Biochemical Reactions & Energy — Practice · Grade 10 Science

Chapter: Investigating the Chemistry of Life

What this practice covers

By the end of Biochemical Reactions & Energy you should be able to:

10 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: chemical reaction, reactant, product, synthesis reaction, decomposition reaction, ATP, metabolism.

Problem 1

A reaction that combines small molecules into a larger one is a
  1. freezing
  2. melting
  3. decomposition reaction
  4. 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

What molecule do cells use as their main energy 'currency'?
  1. ATP
  2. Water
  3. DNA
  4. 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

When ATP releases energy for the cell to use, it becomes
  1. DNA
  2. ADP + a phosphate group
  3. glucose
  4. 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

An active-transport pump uses 1 ATP to move 3 sodium ions out of a cell. How many ATP molecules are needed to pump 3,600 sodium ions?
Answer and explanation

1200 ATP

3,600 ions ÷ 3 ions per ATP = 1,200 ATP.

Problem 5

Explain how a decomposition reaction and a synthesis reaction can be linked together by ATP inside a cell.
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

Explain why saying ATP provides energy because its bond is broken is incomplete. Explain the reasoning in your own words.
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

Why do living cells need ongoing inputs even though energy is conserved? Show the reasoning, not just the conclusion.
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

A pathway occurs faster with an enzyme. Does this prove its equilibrium shifted?
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

A favorable total is useful only when the processes are connectedA favorable total is useful only when the processes are connectedSupplied magnitudes of free-energy changeSynthesis alone: needs12Hydrolysis: can supply20Coupled net: +12 + (−20) = −8 model units.A reaction mechanism must connect the processes.
Original signed free-energy teaching cases with invented units, not a physiological ATP constant.
Interpret the supplied visual. If synthesis needs +25 and the coupled hydrolysis provides −20, is the supplied net favorable?
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

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.
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.

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