4 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 4
Which statement about the energy requirements of living cells is correct?
Answer and reasoning
ACells that are growing or dividing need an energy input, but resting cells need none. A student who thinks only active or growing cells use energy picks this. Resting cells still carry out processes such as maintaining their membranes and gradients, so they still need an energy input.
BCells recycle their energy, so they need an input of energy only to start their processes. A student who thinks energy is recycled within cells picks this. Some energy is lost as heat in every transformation, so cells need a continual input, not just a start-up supply.
CEvery living cell needs a continual input of energy, as some energy is always lost as heat.Correct Every living system needs energy coming in. Cells use energy to build molecules, move substances and maintain order, and every energy transformation releases some energy as heat, so energy must be continually supplied.
DAnimal cells need an energy input, but plant cells make their own energy by photosynthesis instead. A student who thinks plants make their own energy picks this. Photosynthesis captures energy from sunlight, an energy input; it does not create energy. Plant cells without chloroplasts, such as root cells, rely on sugars made elsewhere.
Which of the following is an example of an energy-releasing process coupled with an energy-requiring process in a cell?
Answer and reasoning
AHeat released by respiration supplying the energy to join amino acids into proteins A student who thinks cells use heat as an energy source picks this. Joining amino acids is driven by coupling with ATP hydrolysis and related reactions, not by heat.
BMitochondria creating new energy that is then used for the contraction of muscles A student who thinks living things create energy picks this. Mitochondria transfer chemical energy from food molecules to ATP; they do not create energy.
CATP hydrolysis supplying the energy for O₂ to diffuse into a cell across its membrane A student who thinks all movement across membranes needs ATP picks this. Diffusion is passive: O₂ moves down its concentration gradient without energy from ATP.
DATP hydrolysis supplying the energy needed to pump ions against their concentration gradientCorrect Pumping ions against their concentration gradient (active transport) requires energy. It is coupled with ATP hydrolysis, which releases energy, so the coupled process as a whole releases energy and can proceed.
The two energy diagrams shown compare the energy released when glucose is burned in a flame with the energy released when glucose is broken down to the same products in a cell, through a sequence of enzyme-catalyzed steps. Which statement is supported by the models?
Answer and reasoning
AThe cell releases more total energy, as each enzyme adds some energy at its own step. A student who thinks enzymes supply energy picks this. The models show the same starting and ending levels, so the total energy released is the same; enzymes lower activation energies but add no energy.
BBoth release the same total energy, but the cell releases none of it as heat. A student who thinks cells use energy without losing any as heat picks this. The models show the same total drop in energy, and every energy transformation in a cell releases some energy as heat; releasing energy in small steps lets the cell capture part of it in a usable chemical form.
CBoth release the same total energy, but the cell releases it in smaller amounts per step.Correct Both diagrams start at the same energy level and end at the same level, so the same total energy is released. In the flame it is released in one step; in the cell it is released in a sequence of small steps, which allows a more controlled transfer of energy.
DBurning is more useful to a cell, as cells run on heat, which it releases all at once. A student who thinks cells use heat as their energy source picks this. Cells cannot power their processes with heat; releasing energy in small steps lets cells capture part of it in a usable chemical form.
AIn organisms of all three domains, including bacteria and archaeaCorrect Glycolysis is a core metabolic pathway conserved across all three currently recognized domains, Archaea, Bacteria and Eukarya, which is evidence of their common ancestry.
BOnly in eukaryotes, inside the mitochondria found within their cells A student who thinks glycolysis happens in mitochondria picks this. Bacteria and archaea, which have no mitochondria, carry out glycolysis.
CIn organisms that live where O₂ is available, as it requires oxygen A student who thinks glycolysis requires oxygen picks this. Glycolysis is found in organisms of all three domains, including many bacteria and archaea that live where there is no O₂.
DIn animals and fungi, but not in plants, which photosynthesize instead A student who thinks plants do not carry out cellular respiration picks this. Plant cells break down sugars by glycolysis and the rest of cellular respiration, as animal and fungal cells do.
In preparation: 0 of 4 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
3.3.A.1 Energy input Fix
Energy input
Energy that a living system takes in from outside, as light (for photosynthetic organisms) or as chemical energy in food or other molecules. Every living system needs a continual input of energy.
Students often think Only organisms that are active, growing or dividing need energy; dormant or resting organisms, such as dry seeds, use none. In fact Yes. Dormant seeds and resting organisms carry out cellular processes at a low rate, so they still use energy and still need an energy input, although less than when they are active or growing.
Students often think Plants make their own energy by photosynthesis, so plants do not need an energy input from outside and do not carry out cellular respiration. In fact No. Plants capture energy from an input, sunlight, and store it in sugars; they do not create energy. Plant cells, including those of roots and seeds, also break down sugars in cellular respiration, in light and in darkness.
3.3.A.2 First law of thermodynamics Fix
First law of thermodynamics
Energy is neither created nor destroyed; it can be transferred and transformed from one form to another, such as from chemical energy to heat.
Second law of thermodynamics
Every energy transformation increases the disorder (entropy) of the universe, and in every transformation some energy is released as heat that cells cannot use to do work.
Order in living systems
Cells build and maintain highly ordered structures and concentration gradients from less ordered materials. This is possible without violating the second law because cells use an energy input and release heat and simpler molecules, increasing the disorder of their surroundings by more than their own order increases.
Energy budget
An account of the energy an organism takes in and where it goes: energy lost in wastes, energy lost as heat, and the remainder, which is available for growth, reproduction and storage. For an organism to maintain its order and power its processes, its energy input must exceed its energy loss.
Energy coupling
The linking of an energy-releasing (exergonic) process with an energy-requiring (endergonic) process, so that energy released by the first drives the second. The coupled process as a whole releases energy.
Exergonic and endergonic reactions
An exergonic reaction releases energy: its products have less energy than its reactants. An endergonic reaction requires an input of energy: its products have more energy than its reactants.
ATP hydrolysis
The reaction ATP + H₂O → ADP + Pᵢ, which releases energy. In cells it is coupled to energy-requiring processes such as building molecules, active transport and movement.
Death from loss of order or energy flow
When a cell or organism can no longer obtain or use enough energy, or loses the order its processes depend on, such as intact membranes and concentration gradients, its processes stop and it dies.
Students often think Energy is recycled within a cell or organism, so once energy has been obtained it can be used again and again, and new energy is needed only to start processes. In fact No. Matter can be recycled, but energy cannot: each time energy is transformed, some is released as heat that cells cannot use to do work, so a continual input of energy is needed.
Students often think Once a cell's ordered structures, such as membranes and concentration gradients, have been built, they last without any further energy input. In fact No. Ordered structures and gradients tend to break down, so a cell must keep using energy to repair and rebuild them and to keep pumping substances against their gradients. Without an energy supply, its order is lost.
3.3.A.3 Metabolic pathway Fix
Metabolic pathway
A series of enzyme-catalyzed reactions in which the product of one step is typically the reactant (substrate) of the next. Releasing energy in a sequence of small steps allows a more controlled transfer of energy than a single large step.
Intermediate
A compound in a metabolic pathway that is the product of one step and the reactant of the next.
Students often think Each step of a metabolic pathway runs on its own, so blocking one step affects only that step's product. In fact No. In a metabolic pathway the product of one step is typically the reactant of the next, so if one step is blocked, the steps that follow it lack their reactant and slow or stop.
Students often think When a cell or organism needs a product or an ability, it produces a new enzyme or pathway to provide it. In fact No. A cell cannot produce a new enzyme or pathway because it needs one. New features arise from random genetic changes, and their spread in a population depends on inheritance and selection, not on need.
3.3.B.1 Core metabolic pathway Fix
Core metabolic pathway
A fundamental pathway, such as glycolysis or oxidative phosphorylation, that is shared by organisms in all three currently recognized domains, Archaea, Bacteria and Eukarya.
Conserved process
A process or feature that has been retained, with relatively little change, in many lineages descended from a common ancestor. Conserved core metabolic pathways are evidence that all organisms share common ancestry.
Domains
The three broadest groups of living organisms currently recognized: Archaea, Bacteria and Eukarya.
Students often think One modern group gave rise to the others: for example, bacteria and archaea descended from eukaryotes, as the most advanced group. In fact No. Living groups share common ancestors but did not descend from one another. Bacteria, archaea and eukaryotes are all modern groups that descend from a common ancestor of all three.
Students often think Organisms develop traits, such as a metabolic pathway, by using them during their lives and then pass these acquired traits to their offspring. In fact No. Traits that an organism develops by use during its life are not inherited. Inherited traits, including the enzymes of metabolic pathways, are encoded by genes passed from parents to offspring.
14 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 14
The graph shows the volume of O₂ taken up over 20 minutes at 25 °C by germinating seeds and by dormant (not germinating) seeds of a hypothetical plant species, and by glass beads of the same total volume. Which statement is supported by the data?
Answer and reasoning
ADormant seeds took up no more O₂ than the glass beads did in 20 minutes. A student who thinks dormant seeds use no energy picks this. The dormant seeds took up about 0.20 mL of O₂, while the glass beads took up none.
BSeeds take up O₂ only until they can photosynthesize, and then stop respiring. A student who thinks plants do not carry out cellular respiration picks this. The data show only that seeds take up O₂; plant cells, including those of photosynthesizing plants, carry out cellular respiration throughout their lives.
CThe seeds' rate of O₂ uptake fell over time as they began to recycle energy. A student who thinks cells recycle their energy picks this. Both seed lines rise in a straight line, so the rate of O₂ uptake stayed constant over the 20 minutes.
DDormant seeds took up O₂, but at a lower rate than germinating seeds did.Correct The dormant seeds took up about 0.20 mL of O₂ in 20 minutes, while the glass beads took up none, so the dormant seeds were carrying out cellular processes. The germinating seeds took up about 1.00 mL in the same time, about five times as much.
A student puts germinating pea seeds into one insulated flask and the same mass of pea seeds killed by boiling into a second insulated flask. Both sets of seeds were first rinsed in a disinfectant that kills microorganisms. A thermometer in each flask reads 20 °C at the start, and the room stays at 20 °C. Which result after 48 hours is best predicted?
Answer and reasoning
ABoth flasks stay near 20 °C, and the germinating seeds release no heat to the air. A student who thinks cells use energy without releasing heat picks this. Energy transformations in living cells always release some heat, which warms the insulated flask of germinating seeds.
BThe flask of germinating seeds warms above 20 °C, and the other stays near 20 °C.Correct The living seeds carry out cellular respiration and other processes, and every energy transformation releases some energy as heat, so their flask warms. The killed seeds carry out no cellular processes, and the disinfectant has removed microorganisms, so their flask stays near room temperature.
CBoth flasks warm by the same amount, as both hold seeds with the same stored energy. A student who thinks food releases its energy on its own picks this. Energy is released from stored food only by the cellular processes of living cells; the killed seeds release none.
DThe flask of germinating seeds cools below 20 °C, as living cells absorb heat to grow. A student who thinks cells use heat as an energy source picks this. Cells cannot use heat to power their processes; their energy transformations release heat, so the flask warms.
The model represents the energy transformations when a hypothetical cell breaks down glucose. Values are in arbitrary units. Which statement correctly relates the model to the laws of thermodynamics?
Answer and reasoning
AThe energy leaving equals the energy entering, and part of it is converted to heat.Correct The model shows 100 units entering and 40 + 60 = 100 units leaving, so energy is conserved (first law). The 60 units released as heat show that the transformation is not 100% efficient, as the second law requires: some energy is always released as heat.
BThe cell creates energy, so it can release more energy than it takes in from glucose. A student who thinks cells create energy picks this. The model shows 100 units leaving and 100 units entering; energy is transformed, not created.
CThe heat is energy destroyed in the cell, so it is not counted in the total energy out. A student who thinks used energy is destroyed picks this. Heat is a form of energy that leaves the cell; the 60 units are part of the 100-unit total.
DThe heat is recycled by the cell into glucose, and in the end no energy leaves it. A student who thinks energy is recycled in cells picks this. Heat leaves the cell and cannot be turned back into chemical energy by it; the model shows 60 units leaving as heat.
A seed grows into a large plant whose cells contain highly ordered structures built from simple molecules. A student claims that this growth violates the second law of thermodynamics, which states that energy transformations increase the disorder (entropy) of the universe. Which reasoning best refutes the student's claim?
Answer and reasoning
AThe laws of thermodynamics describe nonliving systems, so they do not apply to a growing plant at all. A student who thinks living things are exempt from the laws of thermodynamics picks this. The laws apply to living things; the plant is consistent with them because it uses an energy input and releases heat.
BThe plant uses an energy input and releases heat, so the disorder of the plant and its surroundings increases.Correct The plant builds order using energy from sunlight, and its energy transformations release heat and simple molecules to its surroundings. The disorder of the surroundings increases by more than the plant's order increases, so the total disorder of the universe still increases, as the second law requires.
CThe plant creates the energy it needs by photosynthesis, so the second law does not limit how it grows. A student who thinks plants create energy picks this. Photosynthesis transforms light energy into chemical energy; it does not create energy, and the plant's growth is consistent with the second law.
DThe plant recycles the energy released by its own cells, so it needs no input from outside to build order. A student who thinks energy can be recycled within an organism picks this. Energy released as heat cannot be reused; the plant needs a continual energy input, sunlight, to build and maintain its order.
The table shows the energy budget of a young hypothetical mammal over one day. Assume that all of the energy in the food eaten that does not leave the body is stored in new body tissue. What percentage of the energy in the food eaten is stored in new body tissue?
Answer and reasoning
A74% A student who thinks energy is recycled within an organism treats the 155 kJ released as heat as still available to the body and counts it as stored: (250 − 65)/250 × 100 = 74%. Heat released leaves the body and cannot be reused.
B82% A student who thinks cells use heat, whether released by their own reactions or taken from their surroundings, to power growth counts both the 155 kJ released as heat and the 20 kJ absorbed while basking as built into new tissue: (250 − 65 + 20)/250 × 100 = 82%. Cells cannot use heat to build tissue; the heat leaves the body, and only the 30 kJ of food energy that remains is stored.
C12%Correct Of the 250 kJ eaten, 65 kJ leaves in feces and urine and 155 kJ is released as heat and leaves the body, leaving 30 kJ stored in new tissue. 30/250 × 100 = 12%.
D30% A student who reports an amount directly as a percentage picks this. 30 kJ must be divided by the 250 kJ eaten: 30/250 × 100 = 12%.
Working Energy stored in new tissue = energy in food eaten − energy in feces and urine − energy released as heat (which leaves the body) = 250 − 65 − 155 = 30 kJ. The 20 kJ of heat absorbed while basking is not food energy and cannot be used by cells to build tissue, so it does not enter the calculation. Percentage = 30/250 × 100 = 12%. Distractors: counting the heat released as still in the body and reused gives (250 − 65)/250 × 100 = 74%; counting both the heat released and the heat absorbed as used to build tissue gives (250 − 65 + 20)/250 × 100 = 82%; reporting 30 kJ as a percentage gives 30%.
Three groups of a hypothetical rodent species (n = 10 per group) were fed diets supplying different amounts of energy for four weeks. The graph shows the mean change in body mass for each group, with error bars representing ±2 SE of the mean. Which conclusion about the group fed 45 kJ/day is best supported?
Answer and reasoning
AIts members' masses stayed exactly the same, as the bar crosses zero. A student who reads an error bar that includes zero as proof of no change picks this. The data show that a change has not been demonstrated, not that every animal's mass stayed exactly the same.
BIts members gained a significant amount of mass, as its mean is above zero. A student who treats any mean above zero as a real effect picks this. The ±2 SE bar includes zero, so the gain of 0.4 g may be due to chance variation.
CEvery animal's change in mass lay between −0.5 g and 1.3 g. A student who thinks an error bar shows the range of individual values picks this. A ±2 SE bar shows the uncertainty in the mean; individual animals' changes may lie well outside it.
DIts mean change in mass did not differ significantly from zero.Correct The 45 kJ/day group's mean change was +0.4 g, but its ±2 SE error bar runs from −0.5 to +1.3 g and includes zero. A change in mass has not been shown for this group.
Working No test statistic is calculated; the decision rests on the ±2 SE error bars. 45 kJ/day: mean +0.4 g, bar from −0.5 to +1.3 g. The bar includes 0, so the mean change is not significantly different from zero: no change in mass has been shown. This does not show that the mass change was exactly zero, does not show a real gain, and the bar describes uncertainty in the mean, not the range of individual animals. For comparison, the 60 kJ/day bar (2.3 to 3.9) and the 30 kJ/day bar (−3.5 to −2.1) do not include 0, so those groups show a likely real gain and loss.
During its five months of hibernation, a hypothetical species of ground squirrel eats nothing, and its body temperature falls close to that of its burrow. Over the winter it loses about a third of its body mass. Which statement best explains this loss of mass?
Answer and reasoning
AIts cells break down stored fat in respiration, and the products leave as CO₂ and water.Correct With no food coming in, the squirrel's energy input is less than its energy loss, so its cells break down stored fat to supply the energy they still use. The atoms of the fat leave the body mainly as CO₂, breathed out, and water.
BIts cells use no energy while it hibernates, so the mass is lost as water that evaporates. A student who thinks resting or dormant organisms use no energy picks this. A hibernating squirrel's cells carry out processes at a reduced rate, so it still uses energy, supplied by its stored fat.
CIts stored fat is converted directly into energy, so that mass simply disappears from its body. A student who thinks fat turns into energy picks this. Fat is broken down in cellular respiration; its atoms leave as CO₂ and water, and only its chemical energy is transferred.
DIts cells recycle the same energy all winter, so the mass lost is water from its tissues. A student who thinks energy can be recycled picks this. Energy is lost as heat throughout the winter and must be replaced from stored fat, whose breakdown accounts for the loss of mass.
The model compares the energy of the reactants and products of reaction 1, which joins molecules A and B, of reaction 2, the hydrolysis of ATP, and of the two reactions coupled together in a cell. Which statement about the coupled reaction is supported by the model?
Answer and reasoning
AOverall it releases energy, as reaction 2 releases more energy than reaction 1 requires.Correct Reaction 1 requires 20 kJ/mol and reaction 2 releases 30 kJ/mol. Coupled, the products have less energy than the reactants, so the overall process releases about 10 kJ/mol and can proceed.
BIt can proceed because breaking a bond in ATP releases energy, as breaking any bond does. A student who thinks breaking bonds releases energy picks this. Breaking a bond requires energy; ATP hydrolysis releases energy overall because of the bonds formed in its products.
CCoupling creates the extra energy that reaction 1 needs, so the total energy rises. A student who thinks coupling creates energy picks this. The energy for reaction 1 comes from reaction 2; no energy is created, and the coupled products have less energy than the reactants.
DReaction 1 releases the energy that drives reaction 2, the hydrolysis of ATP. A student who confuses the energy-releasing and energy-requiring reactions picks this. The model shows that reaction 1 requires energy and reaction 2 releases it, so reaction 2 drives reaction 1.
In a respirometer, a change in the volume of gas shows how much O₂ germinating seeds take up; the CO₂ they release is absorbed by a chemical in the tube. A student sets up one respirometer containing germinating seeds and a second containing glass beads of the same total volume, both in the same water bath. Which statement best justifies including the respirometer with glass beads?
Answer and reasoning
AIt should show no volume change, and if it shows any, the whole investigation has failed. A student who thinks a control must show no change picks this. Small changes in the beads' respirometer are expected when temperature or air pressure changes; they are measured and used to correct the seeds' results.
BIt measures the independent variable, the volume of O₂ taken up, in a setup with no seeds. A student who swaps the independent and dependent variables picks this. The volume of O₂ taken up is the dependent variable; whether living seeds are present is the independent variable.
CIt shows any volume change caused by temperature or air pressure, which is then subtracted.Correct Glass beads carry out no cellular processes, so any change in gas volume in their respirometer is caused by changes in temperature or air pressure. Subtracting it from the seeds' volume change leaves the change caused by the seeds taking up O₂.
DIt proves that the seeds are alive, as nonliving things cannot change the volume of gas. A student who thinks a control proves the hypothesis picks this. Gas volume also changes with temperature and air pressure; the beads are included to measure those changes, not to prove anything about the seeds.
A potted plant of a hypothetical species is moved into continuous darkness at 20 °C and is watered normally. Which prediction about the plant over the following weeks is best supported?
Answer and reasoning
AIt stops using energy until light returns, so it survives in the dark with no change in its dry mass. A student who thinks plants do not carry out cellular respiration picks this. Plant cells respire in darkness as in light, using stored sugars, so the plant loses dry mass.
BIt keeps breaking down stored sugars in respiration, so it loses dry mass and dies once these run low.Correct Without light, the plant has no energy input, but its cells still need energy, so they break down stored sugars in cellular respiration and the plant loses dry mass. When its stores can no longer supply enough energy to maintain its order and processes, it dies.
CIt takes in energy from the soil through its roots, so it goes on growing normally in the dark. A student who thinks plants get energy from the soil picks this. Roots take in water and mineral ions, which supply no usable chemical energy; the plant's only energy input is light.
DIt uses the heat of the room as its energy source, so it survives in the dark for as long as it is warm. A student who thinks cells can use heat as an energy source picks this. Cells cannot power their processes with heat; without light, the plant depends on its stored sugars.
Cultured cells of a hypothetical mammal were placed in a medium containing no glucose, and at time 0 a toxin that blocks oxidative phosphorylation was added. The graph shows the ATP content of the cells and the percentage of cells still alive (viable), each as a percentage of its value at time 0. Which statement is supported by the data?
Answer and reasoning
AATP content fell steeply first, and most of the cells died during the next half hour.Correct ATP fell to about 12% of its starting value within 10 minutes, while almost all cells were still viable. Viability then fell, to 55% at 20 minutes and 4% at 40 minutes. Loss of energy flow was followed by cell death. With no glucose and oxidative phosphorylation blocked, the cells could not make enough ATP.
BATP content stayed close to its starting level for a long time after the toxin was added. A student who thinks cells keep large stores of ATP picks this. ATP fell to 40% within 5 minutes; cells hold little ATP and must make it continually.
CMost of the cells stayed viable to the end, as cell structures last without an energy input. A student who thinks a cell's order lasts without energy picks this. Only about 4% of cells were viable at 40 minutes; without enough ATP, cells could not maintain their order.
DATP content recovered as the cells adjusted to the toxin, as they needed ATP to survive. A student who thinks cells produce what they need picks this. ATP stayed at about 5% from 20 to 40 minutes; it did not recover, and most cells died.
In a hypothetical bacterium, compound S is made from compound P by a pathway of three enzyme-catalyzed steps: P → Q → R → S. Enzyme 1 converts P to Q, enzyme 2 converts Q to R, and enzyme 3 converts R to S. A mutation makes enzyme 2 nonfunctional. Which prediction about the mutant bacterium is best supported?
Answer and reasoning
AOnly R is affected; Q and S are still made at their usual rates in the mutant. A student who thinks each step of a pathway is independent picks this. Q is no longer converted to R, so Q builds up; R is the reactant for enzyme 3, so without R, little S can be made.
BQ builds up, and far less R and S are made than in bacteria that lack the mutation.Correct Enzyme 1 still converts P to Q, but Q is no longer converted to R at a significant rate, so Q accumulates. Because the product of each step is the reactant of the next, little R is made, and so little S.
CS is made at the usual rate, as the cell makes a new enzyme because it needs S. A student who thinks cells produce what they need picks this. A cell cannot produce a new enzyme because it needs a product; a nonfunctional enzyme 2 leaves the pathway blocked.
DP builds up and no Q is made, as the whole pathway stops at its first step. A student who thinks a blocked pathway stops at its start picks this. Enzyme 1 is unaffected and still converts P to Q; the block is at the second step, so Q accumulates.
The table shows how widely three metabolic pathways are found among organisms of the three domains. (Photosynthesis here means photosynthesis that uses chlorophyll.) Which explanation of the distribution of glycolysis is best supported?
Answer and reasoning
AIt arose separately in each domain, as every organism needs a way to obtain its energy. A student who thinks organisms produce what they need picks this. Need does not produce a pathway; a pathway found throughout all three domains is most simply explained by a single origin in their common ancestor.
BIt arose in eukaryotes, which later gave rise to the bacteria and the archaea. A student who thinks one modern group gave rise to the others picks this. Bacteria, archaea and eukaryotes are all living groups descended from a common ancestor; eukaryotes are not the ancestors of prokaryotes.
COrganisms that used glucose developed it through use and passed it to their offspring. A student who thinks traits acquired by use are inherited picks this. Pathways are inherited through genes, not developed by use during an organism's life.
DIt was inherited from a common ancestor of all three domains, so all three have it.Correct Glycolysis is widespread in all three domains, unlike photosynthesis and methanogenesis, which are each found in only some groups. A core pathway shared by every domain is most simply explained by inheritance from a common ancestor of all of them, and its conservation is evidence of that common ancestry.
A student draws four diagrams to represent how a cell drives the energy-requiring reaction A + B → AB. In each diagram, an arrow shows a transfer of energy. Which diagram correctly represents energy coupling in the cell?
Answer and reasoning
ADiagram 1 A student who confuses which reaction releases energy picks the diagram with the arrow running from A + B → AB to ATP hydrolysis. The joining of A and B requires energy; it cannot supply energy to ATP hydrolysis.
BDiagram 2Correct In energy coupling, the energy-releasing reaction, ATP hydrolysis (ATP → ADP + Pᵢ), transfers energy to the energy-requiring reaction, A + B → AB, so the arrow runs from ATP hydrolysis to the joining of A and B.
CDiagram 3 A student who thinks cells use heat to power reactions picks the diagram in which energy passes through heat. Heat released by ATP hydrolysis cannot drive A + B → AB; in coupling, the energy is transferred directly between the coupled reactions.
DDiagram 4 A student who thinks an enzyme supplies the energy for its reaction picks the diagram with the arrow from the enzyme. An enzyme lowers the activation energy but supplies no energy; the energy comes from ATP hydrolysis.
Compiled from the AP Biology Course and Exam Description (effective Fall 2025) and our question bank · Specialist review in progress. How these pages are made · Free, no account
