Name: __________________________ Class: __________________ Date: _____________________ Investigating Osmosis in Plant Cells All atoms and molecules are constantly in motion. The kinetic energy of this movement, particularly in liquids and gases, eventually leads molecules to distribute themselves randomly and uniformly throughout their container. This phenomenon is called diffusion. The general rule for movement of particles is: “Particles move from areas of higher concentration to areas of lower concentration.” Over time, the dissolved molecules in this beaker of water will be evenly distributed. Note that they are still in motion, but there is no net change in concentration once equilibrium is reached. Osmosis is the diffusion of water across a selectively permeable membrane such as those found in living cells. Plasma membranes are made of phospholipid molecules. Since phospholipids have both a hydrophobic and hydrophilic part, they arrange themselves as a double-layer sheet, giving the cell a barrier that very few molecules can pass through without help. Water can pass through the plasma membrane through special protein channels called aquaporins. These channels are not gated, so water effectively has free movement across the plasma membrane, in and out of the cell. As a type of diffusion, osmosis follows the same principle of movement from higher to lower concentration. However, it helps to also think of osmosis as a relationship between water and the dissolved solutes in a solution. Consider the “U tube” experiment as a demonstration of osmosis. A U-shaped tube is constructed with a selectively permeable membrane at the bottom. This membrane is permeable only to water. The left side of the U-tube is filled with a low-salt water solution. The right side is filled with a high-salt water solution. Left Side Right Side 10% NaCl 30% NaCl 90% H2O 70% H2O This side of the tube has a lower This side of the tube has a higher concentration of solute (the concentration of solute (the dissolved salt), but a higher dissolved salt), but a lower concentration of water! concentration of water! What direction would you expect water to move in this situation? In this apparatus, why don’t the solute molecules diffuse? Biology Teaching Resources http://www.aurumscience.com/biology.html 1 Left Side Right Side 20% NaCl 20% NaCl 80% H2O 80% H2O This side of the tube has experienced This side of the tube has experienced a net loss of water, the a net gain of water. The concentration of NaCl is now concentration of NaCl is now less, greater, and H2O is less. and H2O is greater. For osmosis, it is useful to consider a slight variation on the diffusion rule: “Water moves from areas of high solute concentration to areas of low solute concentration.” The behavior of water molecules during osmosis has a significant impact on living cells. Consider the example of the human red blood cell. A cell is dropped into one of three types of solution: A isotonic solution, such as Ringer’s solution, that contains the same concentration of solute (…and the same concentration of water) as the cell. A hypotonic solution, such as distilled water, that contains a lower concentration of solute (…and a higher concentration of water) than the cell. A hypertonic solution, such as ocean saltwater, that contains a higher concentration of solute (…and a lower concentration of water) than the cell. In each of these situations, will there be a net movement of water into the cell or out of the cell? Solution Tonicity Effect on Red Blood Cell Isotonic Solution Hypotonic Solution Hypertonic Solution The diagrams below show the physical effects of these different solutions on the red blood cells. Isotonic Solution Hypotonic Solution Hypertonic Solution No net movement of water. Net movement of water into cell, Net movement of water out of the causing it to burst. cell, causing it to shrivel and die. This lab will examine how osmosis affects plant cells when placed in different types of solutions. Procedure 1: Quantitative Measurement of Osmosis in Carrots Purpose: To identify what type of solution would be considered isotonic for a carrot cell. Hypothesis: What percent concentration salt solution would create an isotonic environment for a carrot cell? (e.g. 0%, 0.5%, 1%, or 5%?) Materials: Four carrot sticks Distilled water Sodium Chloride (NaCl) Four test tubes Procedure 1. Label four test tubes #1-4. 2. Record the salt concentrations of each stock solution available. Table 1: Salt concentration of each beaker Beaker 1 Beaker 2 Beaker 3 Beaker 4 3. Take a baby carrot and cut it in half so that it will fit in a test tube easily. 4. Record the mass and length of each carrot piece. Table 2: Initial recorded mass and length of carrot pieces Carrot 1 Carrot 2 Carrot 3 Carrot 4 Initial Mass (g) Initial Length (mm) 5. Place each carrot piece into a test tube. Make sure the carrots are completely immersed. 6. Allow the carrots to sit in the test tube overnight. 7. Record the final mass and length of each carrot piece. Table 3: Final recorded mass and length of carrot pieces Carrot 1 Carrot 2 Carrot 3 Carrot 4 Final Mass (g) Change in Mass (g) Final Length (mm) Change in length (mm) Which carrots experienced a net gain of water? Which carrots experienced a net loss of water? Analysis Create a graph to show the change in size of each carrot. Place a scale for mass on the left side of the graph, and a scale for length on the right side of the graph. Graph 1: Effect of Different Concentration Salt Solutions on Carrot Mass Conclusion Based on your results, describe what happened to the carrot cells in each beaker in terms of net water movement. How would this physically affect the carrot cells? Would the effects be the same as with red blood cells? What is different about plant cells such as those in carrots and animal cells such as ours? Identify each solution as hypotonic, isotonic, and hypertonic.
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