Biochemical Reactions & Energy — Drills · Grade 10 Science

Chapter: Investigating the Chemistry of Life

What these drills covers

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

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

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

Problem 1

Why does ATP hydrolysis support cell work without implying that bond breaking releases energy?
Answer and explanation

The full reaction is favorable overall; breaking a bond alone requires energy. Coupling links that overall change to another process.

Problem 2

A reaction that releases more energy than it consumes overall is
  1. endergonic
  2. exergonic
  3. catalysed
  4. reversible only
Answer and explanation

exergonic

Exergonic reactions release free energy overall; endergonic reactions require a net input.

Problem 3

Breaking a chemical bond, considered by itself,
  1. always releases energy
  2. always requires energy
  3. never changes energy
  4. releases energy only in cells
Answer and explanation

always requires energy

Breaking a bond requires energy. A reaction releases energy overall only when the bonds formed in the products are more stable than those broken in the reactants.

Problem 4

A reaction breaks bonds requiring 620 kJ and forms bonds releasing 800 kJ. What is the net energy change in kJ, as a magnitude of energy released?
Answer and explanation

180 kJ

800−620 = 180 kJ released in the supplied bond-energy account. Free-energy classification additionally depends on entropy and the stated conditions.

Problem 5

A cell hydrolyses 3.0 × 10⁴ ATP molecules for one transport task and 1.2 × 10⁵ for another. How many times more ATP does the second task use?
Answer and explanation

4

1.2 × 10⁵ / 3.0 × 10⁴ = 4 times as much.

Problem 6

Explain the error in the statement "energy is released when the bond between ATP's phosphates is broken."
Answer and explanation

Breaking that bond by itself requires energy. ATP hydrolysis releases energy overall because the products — ADP and inorganic phosphate, each stabilized by surrounding water — hold less free energy than ATP did, so the bonds formed and the interactions with water more than repay the cost of breaking the bond. The energy comes from the whole reaction, not from the bond breaking.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 7

Define energy coupling and give one cellular example.
Answer and explanation

Energy coupling joins an energy-releasing reaction to an energy-requiring one so that the pair proceeds together. In active transport, ATP hydrolysis is coupled to moving ions against their concentration gradient: the exergonic hydrolysis supplies the free energy the endergonic transport needs, and an enzyme or pump protein holds the two reactions together.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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