Lab Assignment #4: Movementof Molecules PDF

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This is a lab assignment for our biology class. If one does not complete their lab assignments then he/she will fail the course. This lab assignment talks about molecules under the microscope. When we look at molecules under the microscope we see living organisms moving around. But why do they move?...

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MOVEMENT OF MOLECULES OBJECTIVES By the end of this exercise you should be able to: 1. Describe and explain Brownian motion. 2. Describe and explain diffusion and osmosis. 3. Name three factors that affect the rate of diffusion and decribe how they affect the rate. 4. Describe the effects of hypertonic, isotonic, and hypotonic solutions on red blood cells and Elodea cells. ________________________________________________________________ _____________________________INTRODUCTION_____________________ All living cells constantly exchange materials with their environment. For example, animal cells obtain nutrients and oxygen and remove waste. This exchange of material between the cell and its environment is critical to survival. In following series of experiments we will study principles of molecular movement in order to understand how materials are distributed within a cell and how they are exchanged between a cell and its surroundings. BROWNIAN MOVEMENT The molecules of any liquid are in constant motion due to their kinetic energy. Because cytoplasm is approximately 70% to 80% water, its molecules are also in constant motion and this motion helps to distribute materials within the cell. We cannot see the movement of water molecules but we can see the effect of their motion on small particles. Carmine is a fine powder that forms a suspension of tiny particles when mixed with water. Your instructor will show you such carmine suspenssion under high power of a microscope. Examine the motion of these nonliving carmine particles. The bouncing movement of small particles suspended in liquid is called Brownian motion and is due to collisions of these particles with the fast-moving molecules of liquid. In our example, water molecules are pushing the carmine particles. Each time a particle is hit, it rebounds, or bounces away. The water molecules that are moving the carmine particles weigh about five billion times less than the smallest particle you can see with the ordinary light microscope. Describe the path of movement of a carmine particle. ____________________ ________________________________________________________________ Effect of Particle Size The kinetic energy of the water molecules is what causes the carmine particles to move. If all other factors are equal then a given amount of force can move a small particle farther than it can a large one. For example, if a player strikes a baseball with 37

a baseball bat it would travel a lot farther than a basketball. Compare the rate of movement of some of the smaller carmine particles on your slide with that of some of the larger ones. How does particle size influence rate of movement ?______________________ ________________________________________________________________ _______________________________DIFFUSION________________________ Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration. It is a very important process in living organisms. For example, in your lungs, oxygen enters the circulatory system and carbon dioxide leaves it by passive diffusion. The constant movement of molecules is responsible for diffusion. Each molecule moves in a straight line until it bumps into another molecule or the side of the container. After the collision, the molecule will move in another direction. When there are more molecules (a substance is in higher concentration), there are more chances of collisions. So in the center of the cluster there are more possible paths away from the center of the cluster. Therefore, although the movements of molecules are random, the direction of movement tends to be from a region of greater concentration to an area of lesser concentration. Several factors influence the rate of diffusion including the state of mater (solid, liquid or gas), particle size, temperature, mixing and concentration. Diffusion through a Gas We can demonstrate the effect of size or molecular weight on diffusion using the gases ammonia (NH3) and hydrogen chloride (HCl). We are unable to see these gases directly, but when they meet they will react to form ammonium chloride (NH4Cl), which forms a white precipitate. Your laboratory instructor will saturate one cotton plug with hydrochloric acid (HCI) and another with ammonium hydroxide (NH4OH). Both plugs will be inserted simultaneously into opposite ends of a long piece of glass tubing. The NH4OH dissociates into water and ammonia (NH3, a gas). You will see a white "cloud" of ammonium chloride forming at the spot where the gases meet. Hydrogen chloride (HCl) has a molecular weight of 36 and ammonia (NH3) of 17. At which end of the tube does the reaction take place? ____________________ What can you conclude about the relationship between a molecule’s rate of diffusion and it’s molecular weight? _____________________________________________

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Diffusion through a Liquid Place a petri dish filled with distilled water on graph paper (each box is 1 mm2). Your instructor will carefully place a single crystal of potassium permanganate in the center of the dish. Every 15 seconds, measure the total distance (number of boxes measureing from the center of the crystal) that the purple dye has diffused. Record your results in the table below and use this data to construct a graph. Draw a line that connects each data point (dot-to-dot). Time (sec) 15 30 45 60 75 90 105 120 135 150 165 180 195

Distance (mm)

The independent variable is _______________ The dependent variable is ________

Was the rate of diffusion constant? ______________ If not, explain why it changed. ________________________________________________________________

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________________________________________________________________ OSMOSIS When there are no barriers to the movement of molecules, they are free to diffuse from a region of higher concentration to a region of lower concentration. But, cells and organnells inside cells are surrounded by membranes. These membranes are differentially permeable (semipermeable), they allow some, but not all substances to pass through. Osmosis is the diffusion of water through a differentially permeable membrane from a region of higher concentration of water to one of lower concentration of water. Or in other words form a more dilute solution to more concentrated solution. Because when a substance is dissolved in water, its molecules fill the space previously occupied by water molecules, thus reducing the concentration of water. For example, a solution that is 20% sugar is only 80% water. Therefore, it may also be said that osmosis is the diffusion of water through a semipermeable membrane from a region where the dissolved substances are less concentrated to a region where they are more concentrated. Consider what happens when the cell is placed into various concentrations of solutions of particles that are not freely permeable through the cell membrane. If an animal cell is placed into a solution with a greater concentration of dissolved particles than the cytoplasm, water will move out of the cell, causing it to shrink or crenate. Such a solution is hypertonic to the cell. On the other hand, if a cell is placed into a solution with fewer dissolved particles than the cytoplasm, water will enter the cell, causing it to swell and perhaps even to burst. This solution is hypotonic to the cell. A cell placed in an isotonic solution, one that has the same concentration of dissolved particles as the cytoplasm, will not gain or lose water. Water molecules will enter and leave the cell at equal rates. A SIMPLE OSMOMETER Osmosis can be demonstrated when two solutions are separated by dialysis tubing. This tubing has microscopic holes in it that are large enough to allow the passage of water molecules but are too small to permit the passage of larger molecules such as sugar. The dialysis tubing serves as a semipermeable membrane because it allows some, but not all, molecules to pass through it. An osmometer is a device for measuring osmotic pressure. To make a simple osmometer, cut a piece of dialysis tubing so that it is approximately 6 inches long, and rub it gently between your fingers under water until it softens and forms a tube. Tie one end in a knot. Slip the other end over a rubber stopper through which a thistle tube or pipette has been inserted. Then slip a rubber sleeve over the outside to hold the dialysis tubing in place. (This "sleeve" is a larger size rubber stopper that has been bored to have a hole the size of the smaller rubber stopper.) Repeat this procedure to form two sacs of equal size. Fill one sac with a 25% sucrose solution, and the second with a 50% sucrose solution. You will have to squeeze each dialysis bag gently to remove the air. Fill each of these osmometers so that the liquid reaches 40

approximately the same point on the lower part of the thistle tube. Mark the initial fluid level on each thistle tube.

Suspend each sac in its own beaker of distilled water. The sacs should not touch the bottom or sides of the container and the water should completely cover each sac. At l0 min. intervals during the laboratory time, measure the height to which the solution has climbed in each thistle tube.

In which tube does the liquid rise highest? _____________

Explain the results. ________________________________________________ _______________________________________________________________ OSMOSIS in Red Blood Cells The effects of osmosis on animal cells can be demonstrated by exposing sheep red blood cells to solutions of various concentrations of sodium chloride. 1. Place a drop of sheep blood on three microscope slides. 2. Quickly add a drop of 0.9% salt solution to the first slide, of 5% salt solution to the second slide, and distilled water to the third. Label the slide with the concentration of the salt solution added. Tip the slide back and forth to mix the blood and the salt solution. Place a coverslip over each drop. 3. Using the microscope, compare the shape of the red blood cells on the three slides. If osmosis is occurring, the cells will gain or lose water depending on the concentration of the added salt solution. A cell that has lost water will appear wrinkled or crenated. A cell that has gained water will swell. This stretching will change the properties of the membrane enough to allow the hemoglobin to escape. In this case, one would observe large transparent cells or "ghosts." Sometimes the swelling may 41

o extreme that the red r blood cell membbrane will rupture, r a process c alled be so hemo olysis. The normal red d blood cell shape, obbserved in an a isotonic solution, iss that of a biconcave b d disc.

Toniccity: Cell:

__ _________ ______ __ ______________ ___________ ______ __ __________ ____

___ _ _________ _ ____ __ __________ _ ____

ch solution would you choose to have h injecteed if you neeeded an in ntravenous Whic addition of fluid to t your bod dy? ______ _ ____________________ a nt Cells OSM OSIS in Pla Elode ea lives in fresh water, which is a hypoton ic environm ment. As water w enterss the cell, the t large central c vacu uole swellss and exertts pressur e (tugor pressure) which w pushe es the cyto oplasm aga ainst the ce ell wall. Thee cell beco omes turgi d (firm) but the cell wall w prevents lysis. Wh hen a plant cell is placced in a hy pertonic p sollution and loses waterr, the centra a l vacuole shrinks and d cytoplasm m pulls awa ay from the e cell wall. S Such a cell is said to b e plasmollyzed; it has undergonne plasmollysis. 1. Make a wet m ount of an n Elodea leaf and focuus under hig gh power. 2. Ad dd a drop or o two of 5% % salt soluti on to the edge of the coverslip. Touch T a pie ece of paper towe e l to the opposite side e of the coveerslip to pu ll the solutiion through and ob bserve. 3. Trry to reverse the proce ess. Add a f ew drops of o distilled water w to the e edge of th he co overslip and pull it thro ough. Observe the cells. 4. In the space below draw w the Elode ea cell beforre and after exposure to 5% NaC Cl. Identifiy the tonicity of th he solution (hypotonic, isotonic or hypertonic) and desccribe ndition (turg gid, flaccid or o plasmolyyzed). the cell’s con

Solut ion:

pond water

Toniccity: Cell:

__ _________ ________ ___________ ________

5% NaCl ________________ _ _______ _________ 42

QUESTIONS FOR FURTHER THOUGHT AND REVIEW 

Why must particles be extremely small to exhibit Brownian movement ?

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A gardener’s favorite plant wilted several days after he applied twice the recommended amount of fertilizer. What probably happened ? Can you suggest a remedy ?

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What is the difference between molecular motion and diffusion ?

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Indicate with an arrow the net movement of water between hypotonic and hypertonic solutions separated by a selectively permeable membrane. Hypotonic Solution

Hypertonic Solution

5. What results do you have to support the conclusion that diffusion is inversely related to molecular weight ?

6. Why are diffusion and osmosis referred to as physical, passive processes ?

7. In kidney dialysis, a patient’s blood is passed through a bath that contains a mixture of several ions and molecules. The blood is separated from this fluid by a dialysis membrane that is permeable to water, small ions and small molecules, but is impermeable to proteins and blood cells. What should the composition of the dialysis fluid be for it to remove urea (small molecule) without changing blood volume (removing water) ?

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