Investigating the Effect of Glucose Concentration On the Rate of Reproduction of Yeast Cells

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INVESTIGATING THE EFFECT OF GLUCOSE CONCENTRATION ON THE RATE OF REPRODUCTION OF YEAST CELLS

Prediction

In this section I will look at all the factors involved in the rate of reproduction of yeast cells, bring them together at the end of the section and deduct a scientific conclusion as to what I would expect to happen.

Yeast Cell Respiration

Yeast cells, like any other organism, need a substrate to respire.

Respiration in general

In respiration, they use the substrate to produce ATP for cell metabolism. The overall equation for respiration using glucose is:

C6H12O6 + 6O2 Þ 6CO2 + 6H2O

Therefore, the more glucose available, the greater the rate of respiration and the easier it is for cell metabolism to take place. The more detailed version of the respiratory pathway is shown in Figure 1. However, one cannot merely assume that a greater rate of respiration can occur with a higher level of glucose concentration because it is a living organism involved. This means that high levels of glucose concentration could cause the yeast cells to become crenated and therefore the reproductive mechanisms would not function because of the physiological strain caused by the low pressure potential. This would mean that the initial rate of reproduction would be much less than that of cells in solutions of glucose concentrations on the limit of the cell’s pressure potential. However, this need not be a worry because I will not use glucose of such a high concentration. Also, because it is an organism, other factors like waste products and method of uptake need to be taken into account. These are discussed below.

Glucose catabolism in yeast

In the case of yeast, something else also needs to be considered. This is that it is an anaerobe. This means that yeast cells first respire the glucose anaerobically and then break down the ethanol produced to manufacture ATP aerobically using the available oxygen in the solution. In "http://esg-www.mit.edu:8001/cgi-bin/biosearch.pl?yeast", anaerobic respiration is called "http://esg-www.mit.edu:8001/cgi-bin/biosearch.pl?fermentation". These organisms are called facultative aerobes, because when oxygen is present, they respire aerobically, but if oxygen is absent, their respiration is purely anaerobic.

In anaerobic respiration, as in aerobic respiration, the first step consists of breaking down glucose into pyruvic acid (see Box 1). However, the two processes differ considerably in the details of this step. In anaerobic respiration, there is no Krebs cycle. The end products of anaerobic respiration

are usually ethyl alcohol and carbon dioxide, as in yeast cells, or "file:///C:/WINDOWS/TEMP/Contents.asp%3Fz=2%26search=anaerobic+respiration%26pg=2%26br=0%26ti=014DA000", as in muscle cells. Another difference between the two methods of respiration is the amount of energy released. In aerobic respiration, 32 ATP molecules of energy are released per glucose molecule (see Figure 1). In anaerobic respiration, only two ATP molecules of energy are released for each glucose molecule.

As the oxygen supply starts to fall short of the demand by the increasing population, it is less easy for the yeast cells to oxidise the ethanol and this collects as a toxic by-product of anaerobic respiration. If left for long enough, this would in the end caused a rapid decline in the population because as the oxygen in the solution is used up, the level of ethanol will increase with the population, and, with time, will reach a level of toxicity where it actually kills off the population.

Transport of Glucose into Cells

In order to understand how glucose is transported into the yeast cell, it is necessary to study glucose and the barrier that it has to cross, the outer cell membrane, as well as the implications of this.

Glucose

The structure of glucose is shown on the right. The many hydroxyl (OH) groups present result it being very polar. It is therefore only soluble in polar substances such as water. It will not dissolve in non-polar hydrophobic environments.

The Fluid Mosaic Model of the phospholipid bilayer

Biological "http://esg-www.mit.edu:8001/cgi-bin/biosearch.pl?membranes" are bilipid layers. Lipid bilayers are fluid, and individual phospholipids diffuse rapidly throughout the two-dimensional surface of the membrane. This is known as the fluid mosaic model of biological membranes (fluid because the molecules move around, mosaic because it includes "http://esg-www.mit.edu:8001/cgi-bin/biosearch.pl?proteins", cholesterol, and other types of molecules besides phospholipids). In a real cell, the membrane "http://esg-www.mit.edu:8001/cgi-bin/biosearch.pl?phospholipids"create a spherical three-dimensional lipid bilayer shell around the cell. However, they are often represented two-dimensionally as:

Each represents a phospholipid. The circle, or head, is the glycerol and negatively charged phosphate group and the two tails are the two highly hydrophobic hydrocarbon chains of the phospholipid. The tails of the phospholipids orient towards each other creating a hydrophobic environment within the membrane. This leaves the charged phosphate groups facing out into the hydrophilic environment. The membrane is approximately 5 nm thick. This bilipid layer is semi permeable, meaning that some molecules are allowed to pass freely (diffuse) through the membrane. The lipid bilayer is virtually impermeable to large molecules, relatively impermeable to small polar molecules and charged ions, and quite permeable to lipid soluble low molecular weight molecules. Its substantial permeability to water molecules is not well understood. The lipid bilayer is impermeable to medium and large-sized polar molecules. These molecules depend on hydrophilic intrinsic protein channels to assist their transport across the barrier by a process called facilitated diffusion.

Facilitated Diffusion

Molecules that can diffuse through the membrane do so at differing rates depending upon the number of channels available for them to pass through the hydrophobic interior of the membrane bilayer.

Facilitated diffusion utilises membrane protein channels in the membrane to allow charged molecules (which otherwise could not diffuse across the cell membrane because it is non-polar) to freely diffuse in and out of the cell. These channels are the way in which medium-sized polar molecules like glucose may pass into the cell. The number of protein channels available limits the rate of facilitated transport, whereas the speed of diffusion is dependent only on the concentration gradient. This means that there is a limit to the rate of reproduction of yeast cells, even in ideal circumstances, determined by the rate of facilitated diffusion (see Figure 2). The mechanism is not yet fully understood.

The Reproduction of Yeast Cell

The way in which yeast cells reproduce also has an effect on their rate of reproduction.

Figure 3 shows the two most commonly studied yeast species, namely budding yeast Saccharomyces cerevisiae (see A and B) and fission yeast Schizosaccharomyces pombe (see C, D, and E).

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Budding yeasts (the ones that I will be using) have an oval shape. They multiply by forming buds, which grow in size, and, finally, pinch off from the mother cell. The digestive organelles, called vacuoles, can be identified inside the cells (see F).

Fission yeasts are the rod-shaped ones. They form a septum in the middle of the cell (see C and D) and divide evenly.

Figure 4 a) shows the basic stages of cell replication. The budding cell is the same size as the original for simplicity. The actual shape is shown in Figure 4 b). The chromosomes ...

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