Class: BE210
Group: T1
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Date: April 2004

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Abstract: 

Saccharomyces ceravisiae, the organism used in our experiment, is a species of budding yeast.  Throughout history, yeast has proven to be very useful to mankind due to its essential role in the production of beer, bread, and wine.  Recently, it has proved useful for the production of enzymes, pigments, antioxidants, and for various other medical applications.

Yeast are single-celled eukaryotic organisms that can undergo respiration either anaerobically or aerobically.  Thus, yeast is a facultative anaerobe.  Saccharomyces ceravisiae is perhaps the most widely used strain of yeast.  It is one of the key strains used in bakery and brewing, and is now being used widely in many different labs as a classic example of an eukaryotic organism in molecular and cell biology.  For example, Saccharomyces ceravisiae had the honor of being the first eukaryote to have its genome sequenced in 1996.  It contains about 13,000,000 base pairs comprising over 6,000 genes.  Of those, 23% are shared with humans.

Yeast can respirate anaerobically producing ethanol, or aerobically producing carbon dioxide.  Aerobically, the yeast can use less sugar in a smaller time frame to generate the same amount of ATP; so this experiment will only be concerned with the aerobic respiration of yeast.  Respiration occurs in 3 steps: Glycolysis, the Kreb’s Cycle, and the Electron Transport Chain.  As the yeast grows, it eventually starts to reproduce by simple cell division, or budding.  A yeast culture will undergo four typical stages of growth if no outside interference during growth is encountered in the lab.  The first phase, called the lag phase, is the stage during which the cells are preparing for division by producing any enzymes or cellular machinery required.  This phase is characterized by the 0 slope on the graph.  The next stage encountered is the log phase, during which the yeast are growing and dividing constantly without any hindrance or impedance in their doubling rate.  A plot of the logarithm of absorbance vs. time of this stage would be linear.  The slope of this line is termed the exponential growth rate constant.  A higher constant correlates obviously to an increased rate of growth, or shorter doubling time.  This phase will undergo the most analysis when determining the relative growth of yeast.  The next stage to follow is the stationary phase.  Eventually, the competition, lack of nutrients, and abundance of waste products slow down the efficiency of reproduction until it ceases.  After which, the death phase will follow as the yeast can no longer reproduce and die off themselves. 

There are many different factors that can influence the growth of yeast such as temperature, atmospheric pressure, osmotic pressure, availability of nutrients and O₂, and the pH of the growth medium.  As seen in previous experiments, the pH of the extra cellular medium decreases with time as yeast grows.  This can be the byproduct of a few different mechanisms.  First, during respiration, H⁺ ions and CO₂ gas are produced.  The carbon dioxide can then react with water to form even more H⁺ ions.  Also, a proposed mechanism for the uptake of glucose into the yeast cells is active transport, with the driving force being the diffusion of H⁺ down its concentration gradient out of the cell.  However, the yeast must still maintain an internal homeostasis in order to grow and reproduce.  The enzymatic activity of the yeast is crucial to its survival for it regulates everything from cell growth and respiration to cell division, and a pH must be maintained at which the enzymes are intact and working at a relatively fast rate.  To accomplish this, proton pumps are constantly pumping H⁺ ions out of the cell using ATP in the process.   If the pH of the medium ever becomes too low or high, the cell will not be able to maintain its optimal internal pH levels, and the enzymatic activity may become hindered if not stopped all together.  This will lead to cell death.