Introduction Cellular respiration (sometimes called aerobic respiration) occurs in all animal and plant organisms (with a few exceptions). This is the process where food substances such as glucose are metabolised (using oxygen) to produce carbon dioxide and energy. It basically consists of three parts ie glycolysis, Krebs' cycle and oxidative phosphorylation linked to electron transport and chemiosmosis. At the end of the process large amounts of energy in the form of ATP are produced, for cell metabolism and growth. Part 1. Experiments on cellular respiration. Experiment 1 In an investigation of aerobic respiration was carried out using isolated mitochondria. Were added to a prepared medium containing succinate and inorganic phosphate. Note: The mitochondria were isolated from liver cells by the following method: 1. The liver tissue was ground with sand in a pestle and mortar, containing a phosphate buffer at pH 7.0, and 0.4 % sucrose solution, at 4°C. 2. The homogenate was spun (centrifuged) for five minutes and the supernatant, containing the mitochondria was poured into another tube at 4°C, and the precipitate was discarded. 3. The supernatant was kept cold (by immersing the tube in a beaker containing ice) until required for use in the experiment. A sample of the isolated mitochondria was added to a prepared medium containing succinate, inorganic phosphate and 0.4 % sucrose solution at 37°C. Note: Succinate is a 4-carbon compound, which occurs in the Krebs cycle, and can be used as a respiratory substrate. The medium was then saturated with oxygen. Once oxygen saturation was reached, equal amounts of ADP were added to the medium at one-minute intervals, and measurements were taken of the oxygen concentration in the medium, using an oxygen meter, over a total six-minute time period. A graph of the results is presented on the next page. ADP added 250 Concentration 200- of oxygen in the medium / arbitrary units ADP added 150- 100 - ADP added 50 Time / minutes (a) Why were the mitochondria isolated at 4 °C? (b) Why was sucrose used in the isolation medium? (c) Why was phosphate included in the reaction medium? (d) Explain why the oxygen concentration in the medium decreased after adding ADP at time X. (e) Explain why the fll in oxygen concentration was approximately the same following the addition of ADP at time X and at time Y. (f) Explain why the fll in oxygen concentration, following the addition of ADP, was slightly less at time Z than at time Y.