Potato Disc Osmosis Experiment Help!?

[Joker37]

Hi there everybody!

My biology class has just completed a practical activity on osmosis. I am supposed to write a practical report for the experiment however my teacher didn't give us any questions and I have to build up by discussion page so that my report will be lengthy, so I am wondering whether anyone would be able to provide me with questions that I could put in the practical report to include, answer and consider myself. Below is what was written on my practical sheet for the experiment.

 

Any help/guidance from you biologists for this activity would be much appreciated and I would be eternally grateful if anyone could help me with things to think about the experiment and any complexities I need to think/consider and put in my practical report. It seems like a pretty simple experiment just involving the placement of potato discs in different sucrose concentrations for 30 mins and recording the changes in mass. What are the expected changes in mass of the potato discs that were put in a lower concentration of sucrose compared to the ones which were put in a higher concentration of sucrose? What do you guys think?

 

Is this a common biology experiment by the way? Anyway, I emphasize any assistance would be highly appreciated!

 

Introduction: This activity focuses on osmosis and the effect of different concentrations of sucrose solutions on plant (potato) cells. All substances passing into a cell have to move through a plasma membrane. The plasma membrane is partially permeable. Movement can occur via a number of processes including diffusion.

 

Materials:

Fresh potato

Scalpel

Plastic ruler

Petri dishes

Measuring cylinder

Distilled water

Sucrose solutions – 0.1M, 0.2M, 0.4M, 0.6M, 0.8M, 1M

Paper towelling

Tweezers

Electronic balance

Felt pen

 

Method:

1. Label 7 Petri dishes (on lids) with the concentrations of sucrose solution.

2. (A cylinder of potato has been cut for you). Trim the ends flat so that no skin is left. Slice into 1-2 mm slices. You need 70 slices of potato for your group. Using tweezers make 7 groups of 10 slices- place gently on paper towel.

3. Weigh the first ten slices. Record the total mass of these ten slices in a table.

4. Place these ten slices into Petri dish 1 and cover slices with 30 mL sucrose solution of 0.1M. Cover dish with labelled lid and leave 30 minutes at room temperature.

5. Repeat procedure for remaining concentrations of sucrose and distilled water.

6. Carefully remove the ten slices from 0.1M solution and place on paper towel. Gently dab the top of slices to remove excess solution. Transfer to new towel when weighing.

7. Weigh the ten slices and record the mass in your table.

8. Repeat for the remaining dishes. Discard potato and clean up.

9. Calculate the change in mass and record findings in your table.

10. Also calculate the percentage increase or decrease in mass and place answers in the third column in the table.

11. Graph results (sucrose concentration x axis, %change in mass y axis). Remember you may have positive and negative changes and thus your y axis should allow for this. The graphs should be a line of best fit graph – in pencil. Remember to fully label the axes and give the graph a heading.

[Rade]

Hello,

 

Yes, the physical process of osmosis is of great importance to living cells. All biologists must completely understand the process of osmosis, the movement of water molecules across semi-permeable membranes.

 

==

 

Here are some questions for you to consider:

 

1. Why did you use 7 Petri dishes ? Why not 2, or 15 ?

2. Why did you use sucrose solution to add to the Petri dishes, why not ethyl alcohol ?

3. Why did you remove the skin from the potato cylinders ? Why did you use potato slices, why not slices from bark of tree ?

4. Why did you cut the potato slices 1-2 cm thick, why not 11-12 cm thick, or 100-110 cm ?

5. Why did you weigh 10 slices for each Petri dish--why not 2 or 35 slices ?

 

 

OK, now you add 10 more questions using the same method I used to give these first 6 questions.

[Joker37]

Here are some questions for you to consider:

 

1. Why did you use 7 Petri dishes ? Why not 2, or 15 ?

2. Why did you use sucrose solution to add to the Petri dishes, why not ethyl alcohol ?

3. Why did you remove the skin from the potato cylinders ? Why did you use potato slices, why not slices from bark of tree ?

4. Why did you cut the potato slices 1-2 cm thick, why not 11-12 cm thick, or 100-110 cm ?

5. Why did you weigh 10 slices for each Petri dish--why not 2 or 35 slices ?

 

 

OK, now you add 10 more questions using the same method I used to give these first 6 questions.

 

Thanks for the help so far! Really appreciate it Rade. Thanks. :)

 

Also, while we're at it do you know what the difference is between a discussion and a conclusion in a typical biology practical report?

 

I have to go somewhere now but I'll definitely be making some extra questions for my practical report later.

[Qfwfq]

Also, while we're at it do you know what the difference is between a discussion and a conclusion in a typical biology practical report?

The conclusion is, quite simply, the end or closing, the last bit, and it should sum up what you have learnt from the experiment. It should be put in the form a a few statements, the main important things that can be inferred and these are called conclusions.

 

You reach your conclusions mostly through the discussion; it is the detailed examination of what you did and the single results you found, organizing them and inferring logical consequences, comparing things and pointing out where they differ and where they have something in common, how one thing depends on others, all in a way that you can finally arrive at the conclusions.

[Joker37]

Also, does anyone know which of the potato discs should have gained the highest percentage mass change? And, would any be expected to lose mass?

(i.e. the potato discs which were put in 0.1 M, 0.2 M, 0.4 M, 0.6 M, 0.8 M, 1.0 M sucrose solutions or the distilled water)

 

I just want to check whether my results are consistent with what's expected. And if they're not then I could probably try finding reasons for why they're not consistent.

 

 

Thanks

[Joker37]

Also, I was wondering whether people could help make me more questions for the discussion and then I could probably make up answers to them. I've also been told that my class will actually be tested on the experiment with a bunch of questions regarding the activity some time next week. So, it would definitely help me a lot in preparing if people could post up some more questions (as much as possible if you can).

 

 

Thanks again to the people who made a post in this thread!

Very helpful and I appreciate it all.

[joekgamer]

Here's two:

1. What was the expected change?

2. Why was it expected?

 

EDIT:

And more:

3. What actually happened?

4. Why?

5. How is this different/similar to what was expected?

Edited March 17, 2011 by The Polymath

[Qfwfq]

And, would any be expected to lose mass?

This isn't easy to predict because the potato might be more or less dehydrated at the start of the experiment; according to this, a greater or lesser number of them would increase mass during the bath.

 

I just want to check whether my results are consistent with what's expected.

The most basic thing I would expect is, for the same initial mass, the ones in the bath of lower concentration would reach a greater final mass.

[Joker37]

Does the hypothesis have to be written down in past tense?

 

I heard that it does but just want to make sure.

[Joker37]

is it just because of the net movement of water molecules that are responsible for the change in mass of the potato discs?

 

ok just made up some rough random answers...don't really know the answers. what are should have i said?!?

 

Here are some questions for you to consider:

 

1. Why did you use 7 Petri dishes ? Why not 2, or 15 ?

 

Using 15 petri dishes would have taken far too long.

Using 2 petri dishes wouldn't have sufficiently eliminated outlying measurements.

?

 

2. Why did you use sucrose solution to add to the Petri dishes, why not ethyl alcohol ?

 

The sucrose cannot pass through a semi-permeable cell membrane because it is too big of a molecule to do so and because of this any change in mass in the potato discs must be due to net movement of water molecules either entering or leaving the potato cells through the process of osmosis. Ethyl alcohol can pass through the semi-permeable membrane of a potato cell and therefore if this was used instead of sucrose it wouldn't be clear whether any osmosis did occur. The purpose of this experiment was to investigate the effects of osmosis in potato cells placed in varying concentrations of sucrose. Usage of ethyl alcohol instead of sucrose places doubts on the extent of osmosis and hence sucrose solution is best to be used.

 

3. Why did you remove the skin from the potato cylinders ? Why did you use potato slices, why not slices from bark of tree ?

 

The skin from the potato cylinders may not allow the passage of water into or out of potato cells and hence had to be removed. This ensures that the processes of osmosis can be undertaken in the experiment and thus the validity of the method.

 

4. Why did you cut the potato slices 1-2 cm thick, why not 11-12 cm thick, or 100-110 cm ?

 

Thinner potato discs means a larger surface area-to-volume ratio. This means more water molecules per unit time will be able to move into or out of the potato discs.

 

5. Why did you weigh 10 slices for each Petri dish--why not 2 or 35 slices ?

 

Essentially, the more slices that are weighed than the more reliable the measurements are going to be due to the reduction of random errors and outlying data (e.g. because of oddly cut potato discs). However at the same time cutting 35 slices would have taken far too long and made the task far more tedious where experimentors would have been more likely to become more prone to make mistakes. On the otherhand if only 2 slices of potato discs were weighed then outlying data may not have got accounted for and thus extreme values from either ends of the scale could have been recorded, leading to inaccuracies.

[Joker37]

Also, is it legitmate to assume that all the potato discs which gained mass were placed in a hypotonic sucrose solution?

(and hence became turgid)

 

And those that loss mass were put in a hypertonic sucrose environment?

(a thus became more flaccid)

[Joker37]

Also, what could be a variable and a limitation to this experiment?

[Joker37]

3. What actually happened?

4. Why?

 

The potato discs which gained mass were put in a hypotonic sucrose solution. Because there was a higher concentration of sucrose molecules within the potato cells a net movement of water molecules was observed coming into the potato cells in an attempt to maintain an equilibrium position between the concentrations of sucrose within and outside the potato cells.

 

The potato discs which lost mass were put in a hypertonic sucrose solution. Because there was a higher concentration of sucrose molecules outside the potato cells a net movement of water molecules was observed coming out of the potato cells in an attempt to maintain an equilibrium position between the concentrations of sucrose within and outside the potato cells.