An investigation on Osmosis to determine the concentration of sugar molecules inside a potato.

Apparatus

1. Potato
2. Measuring cylinders
3. Water
4. Beakers
5. Sucrose  
6. Weighing scales
7. Potato borer

Variables

The variable in my experiment is the concentration of water and sugar solution. The sugar solution is in molarity. I have changed the water and sucrose so they add up to 100ml.To make it easy to convert the sucrose in molarity. For example 100ml is 1 molar, 20ml is .2 molars and 30 is .3 molar etc.

Safety

I will where safety goggles and use a special potato borer to cut my potato.

Scientific ideas

Osmosis is the diffusion of water through a partially permeable membrane and the basic rules of diffusion apply.

However, the direction of water transfer in or out of the potato depends on the difference in water concentration between the two sides of the membrane. Water will move into an area of lower concentration until the two concentrations equal each other.

By weighing my potato before and after soaking it in the solution I can determine if water is moving in or out of the potato. When the potato weighs the same before and after the soaking it means no water has moved in or out of the potato chip. Therefore, the sucrose solution must be equal to the concentration in the potato chip. This concentration of solution in my results tells me the concentration of sugar molecules in the potato.

Information Sources

I have used the following information sources:

1. The GCSE science classbook (Pg 12-13).
2. The Students guide to investigations sheet.
3. AQA science modular double award book. (Pg 24-25).
4. The biology teacher.

An investigation on Osmosis to determine the concentration of sugar molecules inside a potato

Collecting my data

To do all my experiment properly and get accurate results I had to use the right equipment. When I had collected my results I put them into a table. This is shown below. I collected my data carefully and wrote it in my table and I tried to record them clearly so there wasn’t any mistakes. I had to repeat one of my results because I had done it wrongly. This result was the first one.

Analysing and concluding

My Results

The readings I will take are the weight before the experiment and the weight after the experiment. This will show if the weight has increased or decreased by osmosis. I will calculate the percentage change in weight of the potato chip (by dividing the difference in weight by the start weight x 100) and draw a graph of the results. I have decided to use percentage change as this will give me comparable data to analyse.

Analysing my Results

My graph shows a negative correlation. I have found that at 0.2 moles (80% water 20%sucrose) the concentration of sugar molecules in the potato chip is the same as the concentration of the sucrose solution it was soaked in. The evidence for this is that the concentration is the same for the solution and the potato because the potato hasn’t changed in weight, which means that the water has not been exchanged either way by osmosis.

I have added a line of best fit, which indicates that the first half of the graph shows that the percentage change is proportional to the concentration of the solution. However, the second half of the graph shows a weaker correlation this could be explained because I had to do my experiment over two lesson and the second lessons results could have been rushed due to lack of time. My line of best fit does go through the 0.2 mole result which gave me a 0% increase in weight.

Conclusion

I think that my results have supported my predictions that when the potato is placed in a weak sugar solution the potato will gain weight and that when the potato is placed in a strong sugar solution the potato will lose weight.

This is because water enters the potato by osmosis going from a high concentration to a low concentration along a concentration gradient. The water moves out of the potato by osmosis going against a concentration gradient. However, there is no exchange of water when the concentrations of sugar molecules are equal in the potato and sucrose solution. (see diagram on the next page).

Diagram

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Evaluation

I think my investigation has gone well however, there was a problem with the accuracy of the readings which I took in the second lesson because I was rushed. The reading taken at 0.5 moles shows an extreme piece of data shown on the graph as –15%.

If I did my experiment again I would take the readings I already have and repeat them twice and take an average of all three readings to make sure my results are reliable and accurate. I think the first of half of my results were accurate but the second half of my results were not as accurate due to the remaining time of the lesson. I felt I had rushed those second results and therefore they look spread out on the graph.

I think my results and graph support my prediction and conclusion that:

• When the potato is placed in a weak sugar solution the potato will gain weight.
• When the potato is placed in a strong sugar solution the potato will lose weight.

Additionally my results and conclusion support my scientific ideas that:

• In a high concentration of water the water enters the potato by osmosis going from a high concentration to a low concentration along a concentration gradient.
• In a low concentration of water the water moves out of the potato by osmosis going against a concentration gradient.

• There is no exchange of water when the concentrations of sugar molecules are equal in the potato and sucrose solution.