Biology Science Investigation - Osmosis -

Biology Science Investigation – Osmosis – Biology JHN

Planning

Osmosis – Makeans – David Makean

If a dilute solution is separated from a concentrated solution by a partially permeable membrane, water diffuses across the membrane from the dilute to the concentrated solution. This is known as osmosis.

A partially permeable membrane is porous but allows water to pass through it more rapidly than dissolved substances.

Since a dilute Solution contains, in effect, more water molecules than a concentrated solution, there is a diffusion gradient, which favours the passage of water from the dilute to the concentrated solution. In living cells, the cell membrane is partially permeable and the cytoplasm and vacuole (in plant cells) contain dissolved substances. As a consequence, water tends to diffuse into cells by osmosis if they are surrounded by a weak solution, e.g. fresh water. If the cells are surrounded by a stronger solution, e.g. seawater, the cells may lose water by osmosis.

Explanation of osmosis

When a substance such as sugar dissolves in water, the sugar molecules attract some of the water molecules and stop them moving freely. This, in effect, reduces the concentration of water molecules. In Fig. 11 the sugar molecules on the right have captured half the water molecules. There are more free water molecules on the left of the membrane than on the right, so water will diffuse more rapidly from left to right across the membrane than from right to left.

The partially permeable membrane does not act like a sieve, in this case. The sugar molecules can diffuse from right to left but, because they are bigger and surrounded by a cloud of water molecules, they diffuse more slowly than the water.

Artificial partially permeable membranes are made from cellulose acetate in sheets or tubes and used for dialysis rather than for osmosis. The pore size can be adjusted during manufacture so that large molecules cannot get through at all.

The cell membrane behaves like a partially permeable membrane. The partial permeability may depend on pores in the cell membrane hut the processes involved are far more complicated than in an artificial membrane and depend on the structure of the membrane and on living processes in the cytoplasm. The cell membrane contains lipids and proteins.

Anything, which denatures proteins, e.g. heat, also destroys the structure and the partially permeable properties of a cell membrane. If this happens, the cell will die as essential substances diffuse out of the cell and harmful chemicals diffuse in.

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Water Potential

The water potential of a solution is a measure of whether it is likely to lose or gain water molecules from another solution. A dilute solution, with its high proportion of free water molecules, is said to have a higher water potential than a concentrated solution, because water will flow from the dilute to the concentrated solution (from a high potential to a low potential).

Pure water has the highest possible water potential because water molecules will flow from it to any other aqueous solution' no matter how dilute.

Variables

Temperature – The warmer the solution, the quicker the osmosis will go. This is because the water molecules will be moving more quickly. Careful though, if you heat something like the potato chip up too much then you will simply destroy all the cells.
Difference in concentrations – This will speed up the rate of osmosis because there will be more particles that’ll have to change places.
Surface Area – This will speed up the rate of osmosis because there will be more space on the potato for osmosis to take place.
Concentration of water in potato – the higher the concentration of water in the potato the quicker osmosis will happen because less water will have to diffuse into the potato to even the amounts of water inside and outside the potato.

Prediction

I predict that the higher the concentration of the sugar, the lower the mass of the potato at the end of the experiment. This is because with a higher concentration of sugar in the water, there will be less water molecules in the solution, so it’ll take less time for them to diffuse into the potato. This is explained in Fig. 11 above.

Carrying out the Experiment

For the experiment I used:

A ruler - to measure the length of the potatoes so we knew where to chop them. We had to use the millimetres on the ruler to get the most accurate result.
Top-pan balance – we used this to find the weight of the potatoes, and we used it to two decimal places. This gave more accurate results.
Test tubes – we put the potatoes in these
Water/Sugar solution – we used this at 0,2,4,6,8 & 10 concentrations. 0 being pure water and 10 being 10 % sugar solution. We put this in 6 different test tubes and then we put three potatoes in each tube.

We chose the variable: “Concentration”

This was pre-measured by the lab technicians so we don’t actually know if they did it 100% accurately.

In the experiment I will chop thin strips of potato into three. Then I will measure them on the top-pan balance and record their weights. Then I shall put them in a test tube with a sugar solution in. I will do this six times so in the end I will have 6 test tubes with three pieces of potato in each and 5cm cubed of solution in each. The concentrations will vary for 0 – 10. Then I will leave it 24 hours and then come back and find out the results. I will do the experiment twice to get better results.

I will make this experiment a fair test by making sure all the pieces of potato are the same mass and length, I will make each test tube have the same amount of solution in, and I will make sure the temperature where it is kept is always the same. I will not be able to maintain the temperature all the time but I will be able to make sure the mass of the potatoes and the amount of solution will be the same.

I will do this experiment twice, and each time get the averages of the weights of the potatoes at the beginning and end of the experiment. Doing this will get more accurate results.

Obtaining

Table to show the 1st experiment

Table to show the 2nd experiment

Analysis

In the 1st experiment, as the concentration got higher, generally, the mass of the potatoes got lower, but the graph did not end up as a sigmoid curve which it is supposed to look like. In the 2nd experiment, as the concentration go higher, the mass of the potatoes got lower. The graph looks more like a graph with a sigmoid curve in but still, it is not exactly right, even though I did everything to make it a fair test and get the most accurate results.

My prediction is half correct. In some cases it is correct because the concentration is higher and the mass of the potato is lower, but in other cases the mass of the potato gets higher, which I did not predict. For example, on the 1st experiment at 2 and 4 concentrations the mass of the potato goes up and the same happens on the 2nd experiment at the 6% concentration.

Evaluation

We followed the plan correctly, I believe we gained accurate and sufficient enough results to conclude the experiment, and to prove our hypothesis.
My final results were very reliable, due to the precautions I took to make this a fair test.
To make this experiment better, I believe that we could have done one test at a time, so that we can reduce the time difference, when we have to move the potato from the test tube to the balance. Between this, we have to dry the potatoes just enough, and then put it on the balance. When we are doing this for one set, writing down the results at the same time, while the other 5 sets are on the tissue paper, the water outside the potato tissue is going to vary for all. Therefore, we would be able to concentrate more on one of the sets, instead of trying to finish all of them as quickly as we can.
We also could have got more people to do the experiment with us, so that we can organize the tasks, and we would be able to divide the tasks.
Using more types of molar sucrose solutions would have helped us obtain better results, and more accurate results, so that we can make sure the results are totally correct.
Experimenting with one set for a longer period of time, for each set, would lead us to better results, because the osmosis action would reach its maximum capability, and therefore tell us how much water could be transferred for each solution.
Repeating the same tasks many other times wouldn´t have been very useful, since we had already done the result 3 times, and ALL the results were reliable.
Even though we didn´t use these experimental plans, we still got results which were correct, according to my hypothesis, and backed up my predictions.
But overall, given the apparatus that we got to carry out the test, I think this experiment turned out to be very successful, and I´m very please with my results.