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Exercise 3: Movement Across Cell Membranes A selectively permeable barrier is one of the defining features of a living cell. The cel membrane and the associated transport proteins found in the membrane are responsible for regulating the movement of hundreds, if not thousands, of different types of molecules into and out of the cell. All molecular motion is influenced by diffusion, which is the tendency for particles to spread from higher concentrations to lower concentrations until they are evenly distributed, or reach equilibrium. This movement towards equilibrium is the driving force behin a majority of physiological processes, from neuronal impulses to renal function. Today we will investigate the movement of several different types of molecules across a cell membrane, including water, and we will examine the physical properties of these different molecules to see how they influence this movement. Today's Objectives 1. Observe the movement of water across a membrane in model cells (decalcified eggs) and examine the environmental conditions that determine the direction of osmosis. 2. Compare the rate of osmosis when the concentration gradient varies. 3. Observe the effect of molecular size on the movement of solutes across a membrane. 4. Observe the effect of polarity on the movement of solutes across a membrane. 5. Familiarize yourself with the types of IV solutions used in fluid therapy Osmosis When a selectively permeable membrane can inhibit the movement of some types of solutes and a concentration gradient exists, water will diffuse towards the higher solute concentration to equalize the concentration on both sides of the membrane. If you have a hard time remembering which way water moves in the presence of an osmotic imbalance (concentration gradient), just remember that SOLUTES SUCK! Water will always be drawn towards more concentrated solutes. A solution can be described by its tonicity. Tonicity (= tone or tension) describes the extracellular environment (ECF) in relation to the intracellular (ICF) and how that environment will affect the cell volume. A hypotonic solution will cause a cell to stretch and swell as water enters because it has a lower solute concentration (hypo= below) than a cell. A hypertonic solution will draw water out of a cell and make it shrink because it has a higher relative solute concentration (hyper = above). An isotonic solution produces no change cell volume because there is no difference in concentration (iso = same); an isotonic solution is said to be in osmotic equilibrium with the cell. page 23
You have observed this phenomenon when your fingers get wrinkled after soaking in bath water, a hypotonic solution. Your skin wrinkles because the skin cells swell with water an your skin becomes too large to fit smoothly on your fingertips. Conversely, your skin may feel dry and tight after a day swimming in the ocean, a hypertonic solution, as the salt from the sea water draws water out of your skin cells. Tonicity differs from osmolarity in that osmolarity is an actual measurement of a solution's total concentration of particles. Osmolarity typically has the units of osmoles per lite (Osm/L) or milliosmoles per liter (mOsm/L). Tonicity has no units as it only describes the ECF in relation to the ICF. A solution can be both isotonic and isosmotic unless the solute is penetrating, or able to enter the cell. More about this later. In the following experiment, you will be using decalcified eggs as model cells. The eggs have been treated with vinegar to remove the calcium from the shell, leaving behind a membran- that is permeable to water (solvent), but not to other molecules (solutes). Materials: ● ● ● . 5 decalcified eggs 5 weigh boats, one for each egg 3 Beakers or plastic containers with solutions A, B, & C 2 Beakers or plastic containers with solutions 1 and 2 Paper towel Gram scale page 24
Procedure IA: Determining the Tonicity of Extracellular Fluid: 2. 1. Fill three beakers with enough of solution A, B, or C to cover an egg, about 300 ml. Obtain three decalcified eggs. Gently dry and weigh each egg before immersing it in Solution A, B, or C. Dry the egg by gently rolling it on a paper towel. Do not dry the egg for too long because the paper towel will begin to draw out water from inside the will change the weight of the egg. Record the weight of each egg in Table 1. egg and 3. Let the three eggs soak in solutions A, B, and C for 20 minutes. Go on to Procedure IB while you are waiting. The soak time should be at least 20 minutes, but can be longer if it is more convenient. 4. After at least 20 minutes, dry and weigh each egg and record your results. Use a "+" sign to indicate an increase in weight and a "-" sign to indicate a decrease in weight. 5. The change in weight reflects the movement of water into or out of the egg. Based on the movement of water, determine if the Solutions A, B, and C are hypotonic, isotonic, or hypertonic. Table 1. Weight of Eggs and Tonicity of Solutions A, B, and C Weight Before Weight After Soaking (g) Soaking (g) Difference in Weight (g) Weight rela Tonicity of Solution (hyper-, hypo-, or iso-) Isotonic Egg in Solution A 86.12g / 85.989 -0.14g Egg in Solution B12.03 g 67.45g -4.58 g/H. 69.539 +1.549 Egg in Solution C 67.999
Compare the presoak weight for eggs A, B, and C with their weights after the 20-minute soak. Determine the tonicity of each solution based on the gain or loss of water. Describe the physiological process that caused the change in the weight of each egg. What physical conditions are required to cause the water to move in a particular direction, into or out of the egg? Explain why osmotic homeostasis must be closely regulated for the all the body fluid compartments. cedure IP: The