Structures and Functions in Living Organisms · 7 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 18% of your exam marks.
Aerobic and anaerobic respiration equations and comparisons are consistently tested.
uses oxygen. It takes place inside the mitochondria of the cell and represents the complete breakdown of glucose. Aerobic respiration is by far the more efficient form: a single molecule of glucose can be used to make roughly 30 molecules of ATP through the aerobic pathway, compared with only 2 ATP through anaerobic respiration.
glucose + oxygen → carbon dioxide + water (+ energy released)
C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O
In words: one molecule of glucose reacts with six molecules of oxygen to give six molecules of carbon dioxide and six molecules of water, plus a large amount of energy stored in ATP.
Notice that this equation is the exact reverse of photosynthesis. Photosynthesis builds glucose using light energy; respiration breaks glucose back down to release that energy.
Exam note: If a question asks for the symbol equation, you must give the symbol equation. Writing the word equation when the symbol equation is asked for will lose the mark.
Give the balanced symbol equation for aerobic respiration
What comes up: a 2-mark question asks you to write the balanced chemical symbol equation for aerobic respiration.
Write (two marks): (1) correct reactants and products: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O; (2) both sides correctly balanced. An unbalanced but otherwise correct equation scores only 1 mark.
Watch out: the mark scheme gives no credit for the word equation when the symbol equation is asked for — even a perfect word equation scores zero on a "give the symbol equation" question.