Laboratory Experiment No. 11 - Transport Across Membranes 2w5r39

Expt No 11 – Transport Across Membranes Objectives: To describe the changes on the cells exposed to hypotonic, hypertonic, and isotonic solutions. To identify the type of movement manifested by the food coloring. To compare and contrast diffusion and osmosis. Data: Table 1. Color of water in the beaker over time Time Elapsed Color of water 15 mins Light yellow 30 mins Dark yellow Table 2. Observations on the potato strip that was immersed in different solutions Solution Observations Distilled water The potato strip became turgid. 25% salt solution The potato strip became soft but the change is very minute. 50% salt solution The potato strip became soft and bent. It also decreased in size. Interpretation, Analysis and Discussion: In the experiment, water cellophane was used to represent the cell membrane. As shown in Table 1, as time progresses, the color of the water becomes darker. This is because the food coloring molecules move out of the water cellophane. The solute concentration inside the cell is greater as compared to the concentration outside the cell. To achieve equilibrium, the cell undergoes diffusion. According to Steck (2013) this mode of transport occurs from high solute concentration to low solute concentration. Diffusion is a ive transport where ATP and transport proteins are not needed to transport materials across the membrane. Small non-polar molecules through the membrane by diffusion. Thus, the food coloring used in the experiment can be used to represent these molecules. According to Campbell, Reese and Mitchell (1999), each molecule wanders randomly but there will be a net movement of the molecules across the membrane to the side that began as pure water. The spreading of the coloring across the membrane will continue until both solutions have equal concentrations of the coloring. Once that point is reached, there will

be a dynamic equilibrium, with as many food coloring molecules crossing the membrane in one direction as in the other each second. As shown in Table 2, the potato strip became turgid when it was immersed in distilled water. On the other hand, both strips immersed in 25% and 50% salt solution became soft. However, the changes observed in the former were very minute as compared to the latter since the one immersed in 50% salt solution decreased in size and became bent. In the first set-up, the potato strip was exposed to a hypotonic solution since the potato became turgid. Cells become turgid if water from the environment gets into the cells (Farabee, 2007). This occurs when the solution outside the cell has a lower solute concentration (Steck, 2013). This type solution is said to be hypotonic. Since the molecules inside the cell are too large to through the selectively permeable membrane, water will cross the membrane, instead. In effect, the solution with lower solute concentration (hypotonic) has a higher water concentration. Therefore the water will diffuse across the membrane from the hypotonic solution (outside the cell) to the hypertonic solution (inside the cell) to achieve equilibrium. This diffusion of water is a special case of ive transport called osmosis (Campbell, Reese and Mitchel, 1999). In the second set-up, the potato strip was assumed to be exposed to an isotonic solution. However, because the concentration of solute inside the potato cells was only assumed, the results were not exactly accurate. Solutions of equal solute concentration is said to be isotonic (Steck, 2013). If two solutions are isotonic, water moves across a membrane separating the solutions at an equal rate in both directions; that is, there is no net osmosis between isotonic solutions. In the third set-up, the potato cell was exposed to a hypertonic solution. The potato turned soft and bent and decreased in size because water from inside the cells moved out. The solution of solute outside the cell is higher than the inside so, in order to achieve equilibrium, the water molecules inside the cell will cross the membrane and leave the cell. This s for the change in the size of the potato strip since cells shrink when exposed to a hypertonic solution. According to Fulton (), cell transport involves the many ways that wastes and other materials get in and out of the cell. ive transport is the movement of a substance across a cell membrane without the input of the cell's energy. Simple Diffusion involves the movement of molecules across the cytolemma from a region of higher concentration to a region of lower concentration. Small molecules diffuse the easiest across the membrane. No outside chemical energy is needed for simple diffusion. Osmosis is the simple diffusion of water molecules across a semipermeable membrane. It occurs when the concentration of solutes in the two sides of a semipermeable membrane are different. Movement of molecules is from a solution with a higher water

concentration to a solution with lower water concentration. Facilitated diffusion is diffusion with the help of cell membrane proteins that provide a way for atoms or molecules to more easily diffuse across the membrane. Active transport can enable a cell to move items across the membrane against a concentration gradient. Chemical energy in the form of ATP is used to begin this process. A membrane carrier is used and the direction can be from high to low concentration or from low to high concentration. In exocytosis wastes and cell products are packaged by Golgi apparatus in sacs called Golgi vesicles. Gogi vesicles fuse with the cell membrane and the materials in the vesicles are secreted out of the cell. In endocytosis, the cell membrane surrounds desirable macromolecules outside the cell. The cell pinches off a saclike portion of its outer membrane to form a tiny new vesicle. The vesicle moves into the cell where it releases its contents into the cytoplasm. Endocytosis can be pinocytosis, where the cell membrane encloses a droplet of fluid and its solutes and brings the droplet into the cell, or phagocytosis, where the cell engulfs a food particle. The vesicle containing food then fuses with a lysosome carrying digestive enzymes. Conclusion: Based on the observations made, it is concluded that, cells exposed to a hypotonic solution bloats or becomes turgid. When exposed to a hypertonic solution, they shrink and when exposed to isotonic solution, they turn flaccid or there is no change. It is also concluded that, the food coloring has undergone diffusion. Diffusion is the movement of solute molecules across a semipermeable membrane from a high solute concentration to a low solute concentration. Osmosis is the movement of solvent molecules across a semipermeable membrane from a high solvent concentration to a low solvent concentration. Application: According to Ford (2013), when the salt water enters the throat, the solution helps to neutralize acids in the throat, restoring the natural pH balance that had been disrupted by the sore throat. By doing this, the burning sensations are relieved and the mucous membranes become less irritated, which can speed healing. In addition to neutralizing acid, the salt water gargle helps to wash away unwanted mucus and increase the blood flow to the throat. The capillaries then become dilated, which allows for faster circulation of infection-fighting cells.

References:

Campbell, N.A. Reese, J.B. and Mitchell, L.G. (1999). Biology 5 th ed. USA: Benjamin/Cummings. 137-138 Farabee, M.J. (2007). Water and Solute Movement. Retrieved 1.29.2013 from http://www.emc.maricopa.edu/faculty/farabee/biobk/biobooktransp.html Ford, A. (2013). Ease a Sore Throat By Gargling Water: Say What. Retrieved 1.29.2012 form http://www.divinecaroline.com/36/108976-ease-sore-throatgargling-salt Fulton, T.B (). Diffusion and Transport Across Membranes. Retrieved 1.29.2013 from http://biochemistry.ucsf.edu/programs/ptf/prologue%20links/Diff%20& %20Trans%20Membranes.pdf Steck, T.L. (2013). Membrane Transport. Retrieved 1.29.2013 from http://www.biologyreference.com/Ma-Mo/Membrane-Transport.html