To discover the sugar concentration of a potato.

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C.Grundy         26/4/03        Biology coursework

Sugar concentration of a potato

Aim- to discover the sugar concentration of a potato.

Scientific background-

I aim to find out this sugar concentration by placing slices of potato in different concentrations of sugar and finding one concentration where the potato doesn’t change in mass.  Due to osmosis if there is a difference in concentration between the sucrose solution and the concentration of solutes in the potato there will be a change in mass due to the movement of water.

Osmosis is defined as:

 ‘A special type of diffusion where there is a net movement of water across a selectively permeable membrane from a relatively dilute solution to a relatively concentrated one.’

Here is a diagram to show how osmosis works:

Therefore:

  • If the potato is placed in a solution of high solute concentration water will leave the potato, its mass will decrease and it will become soft. (flaccid.) This is because there will be a certain solution inside the membrane and a more concentrated one outside so molecules of water will leave in an attempt to equalise the concentration.
  • If the potato is placed in a solution of low solute concentration solution water will enter the potato, its mass will increase and it will become hard (turgid.)  This is because there will be a certain solution inside the membrane and a more dilute one outside so molecules of water will enter in attempt to equalise the concentration.

Before I made my prediction and finalised my method I carried out a preliminary experiment to test this theory, and to prove to myself that osmosis took place.

Preliminary Experiment 1.

  • I cut two cylinders of potato to 5cm long using a razor and cork borer.
  • I blotted dry their surfaces and discovered their mass.
  • I then placed each one in a test tube filled with a different thing- one in strong sugar solution and one in water.
  • I then placed a bung in then (to prevent evaporation) and left them for one hour.
  • Then I removed them from their tubes, dried them again and discovered their new mass.
  • Here are my results:

My preliminary experiment:

  • Allowed me to use a cork borer and razor as practice for my real experiment
  • Helped me find out the time to leave my potatoes for, because after leaving it for an hour I see a definite difference in mass.
  • Showed me how osmosis works and gave me some scientific information to help me predict what will happen in my experiment.
  • It also showed me that the sugar concentration inside the potato is nearer that of water than that of the strong sugar solution.  I can see this because only a small amount of water entered the potato to equalise the concentration with water, whereas great deals of water left the potato to equalise the concentration with the sugar solution.  Therefore the range of molarities should be of low molarity.

Prediction

  • If the potato is placed in a solution of high solute concentration water will leave the potato, its mass will decrease and it will become soft. (flaccid.) This is because there will be a certain solution inside the membrane and a more concentrated one outside so molecules of water will leave in an attempt to equalise the concentration.
  • If the potato is placed in a solution of low solute concentration solution water will enter the potato, its mass will increase and it will become hard (turgid.)  This is because there will be a certain solution inside the membrane and a more dilute one outside so molecules of water will enter in attempt to equalise the concentration.

From my scientific background information I now know that at a certain molarity, where the concentration of sugar is the same as that inside the potato, there will be no net movement of water. Water will move in and out of the partially permeable membrane, but will not move more in on direction than another because the concentration doesn’t need equalising and there isn’t one solution that is more dilute than the other.

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I find it difficult to estimate at what molarity that will be because I am unsure of the molarity of the 'strong sugar solution’ used in my first experiment.  However, my second preliminary experiment helped me discover roughly where it is- somewhere between 0.3 molarity and 0.4 which means that I will take more readings from around that point to make my graph more accurate.

With regards to a graph I expect mine to look like the one below, with the line crossing the x-axis at x molarity which will be the molarity of sugar inside the potato. ...

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