Investigating the Effect of Temperature on Rate of Respiration in Yeast

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Investigating the Effect of Temperature on Rate of Respiration in Yeast

 

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   ATP. Glucose is the main energy source, and can be broken down in four different stages: Glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. Yeast is an organism that actively respires aerobically. But it can respire anaerobically, when deprived of oxygen. But anaerobic respiration is not as efficient as aerobic respiration, because oxidative phosphorylation requires a constant supply of oxygen for it to occur; also the link reaction and the Krebs cycle need oxygen. Therefore the link reaction, Krebs cycle and oxidative phosphorylation only work in aerobic conditions, whereas glycolysis can work in anaerobic conditions also. Glycolysis means ‘sugar-splitting’. In glycolysis, there is a net gain of 2 ATP molecules. You must start with a hexose sugar, such as glucose. The glucose molecule will undergo phosphorylation. Phosphorylation involves ATP molecules being broken down into ADP and their phosphates are attached to the glucose molecule. The first ATP causes the hexose sugar to be turned into hexose phosphate, and the second ATP molecule makes it hexose bisphosphate. Phosphorylation raises the energy of the glucose, and reduces the activation energy barrier for the pathway. After phosphorylation, the hexose bisphosphate is split into 2 other identical molecules of triose phosphate. Hydrogen is removed from the triose phosphate using dehydrogenase enzymes, and the hydrogen is taken by an NAD molecule. NAD is a hydrogen acceptor or a coenzyme molecule, and is reduced when it takes hydrogen. Also, the phosphate groups on the triose phosphate molecules are taken by ADP molecules to produce ATP. During these final stages of glycolysis 4 molecules of ATP are produced. Since only 2 molecules of ATP are used initially, and 4 are made in the end, there is a net production of 2 ATP molecules.

Pyruvate enters the link reaction next. But this pathway occurs aerobically. The pyruvate passes from the cytoplasm of the cell into the matrix of a mitochondrion by active transport. In the matrix, the pyruvate is decarboxylated using decarboxylase enzymes and it is dehydrogenated also, which produces more reduced NAD. After that it is combined with coenzyme A to produce acetyl co enzyme A.                             
    + ions and electron, the hydrogen ion remains in the matrix. The electron is transferred to a series of electron carriers, the electron is soon transferred to an oxygen molecule (which is present in the mitochondrial matrix), and hydrogen ions combine with the oxygen molecule to reduce the oxygen to water. The transfer of these electrons creates energy which is used to convert ADP into ATP. When an electron passes from a higher energy carrier, to a lower energy carrier, energy is released. On average, 2½ ATP molecules are made for each NAD molecule entering the chain, and 1½ molecules of ATP for every molecule of FAD used in the chain. [Biology 2 book]}  
                                    There are a few factors which affect the rate of reaction with enzymes. I will discuss these, because we now understand that the rate of respiration must also be dependant on the rate that the enzymes work. First we must consider the collision theory. The collision theory states that when two molecules react, they must have sufficient energy on impact to overcome the activation energy barrier and react. This energy should be enough to break the old bonds and form new bonds for the new molecule.

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Enzymes work up to their optimum temperature proportionally to the rate of reaction. Increase in temperature increases the kinetic energy of the particles which causes them to move faster and faster. If they begin to go too fast, atoms begin to vibrate too vigorously and the chemical begins to break down. When this happens the shape of the enzyme is altered and it is said to be denatured, and an enzyme cannot repair itself after it is denatured. When the temperature goes very low, the kinetic of the particles is very low too, and the particles mostly do not ...

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