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An investigation into how the activity of a population of yeast is affected with respect to temperature.

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Introduction

TITLE An investigation into how the activity of a population of yeast is affected with respect to temperature. INTRODUCTION Yeast cells also known as Saccharomyces Cerevisiae are unicellular organisms, which reproduce asexually requiring only one parent. The are seven major forms of asexual reproduction: binary fission, budding, fragmentation, sporulation, vegetative propagation, cloning and parthenogenesis. Yeast cells reproduce by budding. This is a very simple mechanism similar to binary fission, except that the two resultant cells are not of equal size but comprise a smaller bud cell, becoming detached from a larger parent cell. Yeast use oxygen to build membranes components that are essential to replication. These cell-wall building blocks (unsaturated fatty acids and sterols) can sustain yeast growth even when present at very low levels. In anaerobic respiration, when free oxygen is not present, hydrogen cannot be disposed of by combination with oxygen. The electron transfer chain therefore stops working and no further ATP is formed by oxidative phosphorylation. If a cell is to gain even the two ATP molecules for each glucose yielded by glycolysis, it is essential to pass on the hydogens from the reduced NAD that are also made in glycolysis. This pathway is involved in anaerobic respiration of yeast cells. The hydrogen from reduced NAD is passed to ethanal (CH3CHO). This releases the NAD and allows glycolysis to continue. Yeast + Carbohydrate ? Alcohol + Carbon Dioxide The growth of a culture of individual cells e.g. unicellular yeast is in effect the growth of a population. The typical bacterial growth curve for a batch culture consists of four phases1: 1. LAG PHASE This is a period of inoculation where the growth rate increases towards its maximum. ...read more.

Middle

Giving some yeast solutions more time than others would clearly allow more time for cell division and growth to occur. NUMBER AND RANGE OF MEASUREMENTS I intend on using five different temperatures of water bath ranging from 20oC to 60oC at 10oC intervals. A sample of yeast suspension shall be extracted every 2 hours for 24 hours. This shall allow a population/time graph to be plotted for each temperature. I.e. 5 graphs For each of the five environments the number of yeast cells counted in the populations shall be repeated three times. This shall provide three results at each temperature, which inevitably increases the reliability of the results obtained. PRELIMINARY WORK As I planned this investigation I had two possible methods to measure the population of yeast when in different environmental temperatures. 1) Using a microscope and a special slide known as a haemocytometer slide. It is designed so that a known volume of sample covers a ruled grid. A representative sample of cells can thus be counted and estimates can be made of the number in the total sample. A haemocytometer has an etched grid on it. It consists of a 1mm� square known as an A square which is divided into 25 B squares which have an area of 0.04mm�. These B squares are each divided into 16 C squares which have an area of 0.0025mm�. 2) Using a colorimeter. A colorimeter is a machine that is used to see how much light can pass through a liquid. It shows how much light is being transmitted through a sample of liquid. As the number of cells gets higher, less light will be transmitted through the sample. ...read more.

Conclusion

4) Put this conical flask into a water bath set to 20oC for 24 hours. It may be necessary to monitor the temperature of the water bath using a thermometer. This shall improve the reliability of the results, ensuring that the yeast cells are actually in the required environment for the investigation. 5) Every 2 hours remove a sample volume of 1cm3 of yeast suspension using a 1cm3 syringe. (12 samples of yeast suspension for each temperature.) 6) Turn on the microscope and prepare the haemocytometer: (a) Clean haemocytometer and cover slip using 70% ethanol. (b) Place coverslip squarely on top of haemocytometer, lightly moistening polished surface of the slide before pressing the coverslip into position. (c) Gently redistribute cells throughout medium and take up a small sample of cells into the syringe. (d) Load haemocytometer so that the fluid entirely covers the polished surface of each chamber. Take care not to overload the counting chambers. Should overloading take place, excess fluid may be removed carefully from the groove using filter paper. (e) Using the x10 objective of the microscope, locate the upper left primary squares of each grid (10 primary squares). It may be necessary to increase the magnification. This increases ability to distinguish between two different cells, whilst at a lower magnification these two cells would be viewed as just one cell. Use a counter to count the cells in 10 primary squares. Cells that are within or that touch, the left or top boundary are counted, while those that touch or are outside the lower or right hand boundary are not counted. [Refer to fig. 2 for assistance.] 7) Repeat steps 1-6 using four different temperatures of water bath: 30oC, 40oC, 50oC and 60oC. 8) Repeat each temperature three times to increase the reliability of the results obtained. ...read more.

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