Potato Chips

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Biology Coursework

Skill Area P: Planning


Aim:  To investigate the influence of the concentration of the surrounding solution on potato tissue

Background Information:

Osmosis is a special type of diffusion. It is the diffusion of water molecules through a selectively permeable membrane, it goes from a place of a high water concentration to a place of low water concentration to try and make the two concentrations equal – this is called a concentration gradient but in the case of water it is known as water potential.

        

Cells placed in distilled water, absorb water by osmosis.  This happens because the cells contain dissolved chemicals and therefore they have a lower water potential than the surrounding liquid. Eventually the cells stop absorbing the distilled water even though the concentrations inside and outside the cell are not equal. This is because the cell wall of the cells become stretched and prevents any more water entering.  The cell is said to be turgid.

In a turgid cell (left diagram) the vacuole and cytoplasm swell causing the content of the cell to push against the cell wall. The cell wall resists this pressure and pushes back. The cell cannot take in any more water so consequently osmosis stops.

        

In a concentrated sugar solution the cells lose water by osmosis, as there is a higher concentration of water in the cell, water moves out of the cell by osmosis into the sugar solutions. This causes the cell to become limp; this is called a plasmolyzed or flaccid cell.

In a plasmolyzed cell (right diagram) placed in a concentrated sugar solution, the cytoplasm and sap vacuole shrink causing the cell content to pull away from the cell wall.

Prediction:

I predict that as the concentration of the sugar solution around the potato chip decreases, the size of the potato chip will increase.  I have come to this conclusion due to previous work I have done on osmosis in class.  This introductory work helped me make me a prediction on what would happen in this experiment.  To expand on my first prediction, I am also going to predict that as the sugar solution around the potato chip decreases, the weight of the chip will increase due to the amount of water being absorbed because of osmosis.

If we put a potato chip in a higher sugar concentration it will decrease the potato chip size and weight because water leaves the cells due to osmosis therefore, starving the cells of water and making the potato cells become flaccid.  In pure water the potato cells will expand, therefore making the chips increase in size and weight because the cells are absorbing water by osmosis making the potato cells become turgid.

In osmosis water flows from a higher concentration of water to a lower concentration.  For example – (assume the other parts are made up from a sugar solution) if there was one side of a selectively permeable membrane with 8/10 parts water and the other side was 4/10 parts water, the side 8/10 would flow from its higher concentration to the lower concentration in an attempt to even out the concentration on both sides (so both sides will have 6/10 parts water).  The water molecules are able to pass through a selectively permeable membrane because they are small enough to fit whereas the sugar molecules are too large to fit through.  

The blue dots represent water molecules and the red dots represent the sugar solution.



First diagram:
Right hand side:

Blue = 12
Left hand side:
Blue = 4
Red = 9

Second Diagram:
Right hand side:

Blue = 8
Left hand side:
Blue = 8
Red = 9

This diagram shows water molecules passing through a selectively permeable membrane to a lower water potential so as to even up the concentration on each side.

Apparatus:

Method:

  1. Core five potatoes chips using the potato corer.
  2. Using the scalpel and the ruler on the tile cut the potato chips to exactly the same size (30mm in length).
  3. Taking the test tubes label them A – E so as to get accurate results.
  4. Weigh each potato chip and record their weights in a table.
  5. Place each potato chip in a test-tube each containing 20cm³ (this should be measured with a measuring cylinder) of a different sugar solution.
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  1. Test-tube A: 0% (i.e. distilled water)
  2. Test-tube B: 5% sugar solution
  3. Test-tube C: 15% sugar solution
  4. Test-tube D: 20% sugar solution
  5. Test-tube E: 30% sugar solution

6.        Leave each potato chip in the test tube for 30 minutes.

7.        Remove the potato chips.

8.        Dry the chips with the tissues and re-measure the potato chip weights, record the weights in a table.

9.        Re-measure the potato chips to the nearest millimetre and record in a table.

10.        Empty and clean the test-tubes

11.        Repeat this method three times.

To make the sugar solutions you dissolve the percentage of sugar you want in ...

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