Cellular Energy

Unit 3 · Topic 3.3 end-of-topic test

Suggested time: about 51 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.

Answer every question. For each multiple-choice question, pick one option. When you have answered every question, press Submit the test; the feedback then gives the reasoning for each. For the two 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. Then 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).

Question 1

A drawing of an ATP molecule. Two regions are marked, 1 and 2.

The figure shows an ATP molecule with two regions marked, 1 and 2.

Which marked region is adenosine?

A. 1 B. The left part of 1 only C. 2 D. 1 and 2 together

Question 2

A cell hydrolyzes a molecule of ATP.

How does the ADP that is left differ from the ATP?

A. It has one phosphate group instead of three B. It has lost all three phosphates, leaving adenosine on its own C. It has two phosphate groups instead of three D. It still has three phosphates, with a water molecule added onto the outer one

Question 3

A grasshopper's jumping muscle hydrolyzes ATP each time it contracts. With no ATP, the muscle locks stiff.

Where does the energy for the contraction come from?

A. From the bond to the outer phosphate: the energy stored in it is released when it breaks B. From the difference in energy between what went into the reaction and what came out C. From the water molecule, which is split apart and gives up the energy it held D. From the ADP and Pi, which the muscle breaks down further as its fuel

Question 4

Energy profile for ATP hydrolysis, ATP + H₂O → ADP + Pi. Energy in kJ/mol.

The figure shows an energy profile for ATP hydrolysis, ATP + H₂O → ADP + Pi. Energy is in kJ/mol.

What does the profile show about the reaction?

A. It takes in 30 kJ/mol, because energy must be put in to break the phosphate bond B. It releases 100 kJ/mol, the energy that was stored in the phosphate bond C. It releases 70 kJ/mol, the energy left over in the products D. It releases 30 kJ/mol, because the products hold less energy than the reactants

Question 5

A membrane protein moves a solute into a vesicle against its concentration gradient. A researcher gives the protein ADP plus Pi, and then a look-alike of ATP that binds the protein's ATP site but cannot be hydrolyzed: each time, the solute stays outside. Given ATP, the protein moves the solute into the vesicle.

What do the results show about how the protein uses ATP?

A. Binding ATP alone changes the protein; removing the phosphate is unnecessary B. ATP hydrolysis is coupled to the protein's change of shape; binding alone is not enough C. Moving the solute releases energy that turns ADP and Pi into ATP D. The protein uses ADP and Pi directly as its source of energy

Question 6

A pump in a plant root cell's membrane pushes hydrogen ions (H⁺) out of the cell against their concentration gradient. When the pump hydrolyzes ATP, the pump becomes phosphorylated.

What does the added phosphate do to the pump?

A. Unfolds the pump, so that it is denatured B. Splits the pump into two smaller proteins C. Changes the pump's shape, so that it pushes H⁺ out of the cell D. Adds a second binding site for H⁺ to the pump

Question 7

A researcher gives a muscle a poison. The muscle contracts normally for a few seconds and then stops. By then its ATP is gone, and ADP and Pi have built up. The muscle goes on breaking glucose down as before.

Which reaction has the poison blocked?

A. The remaking of ATP from ADP and Pi B. The hydrolysis of ATP to ADP and Pi C. The breakdown of glucose D. The coupling of ATP hydrolysis to the contracting proteins

Question 8

During a 30-second race a greyhound's leg muscles hydrolyze ATP very fast, yet the amount of ATP in the muscles barely falls over the 30 seconds.

Which of the following explains why the amount of ATP barely falls?

A. The muscles store enough ATP for many minutes of racing B. ATP is not used in a race; the muscles use glucose directly C. Each ATP molecule can be hydrolyzed many times before it becomes ADP D. The muscles remake ATP from ADP and Pi almost as fast as they use it, with energy from food

Question 9

A goldfish is kept in a brightly lit tank and given no food. Over several weeks it grows thin and dies.

Why does the goldfish die in spite of the light?

A. The light is too weak for the fish to make its own food, so it starves in spite of it B. A fish cannot take in energy as light; its only input is food, and none was given C. The tank water carries no minerals, so the fish has no matter to build with D. The fish takes in energy from the water’s warmth, and a lit tank is too cool

Question 10

In one day a seaweed frond absorbs 1,200 kJ of light energy and stores about 50 kJ of it in the sugar it makes.

What happened to the other 1,150 kJ?

A. It was destroyed in the reactions that made the sugar B. It was transformed to heat and passed to the water around the seaweed C. It became the matter, the atoms, of the new sugar D. It was stored in the seaweed's ATP, to be used at night

Question 11

A student writes: 'Plants are producers because they make their own energy from sunlight.'

Which statement corrects the student?

A. Plants make energy from carbon dioxide and water rather than from sunlight B. Plants are producers because the energy they make is stored in the bonds of their sugar molecules C. Plants transform light energy into chemical energy in sugar; they create none D. Plants take in their energy from soil water and minerals, so the student names the wrong source

Question 12

In an hour of flight, a pigeon's breast muscles transfer 90 kJ of energy from the fuel they break down. About 30 kJ of that energy is captured in ATP.

What happens to the other 60 kJ?

A. It is destroyed, so the total amount of energy falls B. It spreads out as heat and can no longer do work C. It is stored in the ADP and Pi that are left over D. It stays in the part of the fuel that was not broken down

Question 13

A fallen tree trunk lying on the ground in a wood softens, crumbles and scatters over the years. The living tree beside it keeps its shape, its molecules built and rebuilt every day.

Which of the following ideas from the second law of thermodynamics does the fallen trunk show?

A. With no energy input its matter grows more disordered: its entropy rises B. The energy that held the trunk together was used up when the tree fell, so nothing is left to hold it C. The trunk’s atoms are being destroyed as it crumbles to dust D. The heat the trunk gives off as it decays can be gathered back to rebuild its order

Question 14

A sperm cell carries no food store of its own. In a dish with no sugar it swims for a few hours; then its ion concentration gradients collapse, its membrane leaks and it dies.

Why does the sperm cell die?

A. Swimming used up the matter the cell needed to build new molecules with B. Its heat could no longer leave the cell, so the cell overheated and died C. Without sugar, water no longer entered the cell, so the cell dried out D. With no sugar it makes no ATP, so the pumping that keeps it ordered stops

Question 15

A male emperor penguin stands through the Antarctic winter holding an egg and eats nothing for two months. It uses up its stored fat, then begins to break down its own muscle protein.

Why does the penguin break down its own tissues?

A. Heat still leaves it every moment, and its own molecules are the only energy source left B. It stored ATP in its fat and muscle while it was feeding, and breaking them down releases that ATP C. The energy it took in over the summer was used up and no longer exists, so it breaks down its tissues to create new energy D. Its cells stop needing ATP once food stops arriving, so the tissue is spare to burn

Question 16

A metabolic pathway in a cell. Each arrow is one reaction, catalyzed by the enzyme written above it.

The figure shows a metabolic pathway in which G is converted, step by step, to K.

Which molecule is both the product of enzyme E2's reaction and the reactant of enzyme E3's reaction?

A. G B. H C. J D. K

Question 17

A metabolic pathway in a cell. Each arrow is one reaction, catalyzed by the enzyme written above it.

In the pathway shown, a cell loses the ability to make enzyme E2. E1 and E3 are unchanged, and G keeps arriving.

What happens to the amounts of the pathway's molecules?

A. H builds up; J and K are no longer made B. G builds up; H, J and K are no longer made C. J builds up; only K is no longer made D. H and J build up; only K is no longer made

Question 18

A log burns in a fireplace in one blaze of heat and light. A fungus breaks the same kind of wood down in dozens of small enzyme-catalyzed steps, releasing the same energy from the same amount of wood.

What is the advantage to the fungus of the many small steps?

A. The steps release more energy in total than burning the wood does B. Each step releases a little energy that can be captured in ATP rather than lost as heat C. Many small steps break the wood down faster than a fire does D. The steps release the energy with no loss at all as heat

Question 19

Four molecules are found in a potato cell: starch, oxygen (O₂), carbon dioxide (CO₂) and water (H₂O).

Which of the four is an organic molecule?

A. Carbon dioxide (CO₂) B. Oxygen (O₂) C. Starch D. Water (H₂O)

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

A potato tuber sprouting in a dark cupboard lives on the starch stored in the tuber. In one ten-minute interval, students measured the energy transferred from the tuber's store: 300 J was transferred from stored starch; 120 J of it was captured in ATP and used for the sprout's work, and 180 J left the tuber directly as heat. Assume these values account for all the energy transferred in the interval. The sprout uses its ATP to build new cells and to move materials into them. If it stays in the dark, the tuber's starch will be used up in a few weeks.

(a) Calculate the percentage of the 300 J that was captured in ATP. (1 point)

A full-credit answer: 120 J of the 300 J transferred ended up in ATP.
120 ÷ 300 × 100 = 40%.
So the sprout captured 40% of the transferred energy in ATP.

energy transferred from starch = 300 J

energy captured in ATP = 120 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{120}{300} \times 100

percentage in ATP = 40 %

Check the box for each point your answer earns

120 divided by 300, multiplied by 100, gives 40% captured in ATP.

Do not award the point for 120 divided by 180 (67%, comparing ATP with heat) or for 180 divided by 300 (60%, the share that left as heat).

Common slip: Dividing 120 by 180 instead of by 300. The whole is the 300 J transferred from the starch. The heat is the other part of that whole, not the whole.

(b) Describe how the first law of thermodynamics accounts for the 300 J. (1 point)

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 300 J is accounted for.
120 J was captured in ATP.
180 J left the tuber as heat.
120 J + 180 J = 300 J.

Check the box for each point your answer earns

energy is neither created nor destroyed, only transferred or transformed, so the 300 J is all accounted for: 120 J captured in ATP plus 180 J that left as heat (120 + 180 = 300).

Accept "the energy in equals the energy out". Accept "180 J was lost as heat (to the surroundings)": that is the exam's own wording, and the heat still exists. Do not award the point for a statement that energy was destroyed, disappeared or ceased to exist.

Common slip: Saying that the 180 J of heat was destroyed or ceased to exist. The heat left the tuber, but it still exists; the first law of thermodynamics counts it. Saying it was "lost as heat" is fine: that is the exam's own wording.

(c) Explain why only part of the energy transferred from the starch ended up in ATP. (1 point)

A full-credit answer: Every energy transfer spreads some energy out as heat.
That heat can no longer do work.
So no transfer is fully efficient.
This is the second law of thermodynamics.
The sprout breaks the tuber's starch down in many small steps.
Each step captures part of the energy released in ATP.
The rest, here 180 J of the 300 J, warms the tuber and its surroundings.

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 tuber's starch 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 sprout wasted energy" with no mention of heat, or for "some energy was destroyed".

Common slip: Saying the sprout "could not use" the 180 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.

(d) Explain what happens to the ion concentration gradients across the sprout's cell membranes once the tuber's starch is used up. (1 point)

A full-credit answer: Once the starch is used up, the sprout has no energy input.
It has no food left, and in the dark it has no light.
So it can no longer remake ATP.
The pumps in its cell membranes work only while ATP hydrolysis is coupled to their pumping.
So the pumps stop.
Ions then leak across the membranes until the concentrations on the two sides even out.
So the ion concentration gradients collapse.

Check the box for each point your answer earns

the ion concentration gradients collapse (ions leak across the membranes until the concentrations even out), because with no starch and no light the sprout has no energy input, so it can no longer remake ATP, so the pumps whose work is coupled to ATP hydrolysis stop maintaining the concentration gradients.

Accept "the concentration gradients collapse" or "disappear" with the no-ATP reason. Do not award the point for "the concentration gradients collapse" alone, or for an answer in which the sprout keeps its concentration gradients by living on light it is not receiving.

Common slip: Saying only that the concentration gradients collapse. The point needs the chain that gets there: no input, no ATP, no pumping, ions leak until the concentrations even out.

(e) A student claims that the sprout could keep its cells ordered by gathering back the heat it gave off. Evaluate the student's claim. (1 point)

A full-credit answer: The claim fails.
The heat the sprout gave off spread into the air of the cupboard.
Heat that has spread out cannot be gathered back to do work: the second law of thermodynamics.
Keeping cells ordered needs energy every moment.
Heat leaves at every step, and none comes back.
So a living thing must keep taking in energy.
In the dark, with the starch gone, the sprout has no input.
So its order cannot be kept.

Check the box for each point your answer earns

the claim fails, because heat that has spread into the surroundings cannot be gathered back to do work (the second law of thermodynamics); keeping the cells ordered needs energy continuously, so the sprout must keep taking in energy, because heat leaves at every step, and in the dark with the starch gone it has no input.

Accept an evaluation built on "heat that has spread out cannot be used again" together with "order has to be supplied with energy continuously". Do not award the point for rejecting the claim with no reason, or for an evaluation that has the sprout creating energy from its own tissues.

Common slip: Rejecting the claim because “the heat is gone” and stopping. The point needs the reason: heat that has spread out cannot do work again, so order has to be supplied with fresh energy continuously.

Free-response score: 0 of 5

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

The model shows how a muscle cell powers the calcium pump in its membrane. The pump pushes calcium ions (Ca²⁺) out of the cytosol against their concentration gradient. Two reactions are numbered, arrow 1 and arrow 2. In a living cell the pump keeps working for as long as ATP is available. A researcher gives a cell a look-alike of ATP that binds the pump's ATP site but cannot be hydrolyzed; the pump stops.

A model of how a muscle cell powers its calcium pump. Two reactions are numbered, arrow 1 and arrow 2.

(a) Describe what happens to the ATP molecule in arrow 1. (1 point)

A full-credit answer: In arrow 1, ATP reacts with water.
This reaction is ATP hydrolysis.
Water splits the outermost of ATP's three phosphate groups off.
ADP is left: adenosine with two phosphates.
The phosphate that came off is free inorganic phosphate, Pi.
The reaction releases energy.

Check the box for each point your answer earns

ATP reacts with water (hydrolysis, ATP + H₂O → ADP + Pi): the outermost of its three phosphate groups is removed, leaving ADP (adenosine diphosphate, with two phosphates) and a free inorganic phosphate, Pi, and energy is released.

Accept "water splits the outer phosphate off ATP". Naming the reaction hydrolysis may be counted toward the description but is not required. Do not award the point for "ATP becomes ADP and Pi" alone, which the figure shows, or for ATP described as losing all three phosphates.

Common slip: Reading the figure back: "ATP becomes ADP and Pi". The point needs what happens to the molecule: water removes the outermost of the three phosphates.

(b) Explain why the reaction in arrow 1 releases energy that the pump can use. (1 point)

A full-credit answer: The products, ADP and inorganic phosphate, hold less energy than the reactants, ATP and water.
So the reaction as a whole releases energy.
The pump can use that energy only because the hydrolysis is coupled to its work.
The pump takes the outer phosphate onto itself: the pump is phosphorylated.
The added phosphate changes the pump's shape.
In its new shape, the pump pushes Ca²⁺ out of the cytosol.

Check the box for each point your answer earns

the products, ADP and inorganic phosphate, hold less energy than ATP and water did, so the reaction as a whole releases energy; the pump can use it because the hydrolysis is coupled to the pump's work, the outer phosphate being transferred onto the pump (phosphorylation), which changes the pump's shape.

Accept "the products sit lower in energy than the reactants" for the source. Do not award the point for "energy stored in the high-energy phosphate bond is released when the bond breaks".

Common slip: Saying energy was stored in the bond to the outer phosphate and came out when the bond broke. Breaking a bond takes energy in. The release comes from the whole reaction: its products hold less energy than its reactants.

(c) Represent arrow 2 as an equation, in words or symbols, showing what goes in and what comes out. (1 point)

A full-credit answer: Arrow 2: ADP + Pi + energy → ATP + H₂O.
Arrow 2 is the reverse of hydrolysis.
ATP and water hold more energy than ADP and Pi.
So remaking ATP needs an input of energy.
The cell supplies that energy from the food it breaks down.

Check the box for each point your answer earns

ADP + Pi + energy → ATP (+ H₂O), the reverse of hydrolysis, with an input of energy shown going in.

Accept the equation without water, and accept "energy from food" as the energy term. Do not award the point for an equation with no energy input, or for one with energy shown coming out.

Common slip: Writing the equation with no energy term. Arrow 2 needs an input of energy; without the energy from food it does not happen.

(d) Explain, using the model and the result of the look-alike experiment, why the cell must keep taking in energy. (1 point)

A full-credit answer: The look-alike binds the pump but is never hydrolyzed.
So no energy is released, and nothing drives the pump's change of shape.
So the pump stops: the hydrolysis itself drives the work.
A cell holds only seconds' worth of ATP.
So it must remake ATP (arrow 2) all the time, with energy from food.
The pump keeps the cell's calcium concentration gradient, part of its order.
So the cell stays ordered only while energy from food keeps flowing in.

Check the box for each point your answer earns

the look-alike binds but cannot be hydrolyzed, so no energy is released, nothing is coupled to the pump's shape change and the pump stops, showing that it is the hydrolysis, not the presence of ATP, that drives the work; because a cell holds only seconds' worth of ATP, it must remake ATP continuously (arrow 2) from the energy in food, so the calcium concentration gradient and the rest of the cell's order can be kept only while energy keeps flowing in.

Accept an answer that links the pump stopping to "no hydrolysis, no energy" and the cycle to "ATP must be remade from food all the time". Do not award the point for restating that the pump stopped, or for treating ATP as a long-term store of energy.

Common slip: Stopping at "the pump stops because the look-alike is not ATP". The point needs the chain: no hydrolysis, no energy released, no coupled shape change; and then why the cell must keep remaking ATP from food.

Free-response score: 0 of 4

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Multiple choice checked: 0 of 19 correct.

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