Question

What should I write for my lab report in the Lab: Diffusion Across a Semi-permeable Membrane

Answer 1

SAMPLE LAB REPORT SCI141: Biology Sections
Introduction
All cells are bound by a functional cell membrane. The cell membrane functions to protect a cell and to regulate the movement of molecules and ions into and out of cells. All cell membranes are either semi permeable (permeable to only water) or selectively permeable (permeable to molecules based on some determining factor). Experimental conditions can simulate cell membranes to show their selective permeability. Dialysis tubing can simulate a selectively permeable membrane.
What a selectively permeable membrane can prevent (selectively) is the normal diffusion of molecules. Diffusion is the passive process by which molecules and ions will move from areas of high concentrations to low concentration. Dialysis tubing will be selectively permeable based on the size of the molecule trying to diffuse through the membrane.
This experiment will examine four molecules and their permeability. The molecules are, in order of increasing size, iodine, water, dextrose, and starch. Based on the size permeability of the dialysis tubing, iodine, water, and dextrose will be able to diffuse across the selectively permeable membrane. The large molecular size of starch will prohibit its diffusion.
In order to verify the results of this experiment, will have to be able to detect the diffusion of the molecules. Two tests exist in order to verify the diffusion of our molecules. 1) When starch and iodine interact the form a bluish black color. Therefore, anywhere a noticeable bluish black color change occurs, both starch and iodine will be present. 2) Benedict’s reagent, when applied to heat, will detect the presence of sugar (like dextrose). A positive Benedict’s test at the end of the experiment will verify the presence of sugar. Materials and Method
See attached handout.
Results
Before the Experiment After the experiment
Beaker Contents
- Starch/Iodine
Reaction
- Benedict’s Test
Water, Iodine
- Negative
- Negative
???
- Negative
- Positive
Cell Contents
- Starch/Iodine
Reaction
- Benedict’s Test
Water, Starch, Dextrose - Negative
- Positive
???
- Positive
- Positive
The iodine/starch reaction was only positive inside the cell, indicating that iodine had entered the cell, while starch did not diffuse out of the cell. A Benedict’s test was Positive both inside and outside of the cell. This indicates that dextrose was able to diffuse out of the cell. There was no test that measured the diffusion of water. Visual observation confirmed that the dialysis tube was more turgid after the experiment. This could be possible verification of the diffusion of water.
Conclusion
Water, iodine, and dextrose are all molecules that were able to diffuse across the selectively permeable membrane, based on their established concentration gradients. The results showed several tests that verified the diffusion of those molecules. This verifies the original hypothesis that molecules that fit the size requirement of the selectively permeable membrane (dialysis tubing) will be allowed to diffuse across their concentration gradients. Starch, however, failed to diffuse because it size must have been too large to be compatible with the selectively permeable membrane.

Answer 2

The process of diffusion across a semi-permeable membrane is known as Osmosis:

• Osmosis is the spontaneous passage of water or any other solvent through a semi-permeable membrane.
• Molecules from a less concentrated solution flow to a higher concentrated solution through the membrane.
• When the solution on either side of the membrane has the same concentration, it is known as an isotonic solution.
• When the concentration of the solution outside the membrane is higher, it is known as a hypertonic solution.

• When the concentration of the solution outside the membrane is lower, it is known as a hypotonic solution.