An experiment was set up as illustrated in the diagram below



DIFFUSION AND OSMOSIS EXPERIMENT

AIM: To determine the permeability of the ‘cell membrane’ to different chemicals.

THEORY: The cell membrane determines what substances can diffuse into a cell. This characteristic of a cell membrane is called permeability. Many cells are selectively permeable - some substances can pass through the membrane but others cannot. Dialysis tubing is an artificial semi-permeable membrane with similar properties to the cell membrane.

|Dialysis tubing |Glucose solution |

|Cotton |Sucrose solution |

|3 measuring cylinders (100mL) |Starch solution |

|3 measuring cylinders (10mL) |‘Clinistiks’ |

|Marking pen |Iodine |

|Cling wrap |Sodium hydroxide |

|Distilled water |Hydrochloric acid |

| |Rubberbands (very small) |

MATERIALS:

METHOD:

1. Cut 3 strips of dialysis tubing, each approximately 15cm long. Tightly tie one end of the tubes with a rubber band. Open one tube at the untied end by wetting your fingers and gently rubbing the end of the tube.

2. Pour 50mL of distilled water into each measuring cylinder. Label the cylinders A, B, and C.

3. Carefully pour 10mL glucose solution into one dialysis tube, making sure not to spill any glucose on the outside of the tube. Use a rubber band to tie the end of the tube, and tie the cotton on it. Ensure that there is no air in the tube, so that the tube is firm. Rinse the outside of the tube and then place this ‘cell’ into cylinder A, as pictured below. Cover cylinder with cling wrap.

4. Fill the second tube with Sucrose, using the same method as step 3. Place into cylinder B.

5. Fill the third tube with Starch, using the same method as step 3. Add 10mL of iodine to 40mL distilled water in cylinder C. Place tube into cylinder C.

6. Set the three cylinders aside for 24 hours.

| |GLUCOSE |SUCROSE |STARCH |

|Diagram | | | |

|Function | | | |

| | | | |

| | | | |

|Prediction | | | |

| | | | |

| | | | |

After 24 hours:

1. Measure and record the amount of fluid in each ‘cell’ (tube).

2. Measure and record the amount of water in each measuring cylinder.

3. For Cylinder A, test the water for glucose using a ‘clinistik’ strip.

4. For Cylinder B, test for sucrose by removing a small amount of the water, then add 5mL of HCl then 5mL of NaOH. What happens? Also test for glucose using a ‘clinistik’ strip.

5. For Cylinder C, record the presence or absence of starch by identifying the colour of iodine in the water. Did iodine enter the ‘cell’? Also test for glucose using a ‘clinistik’ strip.

6. Complete the following table:

RESULTS:

|Cylinder |Substance in tube|Volume of liquid in |Volume of liquid in |Movement of molecules |Movement of molecules |Glucose in |General |

| | |cylinder |tube |from cell to water? |from water to cell? |cylinder water? |observations |

| | |Before After | |Y or N |Y or N |Y or N | |

| | | |Before After | | | | |

|A | | | | | | | | | |

|B | | | | | | | | | |

|C | | | | | | | | | |

DISCUSSION:

What has happened in each of the three cylinders (refer to the substance in the ‘cell’, and the water in the cylinder)? Use scientific terminology in your response and try to explain WHY this has occurred.

________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

TEACHER NOTES:

THIS IS WHAT SHOULD HAVE HAPPENED…

Cylinder A: Glucose

An increase in the volume of liquid in the tubing and a corresponding decrease in the volume of liquid in the cylinder (or volumes may have remained the same due to movement of both water and glucose across the ‘membrane’).

We should also have observed glucose in the cylinder water as it was small enough to diffuse through the dialysis tubing.

So, glucose should have diffused out of the tube through the cell membrane; and water should have diffused by osmosis into the tube through the cell membrane.

Cylinder B: Sucrose

An increase in the volume of liquid in the tube and a corresponding decrease in the volume of water in the cylinder.

The water moved across the ‘membrane’ (down the concentration gradient) by osmosis to try to dilute the sucrose.

There should not have been any sucrose outside the ‘cell’ because the sucrose molecules were too large to pass through the holes in the dialysis tubing.

Cylinder C: Starch

Same as what happened in Cylinder B, but substitute sucrose with starch.

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Adapted from “An Australian Biology Perspective, Prac Manual”, p147.

HOMEWORK: Membranes can block molecules based on their size. Research and then compare the relative sizes of glucose, sucrose and starch molecules (include diagrams). What is the main function of each molecule? From this, make predictions of what you think will happen in each cylinder (PTO for response).

Cylinder C

Cylinder B

Cylinder A

10ml Starch

40ml

Water

+ 10ml iodine

50ml

Water

10ml

Glucose

50ml

Water

10ml Sucrose

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