Respirationlabbackground

Jeannie Kopstein November 2014
AP Biology Respiration lab write up Mrs. Beck

Background:
Aerobic cellular respiration is a pivotal process in which organisms carry out in order to sustain life. It is characterized by the release of energy from organic compounds by means of chemical oxidation within the mitochondria of the cell. The reactants are glucose and oxygen, and after a series of complex steps, the products of carbon dioxide, water, and ATP + heat are released. Thus, cellular respiration is an exergonic process, since heat energy is released in order to do cellular work. The overall process can be encapsulated by the following equation: C6H12O6 + CO2 6CO2+ 6H2O+ 586 kilocalories of energy/mole of glucose oxidized. This reaction seems very straightforward, however there are numerous enzyme-mediated reactions that occur within it that are not so perceptible from the simplified equation. Cellular respiration consists of three major stages: The first is Glycolysis; (occurring in the cytosol) in which chemical energy is harvested by oxidizing glucose into two 3 carbon molecules of pyruvate, and thus producing a net of 2 ATP molecules through substrate-level phosphorylation, as well as a net of 2 NADH molecules. Subsequently, the Krebs Cycle commences after 2 pyruvate molecules are converted to 2 Acetyl CoA molecules in the intermembrane space of the mitochondria. During the Krebs Cycle (occurring in the mitochondrial matrix)4 CO2 molecules are released, 1 ATP molecule is formed (for each turn of the cycle), and the reduced forms of 6 NADH and 2 FADH carry the electrons to the next step: the Electron Transport Chain. This occurs in the inner membrane of the mitochondria, and consists of many electron carriers that pass electrons (donated by NADH and FADH2) along through a series of redox reactions. At the end of the chain, oxygen acts as a final electron acceptor and it reduced them to form water. A proton motive force, or H+ gradient, is formed between the matrix and the intermembrane space and then, diffusing through ATP synthase, the H+ ions travel back into the matrix, causing the phosphorylation of ADP, and ultimately ATP is produced, through oxidative phosphorylation.
Pea seeds, soybeans, and mung beans all carry out the process of cellular respiration to produce energy for the endergonic process of photosynthesis, since they are all autotrophs. It is possible to determine the rate of cellular respiration within these organisms by measuring their oxygen consumption, with the proper setup. Because carbon dioxide is being produced, it would make it very difficult to measure the amount of oxygen consumed due to the fact that the amount of oxygen consumed would normally be replaced by the carbon dioxide produced. Hence, one would assume there to be no variation in the volume of gas from this reaction. However, the carbon dioxide created during cellular respiration is removed in this experiment through the use of potassium hydroxide (KOH), soaked in cotton balls in the respirometers, which chemically combines with the carbon dioxide to form crystals (solid potassium hydroxide K2CO3) which have a very small volume compared to gaseous carbon dioxide. This occurs through the reaction: CO2 +2KOH  K2CO3 + H2O. Furthermore, the concentration of gas in the respirometer decreases as the seeds consume oxygen by cellular respiration. The change in gas concentration is directly related to the amount of oxygen consumed and therefore, the use of KOH allows for the measurement oxygen consumption. The principle of gas laws (PV=nRT) shows that as n (the number of molecules of gas) decreases, V (volume) will decrease, assuming that atmospheric pressure and temperature remain constant. Water will move into the pipette of the respirometer as the volume of gas in the respirometer decreases, showing that the movement of water into the pipette is a measure of the volume of oxygen consumed by the seeds.
Germinating and non-germinating seeds differ in their rate of cellular respiration. Non-germinating seeds are in a dormant state and have low metabolic rates. They are doing just enough to sustain themselves and do not need to create energy as they are not going through growth or development, therefore, they have a low rate of cellular respiration. Once seeds begin to germinate, metabolic rates increase due to cell growth and other cellular processes, such as mitosis and the growth of leaves and petals, which require a lot of energy. Thus, there will be a higher rate of cellular respiration to supply energy to carry out these processes.