The most important question is what sugar concentrations should be used, and the concentration chosen are 50%, 100%, 150% and 200%. The 100% solution is made of 3g of sugar, mixed with 100ml of water; 150% is made of 4.5g; 200% of 6g; and 50% of 1.5g respectively.
Prediction
Theoretically, the higher the concentration is, the faster fermentation should happen. However, if osmotic pressure module is to be taken into account, this theory may not work. If the water potential of the environment is low, the cells are going to lose water through osmosis. This would make fermentation less efficient. So there should be a concentration that the osmotic pressure is the same in the mixture, making fermentation most efficient. It seems likely that the 100% concentration would work best, following by 150%, 50%, and 200%.
Apparatus
The equipment used is:
* Durham tubes
* Test tubes
* Yeast-sugar mixture, with different concentration
* Pipette
Methods
* Transfer 2-3 cm3 of the culture into each test tube.
* Fill a small Durham tube with the same mixture.
* Invert the small tube and slide it into the test tube.
* Note the time, which the Durham tube is set up.
* Take the meniscus movement readings every 5 minutes.
Safety precautions
The equipment used in this experiment is quite safe. However, care is needed in handling glassware, as they are easily broken.
Evaluation
The results obtained are sufficient for the conclusion to be drawn about this topic. As they are plotted on to the graph, they are more or less on the same lines of accuracy - there is no anomalous result obtained. However, for the matters concerning the accuracy of the experiment's layout, there are several inaccuracies - which some can be improved; some cannot.
* Carbon dioxide gas is soluble in water - some is not included in the results
* Surface tension of meniscus pressure - carbon dioxide gas may have been compressed
* Growth of yeast culture - the no. of yeast cells are not the same
* Intoxication by ethanol - some yeast cells may be intoxicated and die
* As more CO2 is collected into the tube, there is less yeast inside it
* The end of the Durham tube is hemispherical - no linear relation of rate of reaction at the start
* Measurement of the length of the meniscus - a ruler was used, and it was not in contact with the Durham tube
* The tubes have to be lifted out of the water bath to take measurements - the environmental temperature could have affected the rate of fermentation every time measurements are taken
Some of these errors can be improved if the experiment was to be carried out again. The Durham tube could be substituted with similar equipment that has flat end, instead of being hemispherical, so the change in height of the meniscus would go in linear proportion. Perhaps a scaled glass rod could be used to instead of a ruler. With this, the readings would be taken adjacently to the tube, rather than outside the test tube itself
If a further investigation is to be carried out, the temperature of the environment and the types of the sugar used are the interesting fields to head forth towards.
Background knowledge
FERMENTATION is the breakdown of sugars by bacteria and yeast using a method of respiration without oxygen (anaerobic respiration). It involves a culture of yeast and a solution of sugar, producing ethanol and carbon dioxide with the aid of the enzymes. This is an 8-10 step process requiring different enzymes each time, but it can be simplified like this.
This process can be slowed down by DENATURATION of the enzymes at a certain temperature.
All the ENZYMES are protein chains of amino acids. They exist in the form of ?-helix structure with hydrogen bonds holding the pitches together. On the amino acid molecules, there is R a group. They react with each other to form peptide bonds, transforming the chain into a 3-dimensional structure. Along the chain there are active sites where interaction between the enzyme and the substrate happens. These sites are sensitive to heat, like the hydrogen bonds that hold the 3D molecule together. When heat is applied to the enzyme, energy is given into the molecule. The active sites deform and the hydrogen bonds break, denaturing this enzyme. It would not be able to function as usual, and this is not reversible. This is called DENATURATION. The 3D ?-helix structure would breakdown and the active sites would change in shape; they would not be able to accommodate the substrate any more. The analogy of this is to compare a key to a keyhole. If the keyhole has changed, the same key would not fit in any more, and the lock would not be unlocked. The same thing happens here, and fermentation could not continue after this has occurred. Also when the temperature is too low, the enzymes would not work because there is not enough energy for activities to happen. .
Concentration of the solutions should make the yield of fermentation higher as they become more concentrated. This would be because the sugar molecules are more abundant as the concentration increases, so the probability that the enzymes come into contact with the molecules is higher. However, more concentrated solutions mean less osmotic pressure.
OSMOSIS is the movement of solvent such as water through a semi-permeable membrane separating solutions of different concentrations. The solvent passes from the more dilute solution to the more concentrated solution until the two concentrations are equal. Many cell membranes behave as semi-permeable membrane, and osmosis is vital mechanism in the transport of fluids in living organisms - for example, in the transport of water from the roots up the stems of plants. The region that has less percentage of water is said to have less OSMOTIC PRESSURE, and vice versa. Water permeates the cell wall into the region with low water potential (osmotic pressure) until the osmotic pressure is the same on both sides.
Water is vital to living organisms, and having too high concentration of sugar risks osmosis problem, and vice versa, as a cell cannot work properly when there is too much water in it.
Analysis
First of all, it is apparent that fermentation is a process that occurs in a linear fashion: the lines in which the reactions occur on are straight. The beginning of the best fit lines are omitted because they are not in linear relation to each other, as the equipment used was not in linear proportion of surface are to volume. Conspicuously, the solution with 100% sugar concentration has worked best of out of the three. A comparison of these can be made as the gradients of these lines are calculated. They would represent the rate in which the reactions are taking place. To make the rates comparable, the gradients are taken from the same point of the graph - between 10 and 20 minutes. Thus it is as follow: from www.essaybank.co.uk
Conc. Rate
200 0.92
150 1.38
100 1.70
The rate is the movement of the meniscus in mm per minute. There seems to be a proportion that establishes a connection between them. If more data of other concentration are acquired, perhaps a formula could be derived from the results.
There are explanations for this. At 200% concentration, the osmotic pressure inside the cells of the yeast is higher in comparison to the sugar solution outside, so water is lost into the solution - through osmosis. The cells now contain less water potential. Although the sugar content in the solution is high, increasing the probability of interaction between the enzymes and the sugar molecules, the yeast cells are now less capable at carrying out reactions, because it has less water potential than before. They become less efficient at fermentation. After all it is the osmotic pressure that is important. It was a misfortune that there was not enough time for the 50% concentration fermentation to be done, but it is evident that 100% concentration would work best. This concentration is quite likely to have equal osmotic pressure, or water potential, in the mixture. Although 50% concentration solution would have much higher water potential in the cells, the abundance of sugar molecules would not be high enough to bear as good productivity as the 100% one.
The vital key point here is that the osmotic pressure has to reach equilibrium, which is closer to 100% concentration, of 3g of sugar per 100ml of water, than any others.
The results obtained are similar to the prediction. If the concentration theory is to be considered, it is a failure, because osmotic pressure actually takes major part in this experiment. Apart from that, all went well in accordance with the secondary prediction.
Results of fermentation with different concentration of sugar solution
200% concentration (6 g per 100 ml) Temp. - 30 C
Time in Distance extended by the meniscus in mm
Minutes 1st 2nd 3rd average
0 0.0 0.0 0.0 0.0
5 12.0 11.0 10.0 11.0
10 15.0 15.0 14.0 14.7
15 20.0 20.0 19.0 19.7
20 25.0 26.0 24.0 25.0
25 30.0 30.0 30.0 30.0
30 34.0 35.0 33.0 34.0
150% concentration (4.5 g per 100 ml) Temp. - 30C
Time in Distance extended by the meniscus in mm
Minutes 1st 2nd 3rd average
0 0.0 0.0 0.0 0.0
5 11.0 10.0 11.0 10.7
10 16.0 16.0 17.0 16.3
15 23.0 24.0 23.0 23.3
20 30.0 30.0 30.0 30.0
25 37.0 37.0 36.0 36.7
100% concentration (3 g per 100 ml) Temp. - 30C
Time in Distance extended by the meniscus in mm
Minutes 1st 2nd 3rd average
0 0.0 0.0 0.0 0.0
5 12.0 12.0 12.0 12.0
10 20.0 18.0 19.0 19.0
15 28.0 27.0 28.0 27.7
20 37.0 37.0 36.0 36.7
Rate of fermentation
Concentration (%) Rate in mm/minute
200 0.92
150 1.38
100 1.70