Osmosis bio lab 2 PDF

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SimBio Virtual Labs®

OsmoBeaker®: Osmosis NOTE TO STUDENTS:

 This workbook accompanies the SimBio Virtual Labs® Osmosis laboratory. Only registered

subscribers are authorized to use this material.  Laboratory subscriptions may not be shared or transferred.

Student’s Name: Lauren Hargrave

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Signature: Lauren Hargrave Date: April 3rd 2020

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           This and other SimBio Virtual Labs® are  accessible through SimBio’s SimUText System®. This workbook, or any portion thereof, may not be reproduced or used in any manner without the express written permission from the copyright holder. Students purchasing this workbook new from authorized vendors are licensed to use the associated laboratory software; however, the accompanying software license is non-transferable. This lab is based on work initially supported with funding from the U.S. National Science Foundation under Grant No. 0230740.

SimBio Virtual Labs®: OsmoBeaker®

Osmosis    

   

Mixing Blood and Water: Osmosis and Cells A patient is brought into the emergency room unconscious, with blood gushing from a wound. Among the first things you must do is get fluid into her circulatory system. If too much blood drains out, her veins could collapse. So you stick a needle into a large vein in her arm and hook up an intravenous (IV) system for delivering fluids into veins. You’re probably familiar with IVs from movies and TV shows: they are the bags hanging from a metal pole with a tube running into the patient. But what is in that bag? Is it just water, or does the water contain something? If so, does it matter what else is in the water? Later in the day, another patient comes into your clinic, a man who’s just had his 50th birthday. As standard procedure, you want to check his intestine for polyps, a possible sign of colon cancer. But the intestines are usually a bit obstructed with the remains of the last few meals. You give him pills to take with lots of water the day before his exam. The pills are made of small molecules that are indigestible and therefore will pass right through the digestive system. This causes water to leak out into the intestine and wash it out. It’s not very pleasant for the patient, but much more pleasant for you when you do your exam. But why should some indigestible material cause water to exit his body? These are two of many of possible examples that involve osmosis, a process by which water moves across membranes in response to differences in the concentration of other molecules dissolved in the water. In this lab, you’ll take advantage of a fictional invention from a fictional bio-engineering company called Fictional Science, Incorporated. Their “Cell-O-Scope” simulator allows you to create tiny “SimCells” made out of a material that is similar to the membranes that surround animal cells. The Cell-O-Scope lets you see molecules moving around inside of the SimCells. By conducting experiments on SimCells and

making observations of how the SimCells respond to different extracellular fluids, you will learn how osmosis works, and why it is an important property to consider when designing IV fluids.

Concentration and Diffusion: a Brief Review Before you begin this lab, it is important that the meaning of concentration and the process of diffusion make sense to you. Concentration is the amount of a substance in a particular volume or space. Concentration is a relative measure that is often quantified as a percent. A 20% salt solution contains twice as many salt molecules in any given volume (e.g., a liter) than a 10% salt solution. Diffusion is the movement of molecules from areas of higher concentration to lower concentration. This lab explores osmosis, a special case of diffusion. Figure 1 illustrates the process of diffusion. In each case, molecules move from areas of higher to lower concentration. (Note that in 3A, the initial number of molecules is the same on both sides of the membrane, but the concentration of molecules is different.)

Figure 1. Diffusion of Molecules Across a Permeable Membrane. A membrane that does not restrict the movement of molecules divides each box into two compartments. Comparing A to B, you can see that in each of the three examples, the number of molecules decreased on the side of the box that started out more concentrated, and increased on the side of the membrane that started out less concentrated.

A common misconception about diffusion is that molecules are somehow directed or “want” to move to where there is more space. This is not the case! Groups of molecules tend to spread out over time, but individual molecules are in constant random motion and are equally likely to move in any direction. You will be able to see this for yourself as you conduct experiments in this lab.

Exercise 1: Pressure and Equilibrium [1]

If you haven’t already, start SimUText® by double-clicking the program icon on your computer or by selecting it from the Start menu.

[2]

When the program opens, enter your Log In information and select the Osmosis lab from your Assignments window. The Cell-O-Scope Simulator (from Fictional Science, Inc.) has two monitors that let you see what happens when artificial cells (“SimCells”) are suspended in different extracellular fluids. The Cell View Monitor lets you watch the entire SimCell. The MembraneViewMonitor zooms in on the SimCell membrane (the stripe down the middle of the Membrane  View Monitor), letting you watch molecules move around inside the SimCell (on the left side of the membrane) and in the extracellular fluid (on the right side of the membrane). ฀

NOTE: In the real world, you could never actually see the molecules in a cell. If you could see the molecules, there would be a lot more of them!

[3]

The Settings Panel beneath the monitors identifies the types of molecules present, and lets you see the approximate number of each type of molecule in the cell and in the extracellular fluid. The Control Panel at the bottom of the screen runs the Cell-O-Scope Simulator. Try clicking the GO button to run the simulator. The molecules should start moving. N  otice that the movement of any individual molecule is random. T hen click the STOP button.

[4]

Make sure that the simulator is STOPPED, then push the cell membrane by clicking on the membrane and dragging it to the left. Watch the SimCell in the Cell  View Monitorchange shape as the membrane is compressed. Next, drag the membrane to the right to stretch the membrane. The relative areas you see in the Membrane View are proportional to the relative volumes of the SimCell and extracellular fluid. You can assume that when the membrane position is in the middle (at 20 on the Membrane Position scale), that the SimCell occupies half of the volume of the Cell-O-Scope. [ 4.1 ] What happens to the membrane (visible in the Membrane View Monitor), when you try pushing it all the way to the right? (If you’re not sure what happened, click the RESET button in the Control Panel and try again.)

The membrane position says “N/A”.

[ 4.2 ] What happens to the SimCell (visible in the Cell View Monitor) when you try pushing the membrane all the way to the right?

The SimCell expands then breaks.

[5]

Click the RESET button in the Control Panel. Then click the GO button. The molecules on either side of the membrane will begin to move around in random directions. ฀

Every time a molecule happens to collide with the flexible membrane, the impact causes the membrane to move. The total force of the impacts of all the molecules on the membrane is called pressure.



[ 5.1 ] If you increase the number of water molecules in the extracellular fluid, will the membrane move to the right or the left in the Membrane View? Explain your reasoning in terms of pressure.

I predict that increasing the number of water molecules will cause the membrane to move to the left because the pressure is expanding.

[6]

In the Settings panel, increase the Initial Count of water molecules in the extracellular fluid from 250 to 500. Click the SET button and the simulation will reset with two times as many water molecules to the right of the membrane.

[7]

Next, click the STEP button (to the right of the GO button) to run the simulation 100 time steps. [Note: you can see how many time steps have passed in the Time Elapsed box next to the Control Panel.]

[ 7.1 ]

Was your prediction in Question 5.1 correct? What happened?

My prediction was correct as the membrane moved to the left.

[8]

Adding water molecules to the extracellular fluid results in more molecules colliding with the outside of the membrane. The additional pressure against the membrane should have caused it to move left. ฀ As  the membrane moves to the left and the cell shrinks, there will be less and less space for the molecules inside the cell to move around, and this will cause them to bump into

the membrane more frequently. Meanwhile, the molecules on the extracellular fluid side of the membrane will have more and more space and bump against the membrane less. Thus, i f you run the simulation longer, the pressure on either side of the membrane should eventually equalize. 

[9]

[ 9.1 ]

Think about the above description. Then run the simulator for 500 additional time steps by clicking the STEP button five more times. For each 100 time steps, pay attention to whether the membrane consistently moves in one direction. How can you tell when the pressures have equalized?

You can tell the pressures have equalized when the membrane does not move further to the left.

[ 9.2 ]

What is the approximate “equilibrium” position of the membrane?

13.2 [ 9.3 ] If you continue to run the simulator after the system has reached equilibrium, why does the membrane continue to jiggle back and forth?

The membrane jiggles back and forth because it's unstable. ฀

A system is at a stable equilibrium when it consistently returns to its original state after being pushed slightly in any direction.

[ 10 ] If the simulator is running, click the STOP button, but do NOT reset it. You can test whether the new membrane position is at a stable equilibrium by pushing it a little in one direction and seeing if it returns to that position. Without resetting the simulator, push the membrane by dragging it a little bit to the left or right, being careful not to push it past its breaking point. Then RUN the simulation for at least 500 time steps.

฀

NOTE: If you push the membrane past its breaking point and the membrane disappears, you’ll need to reset the simulator and let it run until the equilibrium position is reached before trying again.

[ 10.1 ]

Was the membrane at a stable equilibrium before you pushed it? How could you tell?

The membrane was not at a stable equillibrium because it did not return to its original state after pushing it.

[ 11 ]

Next, observe how temperature influences how molecules move by moving the temperature slider at the bottom of the screen to the right (high temperature) and to the left (low temperature) and running the simulator. Once you have a feel for how temperature influences the movement of molecules, click the RESET button. [ 11.1 ] Based on how the molecules moved at different temperatures, which of the below choices (a, b, or both) do you think is correct? [Note: you’ll experiment with this in the next step; for now simply make a prediction.]

[ 11.2 ]

a.

at higher temperatures, the equilibrium position of the membrane should shift to the right.

b.

at higher temperatures, the system should equilibrate faster.

Explain your choice(s).

I predict B, that at higher temperatures the system should equilibrate faster.

[ 12 ]

Experiment with temperature to determine whether your answer to Question 11.1 was correct. [ 12.1 ] Describe the experiment(s) you tried and what you figured out. Does temperature influence the equilibrium position of the membrane? Does temperature influence how long it takes for the system to equilibrate? Explain your results in terms of pressure.

The temperature influences the speed at which the system equilibrates. The hotter the temperature the quicker the system will equilibrate.

[ 13 ]

Click the TEST YOUR UNDERSTANDING button in the bottom right corner of the screen. You can try as many answers as you like to make sure that you understand why only one is correct.

Exercise 2: Osmotic Pressure So far, your membrane has been impermeable (nothing can pass through it), and the SimCell and extracellular fluid have only contained water. Real cells contain water and lots of other types of molecules dissolved in the water. Water is referred to as the solvent in cells, because most of the molecules in real cells are water molecules. The molecules dissolved in the water are referred to as solutes. Real cells also have channels that allow water to pass through their membranes. Osmosis is the diffusion of water across a semi-permeable membrane—a membrane that is permeable to some molecules (such as water) but not others (such as dextrose). If water molecules diffuse across a membrane into a cell, the pressure of the added molecules will cause the membrane to stretch and the cell to grow.

[1]

Select Osmotic Pressure 1 from the SELECT AN EXERCISE button

in the upper left-hand

corner of the screen. [2]

The new SimCell contains 200 water molecules and 50 dextrose molecules. The extracellular fluid contains 250 moles of water molecules, so the total number of molecules is the same on either side of the membrane (250). Click the GO button to confirm that the pressure on either side of the membrane is the same. [ 2.1 ]

How can you tell that there is no major difference in pressure on either side of the membrane?

Because the membrane keeps wiggling back and forth.

[3]

Next you will add channels to the membrane that will allow water molecules to pass through. Click the RESET button and then write down a prediction for what you think will happen to the position of the membrane in the Membrane  View Monitor when you make the membrane permeable to water. [ 3.1 ]

Will the membrane move to the right, left, or stay in the same position? Explain your reasoning.

I predict that the membrane will stay the same position because of the different pressure.

[4]

Add water channels by checking the box next to the water molecule icon in the Membrane Permeability column on the right. Then click the GO button to test your prediction. Run the Simulator for at least 1000 time steps. [ 4.1 ] Were you correct? What happened?

I was incorrect, as the membrane moved back and forth then gravitated towards the right.

Since the total number of molecules starts out the same on both sides of the membrane (250), the same number of molecules hit the membrane from each side as before, but this time, some water molecules pass through. If there are 250 water molecules in the extracellular fluid and only 200 inside of the cell, by random chance, more water molecules will move from the extracellular fluid to the cell than vice-versa. This movement results in a greater total number of molecules inside the cell, and thus greater pressure. ฀

Osmotic pressure is the pressure generated by osmosis. Technically, it is the minimum pressure that must be applied to a solution to prevent water from flowing across a semi- permeable membrane into the solution.

If osmotic pressure in the SimCell increases, the membrane will stretch, and the cell will expand. If the cell membrane is stretched too far, it will break, as you saw if you ran the simulator long enough in Step 4.

[5]

Next, reverse the conditions by loading OSMOTIC PRESSURE 2 in the SELECT AN EXERCISE menu. This time, the simulator will start out with the 50 dextrose molecules in the extracellular fluid rather than in the SimCell. Again, make the membrane permeable to water.

[ 5.1 ] Do you predict that the SimCell will expand or shrink? Why?

I predict that the SimCell will shrink because of the water escaping it.

[6]

Make sure the membrane is permeable to water, then run the simulator to test your prediction.

[ 6.1 ] Were you correct? What happened?

My prediction was correct, as the membrane shrunk.

As you’ve seen, osmosis causes SimCells to expand or contract. In the real world, animal cells work similarly, although their membranes are much more complicated and the limits to how much real cells can expand and contract are different. Plant cells, however, are different than animal cells in a critical and interesting way. Plants have evolved cell walls that take advantage of osmotic pressure! An animal cell expands and contracts more or less like a balloon. But a plant cell’s wall is more like a plastic bag that can inflate, but not stretch much. When water flows into a plant cell, the rigid cell wall restricts expansion, generating pressure against the cell wall (called “turgor pressure”). So when you water a drooping plant, osmosis causes the cells to fill up with water, generating osmotic pressure. As water molecules push on the stiff outer cell walls, the resulting turgor pressure allows the entire plant to become stiffer and more upright again.

[7]

After you have read the above paragraph, click the TEST YOUR UNDERSTANDING button in the bottom right corner of the screen. You can click as many answers as you like to make sure that you understand why only one is the correct response. Feel free to use the space below for notes.

Exercise 3: Osmosis and Concentration In your last experiment, you should have seen that the SimCell containing dextrose molecules ruptured in an extracellular fluid containing pure water. Your ultimate goal is to develop a formula for IV fluid, and it probably would be best if that solution does not cause cells to rupture! This exercise explores how to use calculations of solute concentration to control osmosis in the Cell-O-Scope. [1]

Select Osmosis  and  Concentration 1 from the SELECT AN EXERCISE button

in the

upper left-hand corner of the screen. The SimCell again contains 50 dextrose and 200 water molecules. The extracellular fluid is pure water. [2]

Start by calculating the solute concentration in the CELL. The concentration is simply the number of solute molecules (50 dextrose molecules in this case) divided by the total number of all molecules in the cell (200 water + 50 dextrose). Multiply this number by 100 to transform it to a percentage. [ 2.1 ] What is the solute concentration (%) inside the SimCell? Show how you set up your calculations in the space below. [Note: you can use the Calculator Tool at the bottom of your screen.]

The solute concentration formula is 50/250= 0.2(100). The solute concentration is 20%

[ 2.2 ] What is the concentration (%) of water molecules inside the SimCell? Show how you figured this out.

200/250= 0.8(100). The concentration of water molecules is 80%

[3]

Next, add dextrose to the extracellular fluid to give it a solute concentration of 10% and a water concentrat...