Lab 02 Diffusion Osmosis Permeability

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Transcript of Lab 02 Diffusion Osmosis Permeability

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Exercise 2: Diffusion, Osmosis and Membrane Permeability PreLab

1.

In today's experiment, I predict, on the basis of Fick's law, that the order of the diffusion rates for the substances methylene blue, Alcian blue, ruthenium red, and methyl red (from the fastest to the slowest) will be

2.

On the basis of Fick's law, I predict that the order of the diffusion rates for solutions of 0.3 mM methylene blue, 1 mM methylene blue, and 3 mM methylene blue (from the fastest to the slowest) will be

3.

I predict that when red blood cells are placed in 0.9% NaCl, they will

4.

I predict that when red blood cells are placed in deionized water, they will

5.

I predict that when red blood cells are placed in 10% NaCl they will

6.

What causes diffusion? What determines how rapidly it will proceed?

Saturday, January 24, 2009

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BIOLOGY 204: INTRODUCTION TO PHYSIOLOGY LABORATORY MANUAL EXERCISE 2: DIFFUSION, OSMOSIS, & MEMBRANE PERMEABILITY PRE-LAB 7. Choose the correct answer: At a given temperature, the diffusion rate of large molecules is (greater than / less than) the diffusion rate of small molecules. Explain at the molecular level.

8.

Why can we use hemolysis rate as an indication of membrane permeability?

9.

Why is a hyperosmotic solution not necessarily hypertonic?

10.

Indicate whether each of the solutions below is hypertonic, isotonic, or hypotonic to mammalian red blood cells: 0.09% NaCl 0.9% NaCl 9% NaCl

11.

Indicate whether the intracellular fluid of mammalian red blood cells is hyperosmotic, isosmotic, or hyposmotic to each of the solutions below. 0.09% NaCl 0.9% NaCl 9% NaCl

12.

How might diffusion rates affect the maximum size cells can attain, or, why is the maximum distance between a cell and the nearest capillary typically no greater than 0.01 mm?

13.

Choose the correct answer: A frog living in a freshwater pond is in an environment that is (hypertonic, isotonic, hypotonic) to the frog's tissues.

14.

Assume that you are a health care practitioner who is treating an accident victim. The patient had lost a good deal of blood, blood pressure is dangerously low, and the patient is going into shock. You decide to administer an intravenous infusion to restore blood volume and blood pressure. Why don't you choose sterile deionized water?

Saturday, January 24, 2009

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Exercise 2: Diffusion, Osmosis and Membrane Permeability

PART I: DIFFUSION AND OSMOSISINTRODUCTION Because of their kinetic energy, molecules are in constant motion unless they are at absolute zero (the temperature at which molecular motion ceases). The pathways of molecules vibrating with kinetic energy are random (Fig. 2-1). This random motion is responsible for diffusion, the tendency of molecules to move from an area of high concentration to an area of low concentration.

Figure 2-1. The random path a molecule may take due to its inherent thermal energy.

When molecules are distributed uniformly throughout a solution (that is, there are equal concentrations everywhere) they are at equilibrium and show no net tendency to move Saturday, January 24, 2009 Page 19 7:05:05 PM

BIOLOGY 204: INTRODUCTION TO PHYSIOLOGY LABORATORY MANUAL EXERCISE 2: DIFFUSION, OSMOSIS, AND MEMBRANE PERMEABILITY preferentially in any direction. However, when concentrations are not the same we say there is a concentration gradient between the two regions, and the molecules will diffuse spontaneously from the region of high concentration to the region of lower concentration until they reach equilibrium. A concentration gradient is defined as the difference in the concentrations of a substance at two separate points divided by the distance between the points, or (c1 c2)/d, where c1 and c2 are the concentrations at points 1 and 2, and d is the distance between the two points. If a lump of sucrose is placed in a beaker of water (Fig. 2-2A), at first there will be unequal concentrations of sucrose in different parts of the vessel (Fig. 2-2B). Initially, some of the sucrose will dissolve in the water adjacent to it. This will result in a gradient of sucrose in the solution in the beaker (Fig. 2-2B and C). At any given instant, some sucrose will move toward the sides of the beaker, some toward the top, and some toward the bottom. Since the sucrose concentration is initially high near the bottom of the beaker, at first more sucrose molecules will move toward the top of the vessel than toward the bottom. After a while, more molecules will have moved toward the top of the vessel, and the concentration gradient will have diminished. Eventually, the sucrose will be uniformly distributed throughout the solution in the beaker (Fig. 2-2D). When this occurs there are no longer any differences in concentration, and the system will have reached equilibrium.

Figure 2-2. Dissolution of sucrose molecules from a lump of sugar dropped into a beaker of water. In practice, this would take a very long time, for diffusion in unstirred liquid systems tends to be surprisingly slow.

There are two important characteristics of this process that you should remember. When a difference in concentration exists diffusion will always occur, unless some barrier, like an impermeable membrane, is placed in the way. This molecular movement is passive and spontaneous. No work need be done to cause diffusion to occur. The energy for the movement is contained in the concentration difference and does not need to be supplied from the outside.

Saturday, January 24, 2009

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BIOLOGY 204: INTRODUCTION TO PHYSIOLOGY LABORATORY MANUAL EXERCISE 2: DIFFUSION, OSMOSIS, AND MEMBRANE PERMEABILITY The rate of diffusion is affected by molecular weight, concentration gradient, and temperature as follows: Compounds with high molecular weights diffuse more slowly than compounds with low molecular weights. The greater the concentration gradient, the greater the rate of diffusion. The higher the temperature of the system, the greater the rate of diffusion.C P A MW X

These and other ideas are summarized in Fick's law of diffusion:Q=

where Q = net rate of diffusion C = concentration gradient of substance P = permeability of the substance A = surface area of membrane MW = molecular weight of the substance X = thickness of membrane In the first part of this exercise you will investigate the effects of molecular weight and concentration difference on the diffusion of solute molecules. You will also observe evidence of osmosis, the diffusion of water through a semipermeable membrane. A semipermeable membrane acts as a selective barrier, permitting some substances (in this case, water) to pass through it but not permitting others (in this case, certain solutes) to penetrate. The osmotic pressure of a solution is the opposing hydrostatic force developed by the difference in water concentration between two solutions. Although the diffusion of water is denoted by the special name osmosis, this process is simply a result of the tendency of molecules to move from a region of higher concentration to a region of lower concentration. Osmosis is a type of diffusion that occurs when something prevents solute molecules from diffusing freely, even though their concentrations differ. For example, a membrane impermeable to solutes will prevent them from moving from a region of high concentration to a region of lower concentration. However, if water can move freely through the membrane, it moves into the compartment having the greatest concentration of solute (and lowest concentration of water). When solute concentrations differ, so do water concentrations; where solute concentration is low, water concentration is high and vice versa. Water will diffuse from a region where its concentration is high (low solute concentration) to one where its concentration is low (high solute concentration). This movement of water decreases the difference between concentrations on the two sides of the membrane, both in terms of solute and solvent. If there were no opposing forces, the water would freely diffuse until the concentrated solution had expanded in volume and the dilute solution had shrunk in volume enough so that the concentration of solute (and water) on both sides of the membrane would be the same. But there are always opposing forces

Saturday, January 24, 2009

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BIOLOGY 204: INTRODUCTION TO PHYSIOLOGY LABORATORY MANUAL EXERCISE 2: DIFFUSION, OSMOSIS, AND MEMBRANE PERMEABILITY Let us look at an illustrative example in Fig. 2-3. Here we imagine a beaker containing 100 mL of water divided into two compartments by a semipermeable membrane. If 6 g of a solute to which the membrane is not permeable are added to compartment 1 and 4 g of the same solute are added to compartment 2, the concentration of solute will now be higher in 1 (which has 6 g in 50 mL) than in 2 (which has 4 g in 50 mL). Consequently, the concentration of water will be higher in side 2 than in side 1, and water will move by osmosis from 2 to 1 (Fig. 2-3A). But here's where the opposing force comes in. As the volume in compartment 1 increases due to osmosis, its level rises (Fig. 23B). This results in more hydrostatic pressure in compartment 1 than in compartment 2, and this opposes the osmotic flow of water from compartment 2 to compartment 1 by physically pushing the water back from compartment 1 to compartment 2. At equilibrium, the hydrostatic pressure in compartment 1 is equal and opposite to the osmotic pressure created by the difference in water concentration between the two compartments,