Osmosis is defined as the movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Osmosis is considered in terms of water potential and solute potential.
Water potential is a measure of the kinetic energy of water molecules. Here, water molecules are constantly moving in a random fashion. Some of them collides with cell membrane, cell wall, creating a pressure on it known as water potential. The higher their kinetic energy the more they move and hits the membrane, therefore higher the water potential. Water potential also depends on the number of molecules of water present. Solute restricts the movement of water, so a strong sugar solution with lots of solute particles will lower the kinetic energy and hence water potential.
In osmosis water moves down a water potential gradient until the equilibrium is reached between two regions. The amount of solute molecule that lowers the water potential in a solution is called solute potential. The highest water potential is found in pure water, where no solutes restricts the movement of water. The water potential of pure water is given a value of 0 kilopascals. Adding solutes lowers the water potential to a negative value. The more solute particles added, the lower the water potential becomes which means that the more negative value.
Pure water = 0 Kpa (highest water potential)
Dilute sugar solution = -100Kpa
Concentrated sugar solution = -350Kpa (lower water potential)
Therefore, the nearer the value to 0 Kpa, the higher the water potential.
Potato tubers are made of plant cells. Plant cells always have a strong cell wall surrounding them. They also contain partially permeable cell membrane. They both have different way of being permeable to the molecules of water and other substances. Membranes are composed of phospholipid bilayer with proteins scattered amongst them. In watery solution the phospholipid molecules naturally forms a bilayer, two layers of phospholipid with phosphates on the outside and fatty acids pointing inwards. Membranes allow selective things to pass through the bilayer but prevent other things passing through. Cell membrane allows small molecules such as oxygen, carbon dioxide to pass through easily. Water enters the membrane by osmosis avoiding the hydrophobic centre. Plants have strong cell wall made up of cellulose. When they take up water by osmosis they start to swell, but this cell wall prevents them from bursting as the living part of cell inside the cell wall starts to push against the cell wall. This inelastic cellulose support makes it possible for the plant cell to be turgid whereas in animal cell if water enters the cell by osmosis, the cell burst as volume increases and pressure builds up against cell membrane, which is unable to cope with the pressure.
Plant cell wall is made up of cellulose and has an arrangement of fibres around it. It has a very high tensile strength which makes the cell to withstand the large pressures that develop within it as a result of osmosis. It makes the cell wall very rigid and the arrangement of the fibres around the cell helps it to determine the shape of the cell as it grows. Cellulose fibres are freely permeable, allowing water and solutes to reach the plasma membrane. Therefore cell wall is permeable to water. Cell surface membrane is partially permeable as it is made of phospholipid bilayer.
The outside of the layer is attracted to water and the centre of the membrane is lipid allowing specific substances to pass through. This partially permeable membrane allows the free passage of some particles but is not freely permeable to others. Biological membrane are freely permeable to water. Sucrose and starch molecules are large therefore they are not freely permeable to membranes. They are transported across membranes by special process called facilitated diffusion.
Solutions with a lower solute concentration are described as hypotonic solution and solution with higher solute concentration is described as hypertonic solution. If the solution surrounding a cell has the same solute concentration as the cell then water will pass equally between them. When a solution has the same solute concentration as the cell then it is described as isotonic.
Starch is insoluble in water because it is formed when many monosaccharide condense together to form long chains. As it is long chain polymers and complex carbohydrates, the bonds which hold them together are hard to be broken down by water. Therefore starch is insoluble in water. Sucrose is soluble in water because it is a disaccharide sugar. These sugars are formed when two hexose sugar molecules react together by condensation reaction. When water is added in this sugar, it is broken down into hexose sugar (small unit), as bonds holding the disaccharide sugar are spitted up by water.
I did an experiment using sweet potato and sucrose solution. I measured the change in length and the change in mass of the sweet potato. The results are shown below:
The table below shows the volume of water and sucrose solution used to make dilutions
The concentration of sweet potato obtained for the preliminary work is 0.8M. Referring to the graph, the solute potential of the sweet potato cells which I have founded is -2580.
To obtain this value I am relying on the results of mass change against concentration, as the reading obtained seems to show some general trend in my preliminary work. I have also done experiment on the effect on length of potato when the concentration of sucrose is varied. Both mass change and length change gave me a graph but mass change graph was more reliable than length change. The length of the potato cylinder changes because water enters the cell by osmosis down the concentration gradient and therefore volume of cell is increased. As the concentration of solution is increased, the percentage change in length decreases as solute potential in the solution is increased.
To obtain value for solute potential of the sweet potato I have relied on the results of mass change against the concentration as the readings obtained forms a pattern on the graph. The graph produced for length change is a bit odd. This is because it is hard for a cell to push against the cell wall on all sides and increase the length due to the fact that the cell wall is made up of strong cellulose molecule which has high tensile strength, therefore it is hard for a water to push against this support. The effect of this push caused by osmosis is little than the actual push without cellulose in cell wall. Therefore the results obtained for this will not be accurate enough. Hence, I have decided to rely on the mass change of the potato cell. It is better to use change in mass because we get reliable readings about the water being entered and increased its mass.
Sweet potato has high concentration of solute molecules. Sweet potato cell has sucrose which is soluble in water whereas normal potato has starch which is insoluble in water.
When the potato is left in pure water it will gain mass. I base this prediction on the knowledge that I have about osmosis. Osmosis is term given for the movement of water when it moves from higher water potential to lower water potential. In pure water, there is no solute molecule therefore the concentration of water is higher then the potato which have high concentration of solutes than that of water. Therefore there is net movement of water from the region where they are in high concentration (that is in pure water) to region were there is low water potential present (in potato) by osmosis. Hence water enters the potato cell. Therefore there will be increase in the mass of the potato as water enters down the water potential gradient. In pure water the potato tuber swells because water enters their cells by osmosis.
When the potato is left in a high concentration of sucrose, it will lose its mass. This is because there is large number of solutes molecules present in the solution. Here, the water potential is lower than that of potato cell which has high water potential and low solute concentration. Therefore water will leave the potato cell to reach equilibrium with the sucrose solution. Hence, as water leaves the mass of the potato lowers. In rich sugar solution the potato tubers shrink because water leaves the potato by osmosis.
At a certain sucrose concentration, there will be no change in mass. This is because in the cell of potato and the solution surrounding will have equal concentration of solute and water molecule. Hence there would not be any net movement as the water potentials are equal to each other.
-Use a syringe to measure out different amounts of sucrose solution and water, (total volume as 20ml). Then pour it into the test tubes in a percentage ratio which will give various molar concentration.
-With a maker pen, write a molar concentration on each test tube as to not confuse yourself whenever handling them.
-Take one averaged sized potato and check that it is healthy and hard so that tubers obtained are in good condition to use.
-Cut out 6 potato tubers using a borer then measure out the length as 60mm and cut them with razor blade carefully as it is sharp which might lead to injury, and then weigh them to exactly 0.01g accuracy on the electronic balance.
– I am using a borer to cut the potato because I want a cylinder which has same width and circumference to ensure that I can use potatoes of same shape and then be able to compare them.
-Record the initial mass of the potato cylinder
-Place one in each test tube and leave it for 30 minutes but no more than hours because if left longer there is chance of potato becoming soft to handle.
-While waiting, Set out paper towels to dry potato tuber.
-After 30 minutes, come back to the experiment and then drain the solution in sink and place each cylinders on the paper towel according to the test tube concentration as to not confuse yourself wondering the concentration of cylinder and then recording wrong mass for the potato tuber.
-I then dried the cylinders on the paper towel so that no excess water is left on them, as this might have given increased mass if solution was left on the cylinder.
-Each potato cylinder was accurately measured to nearest gram (0.00g) and masses were recorded.
-Graph was plotted of percentage mass change of potato against concentration of sucrose solution.
Factors to be controlled
*Temperature must be at room temperature. This will make sure that the osmosis is not affected by different temperature.High temperature can increase the rate of osmosis and low temperature can decrease the rate of osmosis.
*Using same balance to weigh chip throughout the experiment, as measurement may be slightly different between scales.
*Same type of potato must be used for tubers as different potatoes have different concentrations.
*The size of potato is very important. This is because if the size of the potato varies, so will the amount of mass it can loose or gain .So I am going to try to cut all pieces of potatoes to the same size. Because of the use of Borer to cut potato, I will have cylinders with same width and dimensions but the length will differ. Therefore I have decided to cut it to 6cm in length so that each cylinder has same length and can be compared with each other to see the effect of osmosis.
*The volume of sucrose solution in which potato tubers are immersed must be same. This is because to enable the experiment to be fair and have same effect of volume on the potato.
The mass of potato is variable and will be measured throughout the experiment. The potato chip will be measured before it is put in solution and after. This will show us the difference in mass and whether or not osmosis has taken place and how much osmosis has occurred. The mass be recorded in grams. The sucrose concentration which I am using is 1.0M and to make dilutions, I am going to mix water to this concentration.
I have decided to take 2 sets of results from my classmates, as one set of result may not be accurate because of human error.
The only factor I am varying is the concentration of sucrose solution
Although the experiment is fairly safe, we have to be aware of some points. While using razor blades, we have to take extreme care and precision as it is very sharp and could easily cause serious injury. Therefore care should be taken. We have to make sure the space and surface where the experiment is going to be carried out should be dry and tidy as watery surrounding may affect the results .Any breakage should be reported to teachers straight away as we are handling with glass equipment which are dangerous to handle if broken.
I think that the experiment went very well, there were no big odd results and they produce a good graph. While I was working on the experiment, all necessary precaution were taken into consideration. I think the procedure used is appropriate enough to make overall conclusion of the investigation. But few problems were faced while doing the experiment.
Referring to the graph, I can see that as the concentration of sucrose solution is increased from 0.0M to 0.6 M the percentage change in the mass of the potato is decreasing. Here I mean that the potato is gaining mass. The trend is that it is a negative correlation and there is a high jump at every two points e.g between 0.2M $0.4M. I think I have obtained a bit odd value between the concentration of 0.4M and 0.6M. Between these points there is no high jump with compared to the trend. This could be due to rate of osmosis at these concentration but to notice it is a short jump with compared to long ones.
Some odd results are seen when I am comparing my results with compared to average value.the odd results are underlined in the average table above. The average value of mass change obtained for 0M concentration is 4.99% but the value obtained from me is far more greater than this i.e. 7.58%.I think this difference is due to the following reasons:
As the averages were found from two sets of result from classmates, I cannot be sure about their reliability. Maybe they have done some inaccuracies in the experiment which would have led us to different values. Or maybe I was inaccurate in my measurements. I think the following are reasons which would have affected the results obtained by me.
In the experiment, the concentration of the solution was prepared by the group of students. Therefore I cannot be so reliable on them for making dilutions. Maybe this might have caused some inaccurate results. To be more reliable, I would do dilutions myself if the experiment is to do again as this is a limitation in the procedure as all students have done the experiment in same way.
I cannot be sure about the reliability of the mass of potato when they were dried on the filter paper. Here maybe I have dried them more or maybe I have dried them less. If I have dried them more then there will be slight decrease in mass than the actual mass. If I have dried them less then there will be molecules of solution on them increasing the mass of potato. This was limiting strategy in the experiment as all student have faced same problem as I have. To improve I will be more careful and patient to dry the potato tuber and if possible I will probably find a nice paper which takes up water on anything precisely without drying the substance.
I think I was not precise enough to record the mass of potato tuber. This is because while weighing the potato taken from solutions , I remember that the value on the electronic balance was not stable. Here the mass of potato was changing e.g sometime the mass will weigh 0.67g and other as 0.68g.I think this was limiting strategy in the experiment. To get appropriate value, I recorded the value, which appeared several time. I think this was a source error in the experiment, as the difference of 0.01g can affect the preciseness of the result. This is limiting strategy in the procedure. To minimize this source of error, I will use a balance which has plastic lid on it. This will ensure that there is no interaction of air molecules of my breathing as well as molecules disturbing the stability of the value on the balance.
To ensure fair test, disc of potato used in experiment should be from same potato, which I was not able to conduct for my extension work. This is source of error because the potato, which I have used in my experiment, might have different rate to absorb solution with compared to the potato used, these results were used to extend and compare my results with. This might have cause slight difference between the concentration obtained from both potatoes.
I have done my experiment but there are several things, which I could have done to improve it and make my readings more accurate .I could have used pipette to measure the volume of sucrose and water instead of syringe, to get accurate reading .I could have obtained results, at more exact level by looking at the potato cylinders under the microscope, which would have given me idea about how potato cylinders reacts to different concentration and also be able to see the cells in more detail and draw some observation conclusion. Here, I would have got idea about the cell being turgid, or plasmolysed at different concentration. I found very hard to insert borer into potato, as the potato was hard and dry. If I would do this experiment then possibly I would find machine that ensures same mass and length of the potato.
I think the results obtained from me are reliable. This is because the value of my results were close enough with other two sets of results obtained by my classmates. E.g
Considering the sources of error involved in the procedure, I think the results obtained from the experiment was appropriate enough and valid conclusions can be drawn from it.
The result obtained showed that the potato cell increase mass in solutions with a high water concentration and decrease in mass in solutions with low water concentration.
By referring to the graph, I have found out the average concentration of the potato cells is 0.65M. As this is the point where the potato is not increasing or decreasing in mass. I have found out the solute potential by referring to the graph plotted from the data in which relationship between molarity and solute potential of sucrose soluton were given .The concentration obtained in my results is compared on this graph and then the line is drawn on the concentration obtained suggesting the value for solute potential. Therefore the solute potential for the obtained concentration of 0.65M for the potato cell is -1890kpa . I think my prediction on value of concentration of sucrose is close enough to the value obtained on the graph. Therefore I can reasonably be confident about the results obtained. This prediction was based on the preliminary work obtained result. As sweet potato contains more solute than the normal potato, so I thought that the concentration of normal potato cell will be less than that of sweet potato cell.
The molar concentration of sweet potato cell was 0.8M and the solute potential -2580kPa.Therefore I predicted that 0M, 0.2M, 0.4M will not be suitable value for the molar concentration of potato as these contains very few solute molecules. Therefore I predicted that the value will be between 0.5M-0.7M. Finally I predicted one value as 0.6M which seems very close enough to the molar concentration obtained on the graph. Above this value the mass of the potato decreases more. At 0.8M and at 1.0M the solution contains more solute particles and low water potential. The concentration of water in potato is high than that in these concentration. Therefore water leaves the potato cell to reach equilibrium point with solution of sucrose. As water leaves the cell, the mass of the potato decreases more than the previous concentration. At 0.8M the mass decrease is -2.94% with compared to 1.0M which has high decrease of -7.91%.As in high concentration (at 1.0),few water molecules are present than in the molar concentration of 0.8M.
Courtney from Study Moose
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