Introduction
Diffusion is the movement of particles from a region of high concentration to a region of low concentration. Osmosis, on the other hand, is a special form of diffusion. It involves the movement of solvent particles, through a selectively permeable membrane, from where they are at a higher concentration to where their concentration is lower. Water is the most common solvent in osmosis due to its ability to dissolve a large number of substances (Bienert & Chaumont, 2014).
Biological membranes are impermeable to large, polar substances such as glucose and water. Transport across these membranes is facilitated by the active transport of ions and substances like glucose. An example of this is Na-Glucose pumps. The passive flow of water across the membrane is because of aquaporins. A concentration gradient is required for the movement of water by osmosis. Solutions involved in osmosis are defined as hypertonic, where solute concentration is high, or hypotonic where the solute concentration is lower relative to another solution. Water flows from hypotonic solutions where there is higher water potential to hypertonic solutions. When solutions have equal concentrations of solutes, there is no net movement of water. These solutions are isotonic (Watson,2015).
Osmosis plays a crucial role in organisms. It is important in the uptake and transport of water in plants, reabsorption of water in kidneys, and the opening of the stoma. The turgor pressure that provides support to plants is because of the osmosis process. It is also important in industries in obtaining desired concentrations of solutions. The maintenance of water and ionic balance is crucial in cell integrity, signaling, and excretion. Cells reduce in size when they lose water to hypertonic solutions. In plants, plasmolysis occurs where the cell contents separate from the cell wall. Crenation occurs in animal cells, where they become smaller. A gain of water from hypotonic solutions will lead to swelling of cells. In plants, cells become turgid due to the cell wall. Animal cells will however burst (Nicolson, 2014).
Materials
1.Potato
2.Potato corer
3.Knife
4.Ruler
5.7-50ml conical tubes
6.dH2O (0.00m)
7.0.10M Nacl
8.0.15M Nacl
9.0.20M Nacl
10.0.25M Nacl
11.0.30M Nacl
12.0.50M Nacl
13.Electronic balance
14.Weigh boat
15. Large Beaker for liquid waste
Method
Googles and gloves have to be worn for this experiment.
Then, a plastic cover is removed from the bowl. The wrapping helps prevent potato cubes from drying.
The cubes are then placed into the bowl after drying. After this, the weight of the cubes is determined by using a balance. The weight of different tubes is determined and then these tubes are soaked into their respective solutions and covered.
These cubes will then be soaked for 24 hours.
The weight of the cubes is then determined.
An Elodea slide is prepared and placed on a microscope slide with the coverslip over it.
Observation is then done by the use of a light microscope.
Results
| NaCl Solution (Molarity) | |||||||
| 0.0 | 0.20 | 0.40 | 0.60 | 0.80 | 1.00 | ||
| Time In | 14:00 | 14:02 | 14:04 | 14:06 | 14:08 | 14:10 | |
| Time Out | 14:00 | 14:02 | 14:04 | 14:06 | 14:08 | 14:10 | |
| Total Time in Solution (min) | 1440 | 1440 | 14440 | 1440 | 1440 | 1440 | |
| Final Mass (g) | 47.053 | 41.738 | 37.043 | 33.325 | 30.072 | 29.209 | |
| Initial Mass (g) | 38.763 | 38.721 | 38.875 | 38.707 | 38.713 | 38.669 | |
| Weight Change (g) | 8.29 | 3.017 | -1.832 | -5.382 | -8.641 | -9.46 | |
| Change in weight (%) | 21.39 | 7.79 | -4.71 | -13.90 | -22.32 | -24.46 |
Figure 1: A Graph showing the relationship between the percentage change in weight and Nacl Molarity
Percentage change in height is calculated as follows :
Change in height = final weight- initial weight
Percentage change in height= change in weight ̸ Initial weight ×100
(47.053-38.763)/ 38.763 × 100 = 21.39 %
(41.738-38.721) / 38.721 × 100 = 7.79 %
(37.043-38.875) / 38.875 ×100 =- 4.71%
(33.325-38.707) / 38.707 × 100 = -13.90 %
(30.072-38.713) / 38.713 ×100 = -22.32 %
(29.209-38.669) / 38.669 × 100 = -24.46 %
Water potential of the potato
= 0 bars + (-2 x 1.0 mol/L x 0.0831 L bar/mol K x 298 K)
-49.468 bars
Figure 2: A drawing of when water is added to Elodea cells. De-plasmolysis occurs
Figure 3: A drawing of when NaCl is added to cells. Plasmolysis occurs
Discussion
The potato cells are subjected to varying salt concentrations. They change in size and shape as a result of the movement of water across the cell membrane. Initially, the potato size increased with an increase in concentration. The size then reduced sharply with increasing concentration, with it leveling off at the last two samples. At 0.41 M, the water potential of the potatoes is equal to the water potential of sucrose. There is no increase in weight at this molarity.
This is because of the initial entry of water into the potato cells. The potato cells have a solution that is hypertonic relative to the solution the potatoes are submerged in. Water is lost as concentration increases as the solution in the potato is hypotonic to that in the beaker.
These findings are consistent with the literature. Osmosis is driven by osmotic gradient, temperature, and the membrane’s thinness. Water moves until solutions are isotonic to each other. This is seen in the changes to the size and shape of cells. When a plant cell loses water, it reduces in size, and cell constituents separate from the cell wall. This phenomenon can be reversed by changing the concentration of the solution in which the cells are put. This is called de-plasmolysis (Stein,2012). The Elodea is plasmolyzed by the NaCl solution but the cells regain turgidity with the entry of water after applying the water drop.
Conclusion
The changes in the size of potato cells were successfully observed at varying salt concentrations. Osmosis, a special kind of diffusion, allows the movement of water molecules from region so high water potential to regions of low water potential.
Exercise 2:Starch Test
Labeling organelles
Materials
1.Compund Microscope
2.Microscope slide
3. Lugol’s reagent
4. Potato
5. Potato peeler
Method
The potato’s skin is peeled carefully in one area using a potato peeler.
Next, a thin slice of potato( a wedge) without any skin is cut using the potato peeler. This will ensure that the view is not obstructed.
The thinnest pieces are chosen to reduce the distance traveled by the path of light
After this, the slices are placed on paper towels and 300microlitre of Lugol iodine added
The slice is then placed on a microscope slide with the coverslip being placed on top
The potato cells are then observed through the microscope at different magnifications.
Discussion
Iodine reacts with starch producing the blue-black color. This is because starch contains both amylopectin and amylose. The linear amylose leads to the blue color because of iodine getting stuck in the structure.Starch is stored in amyloplasts. The high concentration of starch is the reason for the deep staining obtained.
Results
Figure 4: A photomicrograph of potato cells. X represents the cell wall.
Figure 5: Another photomicrograph of potato cells. Y represents the amyloplasts where starch is synthesized and stored.
References
Bienert, G. P., & Chaumont, F. (2014). Aquaporin-facilitated transmembrane diffusion of hydrogen peroxide. Biochimica et Biophysica Acta (BBA) – General Subjects, 1840(5), 1596-1604.
Nicolson, G. L. (2014). The Fluid—Mosaic Model of membrane structure: Still relevant to understanding the structure, function, and dynamics of biological membranes after more than 40years. Biochimica et Biophysica Acta (BBA) – Biomembranes, 1838(6), 1451-1466.
Stein, W. (2012). Transport and diffusion across cell membranes. Elsevier.
Watson, H. (2015). Biological membranes. Essays in Biochemistry, 59, 43-69.
