Water Potential Lab PDF

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The Effect of Various Solutions with Varying Molarities on Potato Core Weights from the Species Solanum tuberosum and Determination of the Water Potential of the Potato Derrick Cheung Abstract All plant cells need osmosis to remain healthy; osmosis allows for turgor pressure and that allows the plant to remain stiff. In this lab, water potential will be found by determining the solute concentration that is at equilibrium with potato cells. Through this, the water potential within the potato cells can be calculated. The hypothesis is that the potato cell will gain the most weight in the deionized water with no solute dissolved in it because a higher concentration of water should be on the outside of the potato cells which would make it absorb water through osmosis. The six potato cores were placed in DI water (pure water), 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 M sucrose solutions for one hour. Change in mass was measured and used to calculate how much water had entered or left the potato cells. The pure water had the most water gain at a class average of 7.9% by mass. The potato core in the highest molarity solution, 0.6M sucrose, lost the most weight with a class average of -10.0% change in mass. The optimal molarity for solution for potato cells to be in is just 0 molarity solution, or pure water. The water potential of the potato cells was determined to be Ψw = -7.5. Introduction What makes a cell a cell is the presence of a plasma membrane. This membrane consists of a phospholipid bilayer that separates what is defined as cell and everything that it is not, or the environment. What makes the membrane special is that it is selectively permeable. This means that the membrane allows for certain molecules to pass through while blocking others. A solution is when water is mixed with other molecules. In this case, water is called the solvent, or what the solute is dissolved in. Most solutes are restricted from passing through the plasma membrane and those that can pass need special carrier proteins that are part of the plasma membrane. (Openstax 2013) The use of special carrier proteins require the cell to input energy. Water is different though; it moves freely in and out of the cell and even enhanced through pores called aquaporins. This entire process of free water movement is called osmosis. When the solution outside a cell has a lower concentration of solute, the cell is said to be in a hypotonic solution and the inside of the cell is hypertonic. When this happens in practice, water will, through osmosis, naturally travel from an area of higher concentration to a lower concentration of water. The cell will continually have water diffuse into it until the pressure of water becomes too great for the cell’s membrane and cause it to lyse. (Britannica 2016) In a plant cell, the presence of a cell wall prevents the cell from bursting, rather the increased pressure provides support for the plant. The perfect conditions for an animal cell differ from that of plant cells. If the potato cell is placed in a hypotonic solution, it will gain water because of osmosis and the opposite will happen if it is placed in a hypertonic solution.

Materials and Methods In this experiment, the potato Solanum tuberosum was used to observe the effects of osmosis. 7 large beakers were also obtained and each had a different solution in it; DI water, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6 M sucrose solutions. A cork borer was used to get seven core samples, each at least five cm long and without skin, from the potato. These cores were cut so that they were all the same size relative to each other. Each core was then blot dried, weighed, and recorded. The temperature was also recorded. Each potato core was then placed in its respective beaker and allowed to sit for about one hour, swirling every 15 minutes. After one hour, the cores were removed and the end time recorded in order to find total time. The cores were blot dried and reweighed to find difference in mass, in other words; water gained through osmosis. The potato core in the DI water, water potential equaling 0, gained the most mass as the hypothesis stated. Results Each potato gained or lost a different amount of water depending on which beaker it was in. The potato core in the DI water gained the most water with an average of 7.9% weight increase (Table 1). As the molarity of the sucrose solution went up, the amount of water the potato gained decreased until the molarity went past 0.3. After 0.3 M sucrose solution, the potato started to lose water instead of gaining. Where the potato is at equilibrium would signify the water potential of the potato cells; Ψw = -7.5, (Figure 1)

Table 1 Lab group 1’s change in mass of potato cells in varying sucrose solutions. Sucrose Concentration (M)

Initial Mass (g)

Final Mass (g)

Change in Mass (g)

% Change in Mass

0

4.38

4.70

0.32

7.31%

0.1

4.66

4.91

0.25

5.36%

0.2

4.59

4.73

0.14

3.05%

0.3

4.64

4.67

0.03

0.65%

0.4

4.57

4.44

-0.11

-2.41%

0.5

4.43

4.18

-0.25

-5.64%

0.6

4.51

4.18

-0.33

-7.32%

Table 2. Solute potential at various sucrose concentrations and the class average of % change in weight of potato cores.

Sucrose Concentration (M)

Ψs

Class Average % Change in Mass

0

0

7.9%

0.1

-2.443

6.09%

0.2

-4.886

3.58%

0.3

-7.329

0.44%

0.4

-9.772

- 4.02%

0.5

-12.216

- 7.53%

0.6

-14.659

- 10.0%

Figure 1. The effect of water potential in different molarity sucrose solutions has on mass of potato cores.

Discussion The main question in this lab was what water potential was and how to calculate the water potential of potato cells. The hypothesis was; if the potato was placed in pure water, it will gain

the most mass, or water, and the potato in the highest molarity sucrose solution will lose the most mass. Based on the results, the hypothesis was supported. The potato core in the DI water gained an average of 7.9% mass, or about 0.3 grams. The weight gained was decreasing until the potato was no longer in a hypotonic solution, at around 0.3 M sucrose solution. Past this molarity, the potato cells started to lose weight. Using this data, the water potential of the potato cells was determined to be Ψw = -7.5. Pure water is the best for the plant because the water potential is the highest, 0, allowing for the highest possible absorption of water since water will always flow from high water potential to an area with lower water potential. The potato cells, like all plant cells, have cell walls. This allows plant cells to absorb as much water as they can and not lyse, only stopping when the turgor pressure hits its limit. This pressure is essential to the plant because it provides support and rigidity, things necessary for healthy function. The experiment can be improved on by using more specific molarities of sucrose solution such as 0.32 and 0.34 M sucrose solutions. This will allow for a more accurate representation of what the true value of the water potential of the potato cells is. Further experimentation can include water with fertilizer instead of sucrose. This will allow scientists to know how much fertilizer is appropriate for the plant since too much will lower the water potential of the soil. If the plant is in a hypertonic environment, the result will be water flowing out of the plant instead of in it. This is unfavorable to the plant because without water, the plant cannot effectively perform its necessary functions and it will also lose its form, and eventually die if it remains in the hypertonic conditions.

Literature Cited Dario-Becker, Juville. Biology: Mixed Majors, Part 1 --. Houston, TX: OpenStax College, Rice U, 2013. Print. "Lysis." Encyclopædia Britannica. Encyclopædia Britannica, Inc., n.d. Web. 18 Apr. 2017....