Theory question 101
Respiration(a) Describe briefly the process involved in the breakdown of glucose in the cell of living organisms to produce energy (A T P).
(b) State two conditions under which glycogen can be converted to glucose to produce energy.
(c) Describe an experiment to demonstrate that oxygen is released by green plants during photosynthesis.
Show the model answer
(a) Glycolysis in the cytoplasm: glucose (6C) is broken down by enzymes into two pyruvic acid (3C) molecules with a small gain of ATP; no oxygen needed
Without oxygen: pyruvic acid → ethanol + CO₂ (yeast/plants) or lactic acid (muscle) – little energy
With oxygen (in mitochondria): pyruvic acid → acetyl coenzyme A → Krebs cycle releases CO₂; hydrogen passes along the electron transport chain to oxygen forming water and much ATP (about 38 ATP per glucose)
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
(b) Any two: when blood glucose falls (fasting/starvation); during vigorous exercise; in stress/fright (adrenaline released); under the action of glucagon
(c) Place a water plant (e.g. Elodea) in a beaker of water containing sodium hydrogen carbonate; cover with an inverted funnel and a test tube full of water; set up a control kept in darkness; leave in sunlight
Gas bubbles collect in the tube in light; test with a glowing splint – it relights, showing oxygen; little or no gas in the dark control
In (c), state the test for oxygen (a glowing splint relights).