Stumpf Osmosis Lab Report PDF

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Process of osmosis and demonstrating it with the use of guard cells...

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Olivia Stumpf, Group B 31L

Osmosis Lab Report Introduction The process of osmosis is one in which water diffuses across a semipermeable membrane. The movement of molecules across a cell membrane is determined by the solution’s tonicity. Tonicity describes the relative solute concentrations inside and outside of the cell. The concentration is determined by the percentage of solvent and solutes within the solution. The three variations of tonicity are as follows: A hypertonic solution is classified by having a higher concentration of solutes than the solution it is being compared to. For example, if the extracellular fluid has a higher concentration of solutes than the cytoplasm inside the cell, the extracellular fluid is considered hypertonic. A hypotonic solution is classified by having a lower concentration of solutes than the solution it is being compared to. For example, if the extracellular fluid has a higher concentration of solvent, and the cytoplasm inside the cell has a higher concentration of solutes, then the extracellular fluid is considered hypotonic, and the cytoplasm would then be considered hypertonic because it has a higher concentration of solutes. An isotonic solution is classified as having an equal solute concentration to the solution it is being compared to.

Olivia Stumpf, Group B 31L

The use of guard cells in this experiment allows for the observation of the process of osmosis because they surround the stomata (pores on plants that take in CO2 and expel O2). “The cell walls of the guard cells are specially designed so that when the guard cells swell, an opening between the two guard cells is created...which allows gas and water vapor exchange between the plant and the environment” (UWL Bio Dept., 2018). During osmosis, the guard cells inflate as water diffuses across the cell membrane into the cells. The guard cells swell up due to the increased amount of water inside of the cells (Figure 1). This occurs because the solution on the outside of the cells is hypotonic compared to the solute concentration inside of the cells. Conversely, the guard cells shrink when water diffuses across the cell membrane out of the guard cells, causing the stomata to close. The guard cells shrink because the solution surrounding the guard cells is hypertonic, causing the diffusion of water from hypotonic solutions (inside the cells) to the hypertonic solution (outside the cells).

Olivia Stumpf, Group B 31L

Figure 1: The photo on the right shows two shrunken guard cells, closing the stoma. The photo on the left shows two swollen guard cells surrounding an open stoma (Wikipedia, 2020).

Passive transport is the diffusion of molecules across a cell membrane to an area of high concentration from an area of low concentration. The process of passive transport does not need energy to occur. However, active transport requires energy in the form of ATP which is synthesized in the cell’s mitochondria, and the chloroplasts in plant cells (Foster et. al 2020). Active transport is a process in which molecules move across a cell membrane from an area of low concentration to an area of high concentration (against the concentration gradient). However, the introduction of dinitrophenol (DNP) inhibits the production of ATP by both energy synthesizing organelles. Therefore, osmosis

Olivia Stumpf, Group B 31L

does not require energy because it undergoes passive transport and diffuses across the cell’s semipermeable membrane.

Materials The experiment will observe the amount of open and closed stomata on the epidermis from Tradescantia. The solutions needed for this experiment are a 20% NaCl solution, 20% NaCl solution plus DNP, distilled water, and distilled water plus DNP. To observe the stomata on the Tradescantia epidermis when exposed to the different solutions, microscope slides to test each solution and a microscope are essential as well.

Methods First, put 2-3 drops of each solution on their own slide. Then, carefully tear off a small piece of the Tradescantia epidermis that has a thin, clear portion exposed (this will be where the stomata are most visible). Place the pieces of Tradescantia on four slides, each with one of the solutions on them, and cover them with the slide covers. Place the slide on the microscope and secure with the slide arm. After adjusting the microscope strength to where the stomata are

Olivia Stumpf, Group B 31L

visible, count the amount of open and closed stomata you observe (about 15-20 in total). The open stomata are observed as having light passing through the middle, and the closed stomata have a dark line down the middle of them. Finally, record the data and report it to the lab instructor. After completing the experiment, sanitize the lab station and the equipment used, and dispose of the Tradescantia samples.

Results At the end of the experiment, the only the solution with more open stomata than closed was the distilled water (Table 1). The distilled water with DNP almost had equal results of open and closed stomata, but about 51% of the 90 stomata in distilled water with DNP were observed as closed.

Solutions Distilled water Distilled water + DNP 20% NaCl 20% NaCl + DNP Total

Group data 17 6 5 5 33

Open stomata Class data 66 44 18 17 145

Closed stomata Group data Class data 3 31 9 46 15 73 16 82 43 232

Table 1. Table for number of open and closed stomata counted in each corresponding solution

Olivia Stumpf, Group B 31L

The other two solutions had a much greater number of closed stomata compared to the solutions with distilled water. There was an average of about 81.5% of closed stomata between the two solutions (20%NaCl and 20%NaCl with DNP) (Table 2).

Solutions Distilled water Distilled water + DNP 20% NaCl 20% NaCl + DNP

Percentage of open stomata Group data Class data 85% 68% 40% 49% 25% 20% 24% 17%

Table 2. Table for percentage of open stomata counted in each corresponding solution

Discussion The data shows that more stomata were open in distilled water, this is because the guard cells were exposed to a hypotonic solution which, by the simple diffusion of water across a semipermeable membrane (osmosis), caused the guard cells to swell and take in more water to balance the concentration of solutes within the cell and the extracellular solution of distilled water. With the introduction of DNP, an ATP inhibiting compound, to the distilled water solution there should be little change in the amount of open and closed stomata because the process of osmosis does not require ATP to be carried out. However, the

Olivia Stumpf, Group B 31L

solution of distilled water with DNP resulted in 32% less open stomata. This could be because the DNP is a solute in the solution causing it to be hypertonic to the cellular fluid. So, the guard cells will shrink due to the movement of water across the cell membrane into the hypertonic extracellular solution. The Tradescantia had a significant number of closed stomata in the 20% NaCl and 20% NaCl with DNP solutions. The solutions with NaCl are hypertonic to the guard cells since the guard cells do not have a large concentration of NaCl within their cellular fluid. “NaCl inhibits plant growth and limits water uptake” (Yu et. al 2015). Because of this, the guard cells will become shrunken from the water inside of the cells moving out of the cell and into the extracellular fluid, causing the stomata to be closed (80% of stomata in 20% NaCl closed, 83% of stomata in 20% NaCl with DNP closed (Table 2)). The solutions with added DNP had more closed stomata (Table 1); however, both solutions with NaCl had a greater number of closed stomata than open ones. This shows that DNP did not directly affect the process of osmosis in the experiment, but it effected the tonicity of the solutions and in turn caused the process of osmosis to be different than predicted. For example, the distilled water with DNP had more closed stomata than open because the DNP caused the

Olivia Stumpf, Group B 31L

tonicity of the solution to be hypertonic compared to the guard cells surrounding the stomata.

Conclusion The results of the experiment do not reject the hypothesis that osmosis does not require energy because it undergoes passive transport and diffuses across the cell’s semipermeable membrane. With the knowledge of tonicity and how it relates to osmosis and the direction in which water moves in relation to the tonicity of the solutions, it is known that ATP is not required for osmosis to occur and therefore the cause of more closed stomata in the distilled water with DNP came from the DNP causing the solution to be hypertonic to the guard cells.

Olivia Stumpf, Group B 31L

Literature Cited Foster, K.J., S.J. Miklavcic. 2020. A comprehensive biophysical model of ion and water transport in plant roots. III. Quantifying the energy costs of ion transport in salt-stressed roots of Arabidopsis. Frontiers in Plant Science. 11:1. University of Wisconsin-La Crosse Biology Department. “Biology 105 Laboratory Manual 5E”. Lab handbook. University of Wisconsin-La Crosse. La Crosse, WI. 2018. Print. Yu, Y., S.M. Assmann. 2015. The effect of NaCl on stomatal opening in Arabidopsis wild type and agb1 heterotrimeric G-protein mutant plants. Plant Signaling & Behavior. 11(2). Open and closed stomata diagram, Retrieved from: https://en.wikipedia.org/wiki/Guard_cell...