Investigating the Effect of Temperature on Rate of Respiration in Yeast

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Zaka Ahmad

Investigating the Effect of Temperature on Rate of Respiration in Yeast

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Yeasts are unicellular organisms, which belong to the fungi group. {Autotrophs are organisms that can produce their own stored energy resources, they can capture sunlight and use it in the process of photosynthesis. Animals, fungi and most other organisms cannot produce their own food, they can only consume their energy sources, these organisms are given the name heterotrophs [taken from resource 3]}. Yeast can be used for baking, because the carbon dioxide released causes the food to expand or ‘rise’. Yeasts also has other uses such as making alcohol.

In the mitochondria of the yeast various biological pathways take place, including the Links reaction, Krebs cycle and the electron transport chain. The structure of the mitochondria is fairly simple, there are basically 4 main parts to it. There is the envelope, which consists of the outer and inner membrane. {The outer membrane is slightly permeable to small molecules, but the inner membrane is less permeable. The inner membrane contains proteins, which are necessary to carry out the electron transport chain and oxidative phosphorylation (the inner membrane is also the site for the electron transport chain and oxidative phosphorylation). The inner membrane also has small spheres attached to it, which are ATP synthase enzymes. There are the cristae, which is parts of the inner membrane that have been folded inwards. In active cells, the cristae are densely packed. The matrix is the site of the links reaction and the Krebs cycle, and also contains the relevant enzymes for these pathways. The matrix also contains ribosomes. The intermembrane space works in conjunction with the matrix to produce ATP, as the energy required to produce the ATP comes from the hydrogen ion gradient between the intermembrane space and the matrix [taken from biology 2 book]}.

{Under aerobic conditions, yeast mitochondria are involved in ATP synthesis coupled to oxidative phosphorylation. Under anaerobic conditions, mitochondria seem to be dispensable, at least for respiratory function. Mitochondria perform other functions in yeast cell physiology, implicating that mitochondria are relevant to intact cell metabolism even under anaerobic conditions, such as synthesis and desaturation of fatty acids and lipids, or stress responses and adaptations to stresses.

[http://biochemie.web.med.uni-muenchen.de/Yeast_Biology/02_Cellfunction.htm]}

{The ‘energy currency’ for cells is known as ATP (adenosine triphosphate). ATP contains adenine and ribose which make up adenosine. And the adenosine is combined with three phosphate groups. When a phosphate group is removed energy is released and ADP (adenosine diphosphate) is formed. Removing of a second phosphate releases the same amount of energy, and AMP (adenosine monophosphate) is formed. Removal of the final phosphate group releases about half the energy, and leaves only adenosine. These reactions are all reversible, but require the same energy to convert them back to the original state. [Biology 2 book]}

In respiration organic molecules are broken down in different steps which release chemical potential energy, which is used to synthesise 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. [Biology 2 book]}

The Krebs cycle is also known as the citric acid cycle, and it also occurs in the matrix of a mitochondrion. Oxygen is not required for this pathway, but it is part of aerobic respiration, so it must be carried out in aerobic conditions.

There are a few main products of the Krebs cycle. Acetyl coenzyme A, from the link reaction, combines with oxaloacetate to form citrate; the citrate is decarboxylated and dehydrogenated, carbon dioxide is a waste gas and is diffuses out of the cell and is excreted, there is production of 2 carbon dioxide molecules per cycle. But the hydrogens are accepted by NAD and FAD, one FAD is reduced per cycle and three NAD molecules are reduced. Oxaloacetate is then formed and the cycle starts again. Reduction of NAD and FAD is a key factor for the final pathway, Oxidative phosphorylation.

The electron transport chain is a system of electron carriers held within the inner membrane of the mitochondrion. {Here, the FAD and NAD molecules lose their hydrogens and give them to the hydrogen carriers, the hydrogens are split into its constituent H+ 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]}

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Enzymes are also present in respiration. They are in the form of dehydrogenase enzymes, decarboxylase enzymes and ATP synthetase enzymes.

ATP synthetase enzymes are used in the synthesis of ATP from ADP. ATP synthetase is a protein that spans the phospholipids bi-layer. ATP synthetase has 3 binding sites. The 3 binding sites allow the ATP to pass through 3 different stages, binding ADP and a phosphate group, forming a tightly bound ATP molecule, and releasing ATP. As ...

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