Biochemical Reactions & Energy — Print · Grade 10 Science
Chapter: Investigating the Chemistry of LifeBiochemical Reactions & Energy
Learning goals
- I can distinguish synthesis reactions from decomposition reactions.
- I can explain how ATP stores and releases energy for the cell.
- I can describe how chemical reactions transfer energy inside living things.
Readings for this unit
Before this unit
Distinguish reaction rate from net energy change.
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 can cells build material even when synthesis requires energy?
Illustrative evidence case. A supplied synthesis and hydrolysis have a favorable combined change only when connected through a mechanism.
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 can cells build material even when synthesis requires energy?
Your first model
Draw matter inputs/outputs, a signed energy balance and the coupling step.
Build the language from the idea
- chemical reaction
- A process that breaks and forms chemical bonds, changing one set of substances into another.
- reactant
- A starting substance that goes into a chemical reaction.
- product
- A substance formed by a chemical reaction.
- synthesis reaction
- A reaction that combines smaller molecules into a larger one — a building-up reaction.
- decomposition reaction
- A reaction that breaks a larger molecule into smaller ones — a breaking-down reaction.
- ATP
- Adenosine triphosphate — the molecule cells use to store and deliver energy for their activities.
- metabolism
- All of the chemical reactions that take place in a living thing.
Metabolism includes reactions that rearrange atoms by breaking some bonds and forming others. Synthesis builds larger molecules; decomposition splits them. These descriptions of molecular size do not alone tell us whether a reaction releases or requires energy. Bond breaking requires energy; bond formation releases energy. The overall balance and reaction conditions matter.
Cells couple favorable reactions to processes that would otherwise require an energy input. ATP hydrolysis is favorable overall under typical cellular conditions and can be coupled to transport, movement, or synthesis. Its free-energy change depends on the whole reaction and conditions, not simply a bond snapping.
ATP + water can form ADP and inorganic phosphate. Energy-releasing metabolic processes support rebuilding ATP from ADP and phosphate. The ATP cycle transfers energy; it does not create energy or provide long-term bulk storage. Enzymes speed reactions by lowering activation barriers, not by making an unfavorable reaction favorable on their own.
A biochemical reaction can release usable energy overall even though an initial energy barrier must be crossed. Enzymes affect the barrier, while the relative energy of reactants and products affects whether energy is released or required. Keep these two ideas separate.
Cells couple energy-requiring processes to energy-releasing processes. ATP hydrolysis can be linked through a reaction mechanism to molecular building or transport. It is the overall coupled process, not simply breaking one bond, that can provide a favorable energy change.
Matter remains accounted for when cells transform food. Carbon, oxygen and hydrogen can leave as products such as carbon dioxide and water or become new cell material. A system diagram should include energy transfer to work and thermal motion as well as matter transfers.
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 · Account for the whole reaction, not one bond
A reaction changes a set of reactants into products through rearranged interactions. Breaking a bond requires energy; forming a bond releases energy. The supplied bond-energy accounting gives 620−800 = −180 kJ, a net release in that simplified model. Cellular free energy also includes entropy and conditions, so a bond-energy sum alone is not a complete biochemical free-energy calculation. Synthesis versus decomposition names molecular transformations, not an automatic energy sign. Track the whole reaction and define what the quantity represents.
Read the visual model
Subtract released formation energy from required bond-breaking energy, retaining the sign convention.
Does a release of 180 kJ mean any one broken bond released that energy?
Visual reasoning — after your attempt
No. It is a net outcome of the specified overall bond-energy accounting, not energy from breaking an isolated bond.
Energy bookkeeping in a reaction compares everything on the left with everything on the right. Breaking bonds in the reactants requires energy; forming bonds in the products releases it. The reaction releases energy overall — it is exergonic — when the products are more stable than the reactants, and requires energy overall — endergonic — when they are less so. The sign of the total, not the behaviour of any single bond, tells you which.
This is why the common phrase "energy stored in a bond" is misleading. A bond is a stabilizing interaction, so breaking one always costs energy. Molecules described as energy-rich are those whose rearrangement into more stable products releases a large net amount, and the conditions matter too: concentration, temperature, and the surrounding water all affect the balance.
Pause and explain
Why is "breaking bonds releases energy" wrong even for molecules described as high-energy?
Check your reasoning after answering
Breaking a bond always requires energy input. A high-energy molecule releases energy because the new bonds formed in its products, together with their interactions with water, are more stable than the ones broken; the release is a property of the whole reaction.
Part 2 · ATP as the cell's working currency
ATP, adenosine triphosphate, contains three phosphate groups; ADP contains two. Hydrolysis uses water and can yield ADP plus inorganic phosphate. Under typical cellular conditions, the whole reaction can have a favorable free-energy change. Rebuilding ATP requires energy from other processes and appropriate biochemical steps. Calling ATP an energy currency is an analogy for repeated transfer and use, not literal money or unlimited long-term storage. Hydrolysis must be chemically linked to useful work; released free energy is not automatically captured simply because another process is nearby.
Read the visual model
Follow the directions around the cycle and identify which step requires replenishing input.
Could repeated ATP rebuilding and hydrolysis run indefinitely with no other energy transfer?
Visual reasoning — after your attempt
No. ATP rebuilding requires input and useful energy disperses through cellular processes; the cycle does not create energy.
ATP is adenosine joined to three phosphate groups. Those phosphates carry negative charges held close together, and the molecule's hydrolysis to ADP and inorganic phosphate yields products that are more stable and better stabilized by surrounding water. The reaction is therefore exergonic, releasing roughly 30 kJ per mole under cellular conditions, which is a convenient size: large enough to drive many cellular tasks, small enough not to waste energy on small ones.
ATP is a transfer molecule, not a store. A cell holds only seconds' worth and regenerates it continuously from ADP using energy from the breakdown of fuels. Thinking of ATP as a rechargeable carrier rather than a battery explains why cells both consume and resynthesize enormous quantities every minute without accumulating any.
Pause and explain
If a cell contains only a few seconds' worth of ATP, how does it meet a sustained demand?
Check your reasoning after answering
It continuously regenerates ATP from ADP and phosphate using energy released by respiration or other fuel breakdown, so supply matches demand through turnover rather than through a large stored amount.
Part 3 · Coupling makes unfavourable reactions happen
An endergonic process requires a positive free-energy change under the stated conditions; an exergonic process has a negative change. Coupling links processes through a mechanism so their combined outcome can proceed. Here a +12 synthesis step and −20 hydrolysis contribution sum to −8 model units. This accounting does not say that two unconnected reactions in neighboring beakers drive each other. Enzymes or molecular machines must link the relevant changes, and actual concentrations and pathways determine feasibility and rate.
Read the visual model
Keep signed changes separate from bars that show their positive magnitudes.
If synthesis needs +25 and the coupled hydrolysis provides −20, is the supplied net favorable?
Visual reasoning — after your attempt
The total is +5 model units, so this stated pair alone is not favorable; additional coupling or changed conditions would be needed.
A cell routinely needs reactions that require energy: building polymers, concentrating ions against a gradient, moving cargo. Energy coupling links such a reaction to an exergonic one so that the pair taken together releases energy. The coupling is physical — one enzyme or pump protein holds both reactions, often transferring a phosphate group from ATP onto the other participant, which changes its shape or reactivity.
Coupling also sets a requirement that can be checked arithmetically. If a synthesis needs 46 kJ/mol and each ATP supplies about 30 kJ/mol, one ATP is insufficient and two are needed, with the excess released as heat. Reasoning like this distinguishes a mechanism a cell could actually use from one that merely sounds plausible.
Pause and explain
Why is it not enough to say that ATP "gives energy" to a reaction that needs it?
Check your reasoning after answering
The two reactions must be physically linked, usually by an enzyme that transfers a phosphate group, and the energy released must at least cover what the other reaction requires. Without coupling, ATP hydrolysis would simply release heat and the second reaction would not proceed.
Part 4 · Pathways, enzymes, and control
A metabolic pathway consists of connected transformations; an upstream change can alter available substrate for later steps. Feedback inhibition occurs when a later product reduces an earlier activity, helping adjust flow to demand. A rate graph describes accumulated product per time, not the net free-energy sign of the entire pathway. Different output can arise from enzyme capacity, substrate supply or regulation. To explain a bottleneck, measure intermediate materials and relevant conditions rather than assume the slowest visible product curve identifies one enzyme uniquely.
Read the visual model
Compare slopes in the same time interval and distinguish production rate from stored amount.
Does the faster product curve show that pathway A has a different equilibrium?
Visual reasoning — after your attempt
Not by itself. It shows faster accumulation under the supplied conditions; equilibrium and causes need appropriate endpoint and mechanistic evidence.
Metabolism proceeds in sequences of small steps, each catalysed by its own enzyme. Smaller steps mean lower activation barriers and allow energy to be captured in usable portions instead of being lost as one large release of heat. Synthesis pathways build molecules and generally require an energy input; breakdown pathways release energy, much of it captured in ATP.
A multi-step pathway is also controllable. Because each step has its own enzyme, the cell can slow the whole sequence by inhibiting one of them, and feedback inhibition does exactly that: the pathway's end product binds an early enzyme and reduces its activity, so output tracks demand. Describing the steps is not enough; the control point is what explains the cell's behaviour.
Pause and explain
Why does inhibiting the first enzyme in a pathway control output more efficiently than inhibiting the last?
Check your reasoning after answering
Blocking the first step prevents intermediates from being made at all, so no material or energy is spent on them. Blocking the last step lets intermediates accumulate, wasting the resources already invested in them.
Apply the whole unit · Audit an energy claim
Illustrative figures for a transport task in one cell. Moving one ion against its gradient requires 42 kJ/mol. ATP hydrolysis supplies about 30 kJ/mol. A student writes: "The pump breaks ATP's phosphate bond, and the energy stored in that bond pushes the ion across. One ATP is enough because bond breaking releases a lot of energy."
Your task
Identify each error in the student's explanation, give the corrected account of where the energy comes from, and determine how many ATP molecules the task requires. State what happens to any energy beyond the requirement and name one observation that would support the corrected mechanism.
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
Correct the bond claim: breaking the bond requires energy; ATP hydrolysis is exergonic because ADP and inorganic phosphate, stabilized by water, hold less free energy than ATP.
Correct the mechanism: nothing is "pushed" by a released bond. The pump protein couples the two reactions, usually by accepting a transferred phosphate group, which changes the pump's shape and carries the ion across.
Compute the requirement: 42 kJ/mol needed against about 30 kJ/mol per ATP, so one is insufficient and two are needed, supplying about 60 kJ/mol.
Account for the surplus: roughly 18 kJ/mol is released as heat rather than stored, so coupling is not fully efficient.
Supporting observation: transport should stop when ATP is depleted or when the pump's phosphorylation is blocked, while the ion gradient itself remains, which separates the energy supply from the gradient.
Three claims need correction. Breaking a bond requires energy rather than releasing it, so the energy does not come from the bond; it comes from the whole hydrolysis reaction, whose products are more stable than ATP. Nothing is pushed across by a release of energy either — the pump protein couples hydrolysis to transport, typically by transferring a phosphate group onto the pump and changing its shape. Arithmetically, one ATP at about 30 kJ/mol cannot meet a 42 kJ/mol requirement, so two are needed and about 18 kJ/mol is lost as heat. The mechanism predicts that transport halts when ATP runs out or phosphorylation is blocked even though the gradient persists, which is the observation that would support it.
Scientific references and adapted source figures
Parent / teacher teaching notes
The phrase "energy stored in a bond" appears in many popular explanations and is the error this unit exists to correct, so expect to revisit it. Require the full accounting — bonds broken against bonds formed — before accepting any claim that a reaction releases energy. The ATP arithmetic in the application is the best check on whether the idea has landed: a student who can only recite that ATP releases energy will not notice that one molecule is insufficient. 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
Reaction energy depends on all reactants, products, and conditions. ATP hydrolysis can be coupled to cell work; breaking a bond alone requires energy.
Worked examples
Example 1
- Account for bonds broken and formed and the relative free energies of all reactants and products.
- Hydrolysis products are more favorable under typical cell conditions.
- Coupling connects that favorable overall change to cell work.
Example 2 · Worked application
- Synthesis may be unfavorable on its own.
- A chemically linked favorable process can make the combined pathway energetically feasible.
- Enzymes organize the coupled steps; merely occurring nearby is insufficient.
Energy is transferred through a mechanism, not created by the enzyme.
Example 3
- Bond breaking requires an input of energy.
- The full reaction forms new interactions and products; its overall free-energy change can be favorable and coupled to work.
Example 4 · Find and fix the mistake
- Test the claim against the stated conditions: Energy is transferred through a mechanism, not created by the enzyme.
- Bond breaking requires input. Net ATP hydrolysis is favorable because the full reactant-to-product process has lower free energy under cellular conditions.
Energy is transferred through a mechanism, not created by the enzyme.
Example 5 · Reason from the visual evidence
- An endergonic process requires a positive free-energy change under the stated conditions; an exergonic process has a negative change. Coupling links processes through a mechanism so their combined outcome can proceed. Here a +12 synthesis step and −20 hydrolysis contribution sum to −8 model units. This accounting does not say that two unconnected reactions in neighboring beakers drive each other. Enzymes or molecular machines must link the relevant changes, and actual concentrations and pathways determine feasibility and rate.
- The total is +5 model units, so this stated pair alone is not favorable; additional coupling or changed conditions would be needed.
Original signed free-energy teaching cases with invented units, not a physiological ATP constant.
Avoid these mistakes
- Breaking a bond alone requires energy; ATP hydrolysis releases energy through the full reaction.
Check your understanding
Explain why saying ATP provides energy because its bond is broken is incomplete.
Discuss the answer
Energy availability comes from the whole reaction and coupling mechanism, not bond breaking by itself.
How can a reaction release energy overall yet still need an activation-energy input?
Discuss the answer
A favorable overall endpoint balance can coexist with an activation barrier. Catalysis lowers the barrier without itself changing the reaction’s net free-energy balance.
Return to the phenomenon
Return to the initial case. Why can cells build material even when synthesis requires energy? Revise your drawing or explanation using two named ideas and one stated limitation.
Compare explanations after your attempt
The whole-reaction balance and coupling explain useful work; breaking one bond or adding an enzyme alone is not an energy source.
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 can cells build material even when synthesis requires energy? 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
The whole-reaction balance and coupling explain useful work; breaking one bond or adding an enzyme alone is not an energy source.
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 coupled synthesis needs +18 free-energy units and its linked hydrolysis contributes −26. Calculate the net, then explain why a catalyst alone cannot replace the coupling.
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
The net is −8 units under the supplied conditions. A catalyst can lower the activation barrier but does not make the isolated +18 process favorable by changing its endpoint free-energy balance.
Words to know
- chemical reaction
- A process that breaks and forms chemical bonds, changing one set of substances into another.
- reactant
- A starting substance that goes into a chemical reaction.
- product
- A substance formed by a chemical reaction.
- synthesis reaction
- A reaction that combines smaller molecules into a larger one — a building-up reaction.
- decomposition reaction
- A reaction that breaks a larger molecule into smaller ones — a breaking-down reaction.
- ATP
- Adenosine triphosphate — the molecule cells use to store and deliver energy for their activities.
- metabolism
- All of the chemical reactions that take place in a living thing.
Evaluate a model, not only a slogan
Reaction energy depends on all reactants, products, and conditions. ATP hydrolysis can be coupled to cell work; breaking a bond alone requires energy. Describe one observation that would test a prediction, and one simplification that could change the result.
In the real world
This is why you have to eat: food is the fuel your cells break down to recharge ATP, and you rebuild roughly your own body weight in ATP every single day. It's also why exercise leaves you breathing hard — your muscles are burning through ATP faster than usual and need extra oxygen to recharge it.
Exit check
Correct the statement “breaking a chemical bond releases energy by itself.”
Check your explanation after answering
Bond breaking requires energy. Net reaction energy depends on all bonds broken and formed and the relative free energies of reactants and products; ATP hydrolysis can release free energy overall.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.
Problem 1
- freezing
- melting
- decomposition reaction
- synthesis reaction
Answer and explanation
synthesis reaction
Synthesis joins smaller molecules; its energy balance depends on the full reaction and may require coupling.
Problem 2
- ATP
- Water
- DNA
- Glucose
Answer and explanation
ATP
ATP is the cell's energy currency. Glucose is an energy source, but its energy is banked into ATP before the cell spends it.
Problem 3
- DNA
- ADP + a phosphate group
- glucose
- a protein
Answer and explanation
ADP + a phosphate group
ATP hydrolysis forms ADP and phosphate; the overall reaction can release free energy under cellular conditions.
Problem 4
Answer and explanation
1200 ATP
3,600 ions ÷ 3 ions per ATP = 1,200 ATP.
Problem 5
Answer and explanation
Energy-releasing metabolism supports ATP production. ATP hydrolysis coupled to synthesis or transport can make the combined process favorable. No energy is created, and the full reactions, not bond breaking alone, determine the balance.
Problem 6
Answer and explanation
Energy availability comes from the whole reaction and coupling mechanism, not bond breaking by itself.
Bond breaking requires an input of energy.
The full reaction forms new interactions and products; its overall free-energy change can be favorable and coupled to work.
Response criteria
- Use the stated conditions.
- Show the evidence or calculation.
- Explain the conclusion and any stated limitation.
Problem 7
Hints
Energy is transferred through a mechanism, not created by the enzyme.
Answer and explanation
Energy disperses to surroundings; cells need usable energy to maintain organization and drive processes.
Problem 8
Hints
State what the supplied observations or conditions establish, and separate that from any assumption.
Answer and explanation
No. Catalysis speeds approach to equilibrium without changing the equilibrium determined by the reaction and conditions.
Problem 9
Answer and explanation
The total is +5 model units, so this stated pair alone is not favorable; additional coupling or changed conditions would be needed.
An endergonic process requires a positive free-energy change under the stated conditions; an exergonic process has a negative change. Coupling links processes through a mechanism so their combined outcome can proceed. Here a +12 synthesis step and −20 hydrolysis contribution sum to −8 model units. This accounting does not say that two unconnected reactions in neighboring beakers drive each other. Enzymes or molecular machines must link the relevant changes, and actual concentrations and pathways determine feasibility and rate.
The total is +5 model units, so this stated pair alone is not favorable; additional coupling or changed conditions would be needed.
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
The net is −8 units under the supplied conditions. A catalyst can lower the activation barrier but does not make the isolated +18 process favorable by changing its endpoint free-energy balance.
The net is −8 units under the supplied conditions. A catalyst can lower the activation barrier but does not make the isolated +18 process favorable by changing its endpoint free-energy balance.
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.