Diffusion and Osmosis

Diffusion and osmosis are important concepts that explain how water and other materials that cells need are transported across cell membranes.

Let's talk about diffusion first. It is defined as the net movement of particles from an area of high concentration to an area of lower concentration. The graduated change in concentration between the two areas is referred to as the concentration gradient. Although diffusion is net directional, the particles are constantly moving in both directions due to random motion, so even at equilibrium when the particle density is the same throughout the concentration gradient, particles continue to move in both directions at a constant exchange rate.

Similarly, water moves across cell membranes by diffusion in a process called osmosis, but not everything can freely pass through cell membranes, which is why they are referred to as semi-permeable. This is important, because it means that cells can regulate and maintain different concentrations of solutes inside versus outside their membranes. Depending on the relative solute concentrations of solutions separated by semi-permeable membranes we refer to them as hypotonic, isotonic or hypertonic. Hypotonic is when the solute concentration is greater inside the cell in comparison to outside. Isotonic is when the inside solute concentration equals the outside concentration. Hypertonic is when the outside solute concentration exceeds the inside solute concentration. This can affect the movement of water into and out of the cell as water moves to the area of greater solute concentration. In turn, this can affect the shapes of the cells causing cell bloating in hypotonic solutions, no shape changes in isotonic solutions and the cell shrivels in the presence of hypertonic solutions.

The capacity for water to move into cells is different between plant and animal cells because of the presence of the additional plant cell wall. Cell walls are rigid and only permeable to small molecules. When water moves into plant cells the membrane gets pushed up against the cell wall creating hydrostatic or turgor pressure. This pressure limits the amount and rate at which water can enter the cell.

Diffusion is also a major limiting factor to cell size and helps explain why unicellular organisms are generally very small. Multi-cellular organisms are made up of many small cells giving a greater total surface area to volume ratio and increasing diffusion rates. Many aspects of our physiology, like breathing and digestion rely on diffusion. For example, human lungs have many small alveoli which are like small pockets. This extra surface area makes the lung more efficient at diffusion of gases in and out of the bloodstream.

In this lab, you will use two cell models, agar cube and dialysis tubing to test the principles of diffusion and osmosis.