Investigating the Water Potential of a Potato

Investigating the water potential of a potato

Aim

Osmosis is the movement of water molecules from a region of high water concentration to a low water concentration through a semi-permeable membrane. Our aim is to investigate the water potential of a potato through osmosis, by finding the difference in weight before and after the potato has been set in a solution of water, sugar or salt.

Trial Method

Firstly we collected the equipment. This included a scalpel, potato cutter, and test tubes. Then we cut the potato using the potato cutter into cylinders. Then we used the scalpel to cut each potato strip down to size. To get each strip as close as possible the same length and width we used a ruler get correct measurements. Then we filled one test tube with distilled water and another 5 with different sugar solutions in increasing strength of molar, starting with 0.2 molar. After that we dropped our potato strips into each test tube at the same time and let them sit for 28 hours and 30 minutes. We poured the solutions out to get the potato strips. Then we carried them one by one over to the scales. We then weighed them to see if they had gained or lost weight. Once we had the results we calculated the percentage weight gained or lost.

Risk Assessment

Make sure all bags are out of the way

Make sure books are not obstructing the experiment

Tuck in ties

Do not sit down

Carry knives

with the head facing downwards

Tie back long hair

Wash hands after handling potatoes or equipment

Trail Apparatus List

Test tubes x6

Test racks x1

Water

Potatoes x2

Cylinder cutters x1

Crafting knives

x1

Ruler x1

Weighing machine

Variables To Control

There are several variables to control including volume of water. Firstly the volume of water needs to be kept constant to allow a fair test amongst all the potato strips because different amounts of water will determine how much of it will osmosize into or out of the potato strip. This applies to all the other solutions of sugar and salt. Secondly the length of the potato strips need to be kept the same. This is because the longer the potato strips the more volume and mass it is going to have therefore allowing more solution to enter. Also the width of the potato strips was important to keep the same. As the wider the strip the more volume and mass it has therefore having the same theory as length of strip. To keep the width the same we used the same width potato cutter. Although we kept the length and width of the potato strips equal, we did not bother to get an exact weight because it would have taken to long to weigh each and every strip. The weight of the potato would have made a difference because some heavier potato strips might be less denser than lighter ones therefore more solution could osmosize into it or water out of it. It would have been, no doubt, a waste of time. The average weight was 2.132.

Prediction

I predict that as the strength of the sugar solution increases the weight after the experiment will decrease. In the first few test tubes the potato strip will have a lower water potential than the surrounding solution. As books and the internet state, osmosis is the movement of water from a region of high water concentration to a low concentration through a partially permeable membrane (a diagram is shown below). This is because water will always try to reach equilibrium. The potato strip is the partially permeable membrane which will allow water to pass both ways, so if the water concentration is higher on the outside, the particles will move through the membrane so the net result is the water enters the potato strip and vice versa. Saying this, the water in the surrounding solution will move into the potato strip, therefore making it turgid. Whereas the last few test tubes would have a stronger solution meaning the water flows the other way making the potato strip flaccid, as it tries to achieve equilibrium.

Diagram

Pre-test results table

Molar of sugar Before experiment (g) After experiment (g)

0(distilled water as control) 5.84 6.67

0.2 6.05 6.59

0.4 5.62 4.48

0.6 5.8 3.98

0.8 5.8 3.51

1.0 5.74 3.47

From the table above it can be seen that as the concentration of the sugar decreased the weight of the potato strip after the experiment decreased also. This is because the higher the molar of the sugar, the more water will escape from the potato strip in order to achieve equilibrium.

The table below shows the percentage change of weight from before the experiment to after the experiment.

Molar of sugar Before experiment (g) % change in weight from before exp. To after exp. After experiment (g)

0.0 5.84 14.21 to 2 d.p 6.67

0.2 6.05 8.93 6.59

0.4 5.62 20.28 4.48

0.6 5.80 31.38 3.98

0.8 5.80 39.48 3.51

1.0 5.74 39.55 3.47

Average 5.81 25.64 4.78

Changes to be made to method

We will put a drop or two of oil on top of each solution to stop fungus and bacteria colonies

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