Practice questions · Topic 3.3

Unit 3 · Practice for the Topic 3.3 end-of-topic test

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ATP hydrolysis releases energy and drives the cell's work; energy is transformed, never made, and some always spreads out as heat; a living thing stays ordered only while energy keeps flowing in.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. For the free-response questions, write one short sentence for each step of your reasoning, each on its own line, and show any calculation; make every link clear (so, because, therefore). In the exam, write those steps as a paragraph: a bulleted list is not scored. The first free-response question walks you through the reasoning one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it. Energy is measured in joules (J) and kilojoules (kJ; 1 kJ = 1,000 J); energies of reactions are in kJ/mol.

Question 1

ATP is made of a base, a sugar and a chain of three phosphate groups.

Which of the following parts of ATP make up the adenosine?

A. The base only B. The sugar only C. The base and the sugar D. The sugar and the three phosphate groups

Question 2

Cilia taken from windpipe cells and kept in a dish of fluid beat only when a student adds ATP to the fluid. ADP and Pi added on their own leave the cilia still. The student explains: ‘The bond to ATP’s outer phosphate stores energy, and that energy is set free when the bond breaks.’

Which statement corrects the student?

A. The energy comes from the water molecule that reacts with ATP, which gives up the energy it held B. The student is right, except that the energy is stored in all three phosphate bonds, not only the outer one C. The energy comes from the ADP and Pi themselves, which the cilia break down further as their fuel D. The energy comes from the whole reaction: ADP and Pi hold less energy than ATP and water did

Question 3

A motor protein carries vesicles along a fiber inside a nerve cell. A researcher gives the protein a look-alike of ATP that binds the protein but cannot be hydrolyzed: the protein stays still. Given ATP, the protein steps forward along the fiber, and each step is paired with the hydrolysis of one ATP.

Why does the protein stay still with the look-alike instead of ATP?

A. The look-alike is not hydrolyzed, so no hydrolysis is coupled to the shape change that makes the step B. The look-alike releases its energy as heat, and warmth alone cannot swing the protein forward C. The look-alike sits where the protein grips the fiber, so the protein cannot take hold of the fiber D. The look-alike is hydrolyzed like ATP, but each hydrolysis releases too little energy for one step

Question 4

In which of the following changes does the entropy of the matter or energy described fall?

A. A dead moth lying on a windowsill dries out, crumbles and scatters across the sill as dust B. A snail takes dissolved minerals from the water and builds them into the solid layers of its shell C. A drop of perfume placed on a wrist evaporates and spreads through the whole room D. A hot stone lifted from a campfire cools as its heat spreads out into the night air

Question 5

A ripening banana gives off two gases: carbon dioxide, CO₂, and ethylene, C₂H₄, a molecule the banana's cells make.

Which of the two gases is an organic molecule?

A. Neither of them B. Carbon dioxide only C. Ethylene only D. Both of them

Question 6

In plant cells, two steps make the orange pigment beta-carotene, each catalyzed by its own enzyme: a colorless molecule, phytoene, becomes the red pigment lycopene, and lycopene becomes beta-carotene. In a ripe tomato, lycopene builds up in the cells and makes the fruit red; the cells make very little beta-carotene, and phytoene stays scarce.

Which of the following explains why lycopene builds up in the tomato?

A. The enzyme that turns phytoene into lycopene has low activity B. The enzyme that turns lycopene into beta-carotene has low activity C. Both enzymes have low activity D. The tomato's cells make phytoene faster than other plant cells do

Question 7

A salmon swimming upstream draws 3,000 kJ from its stored fat in a day. About 1,000 kJ of that is captured in ATP and used for its swimming.

How much of the 3,000 kJ left the fish directly as heat, and why?

A. 2,000 kJ: in every energy transfer some energy spreads out as heat B. 1,000 kJ: the energy captured in ATP is the part that leaves as heat C. None: the 2,000 kJ not captured in ATP stays stored in the fish’s fat D. 2,000 kJ: the energy not captured in ATP was used up as the fish swam, so it no longer exists

Question 8

Red blood cells stored in a blood bag live on the glucose in the storage fluid. After several weeks the glucose is used up; the cells then lose their ion concentration gradients, swell and burst.

Why do the cells burst once the glucose is gone?

A. The glucose was holding the cell membranes together B. With the glucose gone, water enters the cells faster because the fluid is more dilute C. With no glucose the cells make no ATP, so the pumping that kept their contents ordered stops D. The cells create energy from their own membranes, which weakens them

Question 9

A four-molecule pathway in a yeast cell that makes a flavor compound, M. Each arrow is one reaction, catalyzed by the enzyme written above it.

The figure shows a pathway in a yeast cell in which J is converted, step by step, into a flavor compound, M. A student gives the cells L, and no J.

Which molecules of the pathway can the cells now make from the L?

A. M only B. K, L and M C. J, K and M D. None of them, because the pathway must start from J

Question 10

A student proposes that a cell would do better with a single enzyme that turned glucose and oxygen straight into carbon dioxide and water in one step, releasing all of the glucose's energy, about 2,870 kJ/mol, at once.

What would happen to that energy in the proposed cell?

A. It would all be captured in ATP, since a single step wastes nothing at all B. It would be stored inside the enzyme until the cell needed it C. It would be released as light, as when a spoonful of sugar burns in a flame D. Most of it would spread out as heat, with no step in which ATP could capture a share

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 · Conceptual Analysis · 6 points

Frog eggs develop in a dish of pond water kept in the dark, in a room at 22 °C; each embryo lives on the yolk stored in its egg. In one day, students measured 40 J of energy transferred from the yolk of each egg: 14 J of it ended up in newly made ATP and 26 J left the egg as heat. Assume these values account for all the energy transferred. The embryo uses its ATP to build new cells and to pump ions across its membranes. The yolk lasts about ten days; after that the tadpole must feed.

(a) Identify the source of the energy the embryo uses. (1 point)

Hint: Living things take in energy as light or as food. Which of the two can reach an embryo inside an egg in a dark room?

A full-credit answer: The embryo’s energy comes from the yolk stored in its egg.
The yolk is food.
Food holds chemical energy.
The dish is kept in the dark, so no light reaches the embryo.
An animal cannot use light in any case.
So food is the embryo’s only energy input.

Check the box for each point your answer earns

the yolk stored in the egg, which is food (chemical energy).

Accept ‘stored food in the yolk’ or ‘the chemical energy of the yolk’. Do not award the point for light, for the warmth of the room or for the pond water.

Common slip: Naming the warmth of the room or the pond water as the energy source. Living things take their energy in as light or as food. Warmth speeds the embryo's reactions up, and water supplies matter; the input here is the food in the yolk.

(b) Calculate the percentage of the 40 J that was captured in ATP. (1 point)

Hint: A percentage compares a part with a whole. Which of the three values is the part, and which is the whole?

A full-credit answer: The percentage captured in ATP is 35%.
14 J of the 40 J transferred ended up in ATP, and 14 ÷ 40 × 100 = 35%.

energy transferred from the yolk = 40 J

energy captured in ATP = 14 J

energy released as heat = 26 J

percentage in ATP = \frac{energy in ATP}{energy transferred} \times 100

percentage in ATP = \frac{energy in ATP}{energy transferred} \times 100

percentage in ATP = \frac{14}{40} \times 100

percentage in ATP = 35 %

Check the box for each point your answer earns

14 divided by 40, multiplied by 100, gives 35%.

Do not award the point for 14 divided by 26 (54%, comparing ATP with heat) or for 26 divided by 40 (65%, the share that left as heat).

Common slip: Dividing 14 by 26 instead of by 40. The whole is the 40 J transferred from the yolk. The heat is the other part of that whole, not the whole.

(c) Explain how the first law of thermodynamics accounts for the 40 J. (1 point)

Hint: The first law of thermodynamics is about the total. Which three values in the setup does it connect?

A full-credit answer: The first law of thermodynamics says that energy is never created or destroyed.
Energy is only transferred or transformed.
So every joule of the 40 J is accounted for.
14 J was transformed into chemical energy in ATP.
26 J was transferred to the surroundings as heat.
14 J + 26 J = 40 J.

Check the box for each point your answer earns

energy is neither created nor destroyed, only transferred or transformed, so all 40 J is accounted for: 14 J transformed into chemical energy in ATP plus 26 J transferred as heat (14 + 26 = 40).

Accept 'the energy in equals the energy out'. Accept '26 J was lost as heat': that is the exam's own wording, and the heat still exists. Do not award the point for a statement that 26 J was destroyed, disappeared or ceased to exist.

Common slip: Saying that the 26 J of heat was destroyed or ceased to exist. The heat left the egg, but it still exists, spread into the water and the air; the first law of thermodynamics counts it. Saying it was 'lost as heat' is fine: that is the exam's own wording.

(d) Explain why some of the 40 J left the egg as heat instead of being captured in ATP. (1 point)

Hint: Which law describes what happens to energy in every transfer?

A full-credit answer: By the second law of thermodynamics, every energy transfer spreads some energy out as heat.
That heat can no longer do work.
So no transfer is fully efficient.
The embryo breaks the yolk's food down in many small steps.
Each step captures part of the energy released in ATP.
The rest, here 26 J of the 40 J, warms the egg and the water around it.

Check the box for each point your answer earns

by the second law of thermodynamics, in every energy transfer or transformation some energy spreads out as heat that can no longer do work, so no transfer is fully efficient; the pathway that breaks the yolk's food down releases the energy in many small steps and captures a portion of each in ATP, while the rest leaves as heat.

Accept an answer that names the second law of thermodynamics and heat without mentioning the steps. Do not award the point for 'the embryo wasted energy' with no mention of heat, or for 'some energy was destroyed'.

Common slip: Saying the embryo 'could not use' the 26 J and stopping. The point needs the idea of the second law of thermodynamics: some energy spreads out as heat at every transfer, so no transfer is fully efficient.

(e) Make a claim about what will happen to a tadpole that finds no food once its yolk is used up. (1 point)

Hint: Once the yolk is gone and there is no food, think about what the tadpole can still take in.

A full-credit answer: The tadpole will die.
With the yolk gone and no food, its energy input is zero.

Check the box for each point your answer earns

a correct, specific claim: the tadpole will die (its cells will lose their order), because with no yolk and no food it has no energy input.

Make a claim earns the point for the assertion; the reasoning is scored in part (f). Do not award the point for 'it stops growing' alone, or for a claim that it can live on warmth or water.

Common slip: Claiming only that growth slows or stops. The claim has to name the end of the chain: with no input, the tadpole dies.

(f) Support your claim with reasoning from the laws of thermodynamics. (1 point)

Hint: Think about where some of the energy goes at every step, and whether it ever comes back.

A full-credit answer: Keeping a cell ordered needs energy every moment.
The tadpole supplies that energy through ATP hydrolysis coupled to building cells and pumping ions.
At every step some of the energy leaves as heat, and none of it comes back.
So a living thing must keep taking in energy.
Once the input is zero, the heat loss continues.
So the order cannot be maintained, and therefore the tadpole dies.

Check the box for each point your answer earns

the evidence (the input is zero) AND the reasoning that links it to the claim: keeping order needs energy continuously because heat leaves at every step (the second law of thermodynamics), so with no input the ATP cannot be remade, order cannot be maintained, and the tadpole dies.

Support a claim needs the evidence and the reasoning that links it to the claim. Do not award the point for restating the claim, or for naming the heat loss without linking it to the need for a continuous input.

Common slip: Restating the claim. Supporting it means naming the rule, that energy must keep coming in because heat leaves at every step, and applying it to an input of zero.

Free-response score: 0 of 6

Free response 2 · Analyze Model or Visual Representation · 4 points

The model shows what a liver cell does with glucose as soon as the glucose has entered it: enzyme H catalyzes arrow 1. Glucose-phosphate cannot pass back out through the cell membrane, so the glucose is trapped inside the cell. On its own, glucose + Pi → glucose-phosphate + H₂O would take in 14 kJ/mol; ATP hydrolysis, ATP + H₂O → ADP + Pi, releases 31 kJ/mol. In a cell-free extract containing enzyme H, glucose and Pi alone give no glucose-phosphate; glucose and ATP together give it quickly. Arrow 2 shows how the cell remakes ATP.

A model of the first step a liver cell takes with glucose (arrow 1, enzyme H) and of how the cell remakes its ATP (arrow 2).

(a) Explain why the products of arrow 1 are glucose-phosphate and ADP, rather than glucose-phosphate and ATP. (1 point)

A full-credit answer: In arrow 1, enzyme H attaches a phosphate group to the glucose, making glucose-phosphate.
This is phosphorylation.
The phosphate group comes from ATP: it is the outermost of ATP’s three phosphates.
So ATP is left with two phosphates.
Adenosine with two phosphates is ADP.
That is why the products are glucose-phosphate and ADP.

Check the box for each point your answer earns

the phosphate that lands on the glucose is the outermost of ATP’s three phosphates (glucose is phosphorylated), so ATP is left with two phosphates, which is ADP (adenosine diphosphate).

Accept ‘ATP gives glucose one of its phosphates, so it becomes ADP’. Do not award the point for ‘ATP gives glucose energy’ with no phosphate transferred, or for ADP explained as ATP that has lost all its phosphates.

Common slip: Saying ATP 'gives its energy' to glucose with no phosphate mentioned. What moves is a phosphate group. Losing that group is what turns ATP into ADP.

(b) Explain why glucose and Pi alone give no glucose-phosphate, while glucose and ATP together give it quickly. (1 point)

A full-credit answer: Putting a phosphate on glucose would take in 14 kJ/mol.
So with only Pi present, the reaction does not happen.
ATP hydrolysis releases 31 kJ/mol, which is more than 14 kJ/mol.
Enzyme H couples the two reactions: it transfers ATP's outer phosphate straight onto the glucose.
So the combined reaction releases 31 − 14 = 17 kJ/mol overall, and glucose-phosphate forms quickly.
This is energy coupling: ATP hydrolysis drives a reaction that would not happen by itself.

glucose + Pi → glucose-phosphate + H₂O takes in 14 kJ/mol

ATP + H₂O → ADP + Pi releases 31 kJ/mol

energy released by the coupled reaction = energy released by hydrolysis - energy taken in by phosphorylation

energy released by the coupled reaction = energy released by hydrolysis - energy taken in by phosphorylation

energy released = 31 - 14

energy released = 17 kJ/mol

Check the box for each point your answer earns

adding a phosphate to glucose would take in 14 kJ/mol, so it does not happen by itself; ATP hydrolysis releases 31 kJ/mol, more than the 14 kJ/mol needed, so when enzyme H couples the two by transferring ATP's phosphate straight onto the glucose the combined reaction releases 31 − 14 = 17 kJ/mol and glucose-phosphate forms.

Accept 'the hydrolysis releases more energy than the phosphorylation needs, so the coupled reaction releases energy overall'. Do not award the point for 'ATP has energy stored in its bond' or for 'ATP has a phosphate and Pi does not' with no energy reasoning.

Common slip: Saying the energy comes from breaking ATP's phosphate bond. Breaking any bond takes energy in. The release comes from the whole reaction, whose products hold less energy than its reactants. That release drives the phosphorylation only because enzyme H couples the two.

(c) A poison stops the liver cell from remaking ATP (arrow 2). Make a claim about what happens to the trapping of glucose that enters the cell from now on. (1 point)

A full-credit answer: The trapping of glucose stops within seconds.
It goes on only until the cell's ATP is used up.

Check the box for each point your answer earns

a correct, specific claim: the trapping soon stops (within seconds), once the cell's ATP is used up.

Make a claim earns the point for the assertion; the reasoning is scored in part (d). Accept 'it stops' with any short timescale or none. Do not award the point for 'trapping continues normally because the cell has plenty of ATP' or for 'glucose is trapped faster'.

Common slip: Claiming that the trapping carries on for hours on stored ATP. A cell holds only seconds' worth of ATP, so once arrow 2 stops, arrow 1 stops almost at once.

(d) Support your claim using what the reaction in arrow 2 requires and how long a cell's stock of ATP lasts. (1 point)

A full-credit answer: A cell holds only seconds’ worth of ATP.
So when arrow 2 is blocked, the store is gone almost at once.
Making ATP from ADP and Pi is the reverse of hydrolysis.
It needs an input of energy.
The cell normally gets that energy from breaking down food molecules.
The poison cuts off that remaking, so nothing refills the store.
Enzyme H then has no ATP to take a phosphate from.
So the trapping of glucose stops.

Check the box for each point your answer earns

the evidence (a cell's stock of ATP lasts only seconds; making ATP from ADP and Pi needs an input of energy from food, which the poison cuts off) AND the reasoning that links it to the claim: nothing refills the store, so enzyme H has no ATP to take a phosphate from, and the trapping stops.

Support a claim needs the evidence and the link to the claim. Do not award the point for treating ATP as a long-term store, or for the energy of arrow 2 coming from the glucose-phosphate.

Common slip: Supporting with 'the poison stops ATP' and stopping. The point needs the two facts and the link: the store is only seconds deep, refilling it needs energy from food, so the trapping stops almost at once.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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