Over the last few classes we’ve been discussing Passive transport, or the movement of materials from high concentration to low concentration without the use of energy. There are three types of Passive Transport, all of which are related:

1. Diffusion: Movement of ANY material from High to Low concentration
2. Facilitated Diffusion: Diffusion of large materials that requires a transport protein to move across a membrane.
3. Osmosis: Diffusion of water across a membrane. In your cells, osmosis is a type of facilitated diffusion because it requires Aquaporins to move the water across the cell membrane.

Today in class we’ll be watching Osmosis actually happen with the help of the skin of some Red Onions. As you can see in the GIF, Red onion skin cell’s have a large central vacuole that contains purple pigments. In Isotonic environments, meaning there are equal amounts of water inside and outside the cell, the central vacuole stretches through the whole cell touching the cell walls. But! When the cells are doused with salt water…

The central vacuole’s water goes rushing out because it is now in a Hypertonic environment. This means that there is now more water inside the cell than outside the cell, and since water naturally moves from high concentrations to low concentrations, the water moves out of the cell and causes the central vacuole’s to shrink, as you see in the GIF (and in the lab). But now, how do we bring them back to their normal size?

Since there is now little water left in the cell if we place it in a Hypotonic environment, meaning there is more water Outside the cell than inside, the water will come rushing back in and fill the central vacuoles once again. Amazing.