Coupled Reactions and Why ATP Is the Cell's Currency · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

How ATP pays for uphill reactions

Science · Biochemistry & Molecular Biology · ages 19-21
Name ______________________   Date ____________

  1. What links the two reactions in a coupled pair?

    • A parcel of energy passed by hand
    • A shared chemical intermediate
    • Heat flowing from one reaction to the other

  2. Two reactions are coupled. How do you decide whether the pair runs forward?

    • Add the two free energy changes and check the total is negative
    • Run the downhill step first and hope the uphill one follows
    • Compare the speeds of the two reactions

  3. In glycolysis, attaching phosphate to glucose is uphill on its own.

    Circle one:   True   False

  4. Why can the pathway afford its costly opening step?

    • Later steps harvest back more ATP than was spent
    • The opening step is skipped when energy is low
    • The cell borrows phosphate that is never repaid

  5. Glucose plus phosphate is uphill, ATP splitting is downhill, and their total is negative. What happens?

    • Neither step runs
    • Only the ATP step runs
    • The paired steps run forward together

  6. A new fuel releases a giant packet of energy in one split. Why do cells still prefer ATP?

    • ATP is older and cells never change fuels
    • A giant packet cannot be metered to small jobs and few enzymes fit it
    • Giant packets always run uphill and never release energy

  7. A paired reaction has a positive total free energy change. A student adds more of the shared intermediate. What follows?

    • The total is still positive, so the pair still does not run
    • The pair now runs forward on its own
    • Adding intermediate always flips the sign of the total

  8. A friend says coupling means ATP hands a parcel of energy to the uphill reaction. What is wrong?

    • Nothing, that is exactly how coupling works
    • ATP never splits during coupling
    • Coupling shares a chemical intermediate instead of handing over energy
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Answer key

For grown-ups. Fold this page away before handing over the rest.

How ATP pays for uphill reactions W1-mt_1yVJRArCow-s1

  1. A shared chemical intermediate · The steps share an intermediate, often a phosphorylated compound.
  2. Add the two free energy changes and check the total is negative · Only the total decides. A negative total means the pair descends overall.
  3. True · It only runs because ATP splitting is paired with it.
  4. Later steps harvest back more ATP than was spent · The payoff phase repays the investment with interest.
  5. The paired steps run forward together · A negative total pulls the whole pair forward.
  6. A giant packet cannot be metered to small jobs and few enzymes fit it · The usable middle beats the biggest blast.
  7. The total is still positive, so the pair still does not run · Amounts shift the balance but the sign of the total still rules.
  8. Coupling shares a chemical intermediate instead of handing over energy · No parcel changes hands. The shared intermediate ties the steps together.
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