Energy — practice questions

1. Universal Energy Currency (2 marks): ATP is readily formed and hydrolysed in all known forms of life, linking energy-releasing (exergonic) reactions, such as respiration, to energy-requiring (endergonic) reactions, such as active transport, muscle contraction, or synthesis of macromolecules. It provides a small, manageable packet of energy suitable for most cellular tasks, making it a universal medium of energy exchange.

2. Structural Contributions (3 marks):

• High-energy phosphate bonds: The bonds between the three phosphate groups are relatively unstable and contain a significant amount of chemical potential energy. Hydrolysis of the terminal phosphate bond releases a substantial, yet controlled, amount of energy (~30.5 kJ mol⁻¹) that can power cellular processes efficiently without excessive heat loss.
• Small and Water-Soluble: ATP is a relatively small molecule and is soluble in water. This allows it to be easily transported throughout the aqueous environment of the cytoplasm to wherever energy is needed within the cell.

Step 1: Calculate the total energy captured in ATP. The process generates 32 moles of ATP. The energy released from one mole of ATP is 30.5 kJ. Total energy captured = (Number of moles of ATP) × (Energy per mole of ATP) Total energy captured = 32 mol × 30.5 kJ mol⁻¹ Total energy captured = 976 kJ

Step 2: Calculate the overall efficiency. Efficiency is the ratio of useful energy output to the total energy input, expressed as a percentage. Efficiency = (Total energy captured in ATP / Total energy in one mole of glucose) × 100% Efficiency = (976 kJ / 2870 kJ) × 100% Efficiency ≈ 0.340069... × 100% Efficiency ≈ 34.0%

Answer: The efficiency of energy transfer from glucose to ATP during aerobic respiration is approximately 34.0%. The remaining ~66% of the energy is lost as heat, which contributes to maintaining body temperature in endotherms.