Practice questions · Topic 3.3

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

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 tells you what a wrong choice assumed. For the free-response questions, write your answer in full sentences and show any calculation. 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

A researcher describes a molecule found in every cell: the base adenine joined to the sugar ribose, with a chain of two phosphate groups attached to the ribose.

Which molecule is being described, and what is the adenine-plus-ribose part called?

A. ATP; the adenine and ribose together are adenosine B. ADP; the adenine and ribose together are adenosine C. ADP; the adenine and ribose together are inorganic phosphate D. ATP; the adenine and ribose together are a phosphate group

Question 2

Cells lining the windpipe carry cilia, hair-like projections that beat to sweep mucus upward. In a cell-free preparation the cilia beat only when ATP is added to the fluid; ADP and Pi added on their own leave them still.

Where does the energy for the beating come from?

A. From the bond to the outer phosphate, which stores energy that is set free when it breaks B. From the water molecule that reacts with ATP, which gives up the energy it held C. From the ADP and Pi themselves, which the cilia burn as fuel D. From the whole hydrolysis 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. Each step it takes is paired with the hydrolysis of one ATP: the outer phosphate is transferred onto the protein, which then changes shape and swings forward. Given ADP and Pi with no ATP, the protein binds the fiber and stays still.

How does the protein use ATP to move?

A. ATP hydrolysis is coupled to the step: phosphorylation changes the protein's shape so it moves B. The protein absorbs heat released by the hydrolysis and uses the warmth to swing forward C. Binding ATP alone changes the protein's shape; the hydrolysis itself plays no part D. The moving protein releases energy that turns ADP and Pi back into ATP

Question 4

A bumblebee's flight muscle holds enough ATP for about two seconds of flight, yet the bee flies for twenty minutes at a time, fueled by the sugar in the nectar it has drunk.

How does the muscle keep working for twenty minutes?

A. The muscle stores extra ATP in advance whenever the bee rests on a flower B. Each ATP molecule is hydrolyzed many times before it becomes ADP C. The muscle remakes ATP from ADP and Pi as fast as it uses it, with energy from the sugar D. After two seconds the muscle switches to using the nectar sugar directly in place of ATP

Question 5

A mushroom grows in a dark cellar on a rotting log, with no light at all. Over several weeks it builds new cells and pushes up through the surface of the log.

Where does the energy for its growth come from?

A. The mushroom creates its own energy as it grows B. The warmth of the cellar air, which the mushroom takes in C. The water it absorbs from the damp log D. Chemical energy in the food it takes from the rotting wood

Question 6

A heap of grass cuttings and leaves, left for a week, warms to 60 °C at its center, where bacteria and fungi are breaking the plant matter down.

Where does the heat come from?

A. The bacteria and fungi create the energy as they grow B. The microbes transform chemical energy in the plant matter into heat C. Sunlight absorbed by the heap's surface is trapped at its center D. The plant matter's mass is turned into energy as it rots

Question 7

A salmon swimming upstream draws 3,000 kJ from its stored fat in a day. About 1,000 kJ of that ends up in ATP, which powers its muscles.

What happens to the other 2,000 kJ?

A. It spreads into the fish and the river as heat and can no longer do work B. It is destroyed, so the fish holds less energy in total at the end of the day C. It stays stored in the ADP and Pi left after the ATP is used D. It is saved as fat again, ready for the following day's swim

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 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, S. Each arrow is one reaction, catalyzed by the enzyme written above it.

The figure shows a pathway in a yeast cell in which P is converted, step by step, into a flavor compound, S.

Which molecules are intermediates of this pathway?

A. P and S B. Q and R C. P, Q and R D. Q, R and S

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 · 5 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 embryo's source of energy and state which kind of energy input it is, using one detail of the setup to rule out the other kind. (1 point)

Hint: Living things take in energy in one of two forms. Which detail of how the dish is kept settles which of the two is available here?

A full-credit answer: The embryo's energy comes from the yolk stored in its egg, which is food: chemical energy in the yolk's molecules. Light is ruled out because the dish is kept in the dark, so food is the only kind of input available to it.

Check the box for each point your answer earns

the yolk stored in the egg, which is food (chemical energy); light is ruled out because the dish is kept in the dark (accept: the embryo is an animal and cannot use light).

Accept 'stored food in the yolk'. Do not award the point for light, for the warmth of the room or for the pond water as the energy source.

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 keeps the reactions running and water supplies matter, but 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 out of the 40 J transferred.

Write down the values in the question:

energy transferred from the yolk = 40 J
energy captured in ATP = 14 J
energy released as heat = 26 J

Write down the equation:

                       energy in ATP
percentage in ATP = ────────────────────── × 100
                     energy transferred

Substitute in the values, and calculate:

percentage in ATP = (14 ÷ 40) × 100
percentage in ATP = 35%

Check the box for each point your answer earns

14 ÷ 40 × 100 = 35%.

Do not award the point for 14 ÷ 26 (54%, comparing ATP with heat) or for 26 ÷ 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 it, not the whole.

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

Hint: Where did the 40 J end up? Put the amounts that arrived beside the amount that left the yolk, and ask what the comparison shows.

A full-credit answer: The first law says energy is never created or destroyed, only transferred or transformed. Here every joule of the 40 J is accounted for: 14 J was transformed into chemical energy in ATP and 26 J was transferred to the surroundings as heat, and 14 + 26 = 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'. Do not award the point for a statement that 26 J was lost, used up or destroyed.

Common slip: Describing the 26 J of heat as energy that was 'lost' or 'used up'. It left the egg, but it still exists, spread into the water and the air; the first law counts it.

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

Hint: Compare the 26 J with the 14 J: which law describes what happens to energy in every transfer, and what does it say about where some of it always goes?

A full-credit answer: 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. As the embryo breaks the yolk's food down in many small steps, each step captures part of the energy released in ATP and 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, 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 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 second law's idea: some energy spreads out as heat at every transfer, so no transfer is fully efficient.

(e) Predict what will happen to a tadpole that finds no food once its yolk is used up, and justify your prediction using what a living system must do with energy to stay alive. (1 point)

Hint: What does a living thing have to keep doing with energy just to stay as it is, and what changes about its supply once the yolk is gone?

A full-credit answer: The tadpole will die. A living system stays ordered only by taking in more energy than it loses: order costs energy every moment, paid for by ATP hydrolysis coupled to building cells and pumping ions, and heat leaves at every transfer. With the yolk gone and no food, the tadpole's input is zero while its losses go on, so it can no longer remake ATP, the pumping and building stop, its order breaks down, and a significant loss of order results in death.

Check the box for each point your answer earns

with no yolk and no food the tadpole has no energy input, so it can no longer make ATP; the building and pumping that keep its cells ordered stop, its order breaks down and it dies, because a living system must take in more energy than it loses as heat, and its losses continue while its input has fallen to zero.

Accept 'it dies because it can no longer make the ATP that keeps it ordered'. Do not award the point for 'it stops growing' alone, or for a prediction that it can live on warmth or water.

Common slip: Predicting only that growth slows or stops. The prediction has to follow the chain to its end: no input, no ATP, order lost, death.

Free-response score: 0 of 5

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

The model shows the first step a liver cell takes with glucose that has entered it. Enzyme H transfers the outer phosphate of ATP onto the glucose, giving glucose-phosphate and ADP (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; if none were remade, the cell's ATP would last only a few seconds at its normal rate of use.

A model of the first step a liver cell takes with glucose. Arrow 1: enzyme H transfers the outer phosphate of ATP onto glucose, giving glucose-phosphate and ADP. Arrow 2: the cell remakes ATP from ADP and inorganic phosphate (Pi) with energy from food.

(a) Describe what happens in arrow 1, naming what is transferred, what receives it, and the products. (1 point)

A full-credit answer: In arrow 1 enzyme H transfers the outermost of ATP's three phosphate groups onto glucose: glucose is phosphorylated. The products are glucose-phosphate and ADP, adenosine with two phosphates.

Check the box for each point your answer earns

the outer phosphate group of ATP is transferred onto glucose (glucose is phosphorylated), giving glucose-phosphate and ADP (adenosine diphosphate) as the products.

Accept 'ATP gives glucose one of its phosphates, leaving ADP'. Do not award the point for products named as 'glucose and energy' with no phosphate transferred, or for ATP losing all three phosphates.

Common slip: Saying ATP 'gives its energy' to glucose with no phosphate mentioned. What moves is a phosphate group; that transfer is how the energy is put to use.

(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 run. ATP hydrolysis releases 31 kJ/mol, more than that. Enzyme H couples the two: it transfers ATP's outer phosphate straight onto the glucose, so the combined reaction releases 31 − 14 = 17 kJ/mol overall and runs quickly. That is energy coupling: a reaction that would not run by itself is driven by ATP hydrolysis happening at the same time.

Write down the values in the question:

glucose + Pi → glucose-phosphate + H₂O takes in 14 kJ/mol
ATP + H₂O → ADP + Pi releases 31 kJ/mol

Write down the equation:

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

Substitute in the values, and calculate:

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 run 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 runs.

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, and it drives the phosphorylation only because the two are coupled.

(c) A poison stops the liver cell from remaking ATP (arrow 2). Predict what happens to the trapping of glucose in the cell over the next minutes. (1 point)

A full-credit answer: Glucose goes on being trapped for only a few seconds, as the cell uses up the ATP it holds. Then arrow 1 stops: with no ATP there is no phosphate to transfer, no glucose-phosphate forms, and glucose that enters the cell can pass back out again.

Check the box for each point your answer earns

the trapping continues only for seconds, until the cell's ATP is used up; then no more glucose-phosphate forms (arrow 1 stops), so glucose is no longer trapped and can leave the cell again.

Accept 'it stops within seconds to a minute'. Do not award the point for 'trapping continues normally because the cell has plenty of ATP' or for 'glucose is trapped faster'.

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

(d) Justify your prediction using the time the cell's ATP would last and what making ATP from ADP and Pi requires. (1 point)

A full-credit answer: The cell's ATP would last only seconds, 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, which the cell normally gets from breaking down food molecules. The poison cuts off that remaking, so nothing refills the store, enzyme H has no ATP to draw a phosphate from, and the trapping of glucose stops along with every other process the cell pays for with ATP hydrolysis.

Check the box for each point your answer earns

the cell's ATP would last only seconds, so once arrow 2 is blocked the store is gone almost at once; making ATP from ADP and Pi is the reverse of hydrolysis and needs an input of energy, which the cell gets from breaking down food, so with arrow 2 blocked no ATP is remade and every process coupled to ATP hydrolysis, including arrow 1, stops.

Accept an answer that links 'seconds' worth of ATP' to 'remaking ATP needs energy from food'. 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 or from Pi.

Common slip: Justifying with 'the poison stops ATP' and stopping. The point needs the two facts that make the effect so fast: the store is only seconds deep, and refilling it needs an input of energy from food.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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