Awesome Osmosis and Osmoregulation Purpose: By the end of this lab students should be able to: 1. Understand osmosis and be able explain the differences between isotonic, hypertonic, and hypotonic solutions. 2. Describe some of the methods of osmoregulation by freshwater and marine organisms. Background information Definitions: Concentration: the amount of stuff dissolved in solution. (Seawater has a higher salt concentration than fresh water). Diffusion: the dispersal of matter within an environment such that it becomes equally concentrated throughout the environment. Hypertonic: a solution containing a greater amount of dissolved stuff than a creature or object in the solution. Hypotonic: a solution containing a lesser amount of dissolved stuff than a creature or object in the solution. Isotonic: a solution containing an equal amount of dissolved stuff than a creature or object in the solution. Osmoregulation: the process of regulating the amount of salt and other dissolved substances to control the loss or gain of water from osmosis. Osmosis: the diffusion of water across a semi-permeable membrane. Salinity: the relative amount of salt dissolved in water. (Seawater has a higher salinity than freshwater). Semi-permeable membrane: a membrane that permits the free passage of water but prevents the passage of a dissolved substance like salt. The fundamental concepts of this are salinity and osmoregulation. Salinity refers to the amount of salt dissolved in water. Freshwater in lakes, rivers, and streams has a lower salt content and thus a lower salinity than seawater. The salinity of the surrounding environment is an important constraint that marine organisms must deal with in order to survive. Diffusion refers to the "desire" of all matter to be equally concentrated in its environment. If a large concentration of something is put into a particular region of the environment, it will disperse until its concentration is uniform throughout the environment, provided it does not encounter any barriers through which it cannot pass. Salt exists in water as sodium ions (Na + ) and chloride ions (Cl - ). Charged ions like sodium ions and chloride ions are unable to pass through most biological membranes. However, water molecules are able to pass through most biological membranes so salinity imbalances within biological systems are naturally corrected via the diffusion of water across semi-permeable membranes to equalize salt concentrations on both sides of the membrane. This process is known as osmosis. 1 P a g e
When the concentration of dissolved solids, such as salt, are equal on both sides a semi-permeable membrane, the solution is said to be isotonic and there is no net flow of water to either side of the membrane. As a result, there is no net change in salinity in an isotonic solution. When the concentration of dissolved solids (salt) is greater inside a semi-permeable membrane than outside the membrane, the solution outside the membrane is said to be hypotonic and water will diffuse across the membrane from outside to inside in an effort to decrease the salinity inside the membrane. When the concentration of dissolved solids (salt) is greater outside a semi-permeable membrane than inside the membrane, the solution outside the membrane is said to be hypertonic and water will diffuse across the membrane from inside to outside in an effort to decrease the salinity outside the membrane. Because water will naturally diffuse across biological membranes from regions of lower salinity to regions of higher salinity, marine organisms and freshwater must take great care to maintain the proper balance of water and salt within their bodies to sustain life, through the process known as osmoregulation. The different salinities of freshwater and seawater present different challenges to the organisms that live in these habitats. Organisms living in seawater must have a means of preventing the loss of water from the body to the highly saline and potentially hypertonic environment. Freshwater organisms must deal with the opposite problem of preventing excessive amounts of water from the potentially hypotonic freshwater environment entering their highly saline bodies. And organisms such as salmon, who are capable of living in both freshwater and seawater, must have a means of dealing with the different salinities of these different habitats. [Note: The seawater environment is not hypertonic to all marine organisms, just as the freshwater environment is not hypotonic to all freshwater organisms. Implications of this and reasons for this will be discussed later in the activity.] Pre-Lab Activities A. Watch the video: Osmosis: Easy Lab Experiment B. Complete the Lab Bench Activity on Diffusion and Osmosis Materials Needed Potato experiment: Sectioned potato slices Graduated cylinder 4 small beakers or clear cups Table salt. Water. Electronic scale Microscope activity: Microscope. Microscope slide with cover slip (see "Preparation and Teacher Heads-Up" below). Leaves of Elodea/Anachris plant (available at any aquarium store). Eyedropper. Water. Table salt. 2 P a g e
Step-by-Step Procedure Procedure for the Potato Activity 1. Set up three 100 ml salt concentration solutions and one control group. Solution A = 3.5g salt dissolved in 100 ml H 2 O Solution B = 7.0g salt dissolved in 100 ml H2O Solution C = 0 g salt dissolved in 100 ml H2O Solution D = no salt and no water 2. Find the mass of three potato slices. Record data on table 1. 3. Place one potato shape in each of the 3 solutions. 4. Wait 10-20 minutes for the experiment to occur. 5. Once the wait period has ended, remove potato slices from solutions and find the mass for each. Record data on table 1. 6. Find the difference in the mass (g) of the potato at the start and end. If there was a loss in mass, then it should be recorded as a negative number. 7. Share change in mass data with class. Record class data in table 2. Find the average difference in mass for each solution. Table 1: Team Data Solution Start: Mass of Potato Slice (g) End: Mass of Potato Slice (g) Change in Mass (g) A B C D Table 2: Class Data Teams Solution A Solution B Solution C Solution D 1 2 3 4 5 Average mass (g) 3 P a g e
Items for Discussion and Conclusion (Potato Activity): 1. Which solution prepared was an isotonic solution? Hypertonic? Hypotonic? 2. What happened to the potato in each of the solutions? Why? 3. What was the control? Why was it set up? Procedure for the Microscope Activity: 1. View the Elodea/Anachris cells under the microscope. 2. Place a few drops of a saline solution on the microscope slide at one edge of the cover slip. At the same time, use a piece of paper towel at the adjacent edge of the cover slip to absorb access water. This will create a system of flow, such that the salt water flows through the cells of the Elodea/Anachris plant and get soaked up by the paper towel. 3. View the Elodea/Anachris cells after 1 2 minutes in the salt-water environment and note any changes that they observe. 4. Repeat steps 2 and 3 using regular tap water instead of salt solution. 5. View the Elodea/Anachris cells after 1 2 minutes in the fresh water environment note any changes that you observe from the previous salt solution condition. Record Observations (be sure to label drawing and include total magnification) 4 P a g e
Items for Discussion or Conclusion (Microscope Activity): 1. Why did the cells shrink when given the salt-water solution? Be sure to use the appropriate vocabulary to describe in detail. 2. Why did the cells expand when given the fresh water solution? Be sure to use the appropriate vocabulary to describe in detail. Osmoregulation in various fresh-water and salt-water organisms: Freshwater Snails: Fresh water snails have an outer shell that protects a large part of their surface from the osmotic inflow of water. The kidneys and excretory system provide additional osmoregulation. Marine invertebrates: Marine invertebrates have body fluids that are isotonic to the surrounding environment. Marine invertebrates like lobsters, crabs, and shrimp taste "salty" because their bodies must contain lots of salt to keep their body fluids isotonic to their salt-water homes! Fresh-water fishes: Fresh water fishes maintain body fluid concentrations live in a hypotonic environment and thus obtain water directly from osmotic uptake from their environment. These fish generally have highly developed kidneys and active excretory systems to keep the osmotic uptake of water from getting out of hand. Marine teleosts (most fish): The salty environment draws water from the fish via osmosis. The fish compensate by greatly increasing their water intake. Teleost gills have chloride-secreting cells that help to put ingested salts back into the environment. Teleosts also have highly developed kidneys and excretory systems. Sharks, Skates, and Rays: The main form of osmoregulation in sharks, skates, and rays is a specialized salt excretion gland (such as that found in the spiny dogfish). Lamprey: The lamprey has osmoregulatory mechanisms similar to those of marine teleosts. Sea Turtle: The sea turtle, like other reptiles, has specialized salt glands that excrete salt. The shell also provides a barrier to the loss of water to the hypertonic environment. Marine mammals: Excretory organs are designed to help conserve water. Urine is excreted as a semi fluid paste. Salmon: Salmon have well designed excretory systems that can adjust to their dual fresh-water and salt-water habitats. 5 P a g e
References Karnaky Jr., Karl, J. (1998). "Osmotic and Ionic Regulation." In The Physiology of Fishes. 2 nd ed. Boca Raton: CRC Press. Krogh, August. (1965). Osmotic Regulation in Aquatic Animals. New York: Dover Publications. Lutz, Peter L. (1997). "Salt, Water, and ph Balance in the Sea Turtle." In The Biology of Sea Turtles. Boca Raton: CRC Press. Vernberg, Winona, B. Vernberg, F. John. (1972). Environmental Physiology of Marine Animals. New York: Springer-Verlag. 6 P a g e