Diffusion Osmosis Lab 2

 

Biology – Diffusion and Osmosis Lab 2 In this lab we will continue our investigation into diffusion and osmosis. Today, we will be observing the effects of these phenomena on living cells and modeling the changes that a cell might undergo under hypertonic, hypotonic, and isotonic conditions. Timeline for Activity (Suggested) Investigation 1:30-1:45 1:45-2:00 1:45-2:00 Part I Set up and (Potato) Part II (Celery) Part III (Egg)

begin

2:00-2:15

2:15-2:30

2:30-2:45 2:30-2:45

Run experiment None

Test None

2:45-3:00 End experiment

None Set up

3:00-3:15

Clean up None

Background You should now be familiar with the concepts of diffusion (movement of solutes) and osmosis (movement of water). Both occur in a certain direction; specifically, diffusion and osmosis always work down the down  the concentration gradient. In other words, molecules move from areas of high concentration (of those molecules) to areas of low concentration. Solutes will diffuse to “space out” or balance both sides of a membrane. Water will move into areas of high solute s olute concentration to lower the effective concentration of those solutes. Living cells are surrounded by semi-permeable membranes. Water is generally allowed to move freely in to or out of a cell. ce ll. Other solutes may or may not be able to diffuse. S Since ince we will be using solutions with solutes too large for most cell ce ll membranes to allow, we will mainly be studying osmosis in this lab. lab. Part I – Osmosis in Potato Cells Note: Potato cells have a high level of starch within them. Water does not “care” what sort of solute is in a cell – if there are any  solutes,  solutes, water will try to balance the concentration inside to what is outside. Materials   (6) 250mL beakers 1. 0M)   Sucrose solution (0.2M, 0.4M, 0.6M, 0.8M, 1.0M)   Distilled water   Potato discs (pre-cut)   Lab scale   Paper towels, forceps 

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Hypothesis Make a hypothesis about what you think you will observe when your potato slices are each submerged in the various sucrose solutions (and one in distilled water) and left for a length of time. Procedure 1.  Read this entire procedure, then make a data table that will contain the appropriate data. 2.  Obtain 6 beakers and fill each with about 200mL of a different sucrose solution. (One will be filled with distilled water, which would be a 0.0M solution.) sol ution.) 3.  Obtain several potato slices. You want to have enough to be able to put 3-4 slices in each beaker. 4.  Divide your slices among your different beakers, but do not submerge them yet.

 

5.  Weigh each group of potato slices and record this initial mass in your data table. 6.  Submerge each group of slices in their respective beakers. Note the time. This experiment will run for 90 minutes. 7.  At 15-minute intervals, remove your potato slices and quickly weigh them. Record these masses in your data table. Continue this process until the end of your test time. 8.  When you are done, throw your potato pieces away and pour your solutions down the drain.  Analysis and Conclusions 1.  Calculate the percent change in mass for each of the 6 samples you tested. Remember that a drop in massthe is adata negative changechange and should have negative sign. 2.  Collect for percent in mass of aeach solution from the other groups, and calculate the averages for each. 3.  Make a graph of the change in mass (y) vs. molarity of solution (x). (This should be a line graph.) You should graph both your data points and the class average data points. You may do this on a computer and submit separately. 4.  From your data, what can you conclude about the t he concentration of solutes inside inside the  the potato cells normally? How did you arrive at this conclusion? 5.  Did your conclusions support your hypothesis? If not, explain where the differences might have arisen.

Part II – Osmosis in Celery Background Plant cells are surrounded by a cell membrane just like animal cells, but outside that is a more rigid cell wall. This wall provides strength and structure to the plant, especially in stems (which often must hold the weight of the plant). Because of this, they can withstand more internal pressure from water contained within – called “turgor pressure” – than – than an animal cell would be able to survive. Hypothesis Make a hypothesis about what you expect to observe in celery soaked in distilled water and celery soaked in a high-concentration NaCl solution when you bend them. Materials Celery stalk soaked in distilled water    Celery stalk soaked in 1M NaCl solution

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Procedure 1.  Remove a stalk of celery from the distilled water, and bend it until it breaks. Record your observations about this piece in your data section (no table required). Note the amount of bend, the force of the break, and the sound produced. 2.  Repeat this step, but this time use a stalk of celery from the NaCl solution. Record your observations.  Analysis and Conclusions 1.  What do your observations indicate about the turgor pressure of each stalk of celery? 2.  Why do you think each stalk behaved as it did in your experiment? 3.  Did your observations support your hypothesis? Explain.

 

Part III – Modeling Osmosis in Living Cells Background Chicken eggs are useful models of living cells because they behave in much the same way when subjected to osmotic pressure. The only problem is that hard outer shell. Fortunately, chemistry can help us here. Egg shells can be easily dissolved by soaking soa king them in vinegar for 24-48 hours. Materials   4 eggs 

Large beaker or bowl    Vinegar

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Procedure Carefully place your eggs in a bowl or large beaker and cover with enough vinegar to just submerge them. You should notice bubbles forming immediately around the shells – this is the vinegar dissolving the shell material and forming gas (among other products). Leave this set up for at least 24 hours. We will continue this investigation at a later date once the eggs are prepared.

Post-Lab Questions You will have a reading about water potential and several se veral questions to answer about diffusion and osmosis after this lab. They will be on Canvas and should be submitted online.