Diffusion and Osmosis Lab Activity

Dr. Alan Rizzo
IB Biology SL Year 1
October 10, 2013

Diffusion and Osmosis Lab Activity

Activity A – Glucose concentration

Time (min) Observation
Glucose strip Lugol’s solution (Starch indicator) Dialysis tube of glucose starch solution
0 Test pad has green color, indicating 300mg/dL of glucose. Dark and transparent amber with tints of red. It is not uniform: contains dark spots of red color. Solution has translucent, pale, milky, cloudy white color.
3 Test pad is yellow with a tint of green in various spots, indicating the concentration of glucose as being less than 100mg/dL.
15 Test pad is very dark with green color, indicating glucose concentration of 1000mg/dL. Transparent, paler yellowish brown with tints of lighter shade of orange. Solution becomes less translucent and less milky.
Data collection

Conclusion and Evaluation
From this experiment, it is perceived that the size of molecules matter when passing through a semi-permeable membrane. The dialysis tube initially contained only starch and glucose molecules. But because it was submerged in Lugol’s solution that had higher concentration of Iodine for a period of time, as a result of diffusion, indicator solution’s molecules that consisted of iodine naturally moved from a region of higher concentration to a region of lower concentration. On the other hand, glucose and starch are both macromolecules that are large in size. Glucose molecules are smaller, and some of them were able to diffuse through the ‘dialysis membrane’ into Lugol’s solution while starch molecules were too large and remained in the dialysis tube. In the end, molecules in Lugol’s solution and glucose molecules were present outside the tube. Inside the tube, there were Lugol’s solution’s molecules, remaining glucose molecules and starch molecules. The concentration of all solutions is balanced inside and outside the dialysis tube consequently.
This experiment showed one systematic error. The dialysis tube, after being in amber-colored Lugol’s solution for a long time, should turn darker or black in color. Becoming darker in color indicates the diffusion of Lugol’s solution’s molecules that pass through the membrane. In order to clarify this error, more trials of this experiment should be conducted on several dialysis tubes, which will cause the data to be less anomalous and more accurate.

Activity B – Potato sticks and salt concentration

Hypothesis: The longer the time period potato sticks are left in salt solution, the more flexible they will become as most potato cells will be in the state of hypertonic solution.

Potato sticks Observation - Effects of salt solution on potato sticks)
Before After
• Rigid, bendable, rough
• Snaps broken
• Pale yellow
• Average length of 3 cm • Extremely flexible
• Paler shade of yellow
• Does not snap broken when bent
• Average length of 2.9 cm
Data collection

The experiment proves my hypothesis correct. This experiment causes potato cells to shrink as a result of osmosis diffusion. Initially, the concentration in potato cells is high, while the concentration of water in salt solution. Osmosis, movement of water molecules from a region of higher concentration to a region of lower concentration, caused water molecules within each cell to diffuse out into the salt solution in order to create equilibrium of concentration both in and outside the potato cells. Consequently of the decrease in amount of water in each cell, the flexible cellular membrane was pulled in and more wrinkled-like; the cell shrank in general. With cells now shrank and less rigid, potato sticks became more bendable and very flexible.

Activity C – Red onion cells and 2% NaCl solution

Hypothesis: If red onion cells are exposed to 2% NaCl solution’s drop longer, cell membranes will shrink further, as a result of becoming hypertonic solution.

This experiment demonstrated the process of osmosis and supports my hypothesis. By using the microscope, it was noticeable that red onion cells’ cell membranes were shrunk after a drop of 2% NaCl solution was put on the coverslip. It was the result of osmosis, which is the movement of water molecules from a region of high concentration to a region of lower concentration. When NaCl solution was dropped on the coverslip and entered red onion cells, the internal environment of each cell is more concentrated in terms of water. Consequently, the water molecules left the cells for the external environment, which had lower concentration of water molecules in order to create equilibriums inside and outside of the cell. The cell walls did not shrink as they serve as structure and are more rigid in comparison to the