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

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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.  Growth is initially slow as bacteria adapt to produce the necessary enzymes needed to utilise the nutrient medium.  The rate of cell division gradually increases during this phase.

  1. EXPONENTIAL PHASE

At this phase, nutrients are in good supply and few waste products are produced, thus the rate of cell division is at its maximum.  The yeast culture is in a state of balanced growth.

  1. STATIONARY PHASE

Overtime the nutrients begin to be used up allowing toxic waste products to accumulate; thus the rate of growth slows.  This changes the composition of the medium resulting in the production of cells of various sizes with different chemical make-up.  There is a state of unbalanced growth.  The rate at which new cells are produced is equal to the rate at which dead cells are broken down.

  1. DEATH PHASE

The total number of cells remains constant but the number of living cells diminishes.  Cells die from lack of nutrients necessary to produce cellular energy.  The yeast cells become poisoned from the release of their own toxic waste.

There are many factors that affect the growth of yeast cells:

∙ Availability of nutrients

∙ Temperature

-Governed by enzymes, which operate at a narrow range of temperatures.

-If temperature decreases, the rate of enzyme-catalysed reactions becomes too slow to sustain growth.

-If temperature increases; denaturation of enzymes causes death.

∙ pH

-Microorganisms can tolerate a wider range of pH than plants and animals.

∙ Availability of oxygen

∙ Osmotic factors

∙ Pressure

∙ Light

- Photosynthetic microorganisms.

- Adequate supply of light to sustain growth.

HYPOTHESIS

This is an experiment to examine how the size of a population of yeast differs in varying conditions of temperature ranging from 20oC to 60oC.

PREDICTION

I predict that as the temperature of the environment increases that the population of yeast cells will also increase.  I believe that this increase shall be up to a certain temperature until the rate of budding slowly declines.  At low temperatures, the cell division takes place only very slowly.  The division of yeast cells is catalysed by enzymes, which operate at a narrow range of temperatures.  The temperature at which an enzyme catalyses a reaction at the maximum rate is called the optimum temperature2.  Most organisms' enzymes have an optimum temperature of approximately 40oC.  It is around this temperature that I believe the most number of yeast cells shall be counted.  This is because if temperature decreases below the optimum then the rate of enzyme-catalysed reactions becomes too slow to sustain growth.  In contrast, if temperature increases above the optimum this can lead to denaturation of enzymes, which inevitably causes death.  

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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 ...

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