ATP & the Flow of Energy — Print · Grade 10 Science
Chapter: Matter & Energy in Living SystemsATP & the Flow of Energy
Learning goals
- I can explain how ATP stores and releases energy through the ATP–ADP cycle.
- I can distinguish autotrophs from heterotrophs and describe how energy flows from the Sun through living things.
Readings for this unit
Before this unit
Recognize an energy transfer and a coupled reaction.
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain. Trace the explained visual parts and write a first model, then attempt the illustrated and changed independent cases. In the third session explain a correction and revise the opening account using evidence.
Start with something to explain
Why do cells need new usable energy if energy is conserved?
Illustrative evidence case. A supplied energy account conserves total input while only part remains in a selected useful output.
Notice and wonder
Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.
Why do cells need new usable energy if energy is conserved?
Your first model
Draw atoms/products separately from work and heat-transfer arrows.
Build the language from the idea
- ATP
- Adenosine triphosphate, used in coupled energy-transfer processes.
- ADP
- Adenosine diphosphate; it can be converted to ATP using an energy source and phosphate.
- autotroph
- An organism that builds organic carbon from inorganic sources using light or chemical energy.
- heterotroph
- An organism that obtains organic carbon from other sources.
- energy coupling
- Linking reactions or processes so a favorable change can support work.
ATP is an energy-transfer molecule used in many cell processes. It has three phosphate groups; ADP has two. ATP hydrolysis uses water and produces ADP and inorganic phosphate. Under typical cell conditions the complete reaction releases free energy, which can support a coupled process.
Breaking a chemical bond alone requires energy. The favorable overall change in ATP hydrolysis depends on the products, new bonds and interactions, and concentrations. Use the whole reaction when explaining energy rather than calling a phosphate bond a packet that releases energy when snapped.
Cells regenerate ATP using energy from metabolism or light-dependent processes. ATP cycles rapidly and is not the main long-term energy store. Autotrophs build organic carbon from inorganic sources; heterotrophs obtain organic carbon from other sources. Many ecosystems receive energy through sunlight, but some producers use chemical energy.
ATP links energy-releasing processes with energy-requiring cell work. It is regenerated from ADP and phosphate rather than functioning as a one-time stock consumed forever. Cells maintain a continually cycling supply with limited stored amounts.
ATP hydrolysis has an overall energy change involving reactants, products and their interactions. Breaking one bond requires energy; favorable overall chemistry and coupling allow work. Use the full reaction rather than the misleading claim that breaking a bond alone releases energy.
Different cell processes use ATP through different mechanisms, including phosphorylation and molecular shape changes. A cell's energy supply depends on rates of production and demand. An energy diagram should include thermal transfer as well as useful work.
Build your understanding
Use three flexible study sessions: core explanation and Parts 1–2; Parts 3–4 with their reasoning checks and models; then the whole-unit application, worked examples, independent practice, and exit check. Allow about 50–60 minutes per session, including diagrams, data work, and notebook explanations. Attempt each check before opening its guidance. Schedule chapter investigations, review, and exams separately; add time where prerequisites are uncertain.
Part 1 · Two different accountings: energy and matter
A matter account traces atoms into products or new cell material. An energy account follows transfers among chemical changes, useful work and thermal motion. Conservation does not mean useful energy stays available in the same form forever. Metabolism organizes transformations to maintain cells, while heat transfer and other outputs disperse energy. Food matter does not vanish into pure energy in ordinary cellular chemistry: carbon, hydrogen and oxygen remain in material products. Define the system boundary before labeling an input a loss or a new source.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Does cellular use of food mean its atoms disappear after providing energy?
Visual reasoning — after your attempt
No. Atoms are rearranged into products or cell material; energy is transferred through reactions and to surroundings.
Living systems require two separate bookkeeping exercises, and conflating them is the usual source of confusion. Matter is conserved and recycled: the carbon atoms in your tissue have passed through air, plants, and other organisms and will do so again. Energy is also conserved in total, but at every transformation part of it becomes heat that disperses, so the amount available to do cellular work decreases step by step.
The practical consequence is that an ecosystem needs a continuous energy input and does not need a continuous matter input. Sunlight arrives every day; carbon does not have to. A closed container of plants, animals, and decomposers can cycle its matter indefinitely but will run down without light.
Pause and explain
Why can an ecosystem recycle its matter but not its energy?
Check your reasoning after answering
Atoms are rearranged rather than used up, so the same carbon and nitrogen can pass through organisms repeatedly. Energy degrades to heat at each transformation and disperses into the surroundings, where it can no longer drive cellular work, so it must be replaced from outside.
Part 2 · Autotrophs capture, heterotrophs transfer
An autotroph builds organic carbon compounds from inorganic carbon using an energy source. Photoautotrophs use light, while some autotrophs use chemical energy. A heterotroph obtains organic matter from other sources and can use its chemical transformations for energy. These categories concern carbon acquisition, not whether the organism has mitochondria or needs ATP. Producers also respire, and consumers can assimilate material into their own tissues. Distinguish energy capture, carbon fixation, feeding and cell-level energy transfer.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Can a photosynthetic producer still need cellular respiration?
Visual reasoning — after your attempt
Yes. Producing organic matter and supplying many cellular processes through respiration are distinct activities.
Autotrophs build organic molecules from inorganic sources using an external energy supply — light for photosynthetic plants and algae, chemical reactions for some bacteria. Heterotrophs obtain organic molecules by consuming other organisms. Decomposers are heterotrophs that break down dead material, releasing inorganic nutrients back for autotrophs to reuse, which is the step that closes the matter cycle.
Almost all the energy flowing through familiar ecosystems entered through photosynthesis. That makes producers the entry point rather than merely one group among several, and it is why removing producers collapses an ecosystem's energy supply while removing one consumer species usually does not.
Pause and explain
Why are decomposers essential to matter cycling but not to the energy input of an ecosystem?
Check your reasoning after answering
Decomposers return elements locked in dead material to inorganic forms that producers can take up again, which is required for matter to cycle. They do not add energy; energy still enters only through producers capturing light.
Part 3 · ATP as the working currency
ATP hydrolysis can support an energetically linked change, such as a transport protein’s conformational cycle. The useful change depends on the overall reaction and coupling, not on energy being released by breaking one isolated bond. ADP and inorganic phosphate can be used to rebuild ATP through other energy-transferring processes. ATP therefore serves repeated short-term transfers rather than a perpetual source. Count a stated ATP demand separately from the food input or the rate of ATP production; molecule count and energy amount are different quantities.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Why does counting ATP molecules not alone give the total energy supplied by food?
Visual reasoning — after your attempt
The pathway, reaction conditions, coupling and other energy transfers matter; ATP count is only one defined output.
Fuel molecules such as glucose and fat store energy over the long term, but cells do not use them directly for work. The energy released by breaking them down is used to attach a phosphate group to ADP, making ATP, and ATP hydrolysis is then coupled to the cell's tasks. This indirection means a cell has one standard energy carrier rather than a different mechanism for each fuel.
A cell holds only a small ATP pool — a few seconds' worth at working rates — and regenerates it continuously, so the quantity consumed in a day vastly exceeds the quantity present at any moment. That is the sense in which ATP is a currency: it circulates rapidly and is spent as fast as it is earned, while the store is held elsewhere.
Pause and explain
Why does a cell not simply store a large quantity of ATP instead of regenerating it constantly?
Check your reasoning after answering
ATP is a poor long-term store: it is chemically reactive and would require an enormous mass to hold a day's energy. Fuels such as glycogen and fat store far more energy per gram, and ATP is generated from them on demand.
Part 4 · Losses at every step, measured
Efficiency is a ratio of a defined useful output to the corresponding input. Here 35 of 100 energy units enters the selected work/storage output, giving 35%. The remaining 65 is not destroyed; it is transferred elsewhere, including thermal energy and other outputs. Whether a transfer counts as useful depends on the task and boundary. Comparing two biological pathways requires the same input/output definitions and conditions. A measured ATP yield is not automatically identical to a whole-organism growth efficiency or an ecosystem transfer efficiency.
Read the visual model
Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.
Does 35% efficiency violate conservation of energy?
Visual reasoning — after your attempt
No. The complete account retains all 100 units; 35% refers only to the selected useful output.
Energy transfers are inefficient in a way that can be quantified. Of the light reaching a plant, only a small percentage is stored as new plant tissue — often cited as roughly one percent — because much is reflected or transmitted, and much of what is absorbed powers the plant's own respiration. Transfers between organisms are commonly around ten percent, the rest being respired, lost as heat, or never absorbed.
These figures are approximations that vary widely between ecosystems, and treating them as constants is a mistake; they are useful for order-of-magnitude reasoning rather than precise prediction. Used that way they explain real patterns: why food chains are short, why predators are rare compared with their prey, and why producing meat requires more land than producing plant food of the same energy content.
Pause and explain
Using a rough ten percent transfer, how much of a producer's 10,000 kJ reaches a third-level consumer, and what does that imply?
Check your reasoning after answering
About 1,000 kJ reaches the first consumer, 100 kJ the second, and 10 kJ the third — a thousandth of the original. This is why food chains rarely extend beyond four or five levels: too little energy remains to support another one.
Apply the whole unit · Audit an energy budget
Illustrative annual figures for one field, in kJ per square metre. Light energy arriving: 3,000,000. Energy stored in new plant tissue: 30,000. Energy in plant tissue eaten by insects: 9,000. Energy in insect tissue produced: 900. Energy in insect tissue eaten by birds: 450. Energy in bird tissue produced: 45.
Your task
Calculate the percentage of incoming light stored by the plants and the percentage transferred at each of the three later steps, state where the largest proportional loss occurs, and explain in terms of matter and energy why the losses do not violate conservation. Then identify what these figures cannot tell you about the field.
What your response should show
- Choose the appropriate biological model or relationship and state the conditions it assumes.
- Show the counts, rates, ratios, or intermediate reasoning the question requires.
- Use the supplied evidence to justify the conclusion and separate observation from inference.
- Identify a meaningful limitation, uncertainty, or next measurement.
Response guidance — after your attempt
Light to plant tissue: 30,000 / 3,000,000 × 100 = 1.0 percent.
Plant tissue eaten to insect tissue: 900 / 9,000 × 100 = 10 percent.
Insect tissue eaten to bird tissue: 45 / 450 × 100 = 10 percent.
Note also that only 9,000 of 30,000 kJ of plant tissue was eaten at all, which is 30 percent; the rest went to decomposers or remained unconsumed.
Identify the largest proportional loss: the light-to-plant step, which retains 1 percent, far less than the roughly 10 percent retained at the consumer steps.
Reconcile with conservation: the missing energy is not destroyed; it was reflected, transmitted, or released as heat during respiration and dispersed into the surroundings, where it cannot drive cellular work. The atoms involved stayed in the system and were recycled.
The plants stored 1.0 percent of the light arriving, while the two consumer steps each retained about 10 percent of what was eaten, so the largest proportional loss by far is at the point where light is captured. A further loss is hidden in the figures: only 30 percent of the plant tissue produced was eaten, with the remainder passing to decomposers. None of this breaks conservation of energy, because the energy that does not appear in the next organism was reflected, transmitted, or released as heat during respiration, and heat dispersed into the surroundings is no longer available for cellular work. Matter is a separate account and stays in the system, cycling through decomposers back to the plants. What the figures cannot show is the field's species composition, whether the transfer percentages are stable from year to year, or how much of the plant energy went to roots rather than leaves, so they support order-of-magnitude reasoning rather than precise prediction.
Scientific references and adapted source figures
Parent / teacher teaching notes
Keep the two accountings visibly separate — energy flows one way and degrades, matter cycles — because almost every later misconception in this chapter and the ecology chapter traces back to merging them. The phrase "plants make energy" is worth correcting every time it appears. The percentage arithmetic is quick and is what makes the shortness of food chains an explanation rather than a fact to accept. Ask for an explanation of each label, arrow and axis before calculation. Keep first and revised models; attempt the changed case before consulting guidance. Distinguish invented instructional records from named historical sources. Source chromosome symbols are allele-category models, not DNA sequences. These independently authored high-school supplements are not the full OpenSciEd teacher editions or an endorsement.
The key idea
ATP hydrolysis and synthesis link energy-transfer processes with cell work. The full reaction determines the energy balance.
ATP hydrolysis (matter accounting)
ATP + H₂O → ADP + inorganic phosphate
The complete reaction can release free energy under cellular conditions. This equation alone omits coupling and the concentration-dependent energy change.
Worked examples
Example 1
- ATP has three phosphate groups; ADP retains two and inorganic phosphate is released.
- Water participates in hydrolysis.
- The complete reaction can release free energy that supports coupled work.
Example 2 · Worked application
- ATP turnover links energy-releasing and energy-requiring processes.
- Hydrolysis is coupled to pump action through molecular mechanisms.
- ATP regeneration requires energy; ATP is a cycling carrier, not a free energy source.
Account for both hydrolysis and regeneration.
Example 3
- Cell work continually converts ATP to ADP and phosphate.
- Available ATP is limited, so metabolic processes must replenish it.
Example 4 · Find and fix the mistake
- Test the claim against the stated conditions: Account for both hydrolysis and regeneration.
- Energy comes from metabolic or light inputs; ATP transfers it through coupled reactions.
Account for both hydrolysis and regeneration.
Example 5 · Reason from the visual evidence
- ATP hydrolysis can support an energetically linked change, such as a transport protein’s conformational cycle. The useful change depends on the overall reaction and coupling, not on energy being released by breaking one isolated bond. ADP and inorganic phosphate can be used to rebuild ATP through other energy-transferring processes. ATP therefore serves repeated short-term transfers rather than a perpetual source. Count a stated ATP demand separately from the food input or the rate of ATP production; molecule count and energy amount are different quantities.
- The pathway, reaction conditions, coupling and other energy transfers matter; ATP count is only one defined output.
Original ATP-work coupling abstraction, not a fixed free-energy-per-molecule measurement.
Avoid these mistakes
- ATP is an energy-transfer intermediate, not a large long-term fuel reserve.
Check your understanding
Why must an active cell keep regenerating ATP?
Discuss the answer
Continuous regeneration keeps production matched to ongoing demand.
Why does an explanation of ATP need the overall reaction and coupling mechanism?
Discuss the answer
Overall hydrolysis depends on reactants, products and conditions; useful work requires a coupling mechanism. Breaking an isolated bond requires energy and does not explain the whole reaction’s favorable change.
Return to the phenomenon
Return to the initial case. Why do cells need new usable energy if energy is conserved? Revise your drawing or explanation using two named ideas and one stated limitation.
Compare explanations after your attempt
Conservation includes dispersed energy; ongoing work depends on suitable input and coupling rather than an indefinitely self-powered ATP cycle.
What should we investigate next?
Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.
Investigate, explain and revise
Complete these tasks after the teaching parts. Preserve your first model and a supported revision; use guidance after a real attempt.
Activity 1 · Return to the opening case
Use the opening question, defined terms, Parts 1–4 and your notebook. The supplied-data route needs no equipment or external account.
Your task
Return to the initial case. Why do cells need new usable energy if energy is conserved? Revise your drawing or explanation using two named ideas and one stated limitation.
Success criteria
- Use relevant stated observations.
- Explain the mechanism with a labeled model.
- Distinguish quantities and assumptions.
- Describe a revision and a useful next test.
Hint
Follow objects, materials and information separately. Check what each measured quantity means.
Response guidance — discuss after your attempt
Conservation includes dispersed energy; ongoing work depends on suitable input and coupling rather than an indefinitely self-powered ATP cycle.
Activity 2 · Transfer to a changed case
Cover the worked guidance and use the unit explanations/figures to solve a changed problem.
Your task
A defined process receives 240 energy units and delivers 72 to the selected useful output. Find efficiency and explain where the remainder belongs in an energy account.
Success criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Hint
Name the stated conditions, select the relevant mechanism and show intermediate reasoning.
Response guidance — discuss after your attempt
Efficiency is 72/240×100 = 30%; 168 units goes to other transfers/storage under the boundary. It is not destroyed, and identifying particular outputs requires a complete measured account.
Words to know
- ATP
- Adenosine triphosphate, used in coupled energy-transfer processes.
- ADP
- Adenosine diphosphate; it can be converted to ATP using an energy source and phosphate.
- autotroph
- An organism that builds organic carbon from inorganic sources using light or chemical energy.
- heterotroph
- An organism that obtains organic carbon from other sources.
- energy coupling
- Linking reactions or processes so a favorable change can support work.
Evaluate a model, not only a slogan
ATP hydrolysis and synthesis link energy-transfer processes with cell work. The full reaction determines the energy balance. Describe one observation that would test a prediction, and one simplification that could change the result.
Check your mechanism
State the location, reactants, products, and evidence supporting a process. Explain what your model leaves out before using it to predict a result.
In the real world
Sports scientists pay close attention to ATP: your muscles store only a few seconds' worth at a time, which is why an all-out sprint fades so fast and why cells must rebuild ATP nonstop during exercise.
Exit check
Explain how ATP links an energy-releasing process with cell work.
Check your explanation after answering
Energy-releasing processes can support ATP production. ATP hydrolysis coupled to another process can make that combined process favorable; the full reaction, not bond breaking alone, matters.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Problem 1
- the organelle where respiration happens
- the cell's main energy currency
- the molecule that stores genetic information
- a green pigment that absorbs light
Answer and explanation
the cell's main energy currency
ATP is the small, spendable energy-carrying molecule cells use to power almost every activity — its nickname is the cell's energy currency.
Problem 2
- ATP contains DNA while ADP does not
- They are identical molecules with different names
- ATP has three phosphate groups and ADP has two
- ATP has two phosphate groups and ADP has three
Answer and explanation
ATP has three phosphate groups and ADP has two
ATP has three phosphate groups; ADP has two. Hydrolysis forms ADP and inorganic phosphate.
Problem 3
Answer and explanation
36.5 kcal
5 × 7.3 = 36.5 kcal under the stated conditions; actual cellular values depend on conditions.
Problem 4
Answer and explanation
When ATP releases energy it becomes ADP plus a phosphate, but it isn't discarded — the cell adds energy from food or sunlight to reattach the phosphate and turn ADP back into ATP. This ATP–ADP cycle repeats over and over, like recharging a battery.
Problem 5
- A gray wolf
- A mushroom
- A human
- An oak tree
Answer and explanation
An oak tree
An oak tree makes its own food from sunlight, carbon dioxide, and water, so it is an autotroph (producer). The wolf, mushroom, and human are heterotrophs that must eat other organisms.
Problem 6
Answer and explanation
Continuous regeneration keeps production matched to ongoing demand.
Cell work continually converts ATP to ADP and phosphate.
Available ATP is limited, so metabolic processes must replenish it.
Response criteria
- Use the stated conditions.
- Show the evidence or calculation.
- Explain the conclusion and any stated limitation.
Problem 7
Hints
Account for both hydrolysis and regeneration.
Answer and explanation
Energy-coupled transport cannot continue normally without sufficient chemical energy supply.
Problem 8
Hints
State what the supplied observations or conditions establish, and separate that from any assumption.
Answer and explanation
No. ATP is repeatedly regenerated and used, so turnover and standing amount differ.
Problem 9
Answer and explanation
The pathway, reaction conditions, coupling and other energy transfers matter; ATP count is only one defined output.
ATP hydrolysis can support an energetically linked change, such as a transport protein’s conformational cycle. The useful change depends on the overall reaction and coupling, not on energy being released by breaking one isolated bond. ADP and inorganic phosphate can be used to rebuild ATP through other energy-transferring processes. ATP therefore serves repeated short-term transfers rather than a perpetual source. Count a stated ATP demand separately from the food input or the rate of ATP production; molecule count and energy amount are different quantities.
The pathway, reaction conditions, coupling and other energy transfers matter; ATP count is only one defined output.
Response criteria
- Read the structures, axes, directions and units correctly.
- Connect the evidence with the biological mechanism.
- Respect stated model assumptions and distinguish an observation from a further inference.
Problem 10
Answer and explanation
Efficiency is 72/240×100 = 30%; 168 units goes to other transfers/storage under the boundary. It is not destroyed, and identifying particular outputs requires a complete measured account.
Efficiency is 72/240×100 = 30%; 168 units goes to other transfers/storage under the boundary. It is not destroyed, and identifying particular outputs requires a complete measured account.
Response criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.