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 Respiration and Fermentation Lab on-line #10


Monitor respiration activity in yeast by observing CO2 production as sugar is metabolized Investigate the effects of temperature on yeast fermentation


Living cells use ATP (adenosine triphosphate) as their energy "currency". The energy released when a molecule of ATP is hydrolyzed (ATP  ADP + PO4) is used to drive cellular reactions. To stay alive, a cell must continually regenerate its supply of ATP (from ADP & inorganic phosphate). A working muscle cell recycles its ATP at a rate of ~10 million molecules per second! Cells harvest potential energy from metabolic fuels such as glucose by catabolizing them and using the energy to synthesize ATP. This process of harvesting chemical energy is called respiration.

Different organisms use different catabolic processes to harvest chemical energy. Most eukaryotic cells use cellular respiration, the most efficient catabolic pathway for energy harvest. Cell respiration is also called aerobic respiration, because oxygen is consumed as a reactant along with an organic fuel:

C6H12O6 + 6 O2  6 CO2 + 6 H2O + ENERGY.

Most of the reactions of cell respiration, during which a molecule of fuel is degraded completely, take place in the mitochondria. During aerobic respiration, a reduced fuel molecule is oxidized as high-energy electrons are stripped away from the fuel by the coenzymes NAD+ & FAD. The electrons are then donated to an electron transport chain in the mitochondria, where their energy is used to drive ATP synthesis. Aerobic respiration allows cells to harvest a large proportion of the potential energy contained in organic fuels (~40-50% of the potential energy in glucose).

Fermentation is a simpler process that results in a partial degradation of an organic fuel. Fermentation does not require the presence of any specialized organelles and does not require oxygen as a reactant, so it can take place under anaerobic conditions. Because the glucose is not broken down completely, fermentation harvests much less potential energy (~2-4% of the potential energy in glucose). Organisms such as humans use lactic acid fermentation as an energy source when oxygen is scarce. Other organisms, such as yeast and some bacteria, use fermentation as their sole means of harvesting energy. In yeast, a molecule of glucose is catabolized via glycolysis to produce 2 molecules of the 3-carbon intermediate pyruvate:

This is the same process as glycolysis in human cells. However, the pyruvate produced by the yeast is then converted to a molecule called acetaldehyde, releasing a molecule of CO2 in the process:
This acetaldehyde is then reduced to form ethyl alcohol. The reduction reaction is coupled to the oxidation of NADH, thereby regenerating the NAD+ required for glycolysis to continue:

For centuries, humans have taken advantage of the metabolic byproducts produced by yeast during fermentation. The CO2 generated when pyruvate is converted to acetaldehyde makes bread dough rise and carbonates beverages; and the ability of yeast to convert sugar to alcohol is the basis of brewing beer and wine.

Observing Respiration with Yeast

Obtain a packet of baker’s yeast (sold in grocery stores).

We’d like you to observe the activity of respiration (metabolism in the presence of oxygen) in the following simple manner:

Mix the yeast with about 1 cup of water and stir until the yeast dissolves. Make a sugar water solution by adding 2 tablespoons (doesn’t need to be exact) sugar to 1 cup of water, stir until dissolved.

Label small paper or plastic cups A through D and fill as follows:

Cup A2 T yeast with 2 T sugar water, place in freezer for 30 minutes

Cup B2 T yeast with 2T sugar water, place in refrigerator for...
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