Your lab group is experimenting with the diffusion of molecules across a membrane. Dialysis tubing is used as a model cell membrane, because is it semi-permeable, allowing small molecules to cross the membrane, but NOT larger molecules, like starch. A starch solution is placed in the dialysis tubing and then the dialysis tubing is placed in an iodine and water solution. Iodine is used as an indicator for the presence of starch. Normally, iodine is a yellow-brown color, but in the presence of starch, it turns a deep purple. After observing the experimental setup for 20 minutes, the iodine solution remains brown, but the starch solution inside the dialysis tubing is purple and swollen. Based on what you learned from the lab, starch is MOST likely to cross the cell membrane at which of the points indicated on the diagram?

In passive diffusion, the small water molecules can move across the phospholipid bilayer seen in blue. This layer acts as a semi-permeable or selectively permeable membrane; its hydrophilic heads are attracted to water (seen facing outwards) while its water-repellent hydrophobic tails face towards each other- allowing molecules of water to diffuse across the membrane along the concentration gradient.

Thus the water will move from an area of high concentration to an area of low concentration, until the system reaches a steady state called equilibrium- after this, there will be no net movement of water. Similarly via osmosis, the water passes through the membrane due to the difference in osmotic pressure on either side of the phospholipid bilayer this means that the water moves from regions of high osmotic pressure/concentration to regions of low pressure/ concentration to a steady state.

The dialysis tubing mimics a semi permeable membrane; it only allows water and small molecules of iodine to cross into the bag containing starch. The tubing is impermeable to starch; these large molecules require the aid of protein omplexes called membrane channels, in order to move across the membrane and against the concentration gradient.