify your prediction using the pH readings. (1 point)

A full-credit answer: The gradient grows. The chain keeps pumping protons out of the matrix, but with the channel through ATP synthase blocked they cannot flow back, so they pile up in the intermembrane space. That is why its pH fell from 7.2 to 6.8: more hydrogen ions, a steeper gradient than before.

Check the box for each point your answer earns

the gradient grows (gets steeper), because the chain keeps pumping protons into the intermembrane space while the blocked ATP synthase no longer lets them return; the fall in intermembrane-space pH from 7.2 to 6.8 (more H⁺) is the evidence.

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 gradient collapsing, or for a prediction about ATP output with nothing said about the gradient.

Common slip: Answering about ATP ('ATP falls') when the question asks about the gradient, or predicting that the gradient collapses. The chain is still pumping; only the return path is shut, so protons accumulate.

(d) Calculate the ATP made per glucose before and after the drug, and explain why the treated cells used so much more glucose and released lactate. (1 point)

A full-credit answer: Before the drug the cells made 30 ATP per glucose; after it, 2 ATP per glucose. With ATP synthase blocked, oxidative phosphorylation stops and the only ATP comes from glycolysis, two per glucose, so the cells need fifteen glucose molecules for the ATP one used to give; they ran glucose through glycolysis more than three times as fast and still made far less ATP. Glycolysis needs NAD⁺, and the chain, pumping into a gradient that is never drained, slows to a stop and no longer regenerates it, so the cells pass NADH's electrons to pyruvate, making lactate: fermentation, which regenerates NAD⁺ and keeps glycolysis going.

Write down the values in the question:

before the drug: glucose used = 12 mmol, ATP made = 360 mmol
after the drug: glucose used = 40 mmol, ATP made = 80 mmol

Write down the equation:

                     ATP made
ATP per glucose = ──────────────
                   glucose used

Substitute in the values, and calculate:

before: 360 ÷ 12 = 30 ATP per glucose
after: 80 ÷ 40 = 2 ATP per glucose

Check the box for each point your answer earns

360 ÷ 12 = 30 ATP per glucose before and 80 ÷ 40 = 2 ATP per glucose after, AND the reason: with ATP synthase blocked, oxidative phosphorylation stops, so ATP comes only from glycolysis (2 per glucose), so far more glucose must be used for the same ATP; fermentation passes NADH's electrons to pyruvate, making lactate and regenerating the NAD⁺ glycolysis needs.

Accept "the cells rely on glycolysis and fermentation" as the reason if both yields are given. For the lactate, accept any of: "NADH can no longer be unloaded", "NAD⁺ runs short", or "the chain has stopped because the protons it pumps can no longer return"; no name for this stalling is needed, and the reason the untouched chain stops need not be given. Do not award the point for the yields alone, or for fermentation itself making the ATP.

Common slip: Saying fermentation makes the ATP. Fermentation makes none; it regenerates NAD⁺ so that glycolysis can keep making its two per glucose.

Free-response score: 0 of 4

Multiple choice checked: 0 of 17 correct.

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